With toggle clamps, the only way to measure tonnage is to measure the strain (stretch) of the tie bar.

How do you measure the tonnage on a toggle molding machine:

My Answer:

With toggle clamps, the only way to measure tonnage is to measure the strain (stretch) of the tie bar. Using the strain measurement of the tie bar, the force on the tie bar can be calculated if the tie bar material is known. If that is multiplied by the number of tie bars, the tonnage is determined. Most newer molding machines use a strain gauge mounted on or in the tie bar. The best measurement can be obtained using an ultrasonic measurement through the tie bar to determine the exact amount of stretch in each tie bar. This equipment is expensive, but there are many machine evaluation consultants you can hire to do this.


The first step in troubleshooting flash is to determine whether it is a filling problem or a packing problem.

Question: I have a part with flash. It ran many times in the past without flash, but recently we have been using a new machine and it is often creating flash. We have tried many adjustments including packing pressure, clamp tonnage, transfer pressure, etc.

Answer: The first step in troubleshooting flash is to turn off packing and determine if you have flash during first stage injection. This will tell you immediately whether the problem is occurring during 1st stage injection or second stage packing. Ultimately, this will focus your attention very quickly.

Additional Thoughts: In many cases, you see technicians adjust parameters such as packing pressure and clamp tonnage when the problem occurs during injection. The problem with this approach is that the technicians almost never return the clamp tonnage or packing pressure to their original settings when they ultimately fix the flash. As a result, the plant gets lots of other defects such as shorts, sinks, and burns afterwards.

Documentation is the first step to good troubleshooting, it is the first step to continuous improvement, it is the first step to process improvement, it is the first step to better maintenance, it is the first step to getting your technicians up to speed faster, etc.

Question: During a recent on-site training session, I was asked by the plant manager: What is the first step we need to take to get better?

My Response: Better documentation is the first step you need to take to get better. With respect to processing, if you do not properly and accurately document the process that is being used to make a good part, there is nothing to use as a basis of comparison when troubleshooting a bad part. Everything that happens to a process, machine, mold, material, or a piece of auxiliary equipment should be documented. This includes everyone from tooling and processing to maintenance and die setters. Essentially everyone who works on the equipment or process must document what has transpired.

For example, if you don't document the time and reason for the swapping-out of a failed water temperature controller during 2nd shift, the 3rd shift will inaccurate information when troubleshooting defects that might be found on that machine at the start of their shift and may not be able to locate the root cause of the problem in a timely manner.

Additional Thoughts: When trying top get new process technicians up to speed faster, good documentation significantly reduces the learning curve. For example, let's suppose a new technician notices delamination on the parts as well as an increase in transfer pressure, but no other significant deviations in the current process from the documented standard. In this case, he/she can place attention on why the pressure has increased. This approach puts the focus on what likely caused the pressure to increase which is much more efficient than trying to remember all the things that might cause delamination. If they correct the cause for the pressure increase and the delamination goes away, the new technician has a very high level of confidence that they corrected the problem and can move on to the next problem.


The purpose of a short shot during 1st Stage Injection is to ensure that all cavities remain short at transfer during normal process variation.

Question: I was told to short the mold around 98% based on final part weight, is that enough?

My Response: Basing any short shot on the final part weight is an inaccurate and misleading process. This is faulty because there are so many factors which contribute to the percentage of material that is added to the mold between when the first cavity fills and the part is completely packed out. Aspects such as wall thickness, material shrinkage, filling imbalance, degree of semi-crystallinity, pressure loss, material consistency, packing pressure, etc. all make a general rule based on final part weight impossible. because of these factors, a short shot of 95-98% based on a fully packed part will likely be full in most if not all cavities at the time of transfer as well as show screw bounce.

The best approach is to use a short shot percentage based on 100% being equal to the weight of the shot when 1) only one cavity fills and 2) no packing is present. We typically recommend 90-95% short under these circumstances. If the packing is established correctly, the process will have enough pressure to keep the screw moving forward to fill and pack out the parts without flash. This will compensate for 1-2 percent check ring variation as well as 2-3 percent material viscosity variation.

Additional Thoughts: If you are processing very unstable material such as post-consumer regrind having as high as 10-20% viscosity variation from shot-to-shot, then a short shot as low as 75% may be necessary to maintain a consistent final part weight.


When troubleshooting a problem which is unique to a particular tool, but not present in a sister tool, then the solution lies in locating the differences.

Issue: Company is encountering an unexpected reduction in part dimensions after conditioning where a growth in dimensions has been seen in all similar parts. The parts which are similar and produce expected dimensions are molded in 'sister molds' (term used for mold which is nearly identical in design and is molding parts of similar design). These parts have an expected dimensional growth across the entire part after conditioning.

Problem Solving Technique: The solution to such a situation lies not in the new part alone, but in what differentiates the new system from the existing parts which produce expected results. This process involves comparing the new part, mold, process, & material until the difference is found. In theory two similar systems should produce similar results. If they do not, you need to find the differences whether it is a hotter core, reduced melt temperature, larger gate diameter, or a lower material drier residence time. Once the differences are found, you can evaluate and test each to determine whether it is causing the unexpected result. For example, if a core is warmer, add an additional temperature controller to that cooling zone to reduce the temperature and determine if that causes a more expected result... Isolating and testing each potential cause is critical to determining the cause of a problem and its ultimate resolution.

I’d like to invite you to a complimentary webinar I’m hosting on Tuesday, June 21st at 2pm EDT.

During the webinar, we'll discuss why the approach makes such a huge, bottom-line difference for plastics companies, and what you need to do to teach all of your techs to use Scientific Molding. You’ll also have a chance to get answers to your questions during a live Q&A at the end.

The program is geared toward managers and engineers, so please forward this email to others you think might benefit from attending.

Learn more and register here: http://www.plasticsnews.com/routsis


The implementation and maintenance of 5S is not just in having 5S events, but in developing 5S skills that can be used on a daily basis.

Question: How do we get everyone in the plant on board with 5S? Our 5S events become very difficult and no one really likes them.

My Response: The beauty of 5S is the continuous improvement aspect which can be implemented as part of one's daily routine. The application of 5S is not complicated, but each constituent part (sorting, sweeping, etc.) are individual skills which must be learned and practiced. The complication in 5S typically arises when it is only viewed as an 'event' in the eyes of your employees. It is best to get everyone in the plant involved in ongoing reviews, sorts, cleaning, etc. to ensure everyone understands. For instance, let all your employees be involved in the sorting and let the lean manufacturing experts approve or adjust their plans before they act.

If you are looking for some assistance in developing the 5S skills of your employees, we do have a new series of Skill Developing 5S SkillSet courses with associated 5S SkillSet Worksheets which are described here:


Maintenance and Repair Organization (MRO) is an important aspect to efficient and timely equipment maintenance.

Issue: In most Lean manufacturing efforts, maintenance and management tend to be the two slowest departments to adopt practices such as 5S. This may be because they are perceived as an indirect or supportive component of Manufacturing, but this is not entirely true. If it takes an additional 20 minutes to find the correct part in maintenance, or 30 minutes to locate the correct document in HR... then it affects will directly affect manufacturing. Every department including management and maintenance need to be involved in lean manufacturing efforts to ensure these practices are adopted company-wide.

Routsis Training in the News:
On the training side, A. Routsis Associates Inc., Dracut, Mass., recently announced a program of five new online courses called The 5S System (Steps One through Five). The system is a method that describes how to manage a workspace for efficiency and effectiveness. The steps are Sorting, Straightening, Sweeping, Standardizing and Sustaining. Each area is covered in detail to ensure that work environments are clean, organized, consistent, properly maintained and up-to-date. The goal is for participants to learn the daily habits of the world’s most efficient molders to establish and preserve workplace organization, according to Routsis. In order to be competitive in general, you need to be efficient and effective,” said Dan Stephens, senior plastics engineer and a trainer with Routsis. -DeRosa, Angie "Firms should see MRO as path to peak performance" Plastics Machinery Magazine. May 2016

You can read more about the concerns of MRO here:

The screw does most of the melting, but the barrel temperature should be maintaining the melt temperature.

Question: How do I know if there is too much back pressure?

My Response: The quick method is to make a significant drop in the back pressure, such as reduce by half (ie. 1000psi to 500 psi) and see if the melt temperature shows a significant drop (>10°F or >5°C). If so, it is likely too high.

The best method for this is to plot back pressure vs. melt temperature to see the trend. For example, you might see results such as 445°F@1500psi, 427°F@1000psi, 425°F@500psi, 422°F@250psi, 420°@0psi. This would indicate that any back pressure 1000psi or less is likely acceptable, though lower back pressures tend to provide better long-term processes.

Additional Thoughts: If you need a higher melt temperature to disperse a colorant or fill the mold, you should add heat using barrel temperature. Using high back pressure is a very unreliable way to add heat to a polymer and will introduce inconsistency to your process.

The molding machine does it's best to achieve the set mold open and close speed as long as there is enough pressure available.

Question: What is the pressure and speed relation for clamp open and close? How can I make sure that I have enough pressure to satisfy the set speed?

Answer: During clamp movement, the molding machine adjusts the pressure (or power when using electric machines) to the clamp in an attempt to achieve the desired speed. If adequate pressure is not available, the machine will not be able to achieve the desired open or close speed because the movement will be pressure limited... This will result in inconsistent clamp movements and part removal times.

It is easy to determine whether you are using enough pressure 1) Time the current part removal time. 2) increase the pressure and time the part removal time. If your part removal time decreases with a higher pressure (2), then you had inadequate pressure.

Additional Thoughts: If you want to determine the optimum pressure: 1) Use the steps above to verify you are not pressure limited 2) incrementally reduce the pressures to determine the minimum pressure necessary to maintain your part removal time 3) once the minimum is determined, add 10% to each pressure to compensate for normal variation. These steps will ensure you have adequate pressure to maintain cycle time, but not excessive pressure... this will reduce the amount of mold and machine damage which might occur in the case of complications.

note: The above discussion does not include mold protect. Mold protect intentionally use a low-pressure, pressure limited clamp movement to protect the mold during the final stages of mold closing.

The fun of molding PP is that it can take a significant amount of abuse during molding and end use, but only if it has been melted properly.

Concern: In the past week, two different customers consulted us for assistance in troubleshooting failures with their molded parts. They were both running PP and were both running back pressures around 1500psi plastic pressure. Not to get into specifics, the parts were exhibiting brittleness during end-use testing. This was not necessarily the only issue, but significantly high back pressures are common issue in this industry.

My Recommendations: There tends to be a misunderstanding of the proper use and setting of back pressure. This value is typically set too high and is adjusted far too often during troubleshooting. In general, you want to use enough back pressure for mixing and melt consistency, but not much more. This pressure should not have to be changed during production unless a significant change such as the composition of the material, new additives, new screw, or the screw or barrel have been modified in some manner (lot to lot changes in material do not qualify for a back pressure change). 

Essentially, i you need more heat for the colorant to properly disperse, then add barrel temperature... likewise, if you need more melt temperature for the material to melt, mix or inject properly, then add barrel temperature as well. This is why it is critical to know the melt temperature which is used to mold a good part as it is critical to troubleshooting melt-related problems. 

Additional Comments: You should never use back pressure to add extra heat to the material during recovery as this breaks polymer chains & fibers as well as degrades many of your additives, colorants, & processing aids. All of this will reduce the strength of your part and make it more brittle.

It is always critical to check your first stage short shot since it affects both mold filling and part packing.

Request: Please provide reasons & remedies of burn-mark or weld line problems in plastic (HDPE) moulded product during the injection moulding process.

My Response: There are many possible causes, but injection should be checked first:

First, the mold must not be full during 1st stage injection.

If the mold fills all the way during 1st stage injection:
  • The gas will be trapped in the mold and may cause burning
  • The screw will bounce back which will cause
    1. Melt pressure loss in the cavity
    2. Reduced pressure at the melt front
    3. Reduced weld or meld line strength

If you short shot the mold, verify the burning is not present, and then finish filling the mold cavity with packing pressure, you may get better weld line and and burn marks.

You should also consider reducing the clamp tonnage to allow the mold to vent better.

Additional Comments: You can also download our reference guide for more assistance:

Cavitation reduction is not as simple as blocking off a couple cavities and continuing to run. The process should be as close as possible to the original process, but this will require an adjustment to the process inputs.

Question: Do you have a good process for blocking cavities?

My Response: The process is almost identical to the original, but these are some of the characteristics of the new process:

Process Inputs/Outputs which should not change from the Standard Process:

1. 2nd Stage Packing Time
2. 2nd Stage Packing Pressure
3. Cooling Time
4. Back Pressure

Process Outputs which should not change from the Standard Process (may require a change to the process inputs):
1. 1st Stage Injection Time
2. Recovery Time
3. Coolant Temperature
4. Cycle Time

Process Outputs which will change from the Standard Process (may require a change to the process inputs):
1. 1st Stage Injection Weight = (Standard 1st Stage Weight)*(New Cavitation)/(Standard Cavitation)
2. 2nd Stage Final Part Weight = (Standard 2nd Stage Weight)*(New Cavitation)/(Standard Cavitation)
3. Clamp Tonnage = (minimum tonnage required)*(1.1)

Additional Comments: Cavitation reduction should always be avoided or remedied whenever possible. Since the cycle time does not change, a 25% reduction in cavitation requires 33% more cycles to produce the same number of parts. Such a process is always less stable than full cavitation, so the defect rate will also increase with reduced cavitation. This can quickly reduce overall production capacity and often make the difference between profit and loss.


Thin wall molding is a relative term that implies the flow length is long with respect to the part thickness and material viscosity...

Question:  Our thick PVC parts require a higher packing pressure than the injection pressure, yet the training states this is typically the case for thin-wall molding. In our case, the parts are often over 0.300" thick... can you please explain this?

My Response: In your case, the walls are thin relative to the flow length for the very high polymer viscosity of rigid PVC. I recommend running a rheology curve (or at least a speed study), take a picture of the shorts and take notes at each speed (ie gate appearance, part appearance, burning, etc.). With PVC (& CPVC), the viscosity is very high, but the polymer is also very sensitive to shear thinning which allows for some reduction.


Keep in mind that clamp maintenance should be routine and comprehensive. Once a tie bar breaks, it is critical that you ensure everything is level, parallel, lubricated, and functioning properly. If you are unsure, have an expert come in to perform a detailed analysis of your tie bars, platen movement, lubrication, bushings, and leveling.

What are the various causes of a broken Tie-Bar on a Toggle-Type Injection Molding Machine?

My Response:
The tie bars are designed to last a long time, but only if ALL the following conditions are met:
All the tie bars are parallel
The mold travels perpendicular to the platen surface
All the tie bars are properly level
The machine is properly leveled and supported
The force is applied evenly across all tie bars
The mold is properly sized and centered
The tonnage requirements are centered (balanced)
The platens and tie bars do not get too hot
Proper lubrication is applied

Below are many of the factors that cause premature tie bar failure 
Ex-centric (off-center) mold
Mold with an off-center tonnage requirement
Machine not level or properly supported
Too much unsupported weight on the movable side 
Travel is not perpendicular to platen surface
Platens not parallel
Hot runner molds without insulator plate
Over-clamping a mold
Uneven mold wear
Bent or twisted machine base 
Mold too small for the machine
Poor initial construction
Poor or uneven lubrication
Excessive platen concavity
Cracked platen
Uneven mold bolting
Excessive platen stoning

Recently we were assisting a customer in troubleshooting a problem with inconsistent recovery via email. This was a great reminder that it is always important to check and ensure there is a short shot at the start of troubleshooting most problems.

Customer Issue (shortened): 
During a rear zone study, the customer was encountering complications with the process. After further evaluation, it was found that the screw recovery was fluctuating nearly 40% from shot to shot.

In our experience, an inconsistent recovery is most often caused by an inconsistent cushion (at end of packing). The customer reported a cushion which was consistently at .038in, so at first glance it appeared this was not the problem. Upon further correspondance, it was found that the transfer position was set to 0.040in and a short shot was not being molded. In this case, the machine determined the 'cushion' as the forward-most position before screw bounce back occurred.

The customer adjusted the transfer up to 0.070in. This obtained a short shot during first stage and provided a consistent recovery time, which was the original intent.


It is always best to isolate the barrel so the check ring is the only thing being tested, and outside influences are minimized.

With a 2-shot machine, how do I perform a check ring capability study on my 2nd shot?

My Response:
You want to run the shot by itself if possible. Using both barrels will result in a less reliable number. If you were forced to use both shots, the fill and pack the first shot and then subtract that weight from the final weight.

Additional Thoughts: 
When using a two shot machine, have a way to segregate the parts from the two cores. If the check ring shows high variation, you can run the numbers again separately. 

For example if the combined variation is 5%, but the variation in shot 1 and shot 2 may only be 1% by themselves. In such a case, the ring is fine, but the tooling needs help.

Adversely if a high variation is present in one or both of the shots when calculated alone, then the check ring is inconsistent.

Just because the mold is flashing, does not mean the part weight has increased...

Question: Hello, we have a part which is flashing, but the part weight is lower than before. Does this make sense?

Answer: When this is the case, it is usually caused by too much material entering the mold during injection. As the mold becomes full, the pressure builds in the mold cavity. When this process transfers from high pressure injection to low pressure packing, the screw bounces back causing material to exit the mold cavity. The high high pressure may cause flash, but the screw bounceback loses material which can not always be compensated for with packing. The result is often a lower part weight. Beware, that it is common for sinks or voids to also be present in such a part. This is a great example of why you always want to document the final part weight as well as the short-shot weight (without packing) for all your processes.

Additional Thoughts:  The best solution for such a problem is to have the process transfer before any of the mold cavities are full. This allows you to use packing pressure and time to more accurately control the final part weight.

Many older electric molding machines (and a few new ones) have a programming glitch which causes the screw to retract if the packing or back pressure is set too low.

Question: When trying to make a short shot, I set my packing pressure to 0 and the machine pulls the screw back during packing. How can I make an injection-only short shot?

Answer: Turn packing time to 0 (or the minimum allowed time) and use additional cooling time (if necessary) to maintain the same overall cycle time.

The Cause: The screw is retracting due to a programming error on the machine. The machine senses that the pressure in front of the screw at the end of injection is higher than the set packing pressure (0) and it begins to retract the screw. The same thing often occurs during screw recovery. This misbehavior also happens on some machines when the mechanical resistance to movement is higher than the pressure setpoint. If the machine is not well lubricated and maintained, the minimum packing pressure and back pressure necessary to prevent screw retraction can often be very high. In such cases you should consider having a service representative review the condition of the machine and the software to determine if something can be done to rectify this situation.

Part removal is the easiest part of the injection molding process to optimize and there is no excuse for basic clamp movement optimization.

Situation: While walking the recent NPE 2015 trade show in Orlando, many of the processes were dialed in beautifully where the mold quickly opens and closes while the part is ejected on the fly. Unfortunately, some machine manufacturers had their molds slowly opening and closing, pausing 1 or 2 seconds before starting ejection, and a couple molds even cycled the ejection system multiple times. Many of these processes lost many seconds of cycle time to clamp movement. This poorly showcases a machine at the show, but loses a lot of productivity at your plant.

My Suggestion: You should always optimize clamp movements You do not have to slap the mold shut or shake the machine violently as the mold whips open to improve your part removal process. A good basic strategy is as follows:

Mold Opening Fast - This takes place after the initial mold breakaway speed. After the mold components are disengaged, the opening speed should be increased since there are no obstructions. The optimal speed is the fastest speed the machine can safely handle without any awkward movements, machine vibrations.You may need a third, moderate final opening speed if the machine has difficulty maintaining a consistent final mold opening position for robotic part removal.

Final Mold Open - The final mold open position should only be enough to allow proper part ejection.

Eject on the Fly - If a robot is not being used, you should start part ejection before the mold is opened. The fact that the mold is moving away from the part tends to help the part drop downward. This feature typically reduces the total amount of distance the mold has to open when properly set.

Mold Close Fast - Just as with mold open fast, the mold should move quickly when the mold components are not engaged. It is critical that the mold closing speed slows down to a safe speed prior to the mold components making contact for the final mold closing.

Hello Readers,

This post is to inform all our readers that all our online content is now deliverable on any device with a high-speed internet connection.

You can learn more about our training options at: http://www.traininteractive.com/training/

A question recently came in regarding an older post about running high-temperature materials in normal machinery: High Temp Machinery

Question: We run high temp resin in newer electric presses. i do not believe our injection units were retrofitted with any special materials. the main resin put through our units is PAEK  with 30% glass reinforcement, which is a tannish color. we have been experiencing "black streaks" on the part surfaces, and no one in house can seem to figure out the root cause. These streaks are usually  dark brown, (despite the symptom name) and really seems to be burnt up resin at the edge of degradation to me... but whether or not our steel can handle the constant high heat cycles has been the main focus of attention. any thoughts from your end that could possibly help us out? we keep the rear zones at about 700*F on these jobs.

My Response: The most important thing to do first is to compare the actual melt temperature with the manufacturer's suggested temperature and verify the material is being melted properly. I have seen machines set at 750°F with a resulting melt temperature of only 675°F. In this example, we were able to increase the cooling time and significantly slow down the recovery speed to get the melt temp up to 710°F. This material was still 15° below the recommended temp, but acceptable parts were able to be produced for the while better heater bands were ordered.

There is no true 'preform temperature' because there is dynamic range of temperatures along the entire length of the preform.

Question: What should the finish temp be on the preform as it leaves the oven?

My Response: There is no specific temperature, but typically a profile this is specific to the material, mold, and equipment you are processing. For this reason, there may be an average temperature recommendation such as 100°C which might be used, but it can be very misleading. There is a range of temperatures which typically vary between 80-110° along the length of the preform. One of the most important aspects is to ensure you are measuring different locations on the preform at the exact same location each time. A hand-held probe will not give you repeatable, reliable, or useful results.

All the Scientific Molding principles apply when developing a process for V-P transfer using a temperature probe as with a process developed with position transfer.

Question: How does the development of a process differ when using a position transfer vs.a transfer via temperature sensor?

My Response:  The purpose of transferring using a signal from a temperature sensor is to establish a consistent mold cavity filling volume each shot. The first step is to build a good process using position transfer. Once you have established a good reliable process, you know the desired injection volume necessary to make a good part. From this point on, you adjust your temperature sensor transfer settings to get the same fill volume as your established process. This technology does not replace good process development, it just just provides you a method of better repeating that acceptable process.

Additional Thoughts: There is a large amount of processing technology available, but they do not replace god processing. The best use of this technology is to build a good reliable process which compensates for most variation, and then use the technology to reduce variation from that established process.

Salaried employees are much more likely to train outside of the office since they can pick locations/times which are much more convenient for themselves.

Question: How common is it to have employees take the online training at home?

My Response: Salaried employees tend to be more motivated to train on their own time. If the training is required for the job function, hourly employees are very unlikely to train on their own time. It is best to make time during their shift or provide access before or after their shift.

After the required training is completed, some companies have had success getting hourly employees to conduct advancement training on their own time... but these company typically have other incentives such as bonuses or pay increases after the training is complete. Such incentives make this additional effort worthwhile.

When using mechanical purging compounds to purge co-extrusion dies, it is important to not purge all the extruders through the die at once to prevent excessive die pressure buildup.

Question: I have been told "For co-extrusion it is important to purge from first to last and not all at once". Could you explain this?

My Response: Every co-extrusion die is constructed differently, but the main point is to avoid purging all the extruders with a mechanical compound at once if the die is somewhat restrictive. Since non-abrasive mechanical purging compounds tend to increase the die pressure, filling the die with compound from each extruder all at once may result in damage to the die. Some companies have had the die blown right off of the extruders when they have purged concurrently.

Safe procedures for using mechanical purging compounds in co-extrusion dies would involve purging one extruder at a time. This can either be purging to either the final material, a virgin resin, or a more neutral material such as polypropylene before moving on to the next extruder. Many molders will also reduce the RPM of all the extruders to help reduce pressure buildup.  This reduces the pressure spikes in the die making it safer and less stressful to the die when purging.

The fill time is not expected to vary much form shot to shot. Typically, large variations indicate a parameter change or a change in machine performance.

Question: We are using fill time as one of the process outputs for monitoring the process consistency. There is hardly any change which is showing variations. My Question is it a right approach to monitor the fill time data with variation of only + 0.04 seconds.

My Response: The lack of variation implies accurately functioning machinery. The purpose of monitoring fill time is to indicate a significant change. This might be a machine variation, pressure limited process, or a parameter change that affects 1st Stage Injection. Typically limits of + 0.5% will identify such changes when they occur.

When your mold is 98% full, packing is already beginning to take place. This makes it nearly impossible to completely separate filling from packing.

Question: I configured my mold with a 98% fill and finish with packing. Why do I still get sinks and short shots?

My Response: We always recommend 90-95% because 98% fill causes the following problems:

1) Since only a small portion of the mold is filling at 98%, packing has already begun to take place in most of the mold cavity. This results in a blending of packing and filling. Whenever the fill varies, the degree of packing will also vary again.

2) Plastics tend to have 5-10% normal variation in viscosity. A 98% fill can only compensate for small variations in material viscosity. In such a process, a moderate drop in viscosity is likely to cause flash, overpacking, overweight parts, warpage, large dimensions, cycling problems, or part sticking. Likewise, a moderate rise in viscosity can cause shorts, sinks, voids, warpage small dimensions, and cycling problems.

3) Common variation in a 'healthy' check ring is typically between 1-3%. This means a process established with a 98% fill can vary between 96.5% and 99.5% full with a 'healthy' check ring. The parts produced from a 99.5% fill are going to be much different than those produced from a 96.5% fill.

4) When the fill reaches 98% full, the mold cavity is beginning to pack. This results in a higher requirement for clamp tonnage since it is required to overcome injection pressure rather than packing pressure.

For more about why 90-95% fill, please feel free to read the following:

We have a new Mobile App that is available for your entire staff at no cost. 

This handy Scientific Molding “Pocket Guide” contains essential processing information for all injection molders.

The guide now available as an app for both Apple™ & Android™ devices, or pdf.

The Scientific Molding Reference Guide includes:

Understanding Plastics
Plastic Materials 
Material Properties, Additives & Preparation
Establishing a Scientific Molding Process
Seven Steps to Scientific Troubleshooting
Molded Part Defects
Basic Mold & Part Design Guidelines
Frequently Used Calculations
The Importance of Training

To download, simply click on the following link:

Spread the word and share this link with everyone.

I'd love to hear your feedback in the comments section.

Polymers are compressible, so there are many ways to increase density of your molded part. This is especially true for semi-crystalline polymers.

Question: Our customer is complaining our product density is lower than other suppliers based on a 3D CT SCAN inspection of our product. How do we increase the density of the PA66+30%GF product.

My Response: There are a few aspects to this, but you should always start with the basics:
1) A higher melt or mold temperature will slow cooling, resulting in more semi-crystallinity.

2) A short shot during fill will allow you to use more packing pressure without flashing the mold.
3) A higher injection speed will reduce the pressure drop during fill and allow more packing to take place.

4) A shorter cooling time will allow the part to have more cooling after molding.


Fixing Mold Sweat...

Andy Routsis
Vote 0 Votes
In a non-climate controlled environment, cold molds will often 'sweat' on humid days due to the mold surface temperature being lower than the dewpoint of the surrounding air.

Question: We have to warm up our molds during the summer to keep the molds from sweating. This results in much longer cooling times. Management will not add climate control to the facility, but can the molds run faster?

My Response: If you cannot control the plant environment, you can still control the mold environment. Many molders will put dehumidifiers next to the machine to provide a dry air environment within the clamp area.

More Comments: If the sweat is mild, you can also gently blow compressed air at the core and cavity surface since this air typically has a dewpoint of -10 to -20 degrees.

Knowledge of a topic or concept is critical to understanding, but practice is the key to skills development.

Question: I want my engineers to learn scientific molding skills, but can't they just take the online courses? Do they really have to bother with the hands-on worksheets?

My Response: If someone reads about Scientific Molding in a book, they may become knowledgeable in the topic. to become skilled in Scientific Molding, he/she must actually go out on the production floor and do it. This gives them a concrete understanding of the application of the knowledge to the production environment. This is why we only perform classroom training in conjunction with hands-on processing equipment.

Additional Thoughts: More According to the Oxford Dictionary, a skill is the ability to do something well.

Essentially, a person has to have a certain level of competency and proficiency to be 'skilled'.

The basics of scientific molding is the same whether you are molding CPVC or PP as it is with large parts vs. small parts. Essentially, the difference between is in the range and scale of parameters and not the scientific approach.

Question: Isn't it dangerous to mold PVC with a cushion since the extra material in front of the screw will degrade?

My Response: This has a negligible affect on the residence time since the cushion is a very small fraction of entire volume of material within the barrel. With PVC , it is vitally important to mold with a cushion since the performance and repeat ability of the smear tip is greatly influenced by the material viscosity. Frequent variation in part weight and quality will occur if a cushion is not used because viscosity of PVC is greatly influenced by small material temperature fluctuations.

Additional Information: Since the screw typically contains approximately 2 times the maximum shot capacity of the molding machine, a 5-10% cushion will not affect the integrity of the PVC within the barrel.

With the right mold preparation, you can use one set of ejector rods in every machine. One this is done, you can keep the rods in the machine and use the same ejection retract position for all your molds.

Issue: Most of your molds use ejector rods to push the ejection system and you always have to change and adjust ejector rods during each mold change.

Resolution: Spacers should be added to each mold which screw into the ejector plate and are flush with the back of the clamping plate. If ties ejection is used, all the spacers should have the same threading for the knockout rods. Then each machine can have ejector rods machined to be flush with the movable platen when the ejection retract is in the back-most position.

Additional Information: As for the rods and spacers, I recommend they be machined out of Col-Rolled Steel to ensure their strength and endurance.

A cooler mold tends to provide larger part dimensions, but this can often be offset by a reduction in cooling time. It is generally worth the effort to perform a quick DOE to verify.

Concern: During an on-site training session, we were discussing how a cooler mold typically provides the best potential for a lower cycle time. In this discourse, one of the participants was explaining that they typically use mold temperature to achieve specific part dimensions.

My Response: As a class, we used one of their dimensionally critical parts and performed a small DOE with 3 different cooling times at 3 different mold temperatures. Once complete, a part molded 4 seconds faster with a mold temperature set 20 degrees F lower had better dimensions than before. This goes to show that just because you made a good part, a better process is not impossible.


Why Train Operators?

Andy Routsis
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You should have training available to all your employees. This will give you the largest available pool of employees to choose from for advancement.

Question: Why would I want to include operators and part inspectors in my training plan?

My Response: You never know where your next technician could come from. Technical proficiency is something you can develop over time. When you expose your employees to more information, some will be indifferent, while others will want to learn more and expand their knowledge. Unfortunately, many of the people who want to learn more do not even realize it until the information is made available. Our customers are often surprised at whom wants to learn and improve.

Whether you are processing PVC, Nylon, PS, or LCP, the basic strategy for filling and packing the mold is fundamentally the same.

Question: I am having a problem on my connector mold having a short shot. I am using lcp as the material and my mold is 8 cavity mold with submarine gate.

My Response:

Regarding Imbalance - You can improve the cavity imbalance either through a different injection speed or a flow balancing technology such as the MeltFlipper®.

Regarding Processing - The following 4 steps will help you maximize the potential of the tool in its current state:

1) Measure the melt temperature and ensure it is within manufacturer’s recommendations.

2) Make all cavities short shot at time of transfer.

3) Use a packing pressure which will fill out the mold and give you acceptable parts.

4) Reduce the clamp tonnage to allow more venting (this must be done after the transfer and packing is complete).

Other aspects can be investigated once the balance and process issues are addressed.


Is SMED Realistic?

Andy Routsis
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Although true SMED (Single Minute Exchange of Dies) is not likely for all molders, significant reductions in mold change time using the same principles are easily attainable.

Question: Is SMED realistic?

My Response: The underlying benefits to the SMED concept is the preparation. Essentially everything necessary to change a mold is already there and ready and easily accessible. This way the mold can be shut down when the run is done, the changeover can take place, and the new mold is up and running in the shortest time possible. Essentially, if you have to waste time looking for paperwork, finding a hose, replacing a connector, locating a thermolator, checking water lines, verifying hot runner systems, etc. you cannot attain an optimal mold change time. All the work is done before the machine is shut down so you attain the maximum efficiency when the machine is not making parts (ie. the company is losing money).

This is a good starting point, but the most important factor is to know the 1st Stage Fill Time and 1st Stage Short Shot Weight. If these are matched, then the outputs match which is critical.

QuestionSay i have a process with .94sec fill time and 36mm screw. When i change to a 50mm screw, what is the formula to match the same fill/shear rate/?

My Response: This is based off of a ratio between the effective surface are of the screw. The basic formula would be as follows:

Machine B Speed = (Machine A Speed) * [(Machine A Radius)^2 / (Machine B Radius)^2]

For your example:

Machine B Speed = (Machine A Speed) * [(18)^2 / (25)^2] = (Machine A Speed) * 0.518

Additional Thoughts: It is typically easier to match the fill time and short shot weight from the previous process. This will not be exaclty correct because it will not compensate for aspects such as decompression and check ring wear.


When these are in the mold surface, small changes such as reduced melt temperature, slower injection speed, and reduced packing pressure tend to help. These reduce the polymer’s ability to duplicate the cavity detail will improve the overall appearance, but do not fix the underlying problems. 

Question: Are Tiger Stripes Solvable? All the processing changes such as increasing melt , mold temperature , adjusting velocity did not yield result to eliminate however improved the condition. 

My Response: 'Tiger Stripes' is a visual condition on the final part related to poor surface finish. Since the location of material-related defects are not predictable and consistent form shot to shot, this is not likely to be caused by the material since material coloring defects are more random and not consistent in appearance. Some high viscosity and low gloss grades of material may show less cavity detail, and therefor hide the problem, but they will not ‘fix’ the problem. Three of the most common causes are listed below:

1) Poor Sandblasting - The texture was not well applied to the mold. In this case, the striping is likely parallel with the edge of the mold base. This is because the employee would have passed their hand  back and forth when sandblasting, causing lines in the finish if they hold the blaster too close or do not spray evenly. If this is the case, the mold surface needs to be repaired and returned to its original condition

2) Poor Venting (New Problem) - When air cannot escape from the mold it becomes trapped against the mold surface. These tend to occur perpendicular to the direction of flow. When this happens, the polymer does not properly adhere to the mold surface causing it to ‘skip’, giving the cavity a rippled appearance. If this is a new problem, improving the venting, reducing the tonnage, and thoroughly cleaning the mold surface with safe solvents should significantly improve the situation. 

3) Poor Venting (Old Problem) - Unfortunately, if this has been occurring for a long period of time, the mold surface will actually become damaged due to the slipping polymer and buildup of corrosive volatiles on the mold surface. If this is the case, the mold surface needs to be repaired and returned to its original condition and much more venting needs to be added.


Weld Line Strength...

Andy Routsis
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Weld line strength is primarily influenced by the amount of interaction that can be caused at the weld line location. Gas traps and low pressure at the weld line location are some of the primary causes for low weld line strength.

Concern: I am having some issues with one of our newer molds. We are using a PPE 20% glass.  We are getting weld lines in around screw wholes. We have added another gate to move the weld line over, which it did not, and still having some issues with the part cracking. So part of the problem we believe is the mold, part processing, and part the material. 

My Response: Although there are many potential problems, I will address the strength of the weld line here.

If you have gas trapped at the weld line location, it displaces the polymer, reducing the interaction. This typically causes a notch in the surface of the part making it a stress concentration and potential fracture point. If you can feel the weld line with your fingertip or a knife edge, you likely have a gas trap issue.

If you have too little pressure at the weld line location, the polymer chains will not interact, resulting in a weaker weld line. There are many ways to lose pressure but the most common reasons are 1) a slow injection speed will result in a high pressure loss during injection 2) Injecting too much material during 1st stage will cause a spike in pressure causing stress in the part, but the screw will bounce back resulting in a less effective packing phase. This is a common problem and may be contributing to the cracking issues you are encountering.


High Temp Machinery...

Andy Routsis
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Just because a machine is capable of processing at high temperatures, does not mean it is optimized for use at these temperatures. A machine which spends most of it's time at high temperatures should be purchased and designed for that intent. A failure to do so will result in potential inefficiencies and premature equipment failure.

Situation: Recently I was working with a company who processes a large number of high-temperature materials. The manufacturer states the machine can operate at high temperatures without significant modifications, but it is not operating optimally. The melt temperature is consistently 30-50 degrees C below set point and the locking check ring is expanding more than the barrel causing a squealing to occur.

My Recommendations: If you occasionally run high temp materials, then a stock machine may be OK. If you are intending to process mostly high temp materials, especially at high speeds, you should consider a machine deigned for that application. Any thermal expansion differentials should be considered with respect to the materials & tolerances used for the screw, check ring, and barrel. The machine should also be fitted with heater bands designed for high temperature use to heat the material quickly and efficiently. There are many other considerations such as proper heat shield design, adequate nozzle heaters, and a pre-heating capable hopper which can improve the overall efficiency of the machine.

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All participants will receive 10-day access to four of our most popular training courses as well as the highly popular Injection Molding Reference Guide & a Scientific Molding Process spreadsheet.

This free webinar being this Wednesday, October 2nd @ 2PM Eastern Time.

The basic principles apply whether you are using valve gates or not. In theory, you try to use one flow rate through the gates whenever possible to fill the mold because it reduces variation in the process due to a consistent shear on the material.

Question:One of your recommendations on scientific processing is to avoid profiling and perform mold filling in 1st stage.
How this more relevant in today's scenario as many molders are molding the molds having Hot runner systems with sequence in gating?

My Response: The recommendation of using one injection speed when possible is based on the goal of maintaining one shear rate (flow rate) through the gate as the material fills the mold.  Not every mold make a good part with one flow rate, but you always strive for this since it simplifies the process and reduces variability. The approach to sequential valve gating is based on the gating layout and purpose. I will lay out 2 common scenarios and how this can be dealt with the same theoretical approach.

Scenario 1 - Gates opens after melt front has passed: In this case, the flow front begins when the first gate in the cavity opens and then more gates open sequentially only after the melt front has passed. In this situation, you should maintain the same injection speed when possible. This is because the material flow front will maintain the same flow rate and will result in a more consistent shear rate across the entire part.

Scenario 2 - Gates open at different locations starting new flow fronts: In this case, the first gate in the cavity opens, possibly to fill a larger area, and then a second gate opens in a different location and begins a new flow front. These flow fronts will eventually meet as the cavity fills. Assuming the gates are of similar size, you would want to double the injection speed as the gate opens since you doubled the number of flow fronts being serviced. This will cause the flow rate though each gate to remain the same resulting in an even shear rate across the part.


Making Change Stick...

Andy Routsis
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Implementing change takes time. Change is a critical aspect of business today, but you cannot just say 'Do This' and expect everyone to jump on board immediately.

Issue: Every time we implement something new, our people go back to the old ways in just a couple weeks.

My Response: It takes 3-6 months to replace an old learned behavior with a new behavior. For this reason you need to view change with an on-going approach. You cannot just tell someone that a different way is better, and expect them to change. This is especially true if they are experienced and have invested many years into doing things a different way. You have to remember that they have been succeeding for years with the old way, so they need to understand the 'Why' as well as the 'How'. Furthermore, you have to reinforce the new behavior for months to prevent them from going back to comfortable.


The Need for Technicians...

Andy Routsis
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The lack of good technical employees to hire is a global issue and must be addressed in-house. You cannot rely on poaching talent from others to fill all your technician needs.

Question: You don't understand how hard it is to hire technicians in our area. Where is the best place to look for technicians?

My Response: There is no place on the planet where plastics technicians grow on trees. Every company has their own equipment, procedures, parts, customers, paperwork, molds, plastics, etc. For this reason, even if you can hire a technician you hope knows how to process, they still need to learn everything else about your company. This takes time to make a poached technician effective in your company and typically result in a technician which can barely get the job done. Your best bet is to determine the skills and knowledge necessary to develop a good technician and have a plan to teach your employees this information so they can advance from within.


Improving Mixing...

Andy Routsis
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Screw speed is not the most consistent method of improving mixing within the barrel. Back Pressure and Melt Temperature are more consistent and reliable methods of mixing your polymer.

Question: I read your post on using a slow screw for long cycle time for processes like ours. My techs tell me they need to use a high screw speed to get the material properly mixed, is this true?

My Response: High rotational speeds will add shear to the material. Excessively high speeds will also cause heating due to excessive polymer chain breakage and degradation, especially if semi-crystalline polymers are being forced through the transition zone before they are ready to melt. Both of these results are not consistent & reliable, and will add variability to your process. Good mixing typically requires shear, temperature, or both. If you need more back flow & shear, then you should add back pressure to the process. If the material needs more heat to mix or melt properly, then you should increase the middle and front zones to increase the melt temperature. In any case, you should document the Melt Temperature (probe inserted into purge), Back Pressure (in plastic pressure), and Recovery Time (in seconds) when the mixing is adequate so your melting profile can be easily duplicated in the future.


Slow Screw Speeds...

Andy Routsis
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On the newer electric injection molding machines, there is typically no detriment to using a slow screw speed for longer cycle times. This will just result in a more uniform and consistent melt.

Question: Your training states that recovery should consume 80% of the cooling time. Some of our cooling times are almost 2 minutes, will this put excessive strain on our new electric machines?

My Response: This actually adds no additional strain on your servo motor. As long as the machine can maintain a consistent recovery time, you should use a slow screw speed when possible. I have seen all-electric molding machines capable of maintaining consistent recovery times with rotational speeds lower than 10 RPM.


The employees need examples of how the documentation makes their job easier and reduces process shift. If they do not understand how the training helps themselves, they are less likely to document the process changes.

Question: I need advice as to how to better enforce the use of process sheets and track/record any changes made. I am hoping that somebody in the blog an contribute as well.

1) The employees need examples of how the documentation makes their job easier and reduces process shift.

No doc example: Shift 1 gets flash and increases tonnage, shift 2 sees burning and reduces injection speed, shift 3 gets sinks at the end of fill and reduces mold temp, shift 1 then finds large dimensions and decreases packing, shift 2 gets sinks and decreases melt temperature, etc.

Doc example: Shift 1 gets flash and increase tonnage, shift 2 sees burning and cleans the vents and reduces the tonnage a little and the process runs well for the next few days.

2) They also need to know how it helps them identify non-processing problems and bring them to the attention of management. For example, if a mold keeps having burning issues, then they can easily justify the need to invest in venting improvements to the tooling.

3) The employees are never reprimanded for documenting their changes. Employees are trying to do a good job, and when they are attacked for documenting their changes and following procedures, they lose all motivation to document any further changes. (ie. Never let management paw through the logs to determine who made the change last night which causes bad parts)

4) The management needs to reprimand those who do not document changes, including engineers and managers. Ultimately, management needs to back the effort for it to succeed. I had this policy instituted at a company I worked for and was deservedly reprimanded a week later for not documenting my changes during an overnight troubleshooting session.

When we do on-site training we constantly reinforce the benefits and helpfulness of documentation with respect to easing the job of processors and technicians.

All readers are welcome to comment with their experience as well.


Sort vs. Standardize

Andy Routsis
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There are many variations in 5S, and many people get confused between some of the terms. Since the terms all revolve around the same Lean Manufacturing principles there is often a confusion of concepts.

Question: How do I explain the difference between Sorting and Standardizing?

My Response: Sorting involves the organization of workplace and the elimination of unwanted materials. Standardizing typically takes place afterwards to make similar workplaces look alike. For example, Sorting may determine that machine #1 and #2 both require a melt temperature probe at the machine; Standardizing will place the melt temperature probe next to the control panel on both machines so it can be easily found.