Originally published April 1996
Ray Products, Ontario, CA
Although comparatively few companies are familiar with pressure forming, this technique is being used increasingly to produce low- volume products that can match the aesthetics of injection-molded, high-volume parts. The process is especially suited to the production of housings and enclosures.
The pressure forming process is still relatively new. Pressure-formed items that resembled injection-molded products first appeared in the late 1950s or early 1960s, but the method has been heavily promoted only during the last 10 to 15 years. Key elements of pressure forming that are attractive to medical device manufacturers include its low initial investment requirements and its ability to create products with injection-molded aesthetics. Becton Dickinson and Johnson & Johnson were among the first medical manufacturers to use pressure forming.
In pressure forming, sheets of material are fed into ovens that heat them to the proper forming temperature. Platens are used to clamp each tool, and the sheet is forced into or over a temperature-controlled tool by shaped plugs. Air is then forced into the plug chamber, pushing the hot sheet into or over the mold.
Equipment. Heavy-duty forming equipment has aided the development of pressure forming. Electric platens, for example, can be closed and exposed to needed air pressure without "backing off" or opening and losing the air-pressure seal. Using these platens, the former can overcome the "hot strength" of rubber-modified materials and push them into sharp corners and ribs. Pressure used during the process can be 2 atm or greater, depending on the material, depth of draw, detail, and other factors. As tools get larger, pressure forming becomes relatively more economical (see Figure 1).
Most pressure forming tools are cast, fabricated, or milled of aluminum. Because mold temperature is a critical factor, temperature control apparatus is added to the completed tool. Typical additions include heaters or copper sleeves for circulating coolants such as water, oil, or antifreeze. Production of such tools usually requires approximately four to eight weeks. Depending on the complexity of their design, pressure forming tools can last almost indefinitely.
Materials. Theoretically, any thermoplastic material can be pressure formed. However, some materials are more difficult to work with than others. Polyethylene, for instance, flows easily and causes few problems for pressure formers. With vacuum alone, polyethylene can be intricately formed. On the other hand, polycarbonate, which chills quickly, can cause manufacturers to be concerned about tool design and plug assists.
Medical device manufacturers usually specify that their products should be formed of a material that passes the Underwriters Laboratories (UL) 94 V0 or 94 5V tests for flammability. The resins most commonly used in pressure-formed medical products are flame- retardant grades of acrylonitrile butadiene styrene (ABS).
In many cases, assists are used to help distribute material evenly and to coin it into sharp or narrow corners. Depending on its complexity, the design of a product's tooling may require the former to use matched heated molds and assists; otherwise, assists can be made of low-heat-transferring materials such as wood.
Processing Parameters. The most important factors in pressure forming are the input and extraction of heat. For example, ABS could be formed at very low temperatures, but this would have a drastically negative effect on its physical properties. Its impact strength could drop to as little as one-tenth of its published value and its thermal distortion temperature could fall dramatically. ABS should be processed at 325°F or more. Heat extraction is important to control warping, shrinkage, and other forming failures. After the forming cycle, most parts are removed from the mold and secondary trimming operations are performed.
Design Considerations. The heavy-gauge forming industry has changed a great deal in the past few years, particularly in the depth of draw ratios. At one time it was common practice to have a 3:1 material distribution/reduction ratio, or a 1:1 ratio (width to depth) on minor dimensions, and a draft of 5° or more was needed for deep draws. Today's advanced designs can employ 5:1 distribution/reduction ratios, a 1:1.5 width-to-depth ratio, and a draft as slight as 0° (or 1/2° for aluminum cast molds). These advances have enabled product manufacturers to use pressure forming without changing their injection molding designs.