Proper resin requirements

Fitness-for-use criteria:

  • Heat requirements
  • Aesthetics
  • Chemical resistance
  • Mechanical properties
  • Flow requirements

Reasonable fill-pressure requirements

Improve the moldability of a design.

Excessive fill pressures can result in challenges with injection molding, including:

  • High clamp-tonnage requirements
  • Reduced life of mold components due to high stress loading
  • Higher ejection-force requirements
  • Tendency for molders to run excessive melt temperatures to reduce fill pressures
Eastman uses mold-filling simulation to estimate required fill pressure for a proposed part design. The target maximum fill pressure for a part design as determined is 15,000 psi or 20,000 psi if the simulation model includes runner and gate.

 
Eastman Tritan™ copolyester MFR (g/10 min, 280°C, 1.25 kg load)
TX711 7
 TX2001 8
 TX1501 18

Reasonable fill pattern

Eastman uses mold-filling simulation to predict the fill pattern of a proposed part design and gate location, which is effective in predicting potential fill pattern problems:

  • Weld lines
  • Air traps
  • Flow front hesitation
These often require costly modifications to correct after tooling construction.

Eliminating areas of excessive shrink

Excessive volumetric shrinkage during the injection molding process can result in defects in part appearance:

  • Sinks on the part surface
  • Vacuum voids (appear as bubbles)
Eastman uses mold-filling simulation to predict volumetric shrinkage levels in proposed part designs, and the guideline is 6% maximum volumetric shrinkage for proposed part designs.

Gate location considerations

Aesthetics
The gate location on an injection molded part leaves a “witness” where the part is separated from the runner system and is considered an appearance defect. It is typically hidden in an area of the part that is not obvious.

Mechanical properties

  • Resin enters the molding cavity at high pressures and temperatures at the gate location. 
  • The part surface in the gate area typically includes defects that can behave as stress concentrations during tensile loading or drop testing. 
  • Gate locations exhibit inferior mechanical properties compared to the molded resin out in the cavity. 
  • Gate locations should be located in areas of the part which are not subjected to externally applied high tensile loading.

Eliminating notches

Impact failures in part designs are frequently initiated by a stress concentration created by a sharp notch. The performance of a part in a drop test can often be significantly improved with a small increase in radii of sharp features.

FAQs

How much mold shrinkage should I design for with Eastman Tritan™ copolyester?
The typical value as determined by ASTM D955 is 0.005–0.007 in./in. (0.005–0.007 mm/mm).

I am designing a box with a living hinge for the lid. The living hinge will see multiple cycles in the product's life cycle. Does Tritan work well for living hinges?
No. Tritan isn't suggested for living hinge applications.

I am designing a part with multiple ribs. How thick should the base of the rib be to avoid visible sinks on the opposite side?
A general guideline range is that the base of the rib be approximately 40%–60% of the nominal wall section. If the part has a nominal wall of 0.100 in. (2.5 mm), a reasonable range for the thickness at the base of the rib would be 0.040–0.060 in. (1.0–1.5 mm).

What is the minimum wall section that can be molded with Tritan?
The minimum wall section for a part molded with Tritan will be dependent on manufacturing considerations as well as end-use, fitness-for-use requirements. First, the part has to be thick enough to be filled with reasonable fill pressures. Second, the part must be able to meet any real-world, end-use physical requirements. Eastman Design Service engineers have the experience and tools in place to evaluate your particular design and provide feedback regarding a reasonable part thickness with multiple factors considered.

What is the minimum draft angle suggested for a part designed to be molded with Tritan?
The suggested reasonable range is 1.0–1.5 degrees per side. Parts with 0.5 degrees or less have been molded, but this is not suggested due to difficulties that may be encountered, such as sticking, drag marks, and extended cycle-time requirements.

I am designing an electrical housing that needs to have V0 flammability rating, and I want the housing to be clear. Would Tritan be a viable candidate?
No. Tritan does not meet the UL V0 flammability requirements.

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