Pick Two: Good, Fast or Cheap

The pressure on design engineers to make the right decisions on prototyping are more important today than ever because of speed-to-market requirements, increasing cost restraints, and the escalating outsourcing of mold design, mold production and manufacturing to remote locations around the world. At the same time, options for prototyping processes are also expanding rapidly. What makes sense for you?

Start with a thorough analysis of what you want a prototype to achieve and what constraints you face. The major attributes are good, fast and cheap. It's virtually impossible to achieve all three. Pick two and establish priorities.

Here is a checklist of issues to consider:

• Time. How quickly do you need to have the prototypes in hand? What is your target date to go to market with production quantities? Is there any chance you may be required to scale to high volume with little notice?

• Budget. How much money can you spend? Is the project enough of a "sure bet" that you can spend money on a production-ready prototype (i.e, a single cavity in a multi-cavity tool)?

• Quality. What exactly will the prototype be used for? Do you want something to distribute internally for look and feel? Do you want something for market testing? Do you want something to test for fit and function? How rigorous are the testing requirements? Do they include weathering and lifecycle testing? Durability and mechanical testing? What standards must be met?

Other issues to consider include: What is the quantity of prototype parts required for each purpose? How likely are engineering change orders? What secondary operations may be required for the prototype? Is a special color or finish required? Must it be assembled or fastened to something else? Can you live with a gate or other blemish?

Once you have answered those questions, then you must prioritize. To help you make your decision, here's a quick look at the pros and the cons of various options available.

Build a preproduction injection mold from tool steel. This is the most expensive, but it also gives you the most performance. Top-drawer tool builders can produce a trial core and cavity set that can be used on a standard mold base in two weeks. With that tool, you can make actual parts using the plastic compound you want to use. You can achieve all benefits of form, fit and function testing, but you score low on the fourth "f": finance. Two more benefits: a) you can produce very large quantities of prototypes quickly and b) you can transition this cavity to the production tool as the mold builder finishes the other core and cavity sets required in the case of multi-cavity tooling. This allows recovery of some of the prototyping cost.

Build an aluminum tool. Aluminum is cheaper than tool steel and machines easily. You can make hand-loaded inserts instead of building complicated slides. You can make a relatively large quantity of parts, but fewer than with tool steel, especially with the more abrasive glass-filled plastics. You can use the real resin for full form, fit and function testing—depending on the plastic compound (including filler package) and the mold temperature requirements (aluminum can deform). This is less expensive than a steel tool, but also won't last as long. It's a good bridge to production, but doesn't get you to full-scale production.

Machine parts from stock plastic shapes. For more information see box above.

Try one of the newer (since 1986) rapid prototyping (RP) technologies that use an automated process to quickly fabricate a scale model of a part or assembly using three-dimensional computer-aided design data. This approach is also called solid free-form manufacturing, computer-automated manufacturing or layered manufacturing, depending on the process used. In general, RP systems make great prototypes for visual inspection, often in just a few hours. If you want very quick feedback on how the design looks, this is a good approach. Many of the processes do not use the final plastic compound you want to use and are not suitable for serious form, fit and function testing.

Solid Modeling. Some OEMs are trying to go all the way from three-dimensional drawing to production tool. This is obviously the least expensive approach, and probably the fastest, but is not a sure thing.

Next: Part Two will take a detailed look at tips on how to make injection molded prototypes (steel or aluminum tools) most effectively.