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Maximum Pressure Setting During Injection…

This question was received earlier this week…

CD
What should be the minimum pressure during switch over time of injection?

My Response
If you are switching over through the use of screw position, then you should ensure the maximum pressure setting is significantly higher than injection pressure at transfer.
Additional Thoughts
It is critical that you have enough pressure available to maintain the desired injection speed. 
More about this can be found here: 
-Andy

The Best Training Method…

I received this question via email yesterday…

NG
Which is the best type of training?
My Response
No single method of training can meet each of the success factors necessary to teach all your employees. For this reason, it is important to use a training plan comprised of multiple training methods targeted to teach your employees the knowledge and skills they need to succeed.
Additional Thoughts
Our approach to employee training most often includes factors such as focused on-the-job instruction, interactive online training, and skill development exercises.
-Andy

Complications from 1st to 2nd Stage Transfer…

In a comment thread in a previous blog entry, a reader had some questions about how to handle the transfer from 1st to 2nd stage transfer. The basic focus of the discussion both in the comments as well as private were about a general approach to establishing the transfer, yet they have many different types and brands of machines.

My Comments
Unfortunately, here is no standard for machine response from fill to pack. I have seen some machines stop the screw immediately when 2nd stage is turned off, yet allow extensive overshoot once it is turned on. Other machines have different settings for how over or under-damped you want the machine to behave. You should determine the behavior of your actual machines and then make a procedure specific for those machines so everyone can setup a process the same way.
-Andy

More About Back Pressure…

I was recently asked about back pressure…

GG
My employees were asking me to explain the following statement ‘Back pressure causes additional material to flow over the flights of the screw’. I wanted to hear your explanation before discussing it with them.
My Response
During screw recovery, the screw is pumping material forward down the flights of the screw. If unhindered, the material will flow to the front of the screw, thus pushing the screw backward to build the next shot. Back pressure provides resistance to the backward movement of the screw, causing some of the material pumped forward to either 1) flow back within the screw channels or 2) flow back over the flights of the screw between the flights and the barrel.
Additional Thoughts
There will always be some degree of backflow within the channels and over the flights, but increased back pressure will significant increase this effect.
-Andy

Differential Cooling for Warpage…

I was recently at a molder’s facility who was having a complication with one part…

Molder
Even though this polypropylene part has a curve, we have a problem with it curving too much thus causing a complication with assembly.
note: when discussing the troubleshooting process, they have investigated most aspects of the process, but it turns out they never considered trying differential mold temperatures.
My Suggestion
Although I try to avoid using differential coolant temperatures when possible, it would be a good option to investigate for this mold/process combination. Since PP has a high degree of crystallinity, decreasing the temperature of one side will cause a tendency to bow towards the opposite side due to differential shrinkage. In this case, the coolant temperature differential between one side and the other might only be 5 or 10 degrees, but it might be enough to counteract the bowing.
Additional Thoughts
Before using differential heats, it would be beneficial to measure both the coolant temperature entering and exiting the mold as well as the temperature of the mold steel. In many cases, you may be using differential coolant temperature, but the result may be similar mold surface temperatures.
-Andy

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