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Caring for Your Material…

Lately I have noticed a great deal of companies ignoring the integrity of the material during screw recovery. During a recent on-site training visit, one student was encountering significant quality issues with a particular product molded from TPE. In reviewing the situation, it appears the material may be damaged or degraded during recovery which will contribute to the quality issues they are encountering. 

When recovering the next shot, it is important that you consider the following factors.
1) Melt Temperature – Don’t rely on your barrel temperatures alone, actually measure the temperature of the polymer melt once the process has stabilized.
2) Rear Zone Temperature – The rear zone temperature has the highest influence on the material conveyance through the feed zone to the transition zone. 
3) Screw Recovery Time – The recovery time should consume about 80% of the overall cooling time to maximize melting efficiency.
4) Back Pressure – The back pressure should remain low unless there are specific mixing issues or inconsistent recovery times.
-Andy

Only 2% Improvement…?

In response to a recent class, a participant encountered this result…

Participant
We have very fast screw recovery times, typically less than 2 seconds. When performing the rear zone temperature study, there was a steady increase in recovery time from 1.78 sec. @ 225°C to a result of 1.82 sec. at 260°C. Does this seem like a valid result?

My Response
This is a perfectly normal result, especially when the screw is properly designed for the application. In actuality, the lower rear zone temperature demonstrates a 2% drop in screw recovery time which usually translates to a 2% drop in the energy required to melt the material.
Additional Thoughts
Screw recovery typically consumes the greatest amount of energy during the molding cycle. A 2% drop in energy consumed during recovery might result in a 1% drop in overall energy consumed… this can become very significant to the bottom line of a high-speed consumer molder.
-Andy

Understanding Delamination…

I occasionally see employees confuse blisters and delamination and wanted to clarify this here.

Delamination
Delamination is the layering of the polymer as it fills and packs the mold. With this defect, different unconnected layers of plastic develop and form within the part. 
Blisters
Blisters are gas bubbles which appear as small bumps on the surface of the part. This is caused by either moisture, air, gases or volatiles being present in the resin or on the mold surface.
Causes
Both defects can be caused by moisture in the polymer or material degradation. Blisters are more commonly causes by poor material handling while delamination is more commonly the result of degraded material or high injection pressures.
-Andy

Learn and Apply…

I wanted to bring up a very important aspect of employee training…

Learn and Apply
Training is most effective when it has at least two components. The initiative should have aspects intended to teach the important underlying concepts to the employee. These are intended to provide the ‘why’ aspect of the learning. In general, people are less likely to implement a concept if they do not understand the underlying concept behind the desired action. For example, if your employees do not understand the basic concepts of material feed and conveyance, it is difficult for them to understand why a rear temperature study should be performed.
Likewise, the training must have a practical aspect to teach the participant the desired skills. This backs up the preceding ‘why’ instruction with the all-important ‘how’ instruction to ensure the employee knows how the information they learn applies to their actual job.
-Andy

Mold Temp & Oversized Parts…

This question highlights the complexity of polymers… 
Saeed
I came across a troubleshooting guide for oversized parts and one of the remedies was to increase the mould temperature. Now I understand it will help shrink the part to the required size, but one thing I do not understand that due to higher mould temperature, melt would be even more compressed in the cavity resulting in heavier parts. Will it not negate the shrinkage we achieved by increasing the mould temperature. I will appreciate your thoughts on this.
My Response
I understand there are two opposing situations occurring here. An increase in mold temperature results in increased part shrinkage overall. Adversely, the increase in mold temperature will result in an decreased pressure loss throughout the cavity and an increase in the gate seal time. In practice, the increase in shrinkage overcomes the other factors resulting in a smaller part overall.
Additional Thoughts
Keep in mind, a lower packing pressure can often be used with an increases mold temperature.
-Andy

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