Bill Evans

September 1, 1996

10 Min Read

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After proving the product design with a single model, multiple copies may be required for pilot manufacturing or beta site testing. Following are some possibilities to help make the transition to small batches of parts.

Casting of Urethane Plastics for Copying Injection Moldings. Once there is a proven version of a product, multiple copies will often be needed for marketing feedback, design evaluation, or clinical trials. When the number of copies rises above four or five, and one wants to quickly and inexpensively mimic production injection- molded plastic parts, then urethane casting is a viable option.

The urethane family of plastics has broad characteristics, ranging from rigid to rubberlike. These plastics are not as strong or thermally stable as typical thermoplastics, but they achieve usable properties for closely controlled beta tests. The procedure for casting multiple copies of a design for pilot production or alpha and beta site testing is as follows.

First, create a pattern (which, in many cases, is the original model used to prove the design). It is invested in a silicone rubber mold to create a cavity, which usually has a limited lifetime of up to about 25 units, depending on the quality of the original tool or on the intricacy of the molded parts. Once the silicone rubber cavity has been made, the appropriate grade of urethane is poured to cast the part. Remember when doing this that it is possible to get UL fire-retardant grades of resin, but be sure to meet the minimum material thickness to qualify for fire retardancy. (Casting of plastics is definitely an art, and as such is typically done best by model makers.) On simple parts it is also possible to skip the pattern stage and direct data machine a cavity into a block of plastic.

One can also combine materials. In mimicking areas of molding that need to be structurally strong, set in small sections of the real material, such as polycarbonate snaps or acetal runners. Another thing to consider is that one can cast the parts in self-color, a process in which pigment is put in with the resin. This is helpful if the fit between the parts is critical and paint layers won't work. Just as in the SL process, it is important to remember that with urethane casting, the material is highly heat-sensitive. Thin wall sections below 2 mm are especially prone to warpage.

Be aware of how lengthy a process casting can be. It takes up to a week to build and cure the silicone rubber mold. This tool can then produce a set of parts only every 24 to 48 hours. Be sure to factor in finishing or painting time after that. Add all this up, and 30 prototypes can take quite some time.

When to use this technique: Urethane casting is a good choice for short runs of any plastic parts that would otherwise have to be made with costly and time-consuming tooling. Check with the leading model-making service bureaus in your area to find out if they have extensive experience with this technique.

Soft Tooling of Plastic Moldings. When people speak of soft tooling, they mean many different things. In a way, silicone rubber molds of plastic parts could be considered the ultimate soft tooling since they actually are physically soft. But sometimes people refer to either soft steel tooling or soft aluminum tooling, which involve either less-hard P20 steel or aluminum and can produce a large volume of parts at modest piece-price increases over conventional injection molding.

Soft tooling can be the solution to some short-run needs. It proves even more meaningful for runs numbering in the hundreds or thousands. This process is typically in the range of 50 to 90% of the cost and schedule of a hard tool equivalent. (Generally, an aluminum tool might be guaranteed for 50,000 to 100,000 impressions, while the full-priced conventional hard tool is good for 500,000 to 1 million.)

When to use this technique: Consider soft tooling of any injection- molded plastic parts when total production volumes are low (less than 100,000) or the market window is very short and every week saved is worth considerable revenue.

Many injection molders now offer this option. Increasing power of solid modeling CAD has enabled a new generation of rapid prototyping bridge the gap between silicone rubber tools and traditional aluminum tooling, and to shorten the tooling cycle to as little as four weeks. To find companies familiar with this technique, check the trade press for toolers who advertise this service, or ask existing molders for referral. Companies such as Phillips Plastics Corp. (Hudson, WI), PTA Corp. (Longmont, CO), and Plynetics Corp. (San Leandro, CA) are just a few places to start. Some directories may also list this service under rapid or fast-turn tooling.

Creating Complex Parts from Multiple Use of a Single Small Part. Another technique in the area of soft tooling plastics is effective, for instance, when making a disposable medical device from a number of repeated small elements. This can perhaps best be explained through the analogy of a comb. To evaluate specific materials for their engineering properties, it may be appropriate to mold one tooth of the comb quickly in a prototype mold and have many copies of it made and then fabricated into a frame combining all the features. The key elements of the part can be evaluated in the correct materials and processes in quickly produced small tools (production in 5 to 6 weeks is possible for a small cavity, as opposed to 12 to 16 weeks for larger production tools). The remaining frame parts are made using a much lower-cost technique such as laser cutting or machining.

Conversion from Prototyping to RIM Production. If the intended pilot or production volume of a product is in the range of 20 to 50 units per year, some of the previously mentioned pilot processes are viable techniques. This is particularly true for the cosmetic enclosure components of many types of medical equipment. As mentioned earlier, urethane casting can work well for making this transition, especially if one plans to scale up the process to RIM (reaction injection molding of polyurethane foam into a modestly priced low-pressure tool).

For example, imagine a part the size of a typical 15-in. computer monitor bezel. In order to make a few copies in cast urethane as previously described, one might spend $10,000 on the pattern, $5000 on a silicone rubber tool, and from $200 to $300 per finished part. Now say one wants to scale that up to RIM. A RIM vendor may charge $10,000 to make another pattern, $15,000 to create the tool, and $150 for each part. Clearly, a large chunk of the tooling cost is a pattern that is being made twice. With a little forethought, the same pattern can be used for both the early handmade urethane castings and a RIM tool.

To get the most out of a pattern, one needs to understand shrinkage rates. Any casting or molding process involves a shrinkage from the size of the pattern or cavity to the size of the final molded part. With urethane casting and RIM, the shrink rate is very similar. Typical shrink factors are somewhere in the region of 0.001 to 0.003 in. per inch (or 0.1 to 0.3% shrink). However, every vendor works with slightly different numbers due to the resins used, so check the shrink factors for both the urethane casting vendor during the early prototype runs as well as for the intended RIM vendor. One note of caution is that intricate thin-wall stereolithographic models have a tendency to break when invested for silicone rubber tools. Make sure your urethane casting vendor knows if you need to keep the patterns intact for future use.

When to use this technique: The RIM technique is useful for any large or complex enclosure parts that would be much more expensive or impossible to machine or bend from sheet metal. The material is strong enough to replace large metal panels. RIM molders such as Design Octaves (Santa Cruz, CA) are generally listed under a seperate category in larger business directories.

Like many things in the business world, picking and nurturing rapid prototyping vendors is about building a relationship, including trust and understanding. Look for vendors who are honest in their appraisal of the job and realistic about their schedules.

Bill Evans is principal of Bridge Design (San Francisco), a product development consulting company.

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The latest entry in the race for rapid prototyping is a ballistic particle manufacturing (BPM) technology first examined by MD&DI in November 1994. Although not commercially available then, a 3-D printer that makes use of this technology is now finally on the market.

Using a patented material-jetting system that shoots molten microparticles of thermoplastic into 3-D models, the printer's five-axis build process--referred to as digital microsynthesis--builds an object by combining thousands of identical dots of material. The nontoxic plastic is shot from a piezoelectric jet head in a round molten state at frequencies as high as 12,000 microparticles per second. These particles flatten upon contact, slightly melting the surrounding plastic and establishing a gluelike bond between drops. Because the jet head can deliver the material in any direction, objects can be built with their true curvature without the stair-stepping effect that can occur from jetting the material straight down.

BPM Technology, Inc. (Greenville, SC), began shipping the $34,900 Personal Modeler system this month.

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While rapid prototyping techniques offer product design engineers a variety of options for making the transition from product concept to production, verifying that the process was done correctly can be both costly and time-consuming. A natural complement to CAD/CAM and rapid prototyping systems, reverse engineering systems can help designers and production engineers to speed the inspection of both tooling and finished parts. A reverse engineering system can digitize a part's internal and external features using automatic, cross-sectional scanning, and then compare this data point set with the original CAD file to identify how far out of dimension any point of the part's surface is. Most commonly used in first-piece inspection, other applications for the system include quality assurance for tooling as well as true reverse engineering--taking an existing part for which there is no accurate CAD data (perhaps because the part is old or was modified after the mold was made), and essentially re-creating it.

In this reverse engineering system by CGI (Minneapolis), users are able to enter the net processing dimensions, horizontal and vertical sampling, and filtering criteria. After selections have been made, the part may be previewed before final processing.

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Many CAD and rapid prototyping vendors talk about being able to transfer data easily and quickly. But all too often a carefully prepared CAD file that has been downloaded is not translatable or there are other compatibility issues, such as those caused by DAT tapes that are not properly formatted for a vendor's system, incorrectly set export parameters in your CAD system, or the incompatibility of file-naming conventions between the DOS, Windows NT, and UNIX worlds. Following are tips for preventing such glitches.

  • Before finalizing a design, use a trial part and test out the transfer process with the vendor.

  • Use a dedicated line and invest in the highest-speed modem available. For those with an FTP site on the Internet, this is a cost-effective way of transferring large files. When compressing files for transfer, make the files self-extracting to avoid compatibility issues on the receiving end.

  • Avoid some UNIX to DOS problems by keeping to an 8.3 naming convention when originally naming the files (for example, use DRAWING.DWG instead of COMPLICATED_DRAWING_NAME.DWG).

  • Consider carefully how data will eventually be used.

  • Don't believe claims of integration until you have tested them with real-world problems that your particular company needs to solve.

  • Remember that, with the proliferation of computer E-mail and courier services, it is possible to work with the best vendors even if they are located in another part of the country.

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