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Inconsistent Screw Recovery…

This question came in as a follow-up to a recent post.
SA
Recently we were processing Acetal material with black masterbatch. During the whole run the screw recovery time kept going up excessively before coming back to normal values after a few minutes. This was repeated throughout the run and it happens on this particular job all the time. we are not allowed to change either screw speed or back pressure excessively. Purging the barrel through with purging material helps for a while but it goes back doing the same thing.could it be the masterbatch causing screw slip. Your thoughts on this please.
My Response
Essentially, the colorant (masterbatch) has a different melting profile than the base resin. In many cases, the additives melt faster, causing slippage on the barrel during recovery. Optimal recovery occurs when the melted material has the highest tact with the barrel causing the best and most consistent conveyance down the screw. To locate this optimal temperature, you must conduct a rear zone temperature study.
Additional Thoughts
For more about this, feel free to read: 

-Andy

Old Notions of Training…

I recently encountered this comment…

Manager
We tried some training back in the 90’s and it helped a little, but no-one remembers any of it now.

My Response
What was being taught 20 years ago is now very outdated and not considered industry best practice. At that time, the industry standards were very different than they are now. There are many techniques available which allow one to systematically establish, optimize, and document a molding process. The best part is, most of these can be implemented without the purchase of specialized equipment.
Additional Thoughts
The approaches to employee development have also advanced greatly. The science of training adult learners has advanced greatly over the years and there are many ongoing training programs designed to ensure your employees are learning and growing professionally.
-Andy

Clamp Open Position…

A recent blog reader send me a question last week…

Reader
We have recently come across multiple part issues relating to end of arm tooling damaging the part.  We’ve found that a few of our {brand removed} machines have a lot more platen drift than our other machines. We tried setting our mold open tolerances tighter but that only causes the machine to go down more often.  Is this inherent in this brand or hydraulic machines all together? 

My Response
There are many ways to improve this, but these are the first two things I would do before making any major changes.
1) Level the machine. Leveling the machines as well as ensuring the platens are parallel will help ensure smooth and consistent movement of the platens. Some machine brands are not constructed as robustly as others and are more susceptible to movement. For this reason, the machine may need to be leveled, and the platens aligned more often.
2) Adjust the final mold open speed. Slowing down the platen near the completion of mold opening will allow the machine controls to better ensure the mold open stop setpoint is achieved.
-Andy

Traditional Molding…

I was asked this question recently…

Technician
I am told that short-shot filling with our older, open-loop, machines will result is more variation not less. Why is this the case?
My Response
Older, pressure-controlled molding machines have no control over the material shear rate. Since the uses a set pressure, any attempt to short shot by time or position will result in 1st stage variation. Every time the material viscosity shifts upward, the part will be significantly short. Adversely, any drop in viscosity will result in a complete fill or flash during 1st stage. The 2nd stage pressure cannot be properly configured to provide the same final product when the mold shorts as well as when the mold fills completely. This condition requires constant adjustment whenever variation occurs.
If you configure the process to completely fill the mold during 1st stage with older, open-loop molding machines, then then you can can have a relatively consistent final part when 2nd stage pressure is applied.
Additional Thoughts
Keep in mind the simplicity of open-loop molding machines can be very appealing, but they have little control over the actual process.
-Andy

Avoid Profiling…

I have recently received different questions and many revolve around the same theme, profiling. Essentially, it does not cost anything to add profiling to virtually any machine parameter since it is all a mater of programming. As a result, manufacturers add it to everything. This does not mean it is always a preferred function.

In general, you should always avoid profiling anything related to the process, but I will include the following for more clarification…

Don’t Profile

Back Pressure
Screw RPM
Screw Decompression
Pack Pressure
Hold Pressure
Injection Pressure

Profile Only When Absolutely Necessary
Injection Speed
For more clarity on this, feel free to read:
-Andy

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