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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
Vote 1 Vote
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
Vote 1 Vote
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
Vote 0 Votes
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
Vote 0 Votes
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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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
Vote 0 Votes
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
Vote 0 Votes
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
Vote 0 Votes
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
Vote 0 Votes
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
Vote 0 Votes
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.

I am often asked which method is the best way to purge. Unfortunately there are so many different types of compounds and applications that there no specific method which is best, but most procedures include the following aspects.

Preparation: One of the most important aspects of purging is the preparation. This includes obtaining and mixing your materials, acquiring the processing parameters, and reviewing the recommended procedures.

Initial Cleaning: The old material must be vacuumed and wiped out to avoid cross contamination.

Initial Purge: The purging material must be run through the machine to remove the old material from the barrel, screw, nozzle, and hot runner system.

Final Cleaning: A second cleaning operation is typically needed to clear out the purge form the hopper and material delivery system.

Final Purge: The final material is run through the barrel, screw, nozzle, and hot runner system to push out the purging compound.

Production: The last step involves getting the machine back into production mode while making acceptable product.

I understand there are many purging products which have specific requirements, but you will find these steps involved in most purging procedures you encounter.

When making a change to a plastics process, it should always be large enough to make a noticable change. It is often difficult or impossible to differentiate between normal variation and the effects of a small change.

Question: Why do you advise against making small gradual adjustments?

My Response: Plastics materials introduce an inherent degree of variability to the process. Additionally, aspects such as temperature, humidity, and equipment wear are always fluctuating over time. For this reason, you will see always see some amount of shot to shot variation in the process as well as process fluctuations. For example, if you increase the mold temperature 2° to adjust a dimension, any change you might see could easily be the result of normal process variation in a different parameter such as material viscosity. If the temperature was adjusted by 10° and the dimension did not change significantly, then you know with certainty that the mold temperature does or does not significantly affect this dimension. 

Semi-crystallinity in polymers is a commonly misunderstood concept since it cannot be demonstrated or easily visualized. Semi-crystalline polymers have all the softening characteristics of an amorphous polymer combined with the melting characteristics associated with semi-crystalline behavior.

Question: What is different between melting amorphous and semi-crystalline polymers during recovery.

note: I have addressed this in two blog posts.

Semi-Crystalline Polymers: Like amorphous polymers, these polymers get much of their strength through chain entanglement and inter-molecular attraction. The difference is that semi-crystalline polymers have small portions of the long polymer chains that align into neatly packed and arranged semi-crystalline sites known as semi-crystalline regions. These small compact sites are points where the polymer has higher strength and increased rigidity contributing to the overall strength of the part. When melting semi-crystalline polymer, these semi-crystalline sites remain in tact until the polymer reaches a specific 'melt' temperature where the semi-crystalline sites dissolve. If not properly heated, the semi-crystalline sites remain in tact and the polymer chains will actually rip apart resulting in polymer chain degradation. For this reason, semi-crystalline polymers prefer to receive a lot of heat first and a small amount of shear in the end to compete the melting.

The melting of amorphous polymers is not like traditional liquids such as water. There is no specific temperature here the material tuns from solid form to liquid form since there is always some amount of polymer chain entanglement and inter-molecular attraction present to provide strength.

Question: What is different between melting amorphous and semi-crystalline polymers during recovery.

note: I will address this in two blog posts.

Amorphous Polymers: All polymers have polymer chain entanglement and some amount of inter-molecular attraction. When heated, the polymer chain mobility increases and the inter-molecular attraction will decrease, causing the polymer to 'soften'. In amorphous polymers, a constant shearing and heating of the polymer chains works best to prepare the polymer for processing. Screws designed for melting amorphous polymers tend to have a short feed zone with long gradual transition zone to provide consistent shear to help melt the polymer.

The circumferential screw speed from one machine does not directly relate to that of another machine, but it is a good starting point. This is because the material melt, shear, and flow characteristics within the screw channels will be different for each screw, so it is a machine-dependent parameter.

Question: Is circumferential screw speed a machine-independent parameter?

My Response: Circumferential screw speed is a neat parameter option for many molding machines as it measures the rotational speed of the outside of the screw. In theory, I like this parameter better than RPM when comparing different diameter screws, but it is not a fully-transferable machine parameter due to the differences from one screw to another when the diameter of the screw changes. Essentially, this data is a helpful reference when establishing the initial process. It is much more important to match the back pressure and recovery time as these are fully-transferable machine-independent parameters.

The purpose of conditioning is to ensure the polymer chains are in a consistent state to ensure the testing is objective. The orientation, crystallinity, strength, and impact resistance of polymers can be easily influenced through conditioning.

Question: My company deals with polymer compounding and we have an testing lab. According to ISO 291, we are required to condition the multipurpose testing bar for 88 hours. Wish to ask what should we do if during the 88 hours, the condition went of the specification. Thank you.

My Response: The standards do provide allowance for deviation and minimum times for compliant conditioning. For example, most materials require the atmospheric pressure must be within spec for 88 hours, but many material classes require less than 88 hours for temperature conditioning, especially when drying or moisture removal is needed. For this reason, I recommend you verify the specific requirements for your material class. If the conditioning does go out of specification, you must begin measuring the conditioning time at the last point when conditions are in specification.

Additional Thoughts: Keep in mind, many companies will test samples under non ISO conditions for practical or comparative purposes, yet the results cannot be considered ISO or ASTM compliant and should not be reported as such. As a compounder, if your intention is to derive objective data for the customer, then you must follow the standards. You should also have a data recorder which records the temperature, relative humidity, and atmospheric pressure in case your results are disputed. For this reason, you should have a standard methodology for processing the test samples and keep accurate records of this as well. 

A good in-mold labeling process must be reliable and consistent to ensure each part and cavity receives the same amount of pressure. Scientific Molding provides the basis for developing a robust and reliable injection molded part to which the label will adhere.

Question: I am working at a location where in mold labeling is growing. Do you have process training for IML?

My Response: I our experience most IML parts are rejected due to inconsistent processing and molding defects. If the robot puts the label in the right place, but the process is not stable and reliable, then the filling profile and cavity pressure will not be consistent; resulting in an inconsistent label location & adherence. For this reason, we would recommend training on scientific molding principles to ensure the process is robust, consistent, and reliable.


It is relatively easy to convert a percentage setting on your controller to the actual pressure applied to the plastic. The most important factor is that you need to know the maximum pressure capacity.

Question: How do I convert a pressure measurement which is provided in percentage? When the panel says 50%, what is that in pressure?

My Response: Percentage is per-cent or per-hundred. This is a ratio with 100 as the maximum and the percentage as a fraction of that. 50% = 50/100 = half of the maximum. To calculate this, convert the percentage to a fraction 50% = 50/100 = 0.50 and then multiply this by the maximum machine capability.

Pressure = (percent / 100) x Maximum

Essentially, you cannot generate reliable machine independent process documentation based on process outputs if the mold cavity fills completely during 1st stage injection.

Problem: I see many companies trying to utilize machine-independent process documentation while still filling the cavity completely during 1st stage injection. Since the mold cavity is both filled and packed during first stage injection  it can only be repeated when the material has the identical viscosity at the time the process was documented. Each time the material viscosity changes, the amount of filling and packing during first stage injection will change, requiring adjustments to the overall process. For this reason, machine-independent process documentation is only a moderately helpful tool when the cavity is 100% full at the time of transfer.

Resolution: Fill the mold 90 to 95% full during first stage injection and then pack out the part using 2nd stage packing pressure. This will allow you the benefit of documenting important repeatable process outputs such as 1st Stage Time, Melt Temperature, and 1st Stage Short Shot Weight.

Since the regrind has variable sized pieces, additional drying time may be necessary to ensure the thicker chunks are fully dried. The regrind process results in irregular-sized pieces which have different cross sections.

Question:  Most of our parts use an industrial grade resin ( recycled PA with GF) and often with 20% of  regrind resin. This means that the viscosity can vary significantly as well as the size of the “pellet”. Any recommendation when molding with recycled PAGF resin?

My ResponseSince the regrind has variable sized pieces, additional drying time may be necessary to ensure the thicker chunks are fully dried. You should always take measures to reduce the shear applied to this material. This will reduce the degree of polymer chain and additive degradation which takes place in the barrel. If the parts have thin walls, you will have to use higher 2nd stage packing pressures than you currently use, this can be more than the transfer pressure for many molds (assuming a 1st stage short shot is used).

When filling the mold completely during 1st stage fill, many processes respond well to pressure transfer when compared to position transfer. In either case, you will see even better results with a 1st stage short shot (90-95%) using position transfer.

QuestionI recently moved to another company and all the setups are using Injection Pressure as trigger to change from fill phase (Velocity controlled) to Holding ( Pressure controlled). Note that there since there is no pressure sensor in the mold but the machine injection pressure is used. A Engineer here tells me that this was done because while using a screw position switching point they had too much variation of the cushion leading to short shot and flash. What are the pros and cons of a pressure switching point versus screw position switch point? 

Regarding Hydraulic TransferHydraulic pressure transfer is used the fills mold completely during first stage. Once the mold fills, the pressure begins to increase rapidly and the pressure transfer method is used to identify this pressure spike and then transfer. When filling the mold completely during 1st stage fill, many processes respond well to pressure transfer when compared to position transfer. In either case, you will see even better results with a 1st stage short shot (90-95%) using position transfer.

My RecommendationsThe process should run with position transfer to best compensate for the viscosity changes. The part should be 90 to 95% short shot based on part weight. If this is a multi-cavity mold, 100% full is the weight of all parts when the first cavity becomes full (this ensures all mold cavities are short shot). The packing pressure should then be used to compete mold filling and pack out the parts without flash. You will likely use a significantly higher packing pressure, but will gain more process consistency as a result.

A 2nd Stage Packing study is a critical aspect to optimizing any molding process. You should perform a gate seal test for each of your molds to ensure you are not using wasting energy with excessive packing time.

Gate Seal Time: To determine the time required for the gate to seal requires a gate seal study. To perform this test, you mold a series of parts using different 2nd stage times and weigh each part (without gates and runners). When graphed, you will see the part weigh increase with packing time until the gate seals. Once the gate is sealed, the part weight should stabilize for a cold runner mold. If a hot runner gates directly into the part, the part weight will stop curving and become linear at the time of gate seal.

Packing Time: When gate seal is desired, the optimal time used for 2nd stage packing is 10% greater than the gate seal time. When a non-seal condition is desired, the 2nd stage packing time should 10% lower than the gate seal or less.

Design issues are very difficult to process around. Your defects indicate a potential venting problem which should be addressed first by reducing clamp tonnage and/or adding more vents.

Question: I am processing both clear and red transparent PMMA of the same grade. For the red part, the thickest area is 4mm compared the average wall thickness 2.5 mm. It's easy to get shrinkage in the 4 mm position. Each component gets one hot runner gate. Thick area is far from gate compare the conjunction area with clear component. Here is the problem. When you increase the holding pressure or holding pressure time, it easily leads to shining line even break in the conjunction area. I also try to set the holding pressure very high at the first 0.3 second till reduced pressure to 13 seconds. But it doesn't work really. The mold gets some problem, but I want to compensate it with the processing.

My Response: You are correct, this is a design problem which is difficult to process around. The shiny line and breakage defects may indicate a potential venting problem which should be addressed by reducing tonnage and/or adding more vents. The best way to attempt processing this part is to fill the mold with a short shot during 1st stage fill, and then use one pressure for the entire second stage packing until the gate is sealed. To determine the best pressure, perform a study to determine the highest and lowest pressure which will provide an acceptable part.

Related Links:

The red colorant will affect the material conveyance within the feed zone of the screw. Different colorants can cause many intentional and unintentional purposes including a plasticizer, lubricant, slip agent, insulator, etc.

Question: I am processing both clear and red transparent PMMA of the same grade. My question is why is the plasticizing of the red material inconsistent. Both the temperature setting is from nozzle the the feedthroat, 240, 240, 230, 190, 60 degree. The plasticizing speed is 0.1 mm/s, back pressure is 15 bar. The decompression after plasticising is ok. However, the red component isn't stable compared to the clear one. Also I hear the screaming when in plasticising. But others for PMMA, it's normal.

My Response: The red colorant may be affecting the material conveyance in the feed zone. In such a case, you will need to perform a rear zone temperature study (currently 190) Essentially, the red color may need a different rear zone temperature to get the desired material conveyance in the feed zone.

Related Link

When to Optimize the Rear Zone


Unless there is a mixing issue, you should use a low back pressure when possible. This approach will minimize the energy consumed in recovering the shot.

Back Pressure: Any additional back pressure will cause the screw to rotate additional turns due to the polymer backflow over the flights of the screw. Using more back pressure than necessary to create a consistent shot can causes a 5-10% increase in the energy consumed during recovery.

Recovery: In most processes, there is more energy consumed in screw rotation than any other aspect of the process. As a result, any reduction in energy consumption during screw recovery will benefit the overall process.


Dry vs. Wet Purging

Andy Routsis
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Materials with poor conveyance tend to be more successfully purged using wet purging option. Materials with good conveyance tend to be purged successfully using either method.

Question: Everybody purges differently, some claim dry purging is best while others tell me it is a bad idea... what is the best method?

note: 'Dry' purging refers to purging all the material possible out of the barrel before adding the next material. 'Wet' purging involves retaining the previous material in the feed zone of the screw when adding the next material.

My Response: There are many theories regarding purging, and most have validity in some cases. For example, some people state that dry purging will increase screw and barrel wear, while many purging companies claim this is not likely as there is always material left in the screw and barrel. In practice we have found the following to be true, dry purging tends to be more successful than wet purging with most materials and purging compounds. The biggest caveat relates to plastics which are difficult to feed. Such materials are tend to purge better with wet purging as the subsequent material will help convey and remove the previous material through the screw. This works especially well with purging compounds which have expanding components which help material conveyance.

In my experience, most molders do not take the time to determine the optimal cooling time. This should be done after the minimal acceptable mold temperature is determined.

Cooling Time Study: Essentially, you should incrementally step down the cooling time and allow the process to stabilize between each step. The parts produced at each step should be tested for quality to determine the lowest time which produces an acceptable part. 

Optimal Cooling Time: The optimal cooling time setting is 10% above the minimal acceptable time. This provides a buffer to compensate for normal fluctuations in mold and melt temperature. You should allow the process to run for a while at this time while checking quality to ensure you have allowed for process stabilization.


Global Skills Gap

Andy Routsis
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In the global picture, employers in the USA actually fair relatively well in locating skilled production employees and engineers. Believe it or not, finding skilled employees is a world-wide phenomena.

Problem: Managers often say..."In our town, it is tough to find skilled production workers" or " There is a significant lack of engineers in our state". Basically, the lack of industry-specific trained personnel is a global issue in most developed countries. In difficulties in finding Engineering talent US ranks 6th behind Brazil, Germany, India, Japan,and the UK (based on research by Debra Auerbach from Careerbuilder.com). Much of this stems from the fact that there is an ever-increasing amount of specialization in each branch of every industry. For example, an employee working as a technician in one packaging company is unlikely to have all the skills necessary to work at another facility without some training.

Solution: It is the responsibility of each company to develop in-house training systems so you can develop talent in-house. This includes both the employees who are currently in your facility who want to advance as well as those who you hire and need to get up-to-speed.The most efficient in-house programs provide metered training and hands-on exercises over an extended period of time. This will instill the desired practices, habits, and behaviors in your employees.

Please feel free to read the article mentioned in this post:

There is a big difference between making good parts and developing a good process. The problem with traditional troubleshooting is the focus on making good parts.

Good Parts vs. Good Process: Just because you are making a good part does not necessarily mean you have a good process. If your technicians only focus on making good parts, then they are just concerned with making an acceptable part at the moment. Unfortunately, if there is not a good process making a good part, then the process may not be robust enough to compensate for variability. A good molding process is designed to compensate for the natural variability of plastics.

It is never a bad time to train your employees. If you are having a problem keeping up with production, then effective, relevant workforce training will help you get more out of your production facility.

Training: Improving the knowledge of your employees will provide them with a better ability to handle adversity and make educated decisions. There is a difference between good parts and a good process. If your employees do not know how to develop a good process, then they are going to constantly adjusting the process to make good parts.