Common manufacturing processes for plastic and metal parts

One important tenet of Design for Manufacturing / Design for Assembly is the idea that the mechanical engineer and the industrial designer should choose the manufacturing processes that will be used to mass produce custom parts before finalizing the design for that part (and everything that it touches). Here are some common processes that are used to produce plastic and metal parts in consumer electronics and industrial automation applications.

Some post-processes for plastic parts

Sometimes plastic parts have to go through additional processes to arrive at the fit and finish and decorations required.  For example, soft touch paint is a common way to render a slick as-molded plastic part pleasant to touch and hold  – it is often used as a cost effective alternative to two-shot molding to add a rubberized finish to a rigid structural part.  Pad printing or tampoprinting is a common technique to apply an indelible (or nearly indelible) logo mark and other artwork on the surface of a plastic part.

Common manufacturing processes for custom plastic parts

For the purpose of this discussion let’s assume you need to make at least 2000 parts.  For lower volume prototyping builds rapid prototyping processes are more appropriate.

Here are some example plastic manufacturing processes that are commonly used to produce parts in consumer electronics and industrial automation.

  • Injection molding: This is the most common process that yields repeatable parts with the lowest part cost.  Appropriate for moderately sized runs.  The up front tooling cost is a concern.  There are services like Protomold that reduces the up front investment by creating a quick and dirty mold that can’t run as fast and won’t last through as many parts (but is perfectly fine for a first build if you already know you have design iterations coming in the form of a quick-turn product refresh).
  • Thermoforming: This is best for large parts that will be too expensive to injection mold (due to the cost of the mold).   Cosmetic parts that are 12″ and above in the longest dimension can be good candidates for thermoforming. The benefits are lower cost tooling and much faster turnaround time, at the cost of less precise part tolerances and less options for surface finish and in-mold decorating.
  • Rotomoting: This is how rubber ducks are made – appropriate for soft squishable parts that have undercuts.
  • CNC machining: Amazingly, this could be a real option for small run parts as you can get good material properties and surface finish. The flip side is intolerably high costs.  It could make sense for very small lots for very high priced products.

Common manufacturing processes for custom metal parts

For metal parts, the part cost will always be higher than plastic parts. Depending on the process used, it can be a little higher or a lot higher.

Here are some common processes to create metal parts.

  • Sheet metal: When origami works, it is wonderful – sheet metal molds are quick and cheap to make and the lead time is very short.  Sheet metal can be a great alterative to injection molded plastic for internal structural parts as it affords much more flexibility during the early stages of the design process when the product may still change or when the forecast for the product is not clear.
  • CNC machining: When the part is highly complex in shape, CNC machining may be necessary. This is prohibitively expensive for even relatively small parts, and the return on investment (ROI) to go to a die casting process can often be within a few hundred parts.
  • Turning: For shafts and bearing surfaces this is often the only way.  There are specialist metal fabrication suppliers who focus only on custom turning shafts – this is the price you will have to pay if you have precision bearing surfaces in a complex piece of machinery.
  • Sand casting: This is used to produce small lots of custom metal parts with complex geometry. There is no tooling – sand is used as the tooling material and a sand mold is formed around a positive model of the part (typically a CNC version of the metal part). The positive is removed, the two halves of the sand mold is put together and molten metal is poured to take the shape of the part in the void inside the mold. When the metal sets, the sand mold is broken to get the part back out.  This process creates a rough surface finish and does not preserve fine features – sometimes there is a post-machining process to create the tolerances needed for critical surfaces.
  • Investment Casting. This is used for cast parts with complex geometry – and is one step above sand casting.  A positive model, or pattern, of the part is made using a wax like substance (these days a variety of 3D printers can print high precision parts in a substrate that can burn away cleanly just like wax).  It is then put through an “investment process” where a coating is formed around the wax that will become the mold. Then the wax is burned away and the metal is poured into the resulting cavity to assume the shape of the wax pattern.  This process also does not keep tight tolerances, and post machining is needed to tighten the tolerance on critical surfaces.
  • Die Casting. This process incurs the highest tooling cost (typically 5 figures per mold) and the longest lead time (typically 5 months turn, as opposed to 3 months for injection molding) but results in the lowest part cost.  Hardened tool steel dies are created to form molds much like those used in injection molding, then molten metal is poured into the mold to form the part.  Post machining is also needed to tighten surfaces with tight tolerances.

Other metal fabrication processes

There are also other techniques like EDM (for burning a complex shape into a metal blank before finishing it off with a machining run – commonly used to make complex molds), wire EDM and water jet (for creating complex shapes for parts that are effectively flat) and the like.

For certain types of metals sometimes there are secondary processes involved. For example, aluminum parts often go through a hard anodizing process to result in an impervious coating that protects the part from oxidation.  Spring steel parts sometimes go through a heat treatment process to improve its material properties.

 

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