Actually, near and far field refers to the distance from the ultrasonic horn to the weld joint. Typically, when this distance is less than 0.25in or 6mm it is considered 'Near Field' or 'Close Range' ultrasonic welding. Distances greater than that are generally considered 'Far Field' or 'Long Range' ultrasonic welding.

As a general rule, the greater the distance, the more difficult it is to get the vibrational energy of the horn transferred to the point of the weld. Materials which are softer often get less polymer chain entanglement at the interface since much of the vibrational energy is absorbed by the part before it reaches the joint.

Parts being designed for ultrasonic welding should take factors such as horn location and joint design early in the design process to ensure success.

-Andy

Basically, the purpose of a higher feed temperature in a reverse profile is to ensure the pellets soften enough to stick to the barrel so they convey to the transition zone for melting.

You should conduct a test to ensure what feed zone temperature is best for your material as different heater bands, screws, and barrels behave differently.

There are many times I have found that the same temperature can be used for both the feed and middle temperature zones.

Processing guides can be a great starting point when setting your temperature zones, but do not always reflect the best settings for your machine.

-Andy

In a D1 (DECOUPLED MOLDING 1) process the first stage is filling and packing, yet a D2 process isolates filling in the first stage. Could you please explain the difference?

A reverse profile is typically used when processing materials such as nylon and LCP. In fact it is quite common for many semi-crystalline polymers. Basically, such polymers have a melting point. The high rear temperature often helps improve conveyance as well as melt the polymer quickly, Once melted, the polymer can often be cooled a little in the front to improve the overall cycle time and meet the desired melt temperature.

Usually the need for a forward or reverse temperature profile should be approached scientifically. The middle barrel temperatures should be set to the desired melt temperature. The front zones are typically adjusted to obtain the desired melt temperature. The rear temperature should be set  to optimize feed and melt consistency.

Historically, low viscosity polymers with high crystallinity tend to process best using a reverse profile... but you should always look to optimize this, since everyone has different screws, barrel, heater bands, thermocouples, and temperature controllers.

Try not to confuse the front temperature with the actual nozzle temperature. The nozzle temperature will be adjusted independently to meet the needs of the specific process, such as avoiding drool or freeze off.

-Andy

We are waiting until things pick up before we start employee training.

Believe it or not, slow times are the optimal time to start your employee training initiative. These are great opportunities to get your current employees up to speed, and prepare them for the next increase in production.

This is especially true if you hire temporary employees since you will rely heavily on your current employees to being new and temporary hires up to speed when things get busy.

-Andy

Basically, there are two areas within the molding process where back-flow typically occurs:

1) Transfer From Fill To Pack: If the mold is completely full during first stage, the mold will begin to pressurize as the machine tries to continue pack the mold with first stage fill. When the machine transfers to pack using a lower pressure, a little amount of the pressurized material within the mold will tend to back-flow. In some cases, this back flow will cause sinks or voids on the molded part... in other cases, you may see delamination occur. If you are using a pressure monitoring system, you may see a sharp drop in cavity pressure at the point where the machine transfers from fill to pack, especially near the gate.

2) At The End Of Pack: Insufficient packing time results in a gate which is not completely sealed. As a result, some of the pressure in the mold cavity will become relieved by a small back-flow of material across the gate into the runner system. This most often results in sinks or voids near the gate area. If you are using a pressure monitoring system, you should see a significant drop in pressure at the cavity pressure sensor near the gate when the packing pressure times out.

Keep in mind... if your process is properly established, you should notice a nice gradual rise and fall of pressure within the mold cavity and little to no back-flow within the mold cavity.

-Andy

A standard cutoff is helpful when initially establishing your fill... but you ultimately want to target a percentage of shot size. When you target a specific cushion value, then you tend to have the same amount of material, and pressure loss, in front of the screw during pack.

Many companies who mold similar products in similar sized machines sucessfully target a specific cushion value. Most molders have the greatest success in targeting a percentage, such as 5% or 10%, of the full shot size.

The transfer position should not have a significant influence on screw and barrel wear... but focusing on a cushion rather than a specific transfer will improve your ability to maintain a solid and reliable injection molding process.

-Andy

I often encounter this question when upper management does not put a priority on employee training. Many managers will state that employee training is important to them... but often neglect to set aside resources such as time, materials, or manpower to make it happen.

For example, many companies try to use their conference room for training. Unfortunately, such rooms are often so busy, that there is never time to schedule employee training. Often training will get cancelled whenever someone in management wants to hold a meeting. As a result, the employees get the impression that training is not important, and therefore will not make the time.

Basically, employee training needs to become a permanent fixture in your company. You may train a couple hours a week, or a few hours a month... but it needs to be maintained and supported by management. In the example mentioned above, if a manager wanted to interrupt training to use a conference room, he or she should offer the use of their own office to help ensure that the training takes place... this would demonstrate that employee development is important and must be maintained.

As with all initiatives, training needs to be both supported and encouraged by the upper management.

-Andy

Immediately after 2nd stage pack, there is typically a large amount of pressure present in front of the screw. If this pressure is not relieved, it will increase the torque applied during screw rotation. This additional force can quickly weaken, twist, or even break the screw within the barrel when screw recovery starts.

This concern is even greater with electric injection molding machines, since the servo-motor can apply a great amount of torque instantaneously.... especially on direct-drive machines where the minute flex of the belt is non-existent.

To relieve the pressure that’s at the front of screw and to prevent unnecessary screw damage, the ‘screw delay’ or ‘screw decompression’ option should be used. The screw delay option allows you to add a delay after 2nd stage packing to relieve the plastic pressure on the screw before recovery. The decompression will actually back up the screw to relieve the pressure... similar to decompression after recovery.

Hydraulic molding machines often avoid this issue since they have an inherent buffer due to the slight buildup of pressure as the valve opens, and a period of pressure stabilization that occurs before the screw reaches full speed. Adversely, the electric servo-motors apply a high amount of torque to the screw virtually instantaneously. People who are are familiar with hydraulic molding machines often have difficulty adjusting to the instantaneous response of an all-electric molding machine.

-Andy

Each process and company has a different tolerance for wear. A company molding medical devices needs much more control over melting and residence time than a company molding commodities.

The benefits to regularly measuring your screw and barrel is that you can determine what level of wear tends to indicate inconsistent processes. Ultimately, an excessively worn screw will increase residence time, decrease melt consistency, and reduce melting efficiency.

Many companies will have a couple 'junk' machines which are not worth replacing the screw and barrel… but can run a handful of jobs well enough. Other companies keep every machine in perfect shape. Remember, you don't need a race car to get your groceries… but you also don't want to enter a race with a Yugo.

-Andy

Training plays an important role in the success of any molding facility, and provides benefits to both the employee and employer. It is important that personnel at all levels participate in a structured in-house training program. Such employees increase their knowledge base and learn applicable skills to enhance their performance in a production environment.

Training also helps the employer satisfy their customers - and ensures future customers that they’ll be able to handle new challenges. Such training is essential to performing your job correctly. In addition, training can help achieve your professional and financial goals by providing job advancement opportunities.

Don't be afraid to be frank, especially with new hires... Any employee who is unwilling to learn and improve should be concerned about their longevity at your company.

-Andy

Basically, there is a way to optimize recovery speed... but it is a two step process:

1) First, determine the optimal feed zone temperature for your process by performing a feed zone temperature study. The purpose of this study is to determine the optimum feed temperature by graphing feed zone temperature versus screw recovery time. Starting with a low feed zone temperature, incrementally increase the temperature and document the screw recovery time at different increments. When graphed, the screw recovery time will drop and then rise as the temperature is increased. The optimal feed zone temperature is the temperature at which the screw recovery time is the lowest. This is the temperature where the polymer sticks best to the barrel, causing it to convey most efficiently.

2) Once the optimal feed zone temperature is determined, you should adjust the rotational speed of the screw so that screw recovery consumes 80 percent of the overall cooling time. Note that the back pressure used during screw recovery should be high enough to provide a consistent recovery time and consistent mixing. Your recovery times should not vary more than 5% from shot to shot, and 10% from material lot to material lot.

Many older molding machines cannot maintain consistent screw speeds at low RPM. In such a case, you may want to consider a longer delay before recovery to ensure the machine can maintain the desired consistency.

-Andy

I will address this question in a few parts...

Regarding Torque Ratings: Many bolt manufacturers will provide recommended torque values. You must remember that these bolts are not manufactured or designed specifically for injection molders. The same bolt you are using to hold your mold in place may also be used to secure the rafters in a stadium. The torque rating is based on what the bolt can safely sustain... not necessarily how it should be used. Since the machine platen is typically cast, the threads are significantly weaker than the threads on the bolt.

Regarding Torque Recommendations: Most injection molders using similar bolts use a torque value around 50-60 ft-lbs. For more on this, please read our past blog: Proper Torque Value for Clamping Mold to Platen

Regarding Lubrication: You should not need lubrication to remove the bolts from the platen unless you are using a small amount of anti-seize. If you are having problems removing the bolts from the platen, it is likely that your die setters are using too much torque on the bolts or your platen threads may already be damaged, burred, rusted, or dirty. If this is the case, you will need to repair or re-tap the platen holes to ensure proper mold clamping. Lastly, always ensure the platen is smooth, flat, and clean each time you change molds.

Your technicians will get much more support through the use of more clamps rather than using more torque. For more on this topic, I recommend reviewing the following post: How Many Clamps To Use?

-Andy

I will first define the two concepts in practical terms, and then address the differences between the two...

Molded-In Compressive Stress - As the polymer fills the mold and cools, the polymer begins to shrink. As the polymer shrinks, additional polymer is then forced, or packed, into the mold to compensate for this shrinkage. As additional polymer is packed into the mold, internal pressure can build up causing general compression. Some molders will refer to a part with too much molded-in compressive stress as over-packed. These stresses most-often occur when the mold temperature, melt temperature, or packing pressure is too high allowing additional packing to occur. 

Molded-In Orientation Stress - As the polymer fills the mold, the long polymer chains tend to orient in the direction of flow. Basically... more of the polymer chains are aligned in the direction of flow than are aligned perpendicular to the direction of flow. Immediately after the polymer fills the mold, the polymer chains try to orient themselves randomly (their preferred state). Since the polymer chains themselves tend to shrink more along the length of the chain, than perpendicular, the differential shrinkages tend to cause stress within the part. Additionally, the polymer chains are not in the preferred random orientation, so there will always be some internal stress due to orientation. Even though these stresses will always exist, they are made worse by factors such as low mold temperatures and high injection speeds.

Basically, compression stress is created during packing while orientation stress is created during mold filing. As a result, you can have a part with both molded-in compressive stress as well as molded-in orientation stress. Although both can cause a bad part, a good molding process is typically a balance of minimizing these affects and maximizing productivity.

There are many applications where a molded-in compressive or orientation stress is beneficial to the products performance. For example, hinges and cable ties get their strength and longevity from a high degree of molded-in orientation. Likewise, many molders improve the useful life and performance of shock absorbing components when they have internal compressive stresses.

In some engineering applications, the molder will heat treat, or anneal, the molded parts to allow the polymer chains to align themselves in a more random state to reduce the internal stresses.

-Andy

Since a clamp tonnage reduction helps your part quality, you have correctly determined that it is a venting issue. Additionally, it is most likely that you only need venting on the parting line to correct the issue.

The easiest way to determine the vent location is to mold a short shot which will allow you to see where the end of the flow front is located. In most cases, this is where the venting should take place.

Since you are molding PC, your application may be optimized with a large vent encompassing most of the parting line.

In any case, your mold should have a deep channel behind the vents to ensure the venting can leave the mold easily and quickly.

-Andy

You are correct in believing that the melt is not properly attaining the shape of the texture. Basically, proper surface texture reproduction is the result of good interaction between the polymer melt and the mold surface during mold filling. If the melt cannot properly reproduce the mold texture, it obtains a shiny appearance. As a result, the primary factors which improve this are an increase in melt temperature or mold temperature at the point of contact.

Regarding mold temperature: There is really only one way to increase mold temperature... by increasing the coolant temperature, or reducing the coolant flow.

Regarding the melt temperature: Since we are only concerned with the temperature of the melt as it contacts the mold surface, there are five ways to improve this: 1) increase barrel temperature 2) increase screw recovery speed 3) increase back pressure 4) increase injection speed and 5) decrease the gate size.

Options 1-3 are most helpful if your melt temperature is too low when measured by a melt pyrometer.

Options 4 & 5 are often the most helpful because they avoid excessive heating and degradation inside the barrel, and only provide heating through shear at the time of injection.

If you strongly believe venting is the cause, you can always seal the parting line and add a vacuum pump to investigate that prospect further.

-Andy

Although there are a variety of nozzles available to the industry, most have a large diameter opening where they attach to the barrel with a significant reduction to the final orifice where it meets the sprue bushing.There are three common ways the internal dimensions are constructed at the sprue bushing end of the nozzle:

1) GP (General Purpose) Nozzle - This nozzle typically uses a straight land area where the polymer enters the sprue bushing. For example, if the nozzle orifice diameter was .100" or 2.54mm the orifice would maintain that diameter for the length of the land. The benefit to this is the polymer tends to be pulled from the nozzle during mold opening, providing a small area for material to drool between cycles. One disadvantage to this design is that the amount of material that is removed from the nozzle can often be inconsistent. The other disadvantage to the long land area is the increased shear rate that occurs in this region. The general purpose nozzles are often helpful for molding materials which tend to exhibit small amounts of drool.

2) Full-Taper Nozzle - These nozzles have a nozzle orifice diameter which is smallest where the nozzle meets the sprue bushing. Unlike the GP nozzle, the polymer gets the least resistance to flow. The advantage to this design is it provides the most flow with the least shear. The disadvantage to this design is that any drool has the potential of causing cycling issues. Most molders use such nozzles for amorphous materials such as PC and ABS. 

3) Reverse-Taper Nozzle - This nozzle uses an orifice which has an opening larger than the inner dimensions of the nozzle. The purpose of this reverse-taper design is to promote the removal of material from the nozzle as the mold opens. This can be very advantageous for low-viscosity semi-crystalline materials such as nylon and PP which are prone to drool.

To answer your specific question, GP nozzles are bad for some materials, great for certain applications and OK for others. It may be a good idea to educate your employees on the differences, and start using more appropriate designs for applications which could benefit from reduced shear and increased flow.

Whenever possble, I opt for a nozzle which is best suited to the application. This decision would incorporate some of the following factors:

1) What is the maximum nozzle orifice diameter I should use?

2) What is the shortest nozzle I can use?

3) Would a full, straight, or reverse taper be best?

4) How long of an orifice land do I need?

-Andy

Although there are a lot of questions, I think it can be handled from a couple different angles.

1) Pressure is created by resistance to flow - During injection, a significant amount of pressure is being created in front of the screw. Since polymers are compressible, there is a counteracting pressure pushing the screw backward. If your hold pressure is less than the polymer pressure in front of the screw, then the screw will move backward.

2) Excessive mold filling during first stage - Ideally, your mold should not be completely full during first stage injection. If you completely fill the mold during first stage, then there is a spike in injection pressure as the polymer begins to pack out the mold cavity. When this spike in pressure is followed by a lower packing pressure, there is often a backflow of material out of the mold cavity... resulting in screw bounceback. Overall, this is not a good approach to processing because changes such as material viscosity or melt temperature will cause significant variation in your process. 

Your best approach is to:

1) Increase your transfer position to obtain a short shot during first stage.

2) Increase your hold pressure until you obtain appropriate part quality.

3) Perform a gate seal study to ensure the 8 sec. hold time is appropriate for the adjusted hold pressure.

-Andy

I was wondering if you help me locate some references on Processing HDPE and PP with 1% chemical foaming agent? All the information I find focuses on Structural Foam which is a low pressure process. We primarily use high pressure injection molding, using the the chemical foaming agent for weight reduction and cosmetics.

Since the banning of foaming agents containing CFC's, the use of chemical blowing agents has obtained an inappropriate stigma in the marketplace. Believe it or not, their use is not as rare as it may seem. Many injection molders will use a small amount of blowing agents to eliminate sinks, lower material costs, and even enlarge the part to help meet dimensional requirements.

In your case, there are many places to find information... when searching online, use terms such as:

or get more specific:

There are also some good articles available online:

or

Since structural foam molders use the largest volume of additives, much of the literature is focused on their needs... but you will find that many of these resources will also provide great high-pressure molding information... click for an example of this from Bergen.

You may have to register to see some of the materials from the Suppliers, but this is typically free.

Keep in mind, your material providers can be great resources if you ask the correct questions. Some additive providers provide agents specifically formulated for your type of application.

Basically, when you are conducting a high pressure molding process with blowing or foaming agents, you would melt and inject the material in a manner similar to a traditional process, but apply many of the packing and cooling strategies of structural foam. Ultimately, the more blowing agent you add... the more the polymer will behave like a structural foam after it is injected. 

-Andy

Your first assumption is most likely correct...

The hydraulic clamp system on your machine uses a large diameter hydraulic cylinder in the center of the movable platen to apply tonnage to the mold. Since the clamping force is centered on the platen... deflection occurs on the perimeter. This deflection reduces the tonnage appied to the perimeter of the mold resulting in improved venting. Adversely, toggle clamps tend to provide a more evenly distributed force to the platen.

Before you make the assumption that the machine is the only cause, keep a few more aspects in mind... 1) The clamp force might be more accurate on the newer machine. 2) If the mold is too small for the electric molding machine (uses less than 2/3 of the platen) then you can cause platen concavity resulting in additional force applied to the perimeter of the mold. 3) The newer machine may also provide a faster injection speed which would also affect the mold venting.

-Andy

Although adjusting the temperature of your hot runner drops can help balance your tool, there are a few alternative ways molders use to help balance their tool.

1) Adjustable drops - In many hot runner tools, the height of the hot runner drop can often be adjusted to increase or decrease the thickness of the specific drops

2) Use true balanced runner systems - Many hot runner manifolds are not built with balance and symmetry in mind. Additionally, most hot runner systems do not use features such as Beaumont's Melt Flipper to balance the shear within the hot runner system.

3) Balance the clamping - All platens deflect, and many older platens will be somewhat concave. Check and measure these conditions to ensure spacing or additional bolster plates are not needed. Additionally, a review of parallelism during clamping can be very helpful.

4) Balance the cooling - In many injection molds, the cooling supply each individual cavity may not be properly balanced. This is very common when the part geometries are not symmetrical... resulting in variations the effectiveness of the cooling from part to part. You may want to measure and compare the coolant temperature and flow going to each region of the tool.

5) Balance of venting - This can be an often overlooked cause of cavity imbalance. The effectiveness of the melt entering the mold cavity is based much on the air's ability to get out the plastic's way. I have seem many molds with virtually no venting to interior cavities... or even caes where the inner cavities actually vent to the out cavities. causing all sorts of complications in gas removal.

When specifying new hot runner systems or new tooling, try to incorporate systems which can be easily adjusted. Many molders see great benefits to the additional control and flexibility brought forth through the use of valve and thermal gate systems.

-Andy

We deal with technical issues within the plastics industry all the time. Although we openly advertise our expertise in creating employee development systems... we routinely visit companies around the globe to provide technical consultation for plastics, processing, and training.

In general, this question can be handled using four common approaches...

1) If you have a unique issue such as troubleshooting, equipment evaluation, or beginning a new program... a consultant is often the best way to resolve the situation. Many companies hire an engineer to help handle a specific situation... and then under use their talents performing routine tasks once the situation is resolved.

2) If you want to create a long term change in behavior such as 5S or process documentation... a consultant can often help you determine the best approach to determining the best behavior, educating your employees, and maintaining the behavior. Once this is established, it will be very easy to determine whether your current employees can implement the strategy... or if additional staff is necessary to ensure the success of the initiative.

3) If there are only one or two routine tasks which take place monthly or quarterly such as a tool design review or a molding trial... having a consultant on retainer may save you money in the long term.

4) If you have an established routine or series of complex tasks which need to be performed, hiring an engineer or technician is most likely the best course of action.

Always try to hire employees and consultants with the intention of exploiting their talents... for example, hiring a consultant to measure and weight a large number of parts may not be cost effective... but hiring a consultant arrange outside testing, or to evaluate the results and help draw conclusions may be very helpful. Adversely, when our consultants help a company develop a strategy for training their employees, their engineers, managers, and supervisors are often the best people to implement and carry out this training with their employees.

-Andy

Mold filling analysis programs such as moldflow can provide a variety of options for filling. For example, you can use the software to profile in order to minimize variations in fiber orientation, shear rate, shrinkage, etc. Many seasoned injection molders will agree that the simulation information is very helpful, but the final process will depend more factors than can be programmed into the computer... The biggest factor being Quality.

Overall, I find these simulations extremely helpful in establishing the mold design, and to determine the general approach to processing the final tool. If such a program suggests an injection profile, then I would use that information to better understand potential complications with the process. For example, if the software recommended a lower speed through a thin section and a faster speed through a thicker section... I would personally evaluate whether to first try one speed through both, or profile my speeds during the initial setup.

One thing to keep in mind when processing... each injection speed is a variable... the more variables you introduce into the process, the more variation you potentially introduce.

All data such as blueprints, analysis, and customer requirements are helpful to the person establishing the process. Additionally, the more complex the part and tool, the more useful this data can become.

-Andy

This is a topic which often occurs when you are processing with various machines.

I recently received this follow-up question regarding an earlier blog entry...

To establish a molding parameter, what is the normal percentage tolerence to be used for the injection pressure and other parameters? Currently my process does not have any tolerence and sometimes this may cause difficulty in troubleshooting which will result short mold and etc.

In general, a well-established process encounters approximately 10% variation. For this reason, it is critical to ensure you have enough room to adjust your process inputs for this. 

For example, if first stage injection becomes pressure-limited, the machine can no longer maintain the desired injection rate, resulting in an inconsistent fill rate and injection time. This generally leads to unwanted short shots, sinks, and flash on the final part.

To avoid a pressure-limited process, you should always have more pressure available to fill the mold than is actually necessary. This will allow the machine to maintain the ‘injection speed set point’, ensuring the highest possible repeatability.

The problem you may encounter is the fact that many machines actually need an additional buffer to perform properly. For instance, a process may reach a peak pressure of 10,000 psi during first stage fill... yet, if the machine has a maximum setting below 10,500 psi, the process could become pressure limited.

The best way to approach this is to do the following...

1) Establish a good process with significantly more pressure than is necessary.

2) Reduce the maximum injection pressure until it affects the injection time by increasing the 1st stage fill time.

3) Increase this maximum by 10-15% to accomodate for material variablity.

I also recommend you review a few of our related posts... including

If you are running a lot of regrind, or off-spec material, you may want to increase this buffer to as much as 20%. In such a case, it is imperative that you use a short-shot during 1st stage fill.

-Andy

When asked 'How many clamps should I use?' one plant manager I know likes to respond with 'As many as you can'. In reality, each clamping system, mold base, and platen is different. When discussing standard adjustable height mold clamps, I prefer to use six clamps per half on smaller molds, and as many as a dozen for larger molds. In general, the spacing of the clamps are usually dictated by the platen hole location and rail size.

When tightening the bolts, I recommend using an alternating pattern which alternates back and forth as well as up and down. For example, when using a four clamp setup, starting with the top right hand corner, you would tighten in this order:

The intent is to balance the force being applied to the mold base to prevent any uneven stress on the mold base or platen.

Ultimately, the more clamps that you use, the less stress that is applied to any specific clamp point. This will increase the life of the clamps, and help prevent damage to the platen holes. 

-Andy 

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

You are close... when you calculate the intensification ratio, you need to add the surface areas of the two cylinders and use that value in your calculations.

For one cylinder:

Determining the intensification ratios for each machine is a critical step in obtaining real process output data from your molding machines.

-Andy

I can't bother to train my staff... they learn what they need to know on-the-job.

I have a problem calculating the material usage on the production floor. When I weigh 10 shots the average weight is 5.2g per shot for an 8 cavity mold. We originally calculated a material requirements of 70.2kg of polypropylene to mold 108,000 parts. When the run was complete, we used more than 70.2kg of material to mold the parts, and all 108,000 parts weighed more than 70.2kg. Can you please advise us what is the real calculation for the material usage?

Plastics materials have a tendency to exhibit variability. Using this scenario, I will suggest a few factors to incorporate, and a few strategies which may also help increase your accuracy.

Compensate for Startup and Shutdown - All processes have some amount of loss when starting up and shutting down the molding machine. Most companies know the average startup time required to initiate a production run as well as the time to shut down the machine. During this time, material is purged, and scrap parts are being generated. A good starting point for losses is to assume the machine is molding scrap parts throughout this time. Remember, if the machine is scheduled to be shut down and re-started during the run, these processes also need to the considered.

Compensate for Scrap - Since virtually every process creates scrap, you should compensate for the expected scrap rate by adding that loss to your expected amount of material usage.

Compensate for Troubleshooting - When troubleshooting, technicians tend to put more material into the mold as time progresses. For instance, when sink marks occur, the most common action is to increase packing pressure... likewise, when flash occurs, they tend to increase clamp force rather than adjust the transfer position. As a result, the part weight of the last part tends to be higher than the first parts that are produced. Therefor, if part weight is not measured and monitored regularly, I suggest you add a 10% variability factor.

Additionally, polypropylene is a highly semi-crystalline polymer. When the initial shots were measured to calculate the material usage, it is likely that the mold temperature had not yet stabilized. As a result, the mold temperature increased a little, causing more material to be packed into the mold cavity.

Ultimately, the best way to control material usage, and limit costs, is to routinely monitor the shot weight as well as minimize scrap and downtime.

-Andy

Basically... Torque is a measure of rotational force. In other words, the force being applied to rotate something is considered torque.

The way torque is calculated is by multiplying the force being applied times the distance it is being applied. This is typically represented as Newton-meters (N-m) in the Metric system or foot-pounds (ft-lb) for Imperial measurements.

For a more detailed definition of torque, please feel free to visit wikipedia:

Technicians and mechanics often use torque wrenches to measure the rotational force being applied to a screw when they are being tightened. This helps prevent the platen threads from becoming stripped or damaged.

For a more in-depth discussion on this topic, please visit my previous blog:

-Andy

In the prototype phase, the quickest, and least expensive way to do this is using either rotational molding or blow molding.

Rotational Molding: The material clamped within a mold, it is rotated about multiple axis while it is heated to melt the material and cover the circumference of the interior. The mold is then cooled and the part is eventually removed. This process is very slow, but is great for very low production prototypes.

Extrusion Blow Molding: This process uses an extruded, tube shaped, parison which is clamped within the mold and blown to size using pressurized air. The mold is more expensive than a rotational mold, but you are able to manufacture many more parts than with rotational molding.

You can learn more about Extrusion Blow Mlding with our blow molding series of training:

You can also produce the part using injection molding along with an assembly process such as ultrasonic welding... but this seems far too expensive for a prototype of a basic decorative consumer part.

-Andy

Note: High RPM grinders typically rotate a set of blades at a very high speed. When the rotating blades pass by the stationary bed, they cut the part giving the familiar rat-tat-tat sound. The motors on these grinders tend to be over sized... and they often consume a significant amount of energy relative to the amount of material they consume since they have to maintain a very high RPM to work effectively.

In most cases, I prefer the low RPM grinders because they consume less energy than high RPM grinders... especially if it is running constantly. Low RPM grinders use a heavier set of stepped blades which operate more by maintaining the momentum of the blades to steadily chew up the plastic. Over time... theses grinders tend to be less expensive to operate and maintain... especially if they are in constant use.

Most grinders are best used in applications where the amount of material being fed into the grinder is close to the maximum amount it can consume. A grinder which consumes only 10-25% of it's capacity can waste a large amount of energy over time.

-Andy

Believe it or not, this is actually a very common situation. The best way to improve this is to have the tooling personnel help cross-train your employees. This makes them partially responsible for the technicians knowledge  of tooling. This gives them a vested interest in helping the technicians rather than insulting them.

A great opportunity to do this would be during mold maintenance. If you have the technicians assist in the mold breakdown or re-assembly the tooling person can explain the name and purpose of each component during the process.

Much of this comes from the defensiveness on the moldmakers part since they seldom have a good knowledge of processing. So... don't forget to reverse the process and have the technicians teach your tooling personnel about processing.

-Andy

First, you are correct that your process will be more solid and reliable if it is short during first stage. This is critical to compensating for material variation... which will always save you money over the long term.

Second, your first stage injection pressure will show an increase as the material reaches the end of fill. If you look at the injection pressure integral for the entire cycle, you will find that the energy applied to the polymer will be virtually equal in either case.

You should always approach your processing in a logical manner... regardless of whether the machine has an all-electric, hydraulic, or a hybrid design.

-Andy

Ultimately, we recommend that you train your employees using a blended learning model which uses the best of a variety of training methods combined in a comprehensive training plan.

Aspects such as interactive training, focused on-the-job exercises, and training for specific skills are some of the best components to be included.

Ultimately, you should determine what specific skills are important… and devise training strategies to address those competencies.

Adults learn in many different ways. You should mix it up and always have a little fun during this process.

-Andy

You should try to get everyone on the production floor involved in your training initiative.

Aside from processing, quality, and tooling... you may also want to include other departments such as sales or engineering.

I often like to relay a story about one company who nearly had a mutiny by the design engineering department because they were excluded from the advanced process training.

Basically, don’t be afraid to ask around… we have many companies who thought people wanted to be excluded; only to find out everyone was hoping to take some of the training.

We always recommend you give your employees access to training. We figure the worst thing that can happen is they could learn something... which is really not a bad thing.

-Andy

note: After further questioning, it was discovered that the quantities of slip, AB, and MB anti-block additives have not changed. Also, they do not currently collect any MFI (melt flow index) or viscosity data from the supplier.

It is very likely that your material supplier has changed material characteristics on you. The efficiency of your additives relies on their ability to 'bloom' or migrate to the surface. Additionally... it appears that overall morphology of the polymer, including crystallinity, has changed since the material no longer exhibits the two week drop in COF as it once did.

If a significant change in either molecular weight, or molecular weight distribution has occurred, then it will change the COF of the base polymer as well as the additive's ability to 'bloom' or migrate to the surface.

You should require the supplier to provide certification for each lot that you receive. This should include some basic data including the Melt Flow Index.

note: the same effect occurs with injection molding with molded-in lubricants and internal release agents.

You should consider performing some basic material tests at your facility... this should include the MFI (Melt Flow Index), but may also include the DSC (Differential Scanning Calorimeter) and capillary rheometer.

Some companies actually purchase a benchtop extrusion line to test and understand the processing characteristics of incoming material lots prior to actual production.

-Andy

Regarding the screw - Most screw manufacturers will recommend that you use an outside micrometer. Since the flights do not match up, you should lay a block gauge block across a couple flights on one side of the screw and deduct the thickness of that block from the overall measurement. For consistency, you should note the distance, from the end of the screw, that the micrometer contacts the screw. As a result you should have a table with lengths and corresponding diameters.

Regarding the barrel - Using an inside micrometer, most companies will follow a similar procedure as with the screw. The diameters should be take at specific distances down the barrel and be listed in a table of lengths and diameters.

You should avoid using a surface plate with a height gauge to measure the screw wear... This will often mask the areas of the screw with high wear.

-Andy

None of the training out there applies to what my plant is doing.

I first asked him if his extruder had a barrel and screw... He said yes. I then asked about the barrel heaters, cooling fans, and temperature controllers... He again agreed.

He also agreed his extruder has an adapter, die, screw motor, reduced, feed throat, and so on. Then he explained that they used unique downstream equipment on their standard extruder. 

I explained that half of their process is the extruder and the second half is the downstream, in which much training is available. Lastly, I asked if his employees could improve their knowledge and abilities such as in math, quality, and problem solving... He again agreed.

The purpose here was not to sell training... but to get people to think about training materials as components of the overall training solution.

-Andy

In my experience, the plastics industry is relatively slow to adopt the latest technology. Four of the biggest factors in adopting a new technology are:

1) Significant Increase in Performance

2) Demonstrated Reliability

3) Ease of Use

4) Lower Cost

As you may know, the earliest all-electric molding machines were very delicate, and did not provide significant value. As the years have progressed, and technology has improved... these machines have begun to outperform hydraulic machines in virtually all these four areas. It is only a matter of time before all-electric molding machines have dominance in all demographics of the injection molding industry... especially as the costs of all-electric molding machines decrease far below the cost of hydraulic machines.

That being said, there will always be injection molders who will prefer to mold with hydraulic molding machines... or have older machines which are still functioning properly. I recently spoke with one molder who said 'I hate electric machines, they are so quiet it just doesn't sound busy around here'.

I have watched the slow adoption of closed-loop process controls over the past few decades. At first, the controls were confusing, complicated, unreliable, and very expensive. Now this technology dominates the market... yet, there are still manufacturers who will sell you a brand-new injection molding machine with an old-fashioned open-loop process controller ~ if you really want one.

-Andy

There are a couple general misconception regarding the benefits of all-electric molding machines...

Misconception #1 ~ All-electric molding machines will not save you money if you have a long cycle time. The truth is... The pump runs constantly on a hydraulic machine wasting energy, and causing component wear. Electric molding machine components do not run when they are not being used saving electricity and wear during idle moments.

Misconception #2 ~ For long cycle times... hybrid machines provide virtually the same energy savings as electric molding machines; since most of the energy is consumed during screw recovery. The truth is... As with #1, the cost and wear associated with running the pump during idle times will usually outweigh the initial savings during the purchase of the machine.

I recently visited a molder who purchased both a hybrid and an all-electric molding machine to compare their performance and energy savings side by side. Both machines ran the same mold and had processes with cycle times over two minutes. He claimed that he could throw away the hybrid machine, buy another electric molding machine, and still pay for both machines in less than four years just from the energy savings. This sounds extreme... buy it demonstrates the money you can save by thinking long term.

The costs savings for long cycle times is part of the reason machine manufacturers are being pressured to build larger and larger all-electric molding machines.

-Andy

The thin probe is the best method. You do not have to preheat it… and the reading is very fast. Wire probes tend to be too brittle for the production environment, while the newer thin probes are much hardier.

If you want to save a few bucks, you can usually skip the extension cable and plug the probe straight into the meter. Personally, I use the probe that way because I can take the reading with one hand.

Although there are a variety of styles, the probe adapters are pretty consistent... so you can usually plug a new probe into an older meter.

-Andy