New Study Benchmarks Resins for Tiny Parts

Data sheets are not necessarily very useful when selecting resins for micromolding. That's one of the conclusions from an interesting study conducted by Accumold, a leading micromolder based in Ankeny, IA. And don't expect to get meaningful results from flow analysis. Even the best programs may show parts to be unmoldable, or may give incorrect data.

Also, don't necessarily believe it when a resin supplier tells you it's impossible to achieve the seemingly impossible when designing very tiny parts.

"This study shows how, in terms of micromolding, picking resins based on data sheets can be misleading or confusing," says Aaron Johnson, marketing manager for Accumold. "It's important to consult with your molder when designing parts that may be considered outside the norm."

A data sheet, may describe a thermoplastic as "easy flow", for example. There's a big caveat, however.

"These data sheets are often calculated using much larger sample bars and their stated parameters may not apply," says Johnson. For example, most recommended gate sizes are larger than many micromolded parts.

The Accumold injection molding test was conducted with 11 commercially available thermoplastics, ranging from high-density polyethylene (HDPE) to polyetherimide (PEI) and polyethertherketone (PEEK). The critical dimension in the test part is a wall thickness of 0.003-inch over a length of 0.1276-inch.

"The goal was to see how far we could push the different resins and take note how they compared," says Johnson. "The mold was not modified for each resin, including the gate or runner system. We used the standard processing windows as specified for each resin as the only variable to give each process a chance to fill the part on its own."

The only critical dimension was to keep the 0.003-inch thickness. Results show how resin selection affects the desired output when designing a part. The mold was built with a thick-to-thin transition to create an optimal opportunity for the parts to fill. That, of course, is not always possible in the real world.

The critical concern in the test was to maintain the 0.003-inch height. Length of fill was not the key issue.  "Simply increasing the pressures and blowing open the mold would not have given us the right data," says Johnson. The mold was not adjusted either for performance or shrink.

The purpose of the study was to show how various thermoplastics stack up in a given scenario.

Click here for a table with detailed findings and click here for a copy of the Accumold report.