Vacuum forming is a method of plastic thermoforming that involves placing a flat sheet of plastic into a frame and applying heat. When the plastic is malleable and the proper temperature has been reached, the plastic sheet begins to sag. The frame lowers, and the plastic starts to cover the mold. After a seal is created around the outside edges of the mold, vacuum removes any air inside the cavity (thus the name “vacuum forming”). The vacuum action causes the heated plastic to be pulled down around the mold.
Once the thermoplastic sheet has hardened, the vacuum is shut off, and the platen that contains the mold is lowered (or raised if running the mold on the top platen) leaving the newly-produced thermoformed plastic part in the vacuum forming machine’s clamp frame. The part is then removed from the frame and all excess plastic is trimmed off around the edges of the part.
Products that are usually made by the thermoforming process include: dashboards, display racks, beverage holders, blister packs, hoods, covers, machine guards, radomes, seats, dunnage trays and many other types of products.
Plug Assist Thermoforming
Plug assist thermoforming (vacuum forming) is used when straight vacuum forming is unable to distribute the thermoplastic sheet evenly to all areas of the mold. To help distribute the sheet thickness more evenly, a device known as a plug is utilized to push the sheet into the mold before the vacuum is applied. This process enables more of the thermoplastic material to reach the bottom of the mold, which makes more material available to fill the corners of the mold and helps keep the plastic from thinning out. A plug can also be used to eliminate webbing on thermoformed parts.
Thermoforming Using A Pre-Draw Box
Customers will often want their part(s) to have a fairly even wall thickness. Sometimes this can’t be achieved through conventional vacuum forming alone, which means that we may need to utilize a pre-draw box. The pre-draw box is used to pre-stretch the hot plastic sheet prior to forming. A pre-stretched sheet will often result in a part with a more even wall thickness than what would have been obtained otherwise. The pre-draw box can also be utilized to help the overall appearance of the part when the mold geometry causes cosmetic imperfections on the surface of the plastic.
Drape forming is similar to vacuum forming. In fact, drape forming can be described as vacuum forming without the vacuum. Instead of relying on vacuum to shape the heated plastic sheet, gravity performs this function. We often use drape forming to minimize thinning, to make a part with a very even wall thickness or to produce a part with good clarity. Drape forming is very limited in the types of parts it can be used to produce and can only be used when compound curves or complex geometry is not present on the desired part. If a plastic part has areas with tight radii, complex contours, etc., then drape forming will not work for molding the part. That being said, some compound parts can still be molded without vacuum using a combination of heat (of course) and mechanical assists…such as when using a double sided tool.
Strip Heating and Bending
Strip heating and bending is a process of forming plastic sheets using only a single heater (usually calrod) and a forming fixture. The straight calrod is used to heat the sheet along a specific area. The heated sheet is then placed onto a forming fixture. The forming fixture is made to a certain angle, and the heated sheet is bent around this angle.
Just as with drape forming, strip heating and bending is very limited in its application. However, for situations when strip heating and bending is appropriate, the customer will end up with a part with a uniform wall thickness and minimal waste, with a minimal investment in tooling.
Vinyl wrapping involves wrapping a preformed (usually a thermoformed) part with a piece of vinyl sheet, and the process requires a few different steps. First, a part is formed from some type of rigid plastic or fiberglass. Then, this part is trimmed and sprayed with adhesive. After the adhesive has been applied, the part is put back on the vacuum forming machine. A piece of vinyl is then heated in the machine and brought out when hot. The part acts as a type of mold and is pushed into the hot vinyl. Vacuum is applied, and the vinyl is pulled against the surface of the plastic part. The excess vinyl is then trimmed off and wrapped around the edges of the part.
We often use this process to produce parts such as instrument clusters and dashboards where our customer wants the final part to have the rigidity of plastic or fiberglass but with a softer feeling surface. Advances in thermoplastics have enabled sheet suppliers to cap their plastic sheet with a soft touch material (typically a thermoplastic elastomer, or TPE) that can be formed in conjunction with the rigid substrate.
The types of vinyl wrapped parts that these multi-layer sheets can be used to make is limited. However, when they can be used, they help to bring the entire vinyl wrapping process together into a single process involving only one machine cycle. The technology for this type of multilayered sheet is still somewhat in its infancy. Thus, material color, type and grain are limited.