Thermoforming Plastic

Pretty much all plastics are formed by first heating plastic pellets and extruding or injecting the hot mess into the desired shape. What most plastics do not share are the heating characteristics required to work with the plastic in a soft or IMG_20130916_163236molten state. For example acrylic sheets absorb water and must be oven-dried first. PVC is one of my favorite materials to thermoform by heating PVC pipes with torches or heat guns and bending to the desired radius. Not only does it save money on fittings but it looks better aesthetically for structures people will see. For an upcoming project, I need a custom enclosure that takes up as little space as possible. Thermoforming, or more specifically vacuum forming, seems like the best way to make a professional and functional case.

The first challenge is to find a plastic source. While I haven’t looked very hard, there seem to be no convenient plastic supply store in the Toledo area. Because of this, I settled on a surprising source: 5-gallon buckets. These buckets are made of HDPE and are somewhat durable albeit a bit too soft for my taste. Using a jigsaw, I cut off the top support rings and the bottom to create a plastic sleeve. Cutting this apart results in a long sheet of plastic.

For a heat source I am using the same toaster oven I use for SMD reflow.

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Since it is already contaminated with heavy metals and fluxes, possibly harmful plastic fumes couldn’t hurt anything. My first test was merely sticking a square into the oven and turning it on.

While it did flatten a bit, the plastic stuck to the pan and had no intention of letting go. As all vacuum forming machines use frames to hold the plastic in place and suspended, so will I.

The frames are made out of 1″ aluminum angle I had scrap from some other project. With a jigsaw and a compound square, I marked and cut out triangles from one face to make a 6×10″ square. Each corner was clamped in a vice and hammered square after the first bend resulted in a soft corner with an unsightly radius. Another frame was cut with an additional 1/8″ to each side so it would fit around the first frame. A small tab was left on one end so it could be bent and screwed into the other end to form a sold square. The inside frame was kept unrestrained.

The first test was conducted with a full piece of plastic pressed between the two frames. he toaster oven was turned on and…

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Not so great a result. The bottom heater gets way too hot way too quickly, resulting in two band of melted plastic with a warm but solid band between. These melted band quickly reach sagging temperature, sagging straight into the mesh shelf and generally making a huge mess. I played around with the temperature knob a little and found the broil setting only turns on the top heater. A second piece of plastic is inserted into the frames and stuck back in the oven. Set on broil, I turn the oven on and watch diligently.

All was going well, the plastic seemed to heat somewhat evenly, when suddenly…

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Everything worked just fine! The entire sheet heated to a soft state. I removed the frames with heavy gloves and stuck the assembly over a bottle of sunscreen I had on the floor. The plastic stretched somewhat over the bottle and hardened in place. Without a vacuum to pull the plastic against the bottle, the softened plastic sagged a bit and looked much like a sheet pulled over a shape rather than the taut elastic I hope to achieve. Interestingly enough, the text of the bottle somehow transferred to the plastic leaving blurry words and separating line in the sheet.

Up next will be to build a small vacuum table using a shop vac and some perforated board. Look for my post coming soon about my next big project update!

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