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Should Quality Learn About Processing…

I received this question during a seminar last week…

MH
When you suggest including everyone in the injection molding training initiative, are you suggesting I include my quality technicians?

My Response
Definitely, the more quality knows about production, the better off you are. They usually don’t need to know all the technical information… but an increase in knowledge will always improve overall communication.
You should include your production employees in the education effort so that they have some responsibility in teaching your quality employees about what they do.
Additional Thoughts
Keep in mind, this is not a one-way street. Cross-training your production employees about quality is an equally critical step.
-Andy

Common Gate Sizings…

One blog reader asked this question…

Larry
Would you please give me the guidelines for runner and gate sizing with respect to nominal wall thickness?  
My Response
Below are some common guidelines…
An edge gate should be approximately 1/3 of the part thickness at the mold cavity.
The fan and flash gate should taper down to approximately one-quarter of the part thickness. 
Disc gates taper down to less than one-quarter of the part thickness at the mold cavity. 
Pin gates are commonly machined to 1/8 of the part thickness.
3 plate pinpoint gates taper down to a diameter of approximately 1/3 of the part thickness as they approach the cavity. 
The submarine gate diameter at the mold cavity should be approximately 1/3 of the part thickness. This gate should have at least a five-degree taper per side so it can be removed without complications. 
Additional Thoughts
Unfilled, semi-crystalline materials typically require smaller than average gates, while hard to process amorphous materials such as polycarbonate may require larger than average gates.
-Andy

Back Pressure Setting…

I received this follow-up question the other day…

Kevin
Once I optimize my rear temperature, what back pressure should I use?
My Response
Back pressure serves two important roles… 1) to help ensure proper material mixing. 2) to help ensure a homogeneous melt compression in front of the screw. Your back pressure should be high enough to provide both of these.
The first factor is easily determined by part quality and inspection.
The second factor is determined by the consistency of your recovery time.
Additional Thoughts
Keep in mind, you may not receive a consistent melt recovery until you first optimize your rear temperature zone.
-Andy

Using Shut-Off Nozzles

In a recent seminar, I was asked the following question…

Participant
We often use shut-off nozzles to prevent drooling, are there any other reasons I should use a shut-off nozzle?

My Response
Shut-off nozzles are a great resource to a technician. When processing low viscosity materials, they can reduce the need for excessive decompression. They can also improve material mixing as they may allow for the use of higher back pressures during recovery. Some molders use shut-off nozzles to improve the consistency of the melt density. Shut-off nozzles can also be used to improve cycle time in high-speed applications because the machine can build the next shot while the mold is open.
Another benefit to shut off nozzles is safety… shut off nozzles help prevent the material from exiting the barrel while the mold is open. This helps when a mold is being serviced as well as when an operator is manually removing a part from the mold.
Additional Thoughts
Shut-off nozzles can also be helpful in applications such as micro-cellular molding, liquid silicon rubber, or when using chemical foaming agents.
-Andy

Identifying A Pressure Limited Process…

This question highlights a common point of confusion…

Bob
How do I determine if my process is pressure-limited using my machine’s pressure curve?
My Response
The pressure limited process will have a plateau on the pressure curve prior to transfer. This will also correspond with a drop in the velocity at the time of pressure limiting.
A good way to test this is to increase the maximum pressure, and see if the peak 1st stage pressure increases. If the peak pressure increases with an increase in maximum allowable pressure, then the process was pressure limited.
Additional Thoughts
For more about this topic, please review: Providing a Buffer To Accommodate for Variation
-Andy

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