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Improving Process Efficiency – Packing Time

A 2nd Stage Packing study is a critical aspect to optimizing any molding process. You should perform a gate seal test for each of your molds to ensure you are not using wasting energy with excessive packing time.

Gate Seal Time: To determine the time required for the gate to seal requires a gate seal study. To perform this test, you mold a series of parts using different 2nd stage times and weigh each part (without gates and runners). When graphed, you will see the part weigh increase with packing time until the gate seals. Once the gate is sealed, the part weight should stabilize for a cold runner mold. If a hot runner gates directly into the part, the part weight will stop curving and become linear at the time of gate seal.
Packing Time: When gate seal is desired, the optimal time used for 2nd stage packing is 10% greater than the gate seal time. When a non-seal condition is desired, the 2nd stage packing time should 10% lower than the gate seal or less.
-Andy 

Molding Issues with Red PMMA

Design issues are very difficult to process around. Your defects indicate a potential venting problem which should be addressed first by reducing clamp tonnage and/or adding more vents.

Question: I am processing both clear and red transparent PMMA of the same grade. For the red part, the thickest area is 4mm compared the average wall thickness 2.5 mm. It’s easy to get shrinkage in the 4 mm position. Each component gets one hot runner gate. Thick area is far from gate compare the conjunction area with clear component. Here is the problem. When you increase the holding pressure or holding pressure time, it easily leads to shining line even break in the conjunction area. I also try to set the holding pressure very high at the first 0.3 second till reduced pressure to 13 seconds. But it doesn’t work really. The mold gets some problem, but I want to compensate it with the processing.
My Response: You are correct, this is a design problem which is difficult to process around. The shiny line and breakage defects may indicate a potential venting problem which should be addressed by reducing tonnage and/or adding more vents. The best way to attempt processing this part is to fill the mold with a short shot during 1st stage fill, and then use one pressure for the entire second stage packing until the gate is sealed. To determine the best pressure, perform a study to determine the highest and lowest pressure which will provide an acceptable part.
Related Links:
-Andy

Recovery Issues with Red PMMA

The red colorant will affect the material conveyance within the feed zone of the screw. Different colorants can cause many intentional and unintentional purposes including a plasticizer, lubricant, slip agent, insulator, etc.

Question: I am processing both clear and red transparent PMMA of the same grade. My question is why is the plasticizing of the red material inconsistent. Both the temperature setting is from nozzle the the feedthroat, 240, 240, 230, 190, 60 degree. The plasticizing speed is 0.1 mm/s, back pressure is 15 bar. The decompression after plasticising is ok. However, the red component isn’t stable compared to the clear one. Also I hear the screaming when in plasticising. But others for PMMA, it’s normal.
My Response: The red colorant may be affecting the material conveyance in the feed zone. In such a case, you will need to perform a rear zone temperature study (currently 190) Essentially, the red color may need a different rear zone temperature to get the desired material conveyance in the feed zone.
Related Link: 

When to Optimize the Rear Zone

-Andy

Improving Process Efficiency – Back Pressure

Unless there is a mixing issue, you should use a low back pressure when possible. This approach will minimize the energy consumed in recovering the shot.

Back Pressure: Any additional back pressure will cause the screw to rotate additional turns due to the polymer backflow over the flights of the screw. Using more back pressure than necessary to create a consistent shot can causes a 5-10% increase in the energy consumed during recovery.
Recovery: In most processes, there is more energy consumed in screw rotation than any other aspect of the process. As a result, any reduction in energy consumption during screw recovery will benefit the overall process.
-Andy

Improving Process Efficiency – Cooling Time

In my experience, most molders do not take the time to determine the optimal cooling time. This should be done after the minimal acceptable mold temperature is determined.

Cooling Time Study: Essentially, you should incrementally step down the cooling time and allow the process to stabilize between each step. The parts produced at each step should be tested for quality to determine the lowest time which produces an acceptable part. 
Optimal Cooling Time: The optimal cooling time setting is 10% above the minimal acceptable time. This provides a buffer to compensate for normal fluctuations in mold and melt temperature. You should allow the process to run for a while at this time while checking quality to ensure you have allowed for process stabilization.
-Andy

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