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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

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Rule of Thumb for Packing…

I am often asked the following question…

JH
What is a good rule of thumb for second stage packing?
My Response
In the old days of pressure-controlled injection molding machines, the first stage pressure was used to completely fill the mold slowly while a lower pressure (approx. 40-60%) was used to compensate for shrinkage while waiting for the gate to freeze.In the newer machines, if the mold is completely filled during first stage, then the old 40-60% rule of thumb is often applicable.
If a scientific or decoupled type of process is being used, where the mold is not completely filled during first stage, then there are many factors which play into the final second stage packing pressure. Essentially, the only rule of thumb is to select a pressure which produces an acceptable part. The first stage injection pressure should have no bearing on the second stage packing pressure. I have seen thick parts using second stage packing pressures as low as 25% of the pressure at transfer, and I have seen thin-walled parts use as much as 125%.
Additional Thoughts
The fundamental underlying basis of good injection molding is to separate mold filling from mold packing. This means you should first establish a good fill, and then establish your part packing.
-Andy

Training Certificates…

I received this comment the other day regarding a recent post…

Manager
Two of our techs recently received training certification, but I don’t see any real change between them and the other technicians.
My Response
Most certifications are based on the participants ability to pass some tests in a class setting. This process is beneficial in introducing new techniques and ideas to the employees, but seldom helps in changing the employees behavior. Most experts believe it can take many months to effectively change behavior. As a result, you must implement a broad reaching approach to training which not only teaches the new concepts, but reinforces the correct behavior over time.
Additional Thoughts
In the case of plastics processing, the employee needs to know how to process, practice the knowledge, and have the right processing and troubleshooting techniques reinforced by those in the workplace.
-Andy

MFI and Regrind…

This question arrived as a follow-up question to a recent post…
NM
I am new to the business and an independent broker. In automobile parts (I deal with travel trailers, RV’s, automotive and such) does MFI make a difference? Recycled or virgin? We are trying to cut costs, and it seems a lower MFI and recycled product is cheaper.
Regrading MFI
MFI is essentially a measurement of how much of a particular material flows through a capillary under a given load during the course of 10 minutes. The stresses and shear rates present in this test are not indicative of most plastics processes. If you want more comparative data you can acquire rheological data from the manufacturers… or request samples and actually preform some comparative rheological tests using your actual production equipment.
Regarding Regrind
You always need to first determine whether the use of regrind poses a liability issue with your customers. You should determine the relative quality of the regrind. Depending on the application, you can choose from a wide range of Rheological, physical property, and performance testing methods.
Some related posts on rheology and physical properties:

Create a Training Portfolio…

I am often asked whether or not it is important to provide employees with printed certificates once they complete their training. As a result, I want to use this post to discuss the importance of creating a training portfolio for your employees.

Building a Training Portfolio
Every time your employees complete a step in their training plan or make an advance in competency, it should be documented and retained by the employee. We suggest that you create a personalized binder that the employee can use to store items such as training certificates, worksheets, training aides, photographs, and documents. The intent is to create a personal portfolio which the employee can use to demonstrate not only what they learned, but what they have applied.
-Andy

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