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Spotlight on Thermoforming

From time to time, Polymerplace will look at a plastic process- what it does and how it is used. Thermoforming is a downstream plastic process however is often viewed as an alternative to injection molding in numerous markets and end-uses.

There are two main types of thermoforming: Thin-gauge and heavy gauge.
Thin Gauge is the process using continuous forming of roll fed thermoplastic sheet through an oven tunnel into a forming station, on through trimming to picking and packaging. Usually thin gauge is a more automated process than heavy gauge. Thin gauge is used extensively in the packaging industry.

The heavy gauge division of thermoforming is feeding cut sheets of thermoplastics into a machine that carries the sheets through one or more heating stations to the forming station for molding and then out of the machine. Here secondary steps are necessary to trim the finished part. Trimming includes sawing, routing, shearing and drilling.

In the early days of heavy gauge thermoforming, an acrylic thermoplastic sheet was hung in an oven to absorb heat. The material was then stretched over a mold. After allowing time to cool, it was removed and trimmed. Drape forming is still being used for various applications in the manufacturing of many products.

Thermoforming Techniques

Heavy Gauge thermoforming has grown considerably in recent years. One of the techniques used today is vacuum forming where the hot sheet seals to the mold perimeter.
A vacuum then removes the resistant air from between the mold and the sheet. This allows the ambient air pressure to force the hot sheet to conform to the surface shape of the mold. The maximum pressure typically is 14.7 pounds per square inch(one bar).

Another technique is to introduce a pressure vessel on the opposite side of the sheet from the mold. When the vacuum is applied, high pressure (28 to 118 pounds per square inch)-two to eight bars)-is introduced into the pressure vessel, thus forcing the hot sheet against the mold surface, including crisp corners, detailed logos and textures. The result is a part comparable in appearance to the best injection molded machine housings, control panels or other detailed appearance parts. With the proper mold design, extruded sheet material and heating technique, high pressure forming produces a part that is virtually stress free. This is an advantage over injection molded parts.

A third thermoforming method is to press the hot sheet between two matched molds. Using no vacuum or pressure, the parts can be easily formed producing crisp details and textured parts. The matched mold method and/or high pressure mechanical shaping of the hot sheets also enables thermoforming of thermoplastic composite materials.

Heavy gauge thermoforming is fast expanding the use of two opposing molds, each forming a hot thermoplastic sheet. The two halves are then pressed together while the sheets are still at forming temperature. Twin-sheet thermoforming allows the production of extremely rigid structural parts for various applications such a pallets and containers.

Applications and related developments

Using these various techniques, heavy gauge thermoforming is serving every industry. New applications include building and construction products, products for the electronics industry (usually replacing injection molded parts), and products for the heavy trucking industry and automotive manufacturing. In automotive, the matched molding system is commonly used for forming doors, headlining panels, wheel well openings and closeout panels in the boot. A variation of drape forming and match molding (slip forming) utilizes a mold with a matching plug assist where the material is free to slip into the mold configuration. Slip forming is commonly used in automotive manufacturing for forming carpeting.

The first thermoplastic composite part was used in a German production model in 2000. In developing nations where the smaller volumes of automotive models are produced, vacuum forming is used. The most widely used materials for these applications are ABS, Polypropylene and wood-filled polypropylene. For interior and exterior trim components, where a decorative finish is required, extrusion laminates are being used for both thin or heavy gauge parts.

Another important industry served by heavy gauge is sanitary wear including bathtubs, shower tubs and stalls and wash basins. The materials generally used are ABS, vinyl, acrylic and/or acrylic over an ABS substrate. Some manufacturers use cell cast acrylic, heating the sheets in the convection oven and using vacuum to form the part. Other methods use ABS with a co-extruded top coat of acrylic. This is usually formed on standard thermoforming equipment. The surfaces are then reinforced with fiberglass (woven or chopped fibers) or sprayed urethane foam. A new process recently introduced, uses thermoformed finished surfaces with a waffled thermoformed material back panel, reinforced with cast urethane foam between the two parts.

The appliance industry uses vacuum forming to produce the interior boxes, and door liners in refrigerators. The common materials used here are polystyrene and ABS. The trimming of refrigerator parts are moving from massive punch die to 5 axis robotic router trimming.

The industrial pallet and dunnage industry is moving rapidly from wood boxes and pallets to twin-sheet thermoformed products. The materials used are generally high density and high molecular weight polyethylene. The automotive and food industries have been driving this change because of the increasing demand for recyclable containers and pallets. This has represented a great opportunity for Plastics and thermoforming.

Areas of Growth

The greatest material growth in thermoforming sheet is in high-density polyethylene and high molecular weight polyethylene. These materials have high-impact, heat, chemical and weather resistance.
Polypropylene is also beginning to appear in heavy gauge thermoforming. In addition higher temperature, higher performance engineering materials like ABS and Polycarbonate are used where standard polyethylene or polypropylene are not sufficient in properties.

Heavy Gauge thermoforming has grown due to four major factors:
Better quality sheet -both better materials and sheet quality
Better machinery with precision controls of the heat and forming cycles
The acceptance of well designed, quality built temperature controlled forming molds which allow tighter control of part dimensions, residual internal stresses and the ability to produce repeatable parts.
The increase in thermoforming conferences and seminars. The SPE Thermoforming Division is an excellent organization which provides a forum to advance the field of thermoforming.
Areas of need:
There is still a need for less expensive molds such as epoxy tooling for parts where tight tolerances and fast cycles are not required.

Insight into Heavy Gauge Thermoforming-World Plastics Technology, 2001
Modern Plastics Encyclopedia.