With any polymer, poor and uneven cooling can result in:
• Increased cycle time
• High levels of residual stress
• Increased warpage
• Sticking and difficulty in ejection
Considering the unique properties of Eastman Tritan™ copolyester, a few important guidelines will improve efficiency and part performance.
Because of the relatively low flex modulus of Tritan, uniform and efficient mold cooling—especially in the cores— is critical for the successful molding and ejection of parts.
When working with Tritan, you need to make sure you are cooling the resin enough to set up the outer skin of the part in order to get good release from the mold. This initial skin formation can take 10 to 20% longer than normal engineering resins, but cooling time throughout the rest of the part will be approximately the same.
Here are a few tips to improve cooling efficiency:
• Design ample mold-cooling channels with proper spacing and sizing.
• Use properly sized pumps and supply lines to deliver a good turbulent flow of water around the cavity. (The flow rate should be high enough to reach a minimum Reynolds number (Nr) of 6,000 in those areas where cooling is desired.)
• Use chillers, if practical, to ensure a proper supply of cool water to molds.
• Remove heat uniformly to avoid hot spots that can cause sticking and local deformation.
• Provide ample cooling near potential hot spots such as sprues and hot runners.
• Pay special attention to cooling heat sinks in pins and thin areas, and around slides.
• The mold should be cooled to 60°C (140°F) before ejection.
For more information, see May 29, 2014 Blog “Tooling design — keys to success”
For best results, involve an Eastman tech services engineer or design services engineer early in your planning. We’re prepared to discuss mold design, including line spacing and core cooling techniques such as baffles and bubblers, and much more.
|TMI TIP: For more information download the Eastman polymers Processing and mold design guidelines.|
The Tritan experts at Eastman are hosting a live webinar on December 10, 2015, from 11:00 to 12:00 EST. This webinar will cover these important considerations to help you design for manufacturability:
• Material selection criteria
• Part design for injection molding
• Tool design for injection molding
• Molding workcell design
• Processing tips for Tritan
Join us live! Register now to participate in “Designing for Injection Molding Manufacturability,” offered by Eastman from 11:00 to 12:00 pm EST on Thursday, December 10, 2015.
To help ensure continued molding success, this blog focuses on three different types of flashing problems, their possible causes, and corrective actions you can take for each.
|Possible cause||Corrective action|
|1. Poor mold spot||Spot mold to ensure fit and vent depths at ~0.025mm (0.001in).|
|2. Injection Pressure too high||Reduce pressure. This may require higher melt temperature or lower injection speed.|
|3. Coost time too long||Shorten boost time, lower boost pressure, or move transfer to an earlier position.|
|4. Insufficient clamp tonnage||Increase clamp pressure or move to a machine with more tonnage.|
Flash and Short Shot in Different Shots:
|Check ring inconsistent; worn barrel||Check and replace assembly ring. Replace/reline barrel as needed.|
Flash and Short Shot in Same Shot:
|Possible Cause||Corrective action|
|1. Core shift||Check wall thickness for core shift. Investigate for source of core shift and repair mold.|
|2. Inadequate clamp tonnage||Increase clamp pressure or move to a machine with more tonnage.|
|3. Fill speed or pressure too high||Reduce speed or fill pressure.|
|4. Melt temperature too low||Check and raise melt temperature if needed.|
|5. Sprue too long too small, "O" dimension||Install correct-size sprue.|
TMI TIP: Tritan can be successfully ultrasonic welded using energy director and shear-joint designs. The resulting device can exhibit outstanding impact strength, clarity, and durability—without weld joint flashing.
For more information about flashing, talk with your Eastman technical service representative. You can also download an Injection Molding Troubleshooting Guide (specify medical or durables).
Expect a balance of important advantages with Eastman Tritan™ copolyester MX711.
- Chemical resistance
- Hydrolytic stability
- Compatibility with sterilization methods
- Secondary operations and assembly
- Heat resistance
Eastman Tritan™ copolyester MX711 is well-suited for medical-device applications that demand superior chemical resistance, clarity and hydrolytic stability, including:
- IV system components
- Blood therapy devices
•Select a compatible gating style for the selected resin. Most conventional cold gating styles work well with copolyesters. For hot runner systems, valve gates should be used.
Design Tooling with good Cooling/Thermal Control
•Copolyesters require good thermal control throughout the cavity for optimal processing.
Design Tooling with a plan for venting
•Poor venting can result in burn marks and incomplete fill.
Design Tooling with a plan for ejection
•Parts should be adequately supported during ejection to avoid part deformation/breakage
Click here for more successful processing tips