Pick Two: Good, Fast or Cheap
The pressure on design engineers to make the right decisions on prototyping are more important today than ever.
This is 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. In addition, 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 handloaded 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.
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.
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.
Machine Parts from Stock Plastic Shapes
An increasingly popular prototyping option is the machining of plastics stock shapes: sheet, rod, tube or custom profile. This approach avoids the high cost of building a mold, and gives you a part in 7 to 10 days.
One of the leading players is Boedeker Plastics in Shiner, Texas, which carries a large inventory of industrial and engineering plastics stock shapes and actively works with design engineers to specify plastics and develop prototypes. Many development projects go to production scale of machined parts when customers don’t need large volumes or want the flexibility to make frequent design changes.
“If you really want to test the prototype in its final application, an excellent approach is plastic stock shape machining,” comments Mike Raindl, general manager of Boedeker Plastics. One caveat is that shapes are extruded or compression molded, and have a slightly different property profile from injection molded parts. Mechanical properties generally decline 25 percent for extruded shapes and another 25 percent for compression molded parts, when compared to injection molded test specimen values on data sheets. The stress histories of the parts are also quite different. And some materials with unusual flow characteristics can’t be extruded and aren’t available as stock shapes.
Raindl offers these tips to design engineers considering machined shapes as prototypes:
- Provide a solid model whenever possible. Most of the cost in prototyping is in the programming and drawing of the part if a 2-D file is all that’s provided. NC programmers also must try to estimate what design engineers want in some cases. “You could have an eight-hour drawing process that could have errors,” says Raindl. “By providing a clean solid model, you reduce any guess work from taking place.”
- Keep in mind the capabilities of machining processes versus what can be achieved in a mold. “There’s no such thing as a 0.250 inch deep by 0.003-inch radius in a corner because there isn’t a 0.006-inch end mill out there with enough cutting flute to reach that deep,” he comments.
- Check with the prototype company or your material supplier to make sure the material you want is available in a stock shape and within the size constraints of your part.