Looking at Converting Metal Parts to Plastics? Here are Some Critical Tips for Success

Converting metal parts to plastic?

Certainly, not a new concept (conversions have been documented back to the 1950’s!), now may be the time to revisit the process. 

The Upside

The excellent article by Mark Crawford of ASME (Metal to Plastic: Design Flexibility) cites some of the following as real positives for the conversion:

• High tensile strength with proper structural design
• Reduced part weight
• Highly repeatable in processing (less scrap)
• Lower manufacturing costs
• Enhanced regulatory compliance
• Greater design flexibility (part consolidation)
• Lower packaging and shipping costs
• Up to six times longer tool life.

Further, additional benefits can be derived from materials analysis and specification by utilizing ribbing, bosses and various material fill combinations of the engineering resin employed. 

From the Field

In an interview with Elk Grove Village, IL, molder Mike Walter of Met2Plastics several of the points reflected in the list above were echoed, along with:

• Significantly reduced production lead times for converted die castings to injection molded parts (12-16 weeks for castings vs. 4-6 weeks for injection molding)
• Virtual elimination of non-environmentally friendly plating required for metal part vs. molded engineering resin parts, which are naturally corrosion-resistant as-molded
• Molded parts can carry with them National Sanitary Foundation (NSF) status with no secondary operations.

An example brought up by Walter was the replacement of not-so-humanly-friendly parts with more benign forms.  In one instance, a solid lead casting used for a spacer/insulator was replaced with an injection-molded part containing tungsten and a polyester engineering resin.  The performance was virtually identical, with a less hazardous component.  This should appeal to those exporting parts into countries with strong RoHS requirements.

 
The Potential Downside

A recent article by Claire Goldsberry in Plastics Today (Converting metal components to plastic takes on new importance) provides some valuable caveats for organization planning such a changeover.

Citing a source at Iowa molder, Accumold,

• The mechanical load on the part - the physical forces that puts demand on the part.

• The environment in which the part is placed in terms of temperature. "If it's a structural part out in the open, it may be subject to gravel bombardment. The drivetrain is a different story because of the high-temperature environment," he explained.

• The chemical environment is also critical to evaluate. "There are a few polymers that are chemically resistant such as PP, POM and PA, as well as some new materials.

Further, other considerations to be factored in, according to Mike Walter of Met2Plastics are:

·Tooling costs associated with molding vs. machining or fabrication

·Knowledge of the design staff to undertake the conversion

·Ability to recast the tolerances, shrinkage factors and proper gating of the new tooling

 
The Bottom Line

Accumold sums the processes up as, “…OEMs want cost savings so they come to the company to evaluate if the part can be converted from metal-to-plastic. We can't do it with every part, but we can do it with most parts. Conversion is a long road and the OEM has to be committed to this in order to be taken seriously by us. We're constantly looking for opportunities and frankly, [the OEM] is always looking for cost reductions. It's very rare that we do a conversion and not save some money…"  Clearly, cost savings is a key goal, other, sometimes more intangible goals, set the stage for a success conversion. 

Working with your molder, or his representative, is the best way to make sure all salient questions are addressed before you make chips!