MTD Highlights Design Mistakes on Micro-Sized Plastics Parts

Custom injection molder and mold maker MTD Micro Molding is proactive in helping clients avoid design mistakes on very small plastic components.

The aim is to manage a customer's expectations on costs and delivery times.

Often, a client ordering a smaller part believes "all of my tolerance and features and specifications can follow along with that part," said John Clark, MTD project manager. Sometimes that thought is applicable, but, inherently, the high-pressure injection molding process can cause flash, mismatching or dimensional instability.

MTD has identified eight common micro design mistakes that can make the project team "put the brakes on," he said.

"We want to identify a future problem and streamline the entire process," he said. Over 12 months, MTD may review 25-50 projects from clients, primarily in the medical market.

Most frequent among the mistakes is a customer's explicit expectation for zero flash.

"With zero flash, we would not be able to manufacture the product," Clark said in a telephone interview.

"Steel pieces wear over time," he noted. "Flash is always going to happen, and it never gets better."

A plastics processor using a tool requiring high-cost maintenance "will spend more time and money in revalidation activities than manufacturing commercial products," he said.

That is highly relevant to MTD, which has tooling and molding capabilities under the same roof in Charlton, Mass.

"The same people and equipment build new tools and maintain tools," Clark said. "If we mitigate problems upfront, it reduces the amount of resources required to mold the product."

The next most frequent mistake involves erroneous gate locations.

"Be flexible, and avoid assigning poor gate locations," MTD says.

That can occur with "rescue projects" where "another molder failed [and the] customer comes to us to pick up the pieces and get the product to market as quickly as possible," Clark said.

"We start over, do a thorough set of verification activities and propose it to the client," he said. "It is up to the client to let us know if the gate location is outside a critical location of the project. Our spec of a gate location may not fit into the functionality of the device."

If so, MTD begins again.

MTD reviews each project's drawings, checks for mismatches and then does a design-for-manufacturability review in a conference call with the client. While sharing computer images with the customer, MTD highlights "anything we don't feel will be an issue" in the color green, Clark said. Yellow highlights aspects needing alternative solutions. Red is "no go 99 percent of the time."

Clark noted that some large OEMs have multiple product design teams that may not interact.

"With a new team, there is not a lot of crossover, and we need to start from square one," he said.

Here are MTD Micro Molding's other recommendations for creating successful designs from the get-go:

> Stay symmetrical and avoid lopsided tolerances; adjust the part model to be at the midpoint of the tolerance range.

> Avoid nearly impossible tolerance for a critical dimension; sustaining an extremely tight tolerance is challenging on long runs.

> Avoid critical dimensions that require cross-sectioning.

> Keep the design real by avoiding critical dimensions that include theoretical points.

> Designate no more than four critical dimensions.

> Keep flash and mismatch separate from your dimension to avoid infringing on manufacturing tolerances.

MTD Micro Molding is certified under the ISO 13485:2003 quality management system for medical devices. The firm has nine mold makers among its 35 employees and does significant work in bioabsorbable and drug delivery micro molding.