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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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Degrading PVC…

I received a question via email last week…

RS
I am using u-pvc injection molding for pvc pipe fittings.
When running a male addoptar which weight about .05 kg we are observing burning in the end of injection, what its cause?
My Response
In such cases, the cause is degraded material. PVC most often degrades whenever it either stagnates in the nozzle or adaptor or remains in the barrel too long.
Additional Thoughts

PVC molders often have a habit of increasing the back
pressure above what is necessary to create a consistent melt. This tends to promote degradation and burning of the material.

-Andy

Shadowing vs. On-The-Job Training…

I would like to relay a common misconception I hear on training and clarify two forms of ‘training’ often used my plastics companies…

Joe
We currently train our people using on-the-job training, we have a new employee follow another employee around for a couple days.
My Concern
Most people equate ‘shadowing’ with ‘on-the-job’, or OJT. According to Merriam Webster, shadowing is ‘to accompany and observe especially in a professional setting‘. This type of training acquaints the learner with the workplace but is not a good form of instruction. You would not want your cars engine to be fixed by someone who watched someone fix an engine, yet many companies put the future of their company in the hands of people who essentially watched someone else work.
Shadowing is a helpful tool, but If you want to train on-the-job, you must actually have training available for your employees. This may be  in the form of worksheets, instructions, guides, visuals, etc. 
For more about on-the-job training feel free to read:

Drying Nylon with a Hot Air Dryer…

This question came from a molder overseas…

JY
We typically process polypropylene and polyethylene but our customer asked us to run Nylon. Can we use a hot air dryer for nylon?
My Response
Hot air dryers are typically used to either remove surface moisture or preheat the polymer.Since they do not significantly reduce the dewpoint of the air, hot air dryers are not an effective method of removing moisture from hygroscopic polymers.
Even if you get the parts to mold, hydrolysis will occur and the parts will not have the strength and impact resistance to be effectively used.
Additional Thoughts
When preparing such a highly hygroscopic material, it is critical that you use a low-dewpoint material dryer such as a desiccant or vacuum dryer.
-Andy

Calculating Part Shrinkage…

This question overshadows a common mistake made by many molders and designers…

Steve
What is the correct way to calculate shrinkage? One engineer insists that you divide the mold dimension by the part dimension and eliminate the one, but this sounds weird.

My Response
Shrinkage is a measurement based on change, and is a represented as either a ratio or as a percentage. The correct method is to determine the difference between the mold and part dimension and then divide that number by the mold dimensions.
ratio = (mold – part) / mold
% = (mold – part) (100%) / mold
Additional Thoughts
The method used by your engineer is close for small shrinkage values, but the disparity becomes increasingly larger as shrinkage values and dimensions increase.
-Andy

Establishing a Fill for Injection Compression…

I was recently asked a few questions via email, this is one part of that email….

Nathan
Are these still correct with Injection Compression?

– Relative viscosity fairly constant during even with low fill speed
– Fill volume 95-98% 

My Response
Basically, you are correct, the first stage fill should use velocity control, but you may need profiling to maintain a low shear rate through the mold cavity. Additionally, the mold should be short shot, but the degree of short shot will be greatly affected by the amount of compression used during the packing phase.
Additional Thoughts
The primary purpose of this process is to reduce the polymer chain orientation within the mold by increasing the effective thickness of the mold during injection. By applying some packing pressure through compression rather than through the gate, chain alignment should not be increased during second stage.
-Andy

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