I won’t go into how vac forming works, because there are plenty of explanations on the internet. But the main concerns are to hold the plastic sheet, to heat it, to hold it on an airtight moulding table (platen) and to reduce the air pressure to make the moulding. In short, the heater softens the plastic sheet that is held in the frame. Then the frame is lowered onto the platen, where the object is. The vacuum pump underneath then draws out the air under the sheet so that the plastic forms the shape of the object; it cools and the job is done.
Step 1: Motor
Inside, the box has two compartments. One compartment houses the vacuum motor so that it blows out of the end of the box. The other end of the motor draws air in from the second compartment, which is the end and top of the box, the top being formed by the platen which has holes to allow the air to enter from under the plastic sheet.
If you found a motor which had a good seal on its output (blowing) end, you could just fix this to blow out of the end of the box and there would be no need for the separate compartments inside.
I found (but have now lost) a website that showed a neat way to hold the plastic. (If this was your idea, send me the link and I’ll add it!) Two identical frames are made from channel section and spring clips hold them together with the open side of the channel facing outward. This leaves the top and bottom of the frame flat and the bottom can seal onto the vacuum box. This seemed a good solution.
The dimensions of A4 are 297 x 210 mm. Allowing for 10mm all round to be gripped by the frames, that left the internal dimensions of the frame at 277 x 190 mm. I found I could buy a U-channel section in extruded aluminium. It is 10mm thick and 15 mm high (on the outside). I planned for the end pieces to go right across and the length pieces to go up to them, so they can be 30mm shorter than the length. Thus each frame requires two pieces at 220mm and two lengths of 277mm. The diagram should make that clear! This adds up to 994mm, which works nicely in a metre length with a small allowance for cutting. You need two frames, so that is 2 metres.
Joining the frame sections at the corners was done by making two cuts in the bottom of the section, right at the sides, and bending the middle part round to form a tongue that links between the sides of the next piece. These were glued together with a two-part glue (Araldite), which I first tested in the oven alongside some plastic card, to make sure it didn’t melt or burn before the plastic melted. In vac forming you don’t go up to the melting point of the plastic.
Step 4: Shopping List
Vacuum cleaner motor
Frame section aluminium U-channel 10 x 15 x 2000
Heater holder aluminium angle 20 x 20 x 1000
1mm thick aluminium sheet
Bottom 310 x 220, any thickness
2 ends each 180 x 220 x 4mm thick (or adjust other measurements to suit)
2 sides each 180 x 303 x 4mm thick (or adjust other measurements to suit)
Bulkhead 155 x 213, any thickness
Platen 310 x 220, thickness to match one frame
Step 5: The Box
I actually constructed the box 10mm smaller all the way round, and used some of the 10x10 to widen the top to the right size. This provided a flat top for the frame to seal on to. I also went around the inside with 10x10, also level with the top, to widen the top edge for the platen to rest on. The platen is the surface the mould stands on, and through which the air gets sucked out.
As the box had to have a separate compartment, I had to make sure I fixed supports to the sides to hold the inner wall before I fixed the box together, otherwise I would not have been able to get inside to fix it. I also had to create the fixings to hold the motor on to the inner wall.
Step 6: Motor Fixings
Step 7: Building the Box
One of the ends needs to have holes to let the air out, and to fit the switches and cables. The holes for these need to be made before it is fastened together. Plywood can be really hard to drill neatly. I used high speed flat drills (with mixed results), but Forstner drills or old fashioned wood bits are better.
The outside rim was attached with screws from the inside, which allowed me to make sure it was aligned precisely. Then I drilled holes from the inside and screwed on the inner rim, making sure to create a flat top surface. This also meant no screw heads show on the outside.
Next I fitted the inner wall to the supports on the sides. I had already put the motor screws and hooks in place, but I left the motor off until the woodwork was complete.
Next came the motor compartment cover. This was made of a single piece of plywood. I wanted to be able to remove this for access to the motor, so I had to size the top carefully so that it would be small enough to come out through the top of the box, but its supports had to be wide enough to get the motor in. Getting the size right was tricky. If you want to copy my machine exactly, the supports allow a gap 149mm wide to get the motor in (diameter 145), and the motor compartment cover is 201 x 177. I screwed in the supports all round (from the outside, so the heads show) and then drilled the cover to allow it to be screwed down, but removable. The cover is chamfered at the top to help it fit through the top rim.
Step 8: Heater
At first I found the heat was too uneven, with a hot stripe in the middle, where the two lobes of the reflector meet. I managed to correct this by carefully cutting the reflector between the two bars (with a craft knife, it is like thick foil) so that the reflector is more flat in the middle. The heat is now mostly even except for a band near one end. I need some inspiration as to how to get rid of this hot band completely, but it works well enough with white plastic.
Step 9: Electrics
I took the power lead from an old kettle base, which gave me a moulded plug and some heavy-duty cable. I brought the end of the lead into the box through a grommet to prevent any chafing. I attached a chocolate block connector in the bottom of the motor compartment. The supply live comes to the connector, where it is connected to two leads that go to each of the switches. The switches I used take 1/4in spade connectors, so I crimped terminals on. I worked it without the heater at first.
The live terminal on the motor needs to be connected to the second terminal of the motor switch, so that the switch connects/disconnects the live power to the motor. The neutral terminal of the motor needs to go to the neutral on the power supply lead at the chocolate block. If your motor has an earth terminal, that should be connected to the earth in the supply too.
If you are in any doubt about the wiring, consult someone who understands electricity. It is too dangerous to take a chance.
Once I completed the heater box I wired the heater with some heavy-duty flex. I changed the heater’s leads so that both tubes were connected together in parallel. I used a heat-resistant chocolate block connector for this. I also connected the chassis of the heater to the earth conductor. I took the flex out of a hole at the back corner using a grommet to protect it. When I was ready to fit the heater, I brought its power lead into the main box through a grommet. The live conductor goes to the second terminal on the heater switch (another crimp terminal) and the neutral and earth to the neutral and earth of the supply cable at the chocolate block. The wires connecting the tubes are heat resistant; the flex I used was not, so it was important to keep it away from the metal surfaces that become hot. If you are in doubt about this you can use heat-resistant sleeving (you might find some inside the heater). Normal cable ties will also melt if they touch the hot surfaces.
You need to make sure that all the wires are securely connected, and that they are not likely to move, especially near the moving parts of the motor (my motor has a small length of rotating shaft exposed at the end near the terminals). If necessary the wires can be held together using cable ties in the box as no great heat is present. Sorry there aren’t any good pictures of the wiring, it is all under the motor and quite difficult to reach.
Step 10: Platen
All the wooden parts were sealed with a diluted polyurethane varnish.
Step 11: Guides / Heater Supports
Some of the pictures show transparent film in use but my best success so far has been with white Polystyrene, Thickness 0.015 - 0.020 inch or 0.4 - 0.5mm.
Step 12: In Use
I used a piece of towelling fixed to the back of a picture frame to make this birthday greeting card.