Many efforts have been undertaken by Original Equipment Manufacturers (OEM’s) to remove costs from their products. Converting metal components to plastic have delivered significant, and in many cases unmatched cost savings. The following lists important aspects of the process.

All metals and plastics have with them a set of convenient property "givens" that are easily understood by designers, rarely change, and are traditionally the first to come to mind when a material is selected for an application. Metals tend to be much more capable of maintaining properties over a broad range of conditions. Plastics are more susceptible to change while in the operating conditions. The key consideration for a possible plastic material in the application is the effect normal operating conditions have on the properties of a plastic component.    

As a result, injection molded plastics offer many more specialized alternatives for designers. Plastics are made up of an expanding variety of characteristics for tensile strength, heat tolerance, and chemical resistance. Because of these specialized characteristics, the number of plastic alternatives also continue to expand into more design critical roles. Designers now have over 70,000 resin alternatives available to them. Finding the right alternative is becoming more difficult then ever. IDES and Matweb are both highly visible and well-respected resin listing websites. Each also provide considerable information to compare the various grades available to you. 

An added measure of flexibility includes a wide range of fillers available to modify the properties of any particular resin. Lets say you like the chemical resistance of Nylon for an application, but you want a little more tensile strength. A wide range of fiberglass filled alternatives are available for you to choose from that will most likely fulfill these goals. You want more specific gravity? Consider stainless steel or brass fillers.

If you need help selecting a plastic resin, go to our Plastic Selection Assistance page.

Three primary plastic processing methods are available to you: injection molding, compression molding, and extrusion/blow molding. Injection molding resins feature the greatest total number of resins for you to select from. Some resins cross over between injection molding and blow molding, but compression molding resins are largely unique to this process.

Before analyzing the potential value of converting metal components to plastic, several initial considerations must be made:

• What temperature will the part see? Although many new resin formulations have been developed over the past several years, some temperatures are simply too extreme for the current family of plastic resins. Temperature in the application is the primary limiting factor for converting to plastic components. Although newer versions of plastics are demonstrating higher and higher heat tolerances, the price per pound for those resins can offset much of the potential savings.
• What chemicals will the part see? Plastics have also shown to be a good selection for applications that are in contact with a variety of chemicals. The specific chemical in the application should be reviewed for compatibility to the resin being considered.  Plastics also do not rust in applications with great amounts of moisture or humidity, which is often a positive aspect however some plastics "swell" with moisture absorption. Some solvents can also affect the properties of the plastic material.
• Does the part need to conduct heat, electricity, or have radio frequency shielding qualities? All these application specific requirements serve to be the "first pass" that narrows the potential plastic grades to a smaller number of candidate plastics.
• What are the cost drivers for the metal part? Are expensive secondary machining steps required? Huge amount of in-process scrap? Could a two piece plastic part be ultrasonically welded to achieve the finished part versus expensive multiple machining steps? A value stream map is a good tool for capturing all the steps in the process and to more visibly demonstrate where opportunities may lie.
• What tolerances are required for form, fit, and function? Dimensional tolerances can be significantly larger on plastic components than metal components, especially when compared to machined metal features. All plastics have different dimensional performance characteristics, and can be sensitive to size and shape. Some grades of resins possess better than average levels of dimensional performance but are still well short of machined metal tolerances. The part drawings will need a close review to determine what tolerances are achievable on a case-by-case basis.
• Tooling costs are also a necessary consideration when converting to an injection molded component. Injection molding tooling can be more expensive than die cast molds, but will last much longer, and will produce a finished part. The tooling is an up-front expense that must be planned for. 

The Primary Reasons to Change from Metal to Plastic

Reduce Weight 

Reducing weight in an assembly can potentially increase automotive fuel efficiency, improve the portability and appeal of a device, reduce component freight costs, or simply in an attempt to improve the convenience of an item for the consumer.

It is relatively easy to see opportunities to reduce weight by converting metal components to plastic by comparing specific gravity values. A plastic alternative could be an improvement in weight of several times over.

To Reduce the Number of Components in an Assembly

The intent is to simplify the product or assembly. Customers constantly want things that are lighter, faster, smaller, and more convenient to use. Injection molding offers great flexibility to form practically many geometric shapes. The only limiting factor is that the part must be able to be removed from the mold. Plastic tool designers are uniquely creative when it comes to part and tool design to enable the reduction of parts in an assembly. Often components like grommets, screws, and bolts can be incorporated into the molded components. Injection molding can be faster and less expensive than machining, casting, or forming metal components.

Reduce Manufacturing Scrap Costs

With this opportunity, you are looking to identify where the yield % of raw materials to finished product diminishes (process scrap). The easiest way to identify this is by using value stream maps if you’re familiar with that technique. If you are not familiar with VSM’s, just go out on the production floor and ask where the scrap goes – You’ll find it. If you can mold the component without all the process scrap that’s got to be a huge win for you.

Improving the Performance of Components in an Assembly or Device

This effort is usually driven by Quality improvement initiatives, weight reduction, application failures, or requests from customers who are seeking alternative solutions in an application. The suitability for improved performance must be judged on a case-by-case basis after all relevant application factors can be reviewed.      

Improve the aesthetics of a product

Designers now have a variety of plastics, all with different "feel" and textures that can be modified to further please the customer as they use the product. Not only are tactile improvements workable, but almost any color also is possible. Colorant suppliers can match practically any color for you. 

Summary

We very much appreciate the opportunity to work with designers to offer our extensive knowledge of resins and their applications. We would be pleased to work with you on your next project. 

If you would like to ask for our opinion on a particular project, please contact us at: sales@indmolding.com, or call us at 806-474-1000.