Tips for metal-to-plastic conversions

Jim Conkey
Application Development Engineer
Eric Sattler
Application Development Engineer
DSM Engineering Plastics
Evansville, Ind.

No question a lot of metal parts have been converted to plastic over the past several years. But there are still many opportunities for conversion that go unevaluated. That's because designers sometimes dismiss plastics simply because a particular part "has always been made of metal." Here are some tips that can help separate designs that are contenders for plastic conversion from those that still must remain metal.

Does it matter?

Metals offer a wide range of performance "givens" that designers readily grasp. In contrast, plastics aren't always as widely understood and in fact may be misunderstood. Key to successful conversion is a good understanding of both the application and the plastic's capabilities.

Advances in material testing let manufacturers better predict how plastic will perform under the loads and temperatures traditionally used when testing metals. Improved CAE software has also helped better define design principles for plastics.

Traditional areas for metals include gears that have significant torque loadings. Such gears made from steel, for example, need lubrication while their plastic analog wouldn't. Weight reduction is another benefit gleaned from metal-to-plastic conversion. Replacing metal throttle body gears with equivalents made from plastic not only lightens the component but also reduces cost, eliminates contained lubricants, and provides for smoother operation. And in some applications plastics reduce noise and boost recycling opportunities.

One method for evaluating the pros and cons of conversion is to assess the primary performance demands. Here are a few critical parameters that must be defined:

- System operating temperature range.
- Maximum load and deflection conditions.
- Creep and fatigue constraints.
- Wear limitations (tribology) and the types of materials to which the parts will mate.
- Impact or shock-load requirements.
- Chemical contact or use inside the system.
- UV or other weatherability requirements.
- Part consolidation potential.

Another important parameter is stress. Designers must determine the type and magnitude of stress parts will see. Also important is the temperature parts are subjected to under these stresses. Engineering plastics can withstand a substantial amount of stress. This is evident in plastics employed in thrust washers, EPS gears, and chain tensioners. Plastics can also hold tolerances tight enough for plastic-to-metal threading where cold creep can be an issue.

Plastic parts can often be thinner, lighter, and smaller than their metal counterparts. That's important as available real estate shrinks in applications such as motherboards, under-the-hood automotive, and cell phones. Plastics offer a lightweight, high-performance package that metals just can't touch in many cases. For the most part, engineering plastics suppliers that are knowledgeable about metals can help designers determine if a part can be converted to plastic. They will be intimately aware of limitations on the various grades of plastics.

But there are no golden rules and the boundaries between the two types of materials are constantly being redrawn. Even designers familiar with plastic forming processes may not have heard about new resin systems. So it's important to contact a resin supplier familiar with metals as early in the design as possible. They will be able to shed light on whether the design is suitable for metal-to-plastic conversion. They can also recommend ways of modifying the design to improve manufacturability via one of the many plastic-molding processes available.

Material pros and cons

Material choice will impact manufacturing and assembly costs. That's because material cost comparisons are typically tabulated on the basis of molded or machined per-part costs. But plastics offer significant savings down the road. The most obvious advantages come from lighter weight, but plastics offer a lot more when manufacturing and assemblies are included in the equation. For example, while the tooling for an aluminum part and plastic part cost about the same, the rate of manufacturing is often slower for metals. Further, tooling life for plastic parts is typically ten times that expected from a tool for casting aluminum.

In most cases it takes numerous steps to turn a metal casting into a near-net shape. But a plastic component is usually ready to go right out of the mold. Many metal parts also need either a coating (paint, oil, etc.) or anodizing for corrosion protection. Plastic materials are often inherently corrosion resistant.

It also takes a lot of energy to produce a metal part. Not only do metals have a much higher melt temperature, successive machining steps -- from stock shape to near-net and screw machining -- all require energy and time. Though CNC machining significantly cuts the risk of failure at each stage of the process, these steps become unnecessary with plastics. Additionally, metal parts generally can't be switched to a less-expensive metal without going through a redesign. In contrast, less-costly plastics can often use the same molds as their more-expensive predecessors.

Over the past decade, engineers began looking at designs as systems rather than individual parts. That's where a lot of design breakthroughs begin. A systems viewpoint also lets traditional metal assemblies be seen in a new light with regard to plastics. A case in point is the valve lifter guide assembly in push-rod engines. Before becoming a one-piece plastic part, the valve lifter guide was a seven-piece assembly. Part consolidation is not just a trend but a way to design-in simplicity that will show up in everything from lower inventory levels to faster time to market.

Material property comparison