urethane casting guide: 5 things you need to know

urethane casting prototype

I like urethane castings.  They always look so clean and pretty.

Use them for the wrong project however, and you’ll be stuck explaining to your client why you dropped a few grand on prototypes and have nothing to show for it but a receipt.

When I evaluate castings for a project I’m working on, I hit a mental checklist: appearance, material, precision, elastomers, and quantity.

Appearance: Pretty

Urethane castings look sharp.  They’re what marketing has in their mind when they ask for something to show focus groups.

The finish on the casting is determined by the finish on the master.  The polish on the master can result in a texture so smooth that you can achieve a transparent effect when using a clear urethane to make the casting.

Or you can cast using a colored urethane (no painting!).  Just supply a paint chip or Pantone color to the shop and have them start mixing.

Material: It’s urethane… and nothing else.

As you might guess, the urethane casting process only allows you to cast, well, urethane.

Sounds really limiting right?  It’s actually not too bad.

Casting houses use different blends of urethanes to mimic common plastics. Wannabe blends for ABS, acrylic, polycarbonate (PC), polypropylene (PP), polyethylene, santoprene, and nylon are all readily available.

Some shop even offer heat and chemically resistant grades of urethane.  They’re cool, but if you intend on performing heat or chemical testing, you are better served in the long run by testing the actual material you will be using in production.

Precision: Not so much

  • Master = original
  • RTV mold = copy
  • Each urethane casting = copy of a copy

Doug Kinney #3 said it best:

“You know how when you make a copy of a copy, it’s not as sharp as… well… the original?”

Same idea with urethane castings.  If you’re prototyping some packaging (an MP3 player housing, for examples) you’ll be fine.  If you are prototyping a tiny mechanism, you’re much better off sending the job to a high-precision machine shop.

Elastomers: Got any?

Urethane castings are your only choice for elastomer prototypes of any decent quality.

Period.

There are other options that can (sort of) do the trick (visually), but to get the right feel, urethane casting is your only option.

Most urethane casting shops will recommend a durometer of 40 to 80 Shore A, but you can often push them as low as 20 Shore A or as high as 90 Shore A.

Quantities: 6-20

The low end of the range is driven by is non-recurring costs.  The master and the mold take time and material to make.  It works out that urethane casting becomes cost-effective at about 6 or 8 parts.

The high end is limited by the durability of the mold.  The RTV we used to make it is cheap, but it wears down after 20 or so castings.

You always have the option of making multiple molds in order to get more castings.  Just keep in mind that as your quantities get higher, rapid injection molding becomes more feasible.

Keep this information in mind and be on your way towards beautiful prototypes.

What about you?  Do you have any rules of thumb for urethane castings?

Related posts:

  1. urethane castings: 3 things to ask before you hit ‘send’
  2. pro|engineer (or creo, or whatever): a few more things
  3. five phases of “done-ness”

Tagged as: prototyping, urethane casting, urethane casting guide

Article by chris loughnane

Chris is Design Engineer + Sustainability Advocate at a Boston area design consultancy and has written 127 sweet posts for the Product Development Notebook.

Need a regular fix of product design goodness? Subscribe via RSS or Email to keep up with the latest posts!

The Product Development Notebook runs the Thesis Wordpress Theme.