Vacuum thermoforming is a process that brings a piece of plastic, like polystyrene (PS) or polymethylacrylate (PMA), into a desired shape by heating it up and then using low pressure (a "vacuum") to press it against a (positive) mold.
Vacuum thermofoming devices are mostly industrial machines, although some hobbyist solutions using a vacuum cleaner exist. I am a using an Erkoform-D precision thermoforming device that is normally used in dental laboratories. I bought the device from a surplus dealer on the Internet. It was already some 20 years old back then but is still in working condition.
The machine processes 12cm discs of plastic with a thickness of up to 5mm. I am mostly using 1mm PS sheets. A circle cutter is a handy tool for creating discs of plastic, although a pair of tailor's scissors would probably be good enough in this case, because the seal on the machine is pretty wide.
Here is the disc after cutting it out of the sheet. There will be a dent in the middle of the disc from the punch of the circle cutter. It is important to put the dented side toward the mold later, or the dent will be on the outside of the thermoformed part.
The Erkoform-D is basically a purely mechanical device (except for the lamp) that creates −0.8 bars of underpressure from 8 bars of overpressure using cascaded venturi tubes. This means that it needs a hefty compressor. The one in the image is a "silent" compressor, but it still sounds as forbidding as it looks.
So, step one: this is an 8-bar compressor and 8 bars is what is needed, so open the regulator (red knob) fully. (It is not a good idea to leave it open when storing the device, because the diaphragm will suffer.) Then attach the hose to the thermoforming machine and the compressor and power both up.
This is the Erkoform-D vacuum thermoforming device. It is a rather small machine (18×25cm at the base) but weights about 10kg. The round part is the thermoforming chamber and the square part on the top is a quartz lamp used for heating up the plastic disc. There are three switches: a master power switch (big switch on the lower right), a lamp switch (small switch next to the big one), and the lamp-off switch close to the lever, which is activated by pushing down the lever when closing the vacuum chamber. More on that later.
Here is the machine with the vacuum chamber open and a PS disc inserted into the frame that will later be pushed down onto the chamber. The frame is now in front of the quartz lamp, which will heat up the pastic disc. The machine is not powered up at this point, and the chamber is still empty.
The vacuum chamber is filled with granulated stainless steel, which allows air to pass without restriction under the mold. The thermoforming process itself takes a fraction of a second and it is important that the air is sucked out quickly and evenly.
Now it is time to power up the Erkoform and heat up the lamp by pressing the "lamp" switch. This is a momentary switch which can only power up the lamp. It is powered down by starting the thermoforming processor or by toggling the master switch.
The quartz lamp powering up.
At this point make sure that the compressor has reached its nominal pressure of 8 bars. My compressor has a 6-liter tank, so it can safely be powered down at this point. The air in the tank is more than sufficient to drive the thermoforming process.
The frame with the PS disk is put in front of the quartz lamp. Note that the frame has been rotated and the "backside" of the disc is being heated. The frame has to be rotated back to its original position before closing the chamber.
Modern thermoforming devices do a lot of these steps automatically, but they are more expensive and have more potential points of failure. All that can break on an Erkoform-D are the quartz lamp or the seals.
When the PS disc reaches its glass transition point, it starts to wobble slightly when you move the frame. It also wraps around the lower part of the frame as can be seen in the picture. The "glass transition point" is the temperature where a hard ("glassy") material becomes "rubbery". Not all materials have this property, but many plastics do.
Note that the frame is now in its proper orientation to start the thermoforming process.
And that's it. As soon as the frame is pushed down on the chamber using the handle, a switch opens the valves and at the same time turns off the lamp. The pressure in the chamber very rapidly drops to 0.2 bars and the plastic sheet is wrapped tightly around the positive mold.
There is a short hose at the top of the device that can be used to cool the plastic sheet with compressed air after thermoforming. This is not really necessary when processing 1mm sheets, though, because they cool down quickly on their own.
At this point the remaining air can be drained from the tank of the compressor. After that the regulator should be closed.
The PS disc after thermoforming. Positive molds have a "draft angle" of 3 degrees, which means that all parts of the mold must tilt "inward" by at least three degress. If this is not the case, the mold will get stuck in the thermoformed part. The part being formed here does not adhere to this rule, so some moderate force is needed to remove the mold.
The part is then removed from the disc using a 1mm milling drill.
A thicker milling drill is used to clean up the part. Further postprocessing of the part is not subject to this description.
In case this is a detour from the Arkonide Robot construction page, you can resume reading here.
In case you did not come here via the Arkonide Robot construction page: the part thermoformed here is the mouthpart of a science fiction robot model.