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The Process of Vacuum-forming Plastic Parts

Aug 27, 2001
Greg Mills

As inventors, we need to have an idea of how various inventions might be best "reduced to practice." Vacuum forming is a manufacturing process that has real cost advantages over injection molding, if the parts you need lend themselves to it.

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I was introduced to the plastic vacuum forming process when I was developing an applicator tool, sort of a hollow shoe for "muddy drywall tape." That was 10 years ago, but I remember watching with amazement as I saw large sheets of hot plastic sucked down around complex molds before my very eyes.... The parts cooled in seconds and then the parts lifted off the molds and a new sheet of hot plastic dropped down and the process repeated.

As inventors, we need to have an idea of how various inventions might be best "reduced to practice." Vacuum forming is a manufacturing process that has real cost advantages over injection molding, if the parts you need lend themselves to it.

If the item you need to replicate is of a relatively uniform wall thickness and has an area that is flat on it, you may be able to vacuum form them. The thickness of the plastic and the size of the original sheet dictate how high the part can project above the plane. When plastic is vacuum formed they say it was "pulled." You can see the plastic stretch as it is sucked down as the vacuum is applied, beginning at the top of the part and progressing down.

A similar process where hot plastic is sucked into a mold that is even more dramatic, is called blow molding. Plastic bottles are manufactured that way.

My part drew a bit over 6 inches. I built a prototype model out of Basswood and Bondo. The molds for for vacuum forming are cheap compared to injection molds. A mould that is $2,000 to vacuum form may be $200,000 to injection mold the same part. The reason for the price discrepancy is that the vacuum form mold consists of a positive model mounted on a flat surface with small holes drilled in the flat surface. The injection mold is a hollow cavity carved out of a solid block of steel.

Molding Process

For short runs the model can be of most anything. Even Plaster can be used. It is important to drill holes at the bottom of any depressions so that the plastic will conform to every detail. If the mold fails to pull into an area an extra hole will allow the air to be vacuumed out.

The plastic used is very expansive and sags when it is hot enough, but does not completely melt or burn until it is even hotter. The plastic stretches like rubber when it is hot and then returns to being rigid when it cools. The typical industrial vacuum forming machine has an electrically heated hood to heat but not melt the plastic due to holding at a median temperature controlled by a thermostat. The parts cool in seconds when cool air is blown on them. They are then lifted off the mold and die cut to free the part.

I was very interested when I heard about a line of mini vacuum forming tools for prototype production. Kingston Vacuum Works of Kingston, NY. sells four different models starting at $99. The plastic is held in a metal frame and heated in the kitchen oven at 300 degrees or by way of a hot air gun and then quickly placed over the part on the "table," a vacuum cleaner hooked up to the device then sucks the air out, just like the big professional machines. See the Web site at WarmPlastic.com . The also sell plastic sheets that are used on the vacuum formers.

Many times vacuum formed plastic sheeting is an important part of the package. Blister packs are preformed plastic shells that are commonly glued to a printed cardboard backing. Compared to other ways to prototype things, vacuum forming is easy and cheap. Check it out.

Andy Gibbs and I are two of the featured speakers at the upcoming 2001 Mississippi Inventors Conference. I will be doing a seminar on prototype production. There will be demonstrations of silicone mold making, vacuum forming and 3D virtual prototype computer files. The conference this year will be held September 8th at Jackson, Mississippi. See ya there!

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