Mold Design- critical factors #1

Factor #1—Polymer melt residence time

Mold design goes hand in hand with part design to determine the manufacturability of a product. One factor that is critical to manufacturability-as well as to preserving the performance characteristics of the material being used-is polymer melt residence time.
 
In this blog, the first of a five-part series discussing critical factors of mold design, we answer these questions about melt residence time: 
      •  What is melt residence time?
      •  How does it affect the integrity of a polymer?
      •  How does this relate to Eastman Tritan copolyester?
      •  What is the residence time recommendation for Tritan?

Melt residence time—know when enough is enough (and why) 
The polymer melt residence time is defined as the total time the resin is in the molten state required for injection molding—from the time it is melted in the injection unit’s barrel through to the mold’s runner system and main cavity.
 
Unreasonable melt residence time can reduce the molecular weight of polymers and affect their desirable performance characteristics. 
NOTE: Be sure to consider the size of the injection unit when calculating total residence time. When possible, pair the mold and injection molding machine to prevent excess residence time being used up in the machine's barrel before the polymer reaches the mold. While this is best practice, it is not always practical for custom molders who produce parts out of multiple molds.

How residence time affects molecular weight 
While the resin remains heated in the molten state, there is a tendency for the molecular weight to decrease. The loss of molecular weight is not visible in the molded part and is only detected by sophisticated testing. Finished parts may look fine aesthetically only to have their loss of integrity revealed in reduced performance and fitness for use.
 
For example, customers select Eastman Tritan copolyester for the advantages it provides in chemical resistance and impact strength. Both properties can be compromised by reduced molecular weight. Therefore, it is important to retain as much of the original molecular weight of Tritan as possible.

Recommended melt residence time
Reasonable melt residence time for Tritan is 5 minutes or less. Good mold design, combined with part design and efficient processes, can help achieve this goal—and help ensure performance from your molded parts.   
 
Brand owners invest a lot of time and energy selecting the material with the right combination of performance attributes. Proper melt residence time is one way for tool designers and molders to ensure the desired performance will be delivered by the finished molded part.
 
Questions about melt residence time? The Tritan experts and the Eastman Design Services group will help you find the answers.