Plastics Training & Technology Blog: Plastics Technology Archives

Screw Recovery Alarm

By Andy Routsis on September 2, 2010 9:59 AM 0 0 Vote 0 Votes

I was just asked…

Jim

"What is the value of having a minimum screw recovery alarm? I can see the maximum happening but the minimum I'm not so sure. I guess if someone puts the screw speed up to purge and does not put it back it may be useful. Is there anything else I might be missing?"

My Response

I believe you are correct Jim. Sometimes companies put alarms and options on the machine just because it's easy to program.

Additional Thoughts
The only benefit to such an alarm may be to monitor tampering of screw RPM but this is extremely rare.

-Andy

Calculating Residence Time

By Andy Routsis on August 27, 2010 10:08 AM 0 0 Vote 0 Votes

I was recently asked the question...

Kevin

"Do you have any way of easily calculating the residence time in screw / barrels?"

My Response

Most general purpose screws contain approximately 1-2 shots within the flights depending on the geometry. To calculate the residence time you will have to know your part volume, the volume of material in the barrel, and your cycle time. You can use a basic calculation for the barrel volume minus the screw volume (ignoring the flights because they are relatively small) to calculate the material your screw holds.

To calculate the volume of the barrel:

3.14 x (Barrel radius)^2 x (Barrel length)

To calculate the volume of the screw:

3.14 x (Root radius of the screw)^2 x (Barrel length)

Volume of material in the barrel/screw: Barrel volume-screw volume

As for the residence time:

If you calculate the volume of material in the barrel as 10 ounces and your part is 5 ounces your screw contains two shots of material. Multiply the number of shots in the barrel by your cycle time and you will get a residence time for the material.

For example: If you have a 25 second cycle time and your screw holds 2 shots the residence time is roughly 50 seconds (two cycle times).

The residence time will be a multiple of your cycle time plus or minus one cycle (depending on the ratio of part volume to barrel volume).

-Andy

Submarine Gating for Polycarbonate...

By Andy Routsis on August 25, 2010 7:45 AM 0 0 Vote 0 Votes

I received this interesting question through linked-in...

Linked-In Member

Currently we have provided edge gating for appx. 25 dia @ 1.3 thk mm PC part.

We are planning for tunnel or submarine gate... When using polycarbonate material, what kind of gating should be provide?

My Response

When utilizing a submarine gate with PC, there are a few things to consider:

The biggest mistake people make when sub-gating with PC is to use too low an angle. The gate should angle into the parting line at least 60º. This will minimize the amount of deflection necessary to remove the gate from the mold.

Another mistake commonly made is to make the gate too long. The gate should get to the mold cavity as quickly as possible. This will reduce the amount of deflection necessary to remove the gate from the mold, thus reducing the chances of the gate remaining in the mold.

Lastly, if you are having success with a current gate size, we recommend using a smaller cross sectional area for the sub gate since it is very easy to remove steel and enlarge the gate later.

Additional Thoughts

Pay close attention to your runner design, many molders will actually step the parting line behind the gate to allow more room for the runner system to bend as the gate is removed.

-Andy

Correcting for Cavity Imbalance...

By Andy Routsis on August 23, 2010 1:55 PM 0 0 Vote 0 Votes

I received this inquiry the last week...

Kevin

I have been using the intelligent moulder worksheets and have found poor fill balance in our hot runner tooling. I have had variations from 14 to 22%. How do you get to 6% or less?

note: The Dynamic Cavity Imbalance Test measures the imbalance by comparing the part weights from each cavity using three different injection speeds.

My Response

You should first look for inconsistencies such as blocked or damaged vents and gates.

You should review your hot runner system and determine if any gates can be adjusted. Often an adjustment can be made, or a him can be placed behind the gate drop to reduce flow in the mold cavities which are not filling well.

If the runner system has sharp turns, you can first try rounding the corners to reduce the stresses.

Additional Thoughts

Each mold can pose it's own unique circumstances. For branched runner systems, the ‘Melt Flipper’ can often be used, even with hot runner molds. This technology, licensed by Beaumont Technologies Incorporated, uses strategically placed inserts at runner branches. The inserts rotate the polymer melt to help manage the uneven stresses resulting from runner turns. I have seen this become very helpful in many different applications.

-Andy

"Steel Safe" Runner Designs

By Andy Routsis on August 18, 2010 11:22 AM 0 0 Vote 0 Votes

I was recently asked the question...

Jeff
We are about to build a 2 plate family mold but are not sure which molding machine we will use it in. We may also process two grades of material in the mold. How will these factors effect the size we should cut our runners?

My Response
Often, the feed system design is uncertain. You have mentioned two common issues which do effect the runner size.
The capability of your molding machine to fill the mold with a material whose flow characteristics are unknown should always be taken into account. If the runners are cut to large your machine may not be able to fill the mold.
There may be variation between the melt flow rates and pressures to properly balance a family mold, especially when two grades of material are being used.

In this situation I suggest you use a runner dimension that is "steel safe". "Steel safe" means your design should call for the removal of less mold steel than may be ultimately required. You may want to round down your runner dimensions by one or two standard sizes than what you calculated as nominal. This will result in a lower volume feed system with a greater pressure drop. There is a chance the smaller feed system will work and you will save on scrap material. However, if the undersized feed system is not optimal, this approach will allow you to easily machine the runners and improve the performance of your mold.

-Andy

The Need for Safety...

By Andy Routsis on August 14, 2010 2:12 AM 0 0 Vote 0 Votes

I am travelling overseas and witnessed a roadside welding facility where the workers were not wearing any type of eye protection. As most people know, this is a sure-fire way to lose your eyesight permanently...

My Concerns

Regardless of the country or region in which you live, it is critical that you promote safe working standards for your employees. I understand that different cultures take varying positions on worker safety, but some situations are just unsafe no matter where you live.

-Andy

Calculating Cooling Time...

By Andy Routsis on July 29, 2010 10:00 AM 7 Vote 0 Votes

I just received this question...

Vishal

How do I estimate the cooling time? I located a chart referencing a proportionality constant of a polymer, is this the right way?

My Response

There are many helpful charts and factors, but these use too many assumptions and are just ways to avoid proper circulations.

Proper cooling time estimations contain the following considerations:

Cooling time is is proportionate to the thickness squared
Cooling time is inversely proportionate to the thermal diffusivity
The temperature of the mold, melt, and part at ejection

One of the most common calculations are as follows:

tc = [(h^2)/( $\alpha\,$ )(π^2)] ln |(4/π)[(Tm-Tw)/(Te-Tw)]|

h = wall thickness

$\alpha\,$ = thermal diffusivity

Tm = melt temperature

Tw = mold wall temperature

Te = part temperature at ejection

If you perform a search on the internet, you can find automatic cooling time calculators which will quickly take all these factors in consideration once entered.

Additional Thoughts

Keep these factors in mind...

Always use your thickest portion of the part for these calculations
Assume your actual mold surface temperature will be higher than the temperature setting
Give yourself a 10-25% buffer based on the level of geometric complexity and amount of plastic shrinkage

-Andy

Getting a Spotting Press...

By Andy Routsis on July 27, 2010 1:30 PM 0 0 Vote 0 Votes

I received this question over the weekend...

Jason

A friend of mine recommended I get a spotting press for the molding department. What are their uses and should I consider getting one?

My Response

A spotting press is great for checking the parting line. This is a large hydraulic press specifically designed to open and close the mold.

Spotting fluid is placed on the parting line and the mold is closed under pressure using the press. Areas where the spotting fluid transferred from one half to the other are areas where the parting line is shutting off successfully. A parting line mismatch exists in areas where the spotting fluid does not transfer from one side to the other.

I recommend that any molder either have a spotting press, or access to a spotting press. If you have a toolmaking department, it is a necessary investment.

Additional Thoughts

If you are a small molder, or have a small tooling department just for repairs, then it might be a price advantage to bring the tool to a local tool shop and borrow their press (this will also expose you to their expertise in evaluating the results).

-Andy

Calculating Apparent Shear Rate...

By Andy Routsis on July 26, 2010 9:20 AM 0 Vote 0 Votes

I received this question form a colleague through Linked-In last week...

Colleague

How do I calculate the flow rate of the polymer from the known gate cross-section area?

My Response

The formula for the appearant shear rate through a rectangular gate is as follows:

Appearant Shear Rate = (6 x Q) / (W x H x H)

Q = polymer flow rate

W = gate width

H = gate thickness

Additional Thoughts

As long as the flow rate and gate dimensions are in the same units (mm, cm, or in) they will cancel out and you will receive a a resultant in the units of reciprocal seconds which will represent the rate of shear.

-Andy

Problems with Drool...

By Andy Routsis on July 22, 2010 10:50 AM 0 Vote 0 Votes

I received this question the other day...

AS

We are having problems with material leakage between the nozzle and the sprue. Is there some way we can adjust this?

Note: after some further questioning, it was determined that sprue break was not being used, the nozzle position has been aligned, and the diameter and radius are correct.

My Response

Basically, you need to increase the contact force between the nozzle and the sprue bushing. This will often relate to the carriage forward force or position.

-Andy

Intensification Ratios for Electric Molding Machines...

By Andy Routsis on July 21, 2010 2:31 PM 0 0 Vote 0 Votes

I received this question on our online chat feature earlier today...

Agustin

Can you please give me the IR (intensification ratio) for an electric molding machine?

My Response

Electric molding machines do not have intensification ratios because they lack hydraulic pressure. Your machine should be configured to provide the actual injection pressure.

Basically, an electric molding machine measures the injection force using a conversion from the torque applied during injection. This conversion provides the actual injection pressure on the control panel.

Additional Thoughts

he only time you may need a conversion is if the screw and barrel were replaced, and the machine was not re-programmed to accomodate.

-Andy

Appropriate Sensor Clearance...

By Andy Routsis on July 20, 2010 10:00 AM 2 0 Vote 0 Votes

This question highlights an often overlooked concern when using pressure sensors mounted underneath ejector pins.

Milan

How much of a gap should be used between the pressure sensor and the ejector pin?

My Response

The general recommendation from most manufacturers is between 0.25-0.5mm or 0.010-0.020in.

In reality, this specification refers to the clearance between the top of the ejector pin head and the counter-bore in the ejector plate. Most mold makers use a similar ejector pin clearance even when a sensor is not being used.

Additional Thoughts

The purpose of this clearance is to ensure there is no preload on the sensor, especially if any dirt gets beneath the sensor. Such a preload will destroy the sensor and provide erroneous pressure readings.

-Andy

Regrinding Sandy Plastic...

By Andy Routsis on July 16, 2010 11:25 AM 0 Vote 0 Votes

I received this interesting question recently...

Joy

I have a client with ten tonnes of material awaiting regrind and re-processing. Unfortunately, the material is contaminated with sand.

I have a couple questions. First, is it unethical to regrind and re-process the material since it is contaminated? Second, what are the financial costs and ethics associated with regrinding ten tonnes of material?

My Response

In general, once a material is contaminated with a hard material such as sand, it will become highly abrasive to the grinder, screw, barrel, hot runner, and mold. To regrind that much material in such a state will most likely cause significant damage, requiring repairs and possible replacement of many components. Likewise, such an abrasive material would damage the screw and most likely require a new check ring after re-processing.

If the reground material is sold or any products are sold molded with this material without the customer's approval such an act would be unethical and most likely illegal.

Since many of the costs associated with regrinding this material depend on the material, grinder, region, as well as repair costs, it is impossible to estimate with any degree of accuracy.

Additional Thoughts

This is one of the reasons it is critical to teach your employees the importance of proper handling of materials, parts, and regrind to ensure that contamination does not occur.

-Andy

Understanding Pressure...

By Andy Routsis on July 10, 2010 3:20 PM 0 0 Vote 0 Votes

Someone was reading one of my archived blog entries and asked me this follow-up question...

Tan

Could you please define pressure in a way that I can explain it easily to my technicians?

My Response

Pressure is a measurement of force spread out over an area.

Pressure = Force / Area

This means, if you apply the same force on a smaller area, you get more pressure.

Additional Thoughts

An easy way to demonstrate this by holding up a piece of paper.

First, ask the employee to push on the paper with the palm of their hand. Explain that they can apply a lot of force without the paper breaking. This is because the force they apply is distributed over the area of their palm.

Second, ask the employee to push on the same piece of paper with a pen or pencil with the same amount of force. Explain that the paper breaks because the force is applied over a much smaller area resulting in a much higher pressure.

-Andy

Moving a Mold from One Machine to Another...

By Andy Routsis on July 7, 2010 10:00 AM 6 0 Vote 0 Votes

I just got this question the other day...

MJ

When I want to move mold to another machine, which machine parameters should I check?

My Response

This question brings up the importance of machine independent documentation that includes part specific data such as part weight, fill time, coolant and melt tempertaures as well as the actual pressures applied to the polymer.

When you do this, you can ensure the actual process output from two different machines match. The most important factor is not the parameters that go into the machine, but the parts and associated outputs that result from the process.

Additional Thoughts

This topic is also addressed in the following blog: The Symptoms Of Machine Dependent Documentation

-Andy

Understanding Velocity-Control...

By Andy Routsis on June 30, 2010 12:15 PM 0 0 Vote 0 Votes

I received this question from a previous contributor...

Rafael

When you write velocity-controlled injection, what do you mean?

My Response

A velocity controlled process uses a velocity setpoint to control the rate at which the polymer is injected into the mold. Usually this is represented on the control panel as in/s or mm/s.

On a hydraulic machine, the hydraulic pressure will adjust to maintain the desired screw velocity. On an electric machine, the power applied to the injection servo-motor will be adjusted to maintain the desired velocity.

This is much better than injection using pressure control where a specific pressure is maintained, and the screw slows down as the mold fills.

Additional Thoughts

For more information on this, please visit: Open Loop vs. Closed Loop Controls

Andy

About Safety Glasses...

By Andy Routsis on June 28, 2010 10:15 AM 0 0 Vote 0 Votes

I was recently at a facility in which the employer did not require safety glasses because 'the machines are already guarded'...

My Thoughts

Although there are many regulations, and interpretations, most manufacturers and regulatory groups can agree that safety glasses should be worn under the following machine conditions...

1) Whenever a safety gate is opened, it is considered violated, and eye protection is required. This includes mold or die changes, machine maintenance, mold or die cleaning, clearing a stuck part, and semi-automatic operation.

2) Whenever polymer is purged from the barrel, high pressures and temperatures are being released. Both eye and face protection should be used in the form of a face shield.

-Andy

Complications With 1st - 2nd Stage Transfer...

By Andy Routsis on June 24, 2010 1:15 PM 2 0 Vote 0 Votes

A technician encountered this interesting problem yesterday...

Kevin

We have been working to achieve 95-97% fill during first stage fill with zero hold time and pressure. As soon as I apply hold pressure and time, the screw continues at speed after transfer. My machine has a hold speed minimum setting, but no option to turn this setting off. I am actually observing a pressure spike from 15 bar to 25 bar at transfer. To get an acceptable part, I need to profile a drop in injection speed before transfer as well as increase the transfer position in 5mm increments until the pressure spike at transfer was eliminated.

Can you offer any thoughts as to why this is happening? Is there something I missed?

My Response

Many machines have a bad habit of allowing the screw to overshoot when 2nd stage pressure is turned on, but stop immediately when 2nd stage is turned off. On most of the newer electric and counter-pressure molding machines you can actually watch the screw stop instantly when the second stage is turned off. Unfortunately, a few of these fancy machines are factory set to overshoot the moment the second stage is turned on... this means that the same screw which stopped on a dime will now overshoot and flash just by turning on the second stage time (even when the pressure setting is at zero).

Basically, this is a silly attempt to mimic older hydraulic machine behavior. In such a case, you should turn on the second stage time to a normal value, then set the lowest value possible for pressure and minimum speed. You should now see the screw overshoot on your particular machine. This will allow you to establish a more representative 1st stage short shot.

On any machine, you must establish a 1st stage short shot which represents the machine's behavior during production... this will allow you to fill and pack out the part more consistently.

note: Whenever possible, you should turn off the ability for these newer machines to overshoot. The more accurately the machine performs the desired actions, the more consistent the process.

Additional Thoughts

Although each machine is different, many newer machines have the option to turn on and off the injection overshoot such as a choice between sharp or gradual transfer. Other machines may have this option buried in the maintenance portion of the controls. You may have to contact the manufacturer to find out where this option is located.

-Andy

Balancing Family Molds...

By Andy Routsis on June 23, 2010 3:05 PM 0 0 Vote 0 Votes

A common blog reader asked this question regarding family molds...

Jim

What is the best way of balancing a family mold?

My Response

When balancing family molds, it is best to approach it in two steps.

Step 1 - Balance the runner system - When possible, try to balance the runner system so that each gate starts filling the mold cavities at the same time. Keep in mind, different rates of fill will affect this balancing.

Step 2 - Balance the gates - After you have determined that all the mold cavities have begun filling at the same time, then adjust the gate dimensions to balance the filling of the mold cavities.

In the best case scenario, you could... 1) short shot the runner system and see all the mold cavities begin to fill at the same time, and 2) end filling with a short shot on each mold cavity, so that all the cavities can complete filling and begin packing under pressure control.

Additional Thoughts

Most people only adjust the thickness of a gate to restrict or promote flow, yet this has a significant impact on the shear thinning of the polymer. Adjustments to the length, width or number of gates can often promote or restrict flow, yet have less impact on the shear heating and thinning of the polymer.

-Andy

Same Supplier... Same Grade... Bigger Dimensions...

By Andy Routsis on June 21, 2010 10:35 AM 0 0 Vote 0 Votes

I received a follow-up question regarding a blog entry last week...

ED

We switched our product line over to 4710 PE 100 extruder grade PE about 2.5 years ago. The material was produced in Belgium. We sized all of our new tooling to this material.In February they started making the material in Texas. The supplier says the material is the same. All of my processes were established with Decoupled II process using cavity pressures sensors.

Machines, mold, melt, and barrel temperatures are the same.

When I match the templets from before the dimensions of the parts are too big. I think they have changed the carbon black. I have suspected that a Nucleating Agent has changed. If it has changed the crystalinaty it will affect the shrinkage. Can this be in the Carbon Black? Or can the Nuculating Agent be some where else?

My Response

Although a change in nucleating agent may have occurred, it is more likely that there is a change in the polymer. During polymerization, high density polyethylene can grow, branch, and propagate in many ways. Different material manufacturing conditions can result in a change in Average Molecular Weight, Molecular Weight Distribution, as well as the amount of branching even though the melt flow index is the same.

First, I would request some rheological data from the material suppliers from both facilities. If you can compare some detailed shear rate and viscosity data, it can help you better determine the differences between the grades.

You may wish to investigate the difference in rheological characteristics between both the old and new. A Differential Scanning Calorimeter, or DSC, can be used to determine the relative degree of crystallinity between the samples from your past and current production lines. This may also give you some data regarding any differences in additives.

Additional Thoughts

If you really want to delve into the material, you can use a capillary rheometer or parallel plate rheometer to measure the viscosity characteristics across multiple decades of shear using different temperatures. These types of tests will provide very accurate information regarding the behavior and characteristics of the material.

-Andy

The Purpose Of Feedthroat Cooling...

By Andy Routsis on June 17, 2010 9:50 AM 0 0 Vote 0 Votes

This question arrived via email yesterday...

John

Why is there water running through the barrel of an injection molding machine?

My Response

The basic purpose of the water is to cool the feedthroat to avoid bridging. The feedthroat is the area of the barrel where the material passes through from the hopper the screw.

If this area gets too hot, the material may clump or stick together, creating a blockage known as bridging. If the area is cooled too much, condensation may occur resulting in visual defects or a change in mechanical properties. Condensation in the feedthroat most often occurs during a production delay, such as a mold or material change.

Additional Thoughts

For a related entry on feedthroat cooling, please review: Feedthroat Temperature Differences...

-Andy

How Colorant Affects Shrinkage

By Andy Routsis on June 15, 2010 10:05 AM 2 0 Vote 0 Votes

I received a detailed question via email the other day, I will do my best to convey the nature of the question in this blog...

Kevin

I am running as part in acetal and am having difficulty in maintaining the same dimensions as natural when running colorant:

When molded with natural: overall length = 49.33

Natural + yellow 130C: overall length = 49.32

Natural + green 326C: overall length = 49.20

To increase the overall length of the green part...

Mold temp was increased from 60 to 70ºC

Hold pressure was increased 65 to 110 bar

Hold time was increased to 6.0 to 7.5 seconds

As a result, the part was the correct length, but the weight increased from 23.96 to 24.29 grams.

My question is, is there any technical information regarding colors, pigments, etc. and their affects on materials?

My Response

There is much information available online, but there are a few things that you can also investigate in-house and with your supplier.

1. Any additive will change the melting characteristics of the polymer. You should always perform a tact temperature study as well as check and document the temperature of your melt with each polymer/additive combination. Some additives may cause the polymer to stick to the screw rather than the barrel, requiring an unnecessarily high screw speed to recover the shot. Adversely, one colorant may cause the polymer to melt very smoothly, resulting in a lower melt temperature. To better explain this, one of my customers (who processes only one base resin) showed me a screw with different colored stains along the transition zone. Each stain indicates the melting characteristic of that material/colorant combination. Some colors caused softening near the feed zone, while others softened closer to the metering zone with a rainbow of colors in between.

2. In additional to documenting the process outputs such as fill weight, peak pressure at fill, back pressure, etc. you should also consider taking a picture of the short so that you can visually match the appearance of the colored first stage short shots to the natural first stage short shot. Although most additives act as plasticizers, some additives will actually increase the viscosity of the polymer melt... especially if it reduces the melt temperature. This consequence can be improved by optimizing the screw recovery for each formulation you use.

3. Contact your material supplier and request any Technical Service Bulletins regarding acetals and/or the colorants you use. Also request drying specifications and annealing suggestions. In many cases, materials like acetals will provide more consistent dimensions when annealed.

Additional Thoughts

In semi-crystalline polymers, such as nylon, colorants and additives can also affect semi-crystalline site nucleation and growth.

For, more information about tact temperature studies, please read: Optimizing Screw Recovery...

For free multimedia presentations on process documentation, scientific molding, and in-mold rheology, please visit: http://www.traininteractive.com/free/webinar/player/

-Andy

Common Gate Sizings...

By Andy Routsis on June 10, 2010 12:35 PM 0 0 Vote 0 Votes

One blog reader asked this question...

Larry

Would you please give me the guidelines for runner and gate sizing with respect to nominal wall thickness?

My Response

Below are some common guidelines...

An edge gate should be approximately 1/3 of the part thickness at the mold cavity.

The fan and flash gate should taper down to approximately one-quarter of the part thickness.

Disc gates taper down to less than one-quarter of the part thickness at the mold cavity.

Pin gates are commonly machined to 1/8 of the part thickness.

3 plate pinpoint gates taper down to a diameter of approximately 1/3 of the part thickness as they approach the cavity.

The submarine gate diameter at the mold cavity should be approximately 1/3 of the part thickness. This gate should have at least a five-degree taper per side so it can be removed without complications.

Additional Thoughts

Unfilled, semi-crystalline materials typically require smaller than average gates, while hard to process amorphous materials such as polycarbonate may require larger than average gates.

-Andy

Back Pressure Setting...

By Andy Routsis on June 8, 2010 10:00 PM 0 0 Vote 1 Vote

I received this follow-up question the other day...

Kevin

Once I optimize my rear temperature, what back pressure should I use?

My Response

Back pressure serves two important roles... 1) to help ensure proper material mixing. 2) to help ensure a homogeneous melt compression in front of the screw. Your back pressure should be high enough to provide both of these.

The first factor is easily determined by part quality and inspection.

The second factor is determined by the consistency of your recovery time.

Additional Thoughts

Keep in mind, you may not receive a consistent melt recovery until you first optimize your rear temperature zone.

-Andy

Using Shut-Off Nozzles

By Andy Routsis on June 4, 2010 9:20 AM 0 0 Vote 0 Votes

In a recent seminar, I was asked the following question...

Participant

We often use shut-off nozzles to prevent drooling, are there any other reasons I should use a shut-off nozzle?

My Response

Shut-off nozzles are a great resource to a technician. When processing low viscosity materials, they can reduce the need for excessive decompression. They can also improve material mixing as they may allow for the use of higher back pressures during recovery. Some molders use shut-off nozzles to improve the consistency of the melt density. Shut-off nozzles can also be used to improve cycle time in high-speed applications because the machine can build the next shot while the mold is open.

Another benefit to shut off nozzles is safety... shut off nozzles help prevent the material from exiting the barrel while the mold is open. This helps when a mold is being serviced as well as when an operator is manually removing a part from the mold.

Additional Thoughts

Shut-off nozzles can also be helpful in applications such as micro-cellular molding, liquid silicon rubber, or when using chemical foaming agents.

-Andy

Identifying A Pressure Limited Process...

By Andy Routsis on May 30, 2010 7:50 PM 0 0 Vote 0 Votes

This question highlights a common point of confusion...

Bob

How do I determine if my process is pressure-limited using my machine's pressure curve?

My Response

The pressure limited process will have a plateau on the pressure curve prior to transfer. This will also correspond with a drop in the velocity at the time of pressure limiting.

A good way to test this is to increase the maximum pressure, and see if the peak 1st stage pressure increases. If the peak pressure increases with an increase in maximum allowable pressure, then the process was pressure limited.

Additional Thoughts

For more about this topic, please review: Providing a Buffer To Accommodate for Variation

-Andy

When to Torque Mold Bolts...

By Andy Routsis on May 28, 2010 5:15 AM 0 0 Vote 0 Votes

A blog reader asked a great 2-part question regarding die setting...

Jim

During the installation of a mold, is it a good practice to build tonnage prior to applying torque to mold bolts, or energizing a magnetic platen?

Is there anything wrong with torquing the bolts or energizing the magnetic platen before torquing the bolts?

My Response

Regarding the first question: You should always secure the mold by torquing or energizing before applying full clamp tonnage. The mold should be fully closed without clamp tonnage when the bolts are torqued or magnetic platen energized.

Regarding the second question: The purpose of the clamps or magnetic platen is used to support the full weight of the mold when it is open or without clamp pressure as well as align the two mold halves. Since both the mold base and platen flex during full tonnage, securing the mold while it is under full clamp tonnage may result in an improperly aligned mold base or inadequate torque.

-Andy

Deciphering the Different Screw Tips...

By Andy Routsis on May 27, 2010 6:00 AM 0 0 Vote 0 Votes

During a discussion with his fellow employees about screw tips, this blog reader was asked and forwarded this question to me...

Tim

We were discussing the different types of screw tips: sliding ring (check ring), poppet valve, ball valve and smear tip. I was asked, under what circumstances would you use each type and I feel I could not give a good answer. I was wondering if you maybe able clarify this for me?

My Response

When discussing non-return screw tips in general, there are two competing aspects: backflow vs. material stagnation.

In general, tips with high backflow are not very well suited for high precision molding or materials with very low viscosity. Adversely, tips which cause material stagnation have dead-spots where material can become caught and remain indefinitely. Material stagnation is avoided when processing heat sensitive materials or products which require frequent material or color changes.

Smear Tips - High Backflow & Low Stagnation - These screws are ideal for high viscosity, heat sensitive materials such as PVC.

Ball/Poppet Valves - Low Backflow & High Stagnation - These tips are ideal for low viscosity materials which are not heat sensitive such as polyolefins.

Check Ring - Moderate Backflow & Moderate Stagnation - These tips are the most widely used non-return valve because the versatility of applications which can process using check rings.

Additional Thoughts

Keep in mind, the discussion above focuses on the general uses of these screw tips. There are a wide array of screw tip designs for each of the general categories above. I have seen some check ring designs which have very little backflow, and some ball valves which have very little stagnation.

-Andy

Can A New Machine Reduce Injection Pressure?

By Andy Routsis on May 26, 2010 9:00 AM 4 0 Vote 0 Votes

I often get asked about the interesting processing claims promoted by machine manufacturers.

Tony

One machine manufacturer is claiming they have a control system which reduces the pressure necessary to fill the mold ad therefore reduce the necessary clamp pressure. Is this correct?

My Response

If you are using a good stable molding process with a slight short shot during first stage fill, then you will not see a significant change with the use of a new machine due to two factors: 1) A specific amount of pressure is required to force the material to the end of fill, regardless of the machine manufacturer, and 2) a short shot during fill allows or variation in the degree of overshoot that occurs during the transition from first stag fill to second stage pack. As a result, the amount of pressure necessary to pack thew mold will not change, thus requiring the same amount of clamping force.

Additional Thoughts

Anyone who prefers to mold using a completely full part (thus packing the mold during first stage) might see a drop in necessary clamping force because the machine controls will provide better control over the overshoot of the injection unit, resulting in a more consistent packing during first stage.

-Andy

Hybrid vs. All-Electric Molding Machines...

By Andy Routsis on May 21, 2010 1:34 PM 2 0 Vote 0 Votes

I received an interesting question from recent webinar attendee...

Tony

We currently have 18 machines, all from one manufacturer, but the local service agent is not providing good service. As a result, we will be looking at purchasing a different brand of machine. As a contract (custom) molder, we run a variety of materials and products including some medical.

We were avoiding all-electrics because some of our molds have hydraulic cores and cylinders, and we also heard that some machines have low nozzle contact force.

Although we are pro-hybrid, (the all-electric is 15-20% more expensive) we would like your thoughts on the debate between electric vs. hybrid molding machines. We have a view that electric machines are ideal for lower running costs and lack of oil, would we be making a mistake if we don't move to electric molding machines now?

My Response

There have been some large advances in hybrid molding machines over the years. These machines gain many of the benefits of electric molding machines, but have a slightly smaller cost. Initially, these machines were believed to be a great compromise, but the smaller initial cost is quickly offset by the increased costs to run the machine. Aside from using more electricity to operate the machine, these machines require 2-3 times more water to keep the hydraulics cool, and require more routine maintenance for items such as filters and hydraulic fluid.

I have seen many molders replace hydraulic mold actions with comparable electric components. If this is not an option, there are many hydraulic units which can be easily added to the molding machine for powering a hydraulic core.

As for nozzle contact force, I recommend you discuss this with the manufacturer and determine how they overcome this potential issue. Also, check with the manufacturer to determine whether the contact force motor can be upgraded if necessary.

Additional Thought

Some molders I know have purchased both hybrid and all-electric molding machines from the same manufacturer. In each case, the all-electric machines became the preferred machines.

-Andy

Sharp Transition In Rheology Curve...

By Andy Routsis on May 19, 2010 4:45 PM 0 0 Vote 0 Votes

After performing an In-Mold Rheology Test, one blog reader had this question...

Jason

When performing the in-mold rheology test, I noticed a sharp drop when reaching a specific shear rate. After this point, the viscosity stabilizes at the lower value for the remainder of the higher shear rates. What is the cause for this abnormal viscosity curve.

My Response

Although this seems strange, it is not uncommon. Since changing the injection rate causes a difference in the amount of material entering the mold during fill, this might occur when molding a part where the highest pressure losses occur near the end of fill. You might see a drop in pressure if the part thickness increases, and a rise in pressure when the thickness decreases at the end of fill.

Additional Thoughts

In most cases, you will see a smooth in-mold rheology curve.

-Andy

Converting ºC To ºF...

By Andy Routsis on May 17, 2010 9:15 AM 0 0 Vote 0 Votes

A technician called our office the other day and asked this question...

Steve

How do I convert 250 degrees Celsius to Fahrenheit?

My Response

This can be done using any calculator, the two basic equations are as follows:

(ºC x 1.8) + 32 = ºF

(ºF - 32) / (1.8) = ºC

Therefor:

(250ºC x 1.8) + 32 = 482ºF

Adversely:

(482ºF - 32) / 1.8 = 250

Additional Thoughts

One way to remember this relationship is 1 degree Celsius equals 1.8 degrees Fahrenheit plus 32 (since water freezes at 32ºF rather than 0ºC).

-Andy

Comparing Machine Capabilities...

By Andy Routsis on May 13, 2010 1:50 PM 7 0 Vote 0 Votes

This query came in the other day through email...

Emailer

Today, I moved a mold from one machine to another. The peak injection pressure was 1450 bar with a 0.6 sec. cycle time. On the second machine, I can only get 1.03 sec. injection time at 900 bar. I cannot fill the mold even though the maximum available pressure is 1800 bar.

My Response

It is likely that your new machine lacks the injection speed in cubic millimeters per second to fill the mold.

The best way to approach this is to first determine the 'Shot Volume Factor' for your two machines (see Understanding Shot Volume Factors... for more about this). This factor converts the movement of the screw into the volume it displaces (this is expressed in mm^3/s).

If you multiply this factor by the injection speed used in your old machine, you can determine the volumetric injection rate in mm^3/s or cubic millimeters per second. If you divide this result by the volume factor of the second machine, you can determine the approximate injection speed necessary on the second machine. This will allow you to accurately compare the injection capacity of the two machines.

Additional Thoughts

Many of the newer molding machines are now providing much more helpful information such as the volumetric injection rate and displacement as well as the actual pressure being applied to the plastic during injection, pack, and recovery.

-Andy

Time Required for Process Stabilization...

By Andy Routsis on May 12, 2010 6:50 AM 0 0 Vote 0 Votes

This question highlights a common point of disagreement within the molding industry...

M.

How long does process stabilization take, from set-up to first good parts?

My Response

With the explosion of SMED, Single Minute Exchange of Dies, there are many companies who have successfully reduced their changeover time to a matter of minutes or even seconds. Unfortunately, the startup time is not always as fast.

Generally speaking, most processes take around 5-10 minutes to stabilize. This typically occurs when the flow length and the ability to fill the mold is not significantly affected by the temperature of specific mold components. You can still shorten this time to stabilization by taking steps like the following:

Pre-conditioning the tool temperature before installing it into the machine
Pre-heat the hot runner system before installing it into the machine
Use insulation between the mold and the machine platens
Purge regularly when the machine is idle to help stabilize the melt temperature

High speed molding, and thin wall molding processes where you are processing with a much tighter window, stabilization can take up to 2 hours. When such a situation takes place, it is important to document the variations that take place and develop a time schedule to adjust for this. For example, if the cores take an hour to stabilize, then you might be have to use one 2nd stage packing pressure and 1st stage speed for the first 15 minutes, and then drop them down each 15 minutes until the process stabilizes.

Additional Thoughts

Always document the process using the process outputs. This will help you better determine when the process achieves stabilization.

For more on this, I recommend: Process Inputs vs. Process Outputs

-Andy

Understanding the Dynamic Load Sensitivity Test...

By Andy Routsis on May 10, 2010 12:35 PM 2 0 Vote 0 Votes

I got this request during the weekend...

Tim

I would like an explanation on how to perform a Dynamic Load Sensitivity Test on Hydraulic Molding Machines.

My Response

Although we have training courses dedicated to this topic, I will try to give a brief overview here:

The Load Sensitivity Test is used to determine how the injection flow rate is altered by changes in molding conditions - such as material viscosity.

To test a machine’s ability to compensate for changes in material viscosity, two shots are made under different loads. This is done by injecting one shot into the mold and one shot into the air.

The objective of this test is to compare the two shots and calculate the load sensitivity of the machine. This is expressed as either:

- A percentage of change per thousand psi hydraulic pressure

- A percentage of change per ten thousand psi plastic pressure

Additional Thoughts

For a webinar about machine, mold, and process evaluation... please review:

Practical Scientific Molding Techniques

http://www.traininteractive.com/free/webinar/player/

For machine, mold, and process evaluation training and worksheets, please review:

The Intelligent Molder Series

http://store.traininteractive.com/ProductDetails.asp?ProductCode=wbt.im.as.ims

-Andy

Processing Old Fashioned Rubbers...

By Andy Routsis on May 9, 2010 7:45 AM 0 0 Vote 0 Votes

This unique question was submitted by one of our more active bloggers...

Milan

What would be the difference in injection molding old fashioned rubber-like materials and the newer polymers like PBT?

My Response

Although I have only had a few opportunities to process natural rubbers, it is an interesting experience.

Processing PBT - Polybutylene Terephthalate, PBT, is a semi-crystalline polymer which is heated above the Melting Temperature (Tm) before processing. In this state, it flows very easily and is usually melted using a straight or reverse barrel temperature profile. Because the polymer is processes above the melting temperature, it flows relatively easily. Most amorphous synthetic polymers such ABS or Styrene, do not have a melting temperature, but are processed above the Glass Transition Temperature (Tg) which is a softening temperature present in all polymers.

Processing Natural Rubbers - Natural Rubber, or Polyisoprene, is an amorphous polymer which is often processed at a temperature near or below it's Glass Transition Temperature. This makes it more difficult to mold, resulting in a behavior similar to the molding of many PVC materials. As with PVC, you tend to use a forward temperature profile, and require a special screw configuration, often with a high flow check ring... or no check ring at all.

Additional Thoughts

The molding of natural rubbers can be a messy business and there are many variations of these materials depending on the degree of polymerization and molecular weight distribution.

-Andy

Understanding Shot Volume Factors...

By Andy Routsis on May 6, 2010 10:55 AM 0 0 Vote 0 Votes

I received this interesting question from a blog reader the other day...

Leslie

I recently came across this term: Shot Volume Factor and the units are in^3/in (cubic inches per inch). I speculate that it is some kind of volume conversion number that when used will allow a user to quickly convert shot size from machine to machine. What is this term and how is it calculated?

My Response
As the screw travels forward, it displaced a specific amount of volume as it travels forward. This factor is a machine-specific value which converts the linear displacement of the screw into the volume displaced within the barrel. The shot volume factor can be calculated by determining the
surface area in front of the screw.

Just multiply pi times the radius squared: π*(r^2)

Imperial Example:
1.2 inch diameter screw
π*(r^2) = (3.14)*(0.6in)*(0.6in) = 1.1in^3/in

Metric Example:
30mm diameter screw
π*(r^2) = (3.14)*(15mm)*(15mm) = 707mm^3/mm

Additional Thoughts
Once determined, just multiply the distance the screw travels times the factor to determine the actual displacement of the screw.

Adversely, if you know the estimated shot volume, just divide the volume by the shot factor to estimate the linear displacement necessary to fill the mold.

Calculating this for each machine can be a great way to help your employees quickly identify the differences between each machine.

-Andy

6-Axis Robotics For Injection Molding...

By Andy Routsis on May 5, 2010 10:25 AM 2 0 Vote 0 Votes

I recently had a discussion with a friend of mine regarding robotics...

Al

We would love to use robotics, but our parts are very complex, and cannot be removed by a simple robot.

My Response

6-Axis articulated robotics are becoming increasingly common because of the following reasons:

1. They can perform complex movements - This allows injection molders to remove complex geometries from the mold, as well as simplify automation by allowing the robot to peroxide some of the secondary operations.

2. They are very reliable - Believe it or not, many 6-axis robotics are as reliable as their 2, 3, and 4-axis counterparts.

3. They are inexpensive - I have seem some 6-axis robots that are less expensive than many popular 4-axis robots.

Additional Thoughts

6-axis robotics tend to be slower than their 2, 3, and 4-axis counterparts. In most cases, the difference in speed tends to be less than 1 second increase for 6-axis robotics.

-Andy

Processing With Older Injection Molding Machines...

By Andy Routsis on April 29, 2010 1:25 PM 0 0 Vote 0 Votes

I received this question today from a blogger...

Blogger

How should I proceed, when I want to adjust the process on an older pressure-controlled machine (not velocity controlled)?

My Response

Although each of these older machines have their own specific limitations... there is a basic strategy to use with open-loop pressure-limited injection molding machines:

1) Fill and pack using first stage pressure - The mold should be completely full with small sinks near the gate area

2) Transfer using time - Using second stage hold to eliminate any sinks at the gate area.

Additional Thoughts

Many of these molding machines have really accurate limit switches on the injection unit. Unfortunately, the variability in mold filling as a result of viscosity changes can often cause a process to bounce between shorts and flash.

-Andy

Load Sensitivity Test On Electric Molding Machines...

By Andy Routsis on April 27, 2010 6:00 PM 0 0 Vote 0 Votes

I was asked this question the other day...

Milan

Does it make any sense to perform a load sensitivity test on a electric machine?. I did few tests, but results were very good, range 0,016% to 0,105%.

Note: The Dynamic Load Sensitivity Test measures how much the injection time shifts with a change in resistance.

My Response

Most all-electric molding machines perform very well in the Dynamic Load Sensitivity Test. In most cases, a variation over 1% is very uncommon in newer machines.

In general, load sensitivity is not a concern with all-electric injection molding machines. Such tests are helpful once or twice a year as part of the general machine maintenance program. This will help you maintain a record of the machine's performance, and will help you monitor or detect any degradation in performance over time.

Additional Thoughts

for more about the benefits of all-electric molding machines, please review: Are Hydraulic Machines Going the Way of the Dinasour...?

-Andy

Transferring on End of Fill...

By Andy Routsis on April 26, 2010 9:30 AM 0 0 Vote 0 Votes

I received a blog response which highlights a growing trend in the molding industry...

Blogger

I am using a pressure sensor at the End of Fill on a new all-electric molding machine. With a DIII process, I am having good success in using the pressure at the end of fill for V/P transfer. This works well with a standard deviation of 0.07mm for cushion.

note: For more on a DIII process, please read: Rules for a DIII Process

Additional Thoughts

The advent of highly-responsive all-electric injection molding machines is allowing people to control the process in very unique ways to fit a specific situation. Using the cavity pressure at the end of fill to transfer from velocity control to pressure control can be a great way of controlling the amount of pressure which is received within the mold cavity during the mold cycle.

The biggest concern with this approach is the response of the machine. Most hydraulic molding machines lack the response time to facilitate the correctly use of an end of fill sensor.

-Andy

Results of Screw Wear...

By Andy Routsis on April 21, 2010 7:20 AM 2 0 Vote 0 Votes

I received this follow-up question in a recent blog and decided to answer it in more detail using the blog...

MJ

Which kind of process parameters vary when the screw becomes worn?

My Response

Basically, the most common affects that arise from screw wear relate to the increased back-flow over the flights of the screw. This back-flow increases mixing, but also lowers material conveyance and increases residence time distribution.

As the screw wear increases, the number of revolutions necessary to recover the shot will also increase... thus resulting in more shear heating, a higher melt temperature, and possible polymer degradation.

Additional Thoughts

Since the melting and conveying properties of the screw become compromised, excessive screw wear can also cause a significant increase in the amount of energy which is consumed during screw recovery.

For a related discussion: Tolerance For Screw Wear

-Andy

Feedthroat Temperature Differences...

By Andy Routsis on April 15, 2010 6:05 AM 0 0 Vote 0 Votes

I received this question the other day from an experienced molder...

Cindy

We have 2 different brands of machines. On one brand, the feedthroat temperature is 85-90ºF, yet the other brand has a temperature of 150-190ºF. Is this difference OK...? and could you comment on what effects the higher temperature will have on the process?

My Response

Regarding the Machine Differences - In reality, there is no standard convention for feedthroat temperature measurement. Some machines place the transducer near the cooling lines, some place them closer to the rear heater bands. Additionally, some machines will measure the temperature of the water passing through the feedthroat. In your case, the best way to compare the effectiveness of the feedthroat cooling is to use a surface temperature probe to measure the water temperature entering and leaving the feedthroat on both brands of machines.

Regarding the Effects of Temperature - The feedthroat temperature should low enough to prevent the bridging of material. Always ensure the feedthroat does not get cold enough to induce condensation within the feedthroat.

Additional Thoughts

When such a discrepancy exists between two different brands, it is typically caused by differences in design.

-Andy

Screw Flight Diameter Along the Screw

By Andy Routsis on April 14, 2010 7:45 AM 2 0 Vote 0 Votes

I received this question as a follow-up question regarding a different blog, and decided it warranted it's own entry.

MJ

Does the screw have the same diameter in the front, middle and back screw flight diameter?

My Response

The typical injection molding screw has the same outer diameter along the entire length of the screw. When measuring for wear, the highest amount of shear, and wear, typically occurs in the transition and metering zone.

I have seen some cases where a company may use variying clearances in the metering zone for mixing, or adjust the clearance in the feed zone to accomodate specific feed issues.

Some manufacturers, especially those for micro-molding, will occasionally use a tapered barrel to facilitate material conveyance in the feed zone.

Additional Thoughts

Keep in mind... if you are using a barrier screw, you will have two melt channels in the transition zone with one flight having more clearance than the other.

-Andy

Packing With High Pressure Molding

By Andy Routsis on April 12, 2010 1:00 AM 4 0 Vote 0 Votes

During a recent consultation, I was asked this question...

Engineer

Could you explain how to pack-out a thin-walled part which requires a large amount of pressure to fill?

My Response

In such a case, the pressure to fill the part is typically very high... therefor, the pressure at transfer is also high. In such a case, you may have to complete the part filling at a pressure approximately the same as the pressure at transfer.

In some cases, this pressure is actually higher than the pressure at transfer... depending on the amount of pressure that is necessary to fill the remainder of the mold cavity. The benefit to this is you are using just enough pressure to fill the mold cavity, which will help reduce the amount of flash which is produced.

Additional Thoughts

Some processors have a tendency to fill the mold completely during first stage because they are not used to using a high second stage packing pressure.

-Andy

Conditions For The Dynamic Check Ring Repeatability Test

By Andy Routsis on April 9, 2010 9:30 AM 0 0 Vote 0 Votes

I received this questions from one of our advanced blog readers...

Milan

Before I perform the dynamic check ring repeatability test, must I optimize the screw recovery, decompression, back pressure, and feed zone temperature.

note: The Dynamic Check Ring Repeatability Test monitors the ability of the check ring to mold consistent short shots by weighing and comparing the part weight of 10 cycles. For more read: Dynamic Check Ring Repeatability Test

My Response

It is always nice to have everything optimized.... though these will typically not cause enough variability to fail a check ring repeatability test. If your variability seems high as a result of such a test, I would suggest purging the barrel and optimizing these parameters before I went through the effort of pulling the screw or making any equipment changes.

Additional Thoughts

Also, for those readers interested in learning more about optimizing you melt, I recommend the following blog entry: Optimizing Screw Recovery...

-Andy

Machine Variation vs. Process Variation

By Andy Routsis on April 7, 2010 10:20 AM 2 0 Vote 0 Votes

I received the following question in an email yesterday.Since it was lengthy, I have done my best to reduce it to a more concise inquiry...

Nathan

We have an LCP process where the tolerances for cushion, charge time, fill time, and pressure are very narrow. We are maintaining a consistent cushion with only 0.2mm deviation, yet we are noticing short shots consistently. The detection capabilities of the machine do not seem to be adequate for detecting short shots, is there anything you suggest?

My Response

The biggest problem I see is that you have set you tolerances for both the inputs and outputs which relate directly to each other. Although LCPs are highly crystalline and have a very sharp melting point, their ability to flow into the mold is highly dependent on the rate of injection. As a result, any shift in molecular weight, molecular weight distribution, or additives during lot changes will result in a non-compliance. Eventually, you need to re-evaluate your tolerances.

Ultimately, it appears your issues are not related to variations in your machine, but a lack of flexibility in your process resulting in variation.

Additional Thoughts

Such a process often requires a short shot with a high packing pressure to complete mold filling. This will ensure that there is always enough pressure available to fill the part completely.

I have a few questions to address this week, but I intend to post a blog about high-pressure molding next week. This should help you better understand how to establish such a process.

-Andy

Always Get Mold Prints...

By Andy Routsis on April 5, 2010 7:10 AM 0 0 Vote 0 Votes

I was recently involved in a consulting job and was asked the following question...

Project Manager

The tooling vendor is charging extra for the mold prints and CAD drawings. Should we purchase these?

My Response

You should always obtain prints and CAD files from your tooling vendor... even if you plan on using this vendor for molding your parts, tooling repairs, design modifications, and regular maintenance. Basically, anything can happen, the vendor can go out of business, management could change their practices, ownership can change, or leaking roof could wipe out the data and leave you with a mess.

Just as you have a backup of your computer's critical data... you must obtain prints and files of your molds to serve as a backup of your mold.

Additional Thoughts

The best approach is to ensure that costs associated with prints and digital files are included in the original quotation. Always spell these specifics out in detail so that there is no confusion.

-Andy

Understanding Back-Flow Within the Barrel...

By Andy Routsis on March 28, 2010 9:20 PM 0 0 Vote 0 Votes

I received an interesting follow-up question regarding one of my earlier posts...

Steve

One of our techs stated that there is back-flow in the barrel. Is this true? How does this happen?

My Response

Yes, back flow does occur within the barrel in three ways:

1) Material will flow back over the check ring - During injection, all check rings will leak a little, causing some back-flow along the screw.

2) Material will flow back down the screw - During injection the material that leaks around the check ring will push polymer back down the screw channel.

3) Material will flow over the screw flights - During screw recovery, polymer will pass over the flights of the screw. This mixing will increase with an increase of back pressure.

Additional Thoughts

All this back-flow exists in virtually all reciprocating-screw molding machines. If you are processing a heat sensitive material... you need to do your best to minimize these affects by maximizing the efficiency of your screw recovery, and monitor the performance of your check ring.

-Andy

Assisting Part Ejection From Long Cores

By Andy Routsis on March 26, 2010 1:00 AM 0 0 Vote 0 Votes

I received this e-mail from a molder overseas...

Nathan

We are molding PS 750 (NL) tubes in an eight cavity mold. When we use a higher mold temperature, part ejection is good, but the parts display sinks near the slides. With a lower temperature, we can mold parts which look good, but are difficult to eject. We believe we have enough draft on the part and extensive polishing has been done to assist part ejection, yet the parts is hard to eject. If we spray mold release, the part will release from the mold, but this is not a good long term solution. Do you believe the fault is in the material?

Note: Nathan also provided many photos and specifications for both the tool and parts which we will avoid displaying these to protect his customers.

My Response

The three biggest factors that hinder part ejection and hold the core to the part are: the force holding the part to the core, the vaccuum forces holding the part to the core, and the coefficient of friction between the core and the part.

First, the hoop stresses due to shrinkage causes the part to contact the core even after it begins moving off the core. The warmer the part is during the time of ejection, the less this stress will hinder part ejection.

Second, highly polished surface of the core can create such a good seal with the core, it can help to maintain the vacuum seal during part ejection. Applying a very slight texture or surface treatment to the core surface can often reduce this seal and help gas get under the part, thus reducing these vacuum forces. Many molders will also use porous steel and air poppet valves to help facilitate this removal process.

Third, the coefficient of friction for most tool steels can be improved significantly through the use of dry lubricating surface treatments such as dicronite. This is one of quickest ways to overcome friction and improve part ejection.

Additional Thoughts

One great benefit to surface treatments such as dicronite, you can see the coating on the core, so any wear or damage to that coating can be easily tracked and monitored.

-Andy

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