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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

Global Skills Gap

In the global picture, employers in the USA actually fair relatively well in locating skilled production employees and engineers. Believe it or not, finding skilled employees is a world-wide phenomena.

Problem: Managers often say…”In our town, it is tough to find skilled production workers” or ” There is a significant lack of engineers in our state”. Basically, the lack of industry-specific trained personnel is a global issue in most developed countries. In difficulties in finding Engineering talent US ranks 6th behind Brazil, Germany, India, Japan,and the UK (based on research by Debra Auerbach from Careerbuilder.com). Much of this stems from the fact that there is an ever-increasing amount of specialization in each branch of every industry. For example, an employee working as a technician in one packaging company is unlikely to have all the skills necessary to work at another facility without some training.

Solution: It is the responsibility of each company to develop in-house training systems so you can develop talent in-house. This includes both the employees who are currently in your facility who want to advance as well as those who you hire and need to get up-to-speed.The most efficient in-house programs provide metered training and hands-on exercises over an extended period of time. This will instill the desired practices, habits, and behaviors in your employees.
Please feel free to read the article mentioned in this post:
-Andy

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