Making parts out of plastic that were once metal is a fast-growing area of injection molding due to recent advances in engineered-grade resins that can produce parts as durable as those of metal. The plastic parts are also more lightweight and cost-effective to produce.

However, designers must consider several factors to ensure a successful conversion. Usually, good results come from part designers working closely with their material suppliers, tool designers, and molders to evaluate materials and production. Because everyone contributes a different expertise, the multi-company team can better identify manufacturability efficiencies and devise ways to cut costs.

OEMs should choose molding and moldmaking partners that know a lot about material selection, design for manufacturability, part size, and weight reduction. It can be difficult to change any of these factors one the material and geometry is qualified. Taking the time to consider all options up-front is less likely to create production headaches down the road.

Cut material costs

A part designer might make the initial recommendation for a new material, but it's important that the raw-material supplier, mold designer, and molder make suggestions as well.

Together, the team should look for the most cost-effective material that the molder has used in the past. Molders often have more expertise working with some materials than others. The mold builder and molder's early feedback significantly improves the likelihood of producing quality parts from the first shot off the mold.

In addition, materials don't always behave in production the way that their data sheets claim. When an experienced molder uses the material, the molder is aware of expected venting conditions, actual shrink rates, and other processing mechanics. The molder can also usually negotiate bulk-material prices on resins it uses most frequently, further cutting part cost.

What's more, experienced team members can determine when a specified resin is a higher grade than necessary. Higher-grade materials usually cost more. Selecting a more suitable material — one that still meets all of the product's end-use specs, but by a narrower margin — lets the team whittle down costs even further.

For example, custom molder Plastikos recently produced a part for a large medical customer. The customer suggested using Vectra A130. But Plastikos and the raw-material supplier identified a high-temperature nylon as an alternative. The nylon was 40% less expensive while meeting the same part-performance requirements as the originally suggested material.

Molders can also examine how much material will be used in the runner, and if there are opportunities to reduce waste. For example, a common way to reduce waste is by converting from a cold to a hot sprue (hot runner) system. Sometimes it is even possible to use reground materials. But make sure the regrind is qualified up-front, because it is difficult to make a change later once parts are validated in the end application.

Design for manufacturability

To injection mold more lightweight parts, engineers must also look closely at manufacturability and consider process conditions with respect to machine limits.

For example, increasing cavitation necessitates boosting injection pressure to overcome the resistance from the mold. Depending on the size of the part and the incorporation of slide action and other components, it's easy to mistakenly build a mold that won't fit into the machine. Making sure the part is moldable and the mold is correctly sized is critical.

Maintaining a consistent wall thickness throughout the part is also important in injection molding. When variations are called for, it's best to avoid abrupt changes in thickness and instead use gradual transitions. This helps eliminate sinks, voids, and warping. It also eliminate stress concentrations that can significantly reduce the overall strength and ultimately the performance of the molded part.

Generally, most machined components don't require a consistent wall thickness because shrink and warp are not machining issues. Therefore, when converting a machined part to plastic, the processor and the part designer must thoroughly understand each other's requirements. Only a thorough understanding of the product's function lets processors suggest workable, alternate part designs. Processors can minimize inconsistencies in wall thickness and eliminate poor tooling conditions without over-complicating the mold design and driving up the cost.

Here are additional considerations for part and mold design:

• Gating

  It's important to determine if the part will have gate vestiges, and if so, the best gate placement to eliminate them. Vestiges are often objectionable in the visible areas of cosmetic parts. To avoid vestiges on the part surfaces, incorporate gate recess pads where possible. Also, gates should be located at the thickest area of the part so the plastic flows from thicker to the thinner sections. Otherwise, it may be extremely difficult or impossible to pack the thick areas.

• Steel thickness

  Ensure that the steel mold is thick enough to withstand the injection-molding process. Interlocking (or bypassing) small and intricate features in the tooling often helps sustain the design intent.

  Mold wall support

  Ensure the molds' walls are supported in high injection-pressure areas. When walls are too thin, cavities can deflect under pressure, causing flash and related defects.

The many challenges reinforce the need for a collaborative team-based effort that includes experienced part designers, material suppliers, mold designers, and molders. When you do the extra work to apply the expertise of all these parties, metal-to-plastic part conversions can be a good way to get the lighter, smaller, and cost-effective parts the market demands.

Written by: Rob Cooney
Manufacturing Manager
Ryan Katen
Engineering Manager
Plastikos Inc.
Erie, PA
plastikoserie.com

Edited by: Leslie Gordon, leslie.gordon@penton.com

ARTICLE FOCUS:

• There are several factors to consider in changing metal parts to plastic.

• OEMs need knowledgeable molding and moldmaking partners.

• Experienced molders can usually negotiate bulk-material rates on resins.

FROM BRASS TO PLASTIC, A COST-SAVINGS SUCCESS

A global medical OEM recently decided to explore more cost-efficient methods to produce a traditionally brass component for one of the company's medical pumps. The brass part was machined, a costly and time-consuming process.

Plastikos evaluated whether converting the part to plastic would provide the cost savings the OEM sought. Plastikos started with part prints and a few options — a single piece with molded threads, or a two-piece molded assembly. The initial review included a face-to-face meeting with the OEM's engineers and material suppliers to develop comprehensive risk assessments and material recommendations for both options.

Further analysis showed that the molded-in thread option would need a tougher, more durable material to help prevent the threads from tearing during the core pull. This option translated into costlier raw materials and a higher-risk tool design. It would also be difficult to fill the threads with resin during molding.

Plastikos recommended molding the part and then installing a separate threaded component. This option let the team use a less costly and more familiar material, incorporate regrind, and use a simpler mold design.