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Key steps for metal-to-plastic conversion

Key steps for metal-to-plastic conversion
The ability to replace a metal component with one molded out of plastic means that extremely complex shapes and geometries can be achieved easily. That also means greater design freedom and more optimum design for manufacturability.

The ability to replace a metal component with one molded out of plastic means that extremely complex shapes and geometries can be achieved easily. That also means greater design freedom and more optimum design for manufacturability.

The 10-port water outlet injection molded out of engineered thermoplastic demonstrates metal-to-plastic conversion. Credit: Industrial Molds Group.
In many cases, multiple metal parts that make up a larger component need to be welded in a secondary operation which adds to the time and the cost. Also, where there is a welded area, there can be a tendency toward a weakness in the metal. By combining several metal parts into one molded plastic part, the resulting product or component is stronger and performs better. Additionally, plastic components are relatively maintenance free as they do not need coating or painting to protect the material.

Kerry Smith, account manager for Industrial Molds Group, a mold manufacturer serving the automotive, fluid handling, medical and packaging markets, offers some sage advice when converting metal parts to plastic components. "The first step on the road to doing a metal-to-plastic conversion is ask the right questions upfront with the collaborating parties all being involved," Smith said.

Those questions might include:

  • What are you trying to achieve by converting from metal-to-plastic? (For example, do you want a better product? A product less costly to manufacture? A product with superior properties?).
  • What's driving the change? Weight reduction? Strength enhancement? Aesthetic improvements?
  • What is the part's function?
  • What plastic material will be used?

"All the parties involved need to drill down and answer these questions and any others that may arise," Smith added.

The next step to making a metal-to-plastic conversion is to understand the application. The product engineer, the part designer, the mold designer and the molder all need to collaborate to determine the material properties required given the component's use, the stresses it will be under, and the environment in which it will operate.

It's critical to understand the customer's objective in converting a product or component in order to provide the optimum solution. If, for example, an OEM just wants to convert a component to plastic because plastic is cheaper, the OEM needs to be educated as to why that might not necessarily be the case.

Next, all the parties need to do a cost-benefit analysis. Typically, a metal component is either fabricated out of metal via the machining process or produced from a die casting. With an injected molded plastic part, you need a mold that is in itself a complex piece of machinery. So there are other questions that arise in this analysis:

  • What will the cost of the mold add to the overall cost of the unit price of the parts?
  • What is the price of the plastic material?
  • Will scrap reduction be achieved by injection molding vs. machining parts from metal (which typically results in significant scrap)?
  • How many parts are required monthly/annually? The higher the volume of parts needed, typically the lower the costs to injection mold the parts.
  • What will spending the money upfront to design and build a multi-cavity mold save in the long run over the life of the product given that the parts are being produced in multiples as opposed to single?
  • Will there be automation involved and how with the cost of the automation save money in parts handling and secondary operations such as welding, coating, painting, etc?
  • What will be gained in terms of quality, cost to manufacture, product life, and functionality?

After the OEM has made the decision to replace a metal component with a plastic component, the next step is working through the design challenges. What works for a metal component might not translate exactly with an injection molded plastic component with respect to design.

Converting from metal-to-plastic allows for faster manufacturing cycles and higher throughput. However, volume is an important consideration for OEMs looking at the viability of metal-to-plastic conversions. "OEMs often forget to do the comparisons of plastic injection molds with tooling required for stamping dies or die-cast dies, for example," noted Smith. "That is why asking the important questions up front and understanding the application are key to ensure the conversion is something the OEM will be happy with in the long run, and get a good return on their investment."                  

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