May 2010 Archives

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
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
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

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
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

 



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
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
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



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

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

For machine, mold, and process evaluation training and worksheets, please review:
The Intelligent Molder Series

-Andy




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
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
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
I was recently forwarded a good article in Industry Week by Jill Jusko entitled: When It Comes to Training, Don't Hope for the Best

Article Excerpt
When it comes to training, "what I often see happen is that people will invest in some training and then hope that the results will shine through," says Ryan Hale, lead consultant with Stroud Consulting. Instead, he says, manufacturers must first understand what results they want to achieve and then pick the training and tools to achieve those results.

My Comments
This article highlights a common pitfall in employee training. Most companies don't approach training with the intent of making concrete improvements... and as a result, they don't structure the training to accomplish these concrete goals.

I recommend you consider reading Jill's' article When It Comes to Training, Don't Hope for the Best as well as Selecting Your Training Metrics to learn more about the importance of monitoring training metrics.

-Andy

About this Archive

This page is an archive of entries from May 2010 listed from newest to oldest.

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