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: 

Making the grade—selecting the right medical grade polymer

Eastman understands that the stakes are high when selecting the material for your next fluid management component, blood contact device, or electronic medical device housing. That’s why we want to help make this complicated process simpler when you consider Eastman Tritan^™ copolyester, beginning with two criteria:
Matching a medical grade of Eastman Tritan^™ copolyester to your needs

Sink marks appear as surface depressions (seen as dimples or grooves); voids are less obvious and may appear as bubbles in clear parts. Both compromise aesthetics and can reduce the confidence a customer has in a part—but the impact is much greater on applications such as fluid management, where leakage from a Luer or stopcock can result in loss of sterility and contamination.

In opaque medical device housing applications where aesthetic is critical, the matte finish and pastel colors that are often used make sinks and voids obvious.   

Sinks and voids are caused by localized shrinkage of the resin at thick sections during the following steps:   

1. When excessively heated material expands to fill the mold cavity, it results in excess space between the plastic molecules.
2. The skin of the material in the mold solidifies (freezes) first.
3. As the remaining resin core cools and shrinks, it pulls the solidified skin with it away from the main mold wall.
4. If the skin is sufficiently stiff, core shrinkage may not cause surface deformation but a void can form within the core of the resin as it shrinks.

Focus on gate design— valve gate systems 

Valve gate systems are recommended for hot runner systems used to mold Eastman Tritan^™ copolyester. Advantages compared with other hot melt delivery systems include:

Uniform mold heating and good heat management 
Hot runner valve-gated systems have been used successfully with amorphous copolyesters like Eastman Tritan^™ copolyester for several years. One benefit of the low mold temperature required by Tritan is shorter molding cycle times—but this benefit depends on good management of both heating and cooling.
 
Best results come from designing a mold that allows good temperature management throughout the drops, mold, sprues, and gates to keep the material in the tool above the glass transition temperature (T_g) until it passes through the gate into the mold cavity—and is ready to fill the cavity on the next shot.
 
Uniform heating and proper cooling improve success by:
•  Eliminating holdup spots that can degrade the material
•  Avoiding excessive heat that can lead to sticking
•  Minimizing shear heating
•  Improving processing efficiency
•  Improving part quality 

Collaborate early to avoid false starts 
False starts are costly—especially when setting up a hot runner system. Making changes after a hot runner mold is released for building are difficult, extremely expensive, and can have lasting effects on system efficiency.  
 

• Reduce product development time
• Ensure processing efficiency
• Improve end-product quality

Material sticking can bring processing to a standstill, regardless of its source. When troubleshooting, keep in mind that the location of a sticky part in the mold is not necessarily the area of the mold that is the cause. Different sections of the mold can hang onto the part and exerted force can cause the part to bind in another section. Carefully evaluate what happens as the mold opens with special attention to four possible causes for sticking at the sprue. 

Noise is a turn-off. Quiet is a value-add. Consumers are willing to pay a premium for small appliances that provide robust performance—and do it with less noise.

This 1-hour webinar shows where noise comes from and how a material change can help reduce it. 

Introduction and advantages

Eastman receives many questions about molding Eastman Tritan^™ copolyester with hot runner systems. It can be a complex topic, loaded with variables that include not only mold design, but also part size, polymer formulation, and part design. Success depends on early collaboration and clear communication between Eastman, molders, tool builders, and hot runner suppliers.
 
We’re usingTritanMoldIt.com as a platform to jumpstart this communication between stakeholders.  During the next few months, the Tritan experts will post a series of blogs, beginning with this overview and advantages of hot runners for amorphous copolyesters, including Tritan. 

We invite you to help shape our series of hot runner blogs. Send your feedback. Ask your questions. Share your success stories. We look forward to hearing from you.