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Thermoforming and Pressureforming: Parts for Virtually Every Industry

Thermoforming and Pressureforming

The experienced manufacturer of thermoformed plastics provides parts for virtually every industry from very simple designs to complex configurations, using a multi-technology for fabricating and forming plastics and metals. This is accomplished through a variety of multiple operations and finishing processes. These processes include injection molding, structural foam molding, pressure and vacuum forming, CNC machining, trimming, painting, EMI/RFI shielding, EDM, grinding, welding, tooling and fixtures, assembly and turning. The processes begin with a block, rod, tube or sheet stock.

Thermoforming involves the controlled heating of a thermoplastic material to a temperature where its shape may be altered to the shape of the mold. The physical change to the preheated thermoplastic is accomplished by vacuum, air, pressure or direct mechanical force. Once the sheet assumes the desired shape of the mold design, it is allowed to cool on the mold. This curing process assures the shape of the finished product and allows the plastic material to retain its physical characteristics. The finished part is then removed from the mold to allow further fabrication and assembly. The advantages of thermoforming are the capability to form large parts without expensive equipment and tooling, ease of producing large quantities, inexpensive mold and design modifications, and laminated or foam-filled parts capability. Thermoplastics most commonly specified are acrylic, ABS, PVC, CAB and polycarbonates. Exotic types include Torlon, Vespel and Teflon. Precolored sheets are also used in addition to the secondary steps of robotic painting, pad printing, hot stamping and silk screening.

Thermoforming techniques are straight vacuum, drape, matched mold, pressure bubble-plug assist vacuum, plug assist, vacuum snapback, pressure bubble vacuum snapback, trapped sheet-contact heat-pressure and air-slip.

Pressure forming is a modification of vacuum forming. In pressure forming the air between the mold and heated sheet is evacuated, but compressed air at up to 100 lb/in2 is used to force the sheet into contact with the mold surface where it cools and solidifies. In vacuum forming both male and female molds are used; in pressure forming female molds are almost invariably used. Stretching is the prime and inherent problem in forming plastic sheet. The greater the stretch the greater the problems. Heavy-gage forming covers materials from 30 thousandths to 3/8" thick. A stretch ratio of 3 to 1 is about the maximum for good design. Among the differences between the two processes, sharper definition and tighter control radii are achievable by pressure forming compared to vacuum forming.

When comparing pressure forming with injection molding a number of disadvantages and advantages surface. One disadvantage is that pressure forming is a one-sided process; changes in wall thickness will occur in the process and cannot be controlled as with two-sided injection molding. Holes, slits and apertures cannot be molded in pressure forming but must be produced in a subsequent step. The advantages of pressure forming over injection molding are that pressure forming thereby eliminates abrupt wall thickness changes and sink marks, lower thermal stress concentrations reduce warpage after demolding, larger and lighter parts can be formed, and there is no need to provide flow channels and increased wall thickness.

Routing, milling, drilling or die punching are used to produce cutouts, slots and holes in finished parts. In the case of parts with multiple openings where an exact relationship is required between a series of holes, die punching or the use of CNC milling or routing equipment is sometimes preferred to drilling.

Another process offered by the experienced source is the twin-sheet method. Two sheets in two sets of tools run concurrently to produce top and bottom parts that are molded and bonded together. A 3/16" maximum sheet thickness can be used.

Overall, parts 5 ft x 8 ft can be made by thermoforming. Examples include 42" x 80" golf cart roofs and trailer truck bed liners. Part sizes are limited only by the size of the forming equipment.

Depth of draw is also another significant consideration in forming. Thickness, geometry and material control the depth of drawing.

Industries served by these manufacturing methods are gear, food processing, textile, tobacco and aircraft.

This technical information has been contributed by
Ray Products Company Inc.

Click on Company Name for a Detailed Profile

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