Converting your customer’s part from metal to plastic? Better ask the right questions

Moving a metal part to plastic often carries with it inherent problems that can be a challenge to moldmakers. One company found a way to emulate a metal case that worked just fine with plastics.

Industrial Molds Group (Rockford, IL) prides itself on being a company that specializes in mechanically and geometrically complex tooling for the automotive, industrial, and medical markets. Many of the parts for which Industrial Molds builds molds are for heavy-duty industrial products, and some customers come to the company wanting to convert their product or components from metal to plastic.

While it may sound simple, converting a metal component to a plastic component isn’t a one-to-one conversion process, as many in the industry know. There are challenges, and Tim Peterson, VP of Industrial Molds, explains what needs to happen when doing a metal-to-plastic conversion.

“The first step if someone wants to convert to plastic is understanding the application and what they’re trying to achieve by converting from metal,” he says. “What’s driving the change? Do they want a lighter-weight component? A stronger component? We typically ask them, ‘Why do you want to change and where is this part going or what is the part’s function?’ As you understand the application and what’s driving the customer’s desire to convert from metal to plastic, you will be able to ask the right questions, to drill down and get the information you need to make intelligent decisions.”

Peterson says that it’s critical to understand the customer’s objective in converting a product or component, so that you can provide the optimum solution. “If a customer just wants to convert to plastic because plastic is cheaper, and not necessarily for other, viable reasons, then he might be disappointed,” states Peterson. “With plastic injection molded parts, you’re talking a tooled up product vs. untooled parts that are fabricated. Most of our applications are for automotive where they’re making parts using diecast methods or machining to get what they need. They think plastic is cheaper but some don’t realize there are upfront costs such as the cost of the mold.”

Making plastic work
There are also design challenges. What works for a metal product might not translate exactly with respect to the design in plastic. One customer came to Industrial Molds with its Rainmaster Case, a waterproof electrical box originally produced using a metal stamping process. The original case had knockout slugs, or areas that were “coined” on the case during die-stamping so they could be easily punched out to allow electrical conduit into the case.

However, a problem occurred when the case was converted from metal to plastic, and the customer was unable to mold effectively in the areas for the knockout slugs. Kerry Smith, Industrial Molds’ account supervisor, worked with the customer and Industrial Molds’ engineering staff on this challenge. The optimum thickness for the knockout ring around the slugs was 0.007 inch. The problem in molding the case was that the material had to flow from thicker areas of the wall section through the thinner areas of the knockout ring, and then fill the slugs.

“The area for the knockout slug breakout was so thin that the customer was getting voids in the ring and short shots in the slug. They couldn’t make consistent parts,” explains Smith. “The short shots meant the cases would have a leak path, and this was supposed to be a rain-tight electrical case for outdoor use.”

The solution was to be able to coin the knockout slug areas like the customer did when making the metal cases, which led to another challenge—how to make the mechanics in the mold to coin the knockout slugs similar to the way it did with the metal stamping process. Additionally, the tool required several lifters to release trapped rib features and two-stage ejection to cleanly release from them. The result was a fairly large (40 inches long), tall, and complex two-cavity mold.

“We addressed the knockout slug issue by presetting the slugs to mold a greater thickness, injecting resin, and then coining the areas through hydraulic-driven mechanics,” says Smith. “In order to identify the correct time to coin, we used RJG pressure sensors to let the operator know when to activate the system. We had to play with it a bit to get it just right. Shifting the coining operation a couple tenths of a second during injection allowed us to identify when the material was soft enough to coin and when there was enough plastic in the area so that when we coined it, the knockout slug would be whole. Direct hot-tip gates were a necessity in controlling the injection process as well.”

Amazingly, Smith notes, “The mold performed very well the first time we ran it. The only thing we had to do was play with the timing of the RJG pressure sensors to help us know when to sequence the coining process.”

When you should convert
Peterson adds that converting from metal to plastic allows faster manufacturing cycles and higher throughput, two really big benefits. However, volume is an important consideration for customers. “If I need 10 parts a year it’s not worth it,” says Peterson. “If I need a half million parts a year, now it’s cost effective to convert.”

Material selection is also important. While metals such as stainless steel, aluminum, and copper are expensive, many plastic materials can also be very expensive, especially engineering grades.

“We did a conversion to an Ultem part several years ago, and it’s not a cheap material,” explains Peterson. “Every year the customer was always wanting the price down; always wanting to reduce the price in some way, such as by thinning out wall sections. But engineering changes to molds cost money. So obviously understanding the plastic materials that are required is important because there’s a cost there, too.”

People often forget to do the comparisons of plastic injection molds with tooling for stamping dies, for example. “Sometimes customers need to be educated in what the whole conversion process entails because they sometimes don’t know what they’re asking for,” Peterson notes. “That’s why we need to ask the important questions up front and understand the application, to make sure the conversion is something the customer will be happy with and get what they want. If they’re not chasing the right thing they won’t be happy, and we’ll have an unhappy customer. That’s something we don’t want.”—Clare Goldsberry