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Once the process and screw have been optimized, the next step you can undertake is to preheat the polymer. Even though this material does not need drying, a simple hot air hopper dryer can be used to preheat the material to reduce he amount of heat necessary to melt the material. The more heat that is added to the material before it enters the barrel, the less energy that is required by the screw and heater bands.

Semi-crystalline polymers benefit most from pre-heating since the polymer will typically endure more preheating due to their sharp melting point.

Always be cautious of your temperature in both the feedthroat and rear zone of the barrel. Since preheating hastens the melting, you may need to repeat the tact temperature study.

-Andy

When the mold cavity is filling, the polymer flow front is considered dynamic, or in motion. The balance of two or more mold cavities at this stage primarily relates to the consistency of pressure loss and shear balancing between the mold cavities. This relates to both hot and cold runner systems. The more the cavities are dynamically balanced, the more consistent the process will be in the long term.

Once the flow front reaches the end of the mold cavity, it becomes static. At this stage, part weight differentials are primarily based on the variability in pressures and temperatures. This information is important for final part quality, but can be significantly different than the dynamic balance.

-Andy

Basically, the procedure is similar to that of an injection molding machine. You would turn off the second and third stage, and then compare the shot weight of the different cavities.

To calculate the % variation is as follows:

100% x (maximum weight – minimum weight) ÷ (maximum weight) = % Variation

-Andy

The parameters and process outputs which affect part size in a DIII process are commonly:

• Melt Tempertaure
• Transfer Setpoint
• Packing Speed
• Hold Pressure
• Mold Temperature

One of the best ways to approach dimensional issues with a DIII process, would be to compare the pack rate and cooling rate to the documented standard and return them to the standard. These are the upward and downward slopes of the in-cavity pressure curves representing the rate of packing and cooling within the cavity.

-Andy

In this case, the customer was buying as many parts as the molder can produce. The ejected parts were sub-gated for part removal while the parts are being dropped onto a conveyor belt. Since the material was a TPO, the parts were flexible and needed a 2 second ejection forward delay to ensure the part completely fell off of the ejector pins before the mold closed. Air blasts were previously tried, but the parts were not falling well and the result was part damage and increased inspection.

To speed up the removal process, we replaced the sprue picked with a high-speed robot. Since the robot can move with the ejection system, the part were picked up during ejection, and the ejection was retracted quickly. We were also allowed to reduce the mold opening distance which resulted in an overall cycle time reduction of 1.5 seconds which quickly justified the cost of the high-speed robot.

Don’t overlook the big things when attempting to tweak a process. Sometimes replacing a major component such as a screw, barrel, robot, or even mold will give you a significant ROI. The key is to cover the small details first.

-Andy