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Slowing Down the Screw

If you have to slow the injection speed down to prevent overshooting or gas entrapment during injection; then you are probably filling the mold too much during first stage fill.

Comment: We can use one speed to fill most of the mold, but we always have to slow the screw down as it gets to the the end of fill to prevent flash and gas entrapment.
My Response: The best approach to complete the filling of the mold is to transfer to pressure-controlled packing and let the screw slow down as the mold fills and the pressure builds in the cavity. Actually, the point where you start your slowdown is likely to be a good transfer position.
Additional Thoughts: Keep in mind, you should transfer from fill to pack at the position where the mold cavity is 90-95% fill. This is based on the mold cavity being 100% full with 1st stage only (when the first cavity fills if multi-cavity).
-Andy

High Injection Pressures

It is always to avoid running a pressure-limited process. Any process which is pressure limited will have and inconsistent 1st stage injection weights and 1st stage injection times.

Question: The company I work for now, wants all injection/boost pressures set at max 2,000 to 2,300 psi. I was told that it insured a more consistent shot and was also told that this would blow through any nozzle tip or gate freezing. Wouldn’t this be harder on the check ring?
My Response: The company uses max pressure to ensure the machine always operates under velocity control which is very important. Not all companies need to use full pressure, typically 20% more pressure than what is necessary will work on most machines. This would not be the cause of the early check-ring wear, it is very possible that other causes such as abrasive additives or inferior check rings could be causing this. 
-Andy

When to Optimize the Rear Zone

The optimal rear zone temperature is specific to each machine and material combination. For this reason, a rear zone temperature study should be performed each time a new material is molded on a particular machine. 

Question: In my case we have machines from different brand, so a study in the rear zone temperature must be performed. So, in a situation like that, how do we perform that study? Under which conditions could we perform the procedure?
My Response: Since the rear zone temperature has no significant affect on the polymer melt, the following test can be performed during production. This can allow you to adequate time to allow the rear temperature zone to stabilize and obtain excellent results.
Rear Zone Temperature Study Procedure:
  1. Set the molding machine to manufacture stable product
  2. Set the rear zone temperature to the manufacturers recommended minimum
  3. Allow the rear temperature to stabilize
  4. Record the ‘Rear Zone Temperature’ and the ‘Recovery Time’
  5. Increase the rear zone temperature incrementally
  6. Repeat steps 3-5, but do not exceed the manufacturer’s recommended maximum
  7. Graph the data: Time (Y axis) vs. Temperature (X axis)
  8. Evaluate the graph and select a temperature with a low recovery time 

-Andy

Always Look for Correlation…

When one process output is inconsistent, check the process data in case there is not correlating data which can help pinpoint the actual cause of the problem.

Problem: Screw recovery time variation in PBT material.
notes: The questioner sent a screenshot of the SPC data for the past 200+ shots. In this data, it was shown that the shots with a low recovery time had a high cushion, very often above the transfer or near position. Along with the inconsistent cushion and recovery time, the transfer pressure and revolutions per recovery were also unstable. Additionally the average cushion is almost 40% of the shot size.
My Response: With only recovery data, one would initially infer a feeding problem, but when you combine all the available data, it is clear that the whole process is inconsistent. The cause could be due to many issues, but the core concern here is that the inconsistent recovery was a symptom of the problem, and not the problem itself.
-Andy
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Checking Your Check Ring Performance

All check rings leak, but over time the check ring will wear and leak more. The most important issue is how consistently it leaks during the velocity-controlled injection phase of the process.

Question: I have an issue with a 360 ton machine. Over the last three months, I have had most of my process’ have had their shot sizes grow. Same set process’ as before but as much as 10% more shot is required to fill part. The same tools in different machines are running spot on. Could this be the check ring or the barrel causing this issue?
My Response:I strongly recommend performing a Check Ring Repeatability test on all your machines as a baseline for comparison.
To perform this test:
1. Mold 10 short shots
2. Weigh each shot
3. Perform the following calculation:
100% x (Heaviest-Lightest) รท (Average) = %Variation
Some high precision molders will change the check ring when it exceeds 1 or 2%, while some lower precision molders might wait for it to reach 3 or 4%.
-Andy

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