I needed to make a whole lot of identical parts. I had been making them one at a time. Eventually I realized I was doing it the hard way, and came up with
something better. Vacuum Forming! It allows me to make duplicates of parts quickly, easily, and cheaply.
I happened to be in Michael's Crafts and saw this shadowbox / wooden canvass. The idea then hit me out of the blue that I could turn this thing
into a vacuum forming machine. I bought it and hurried back to my workshop to get started.
The shadowbox is 1 foot square and about 1 1/4 inch deep. Just right to make the base of a vacuum former. I forget what I paid for it, but it was
cheap enough that I bought it, rather than building something similar. I must have figured the time saved would be greater in value than the cost
of the box.
I started by drilling a large hole in the side of the box for the nozzle of my shop-vac, which would provide the vacuum. Eventually I drilled a second
hole so I could use two vacuums at the same time for increased suction power. You may not need two, depending on how powerful your vacuum cleaners
I glued and screwed the pegboard down onto the top of the box with short drywall screws. The heads of the screws were countersunk so that they don't
stick up above the top surface of the pegboard. This is important to get a good air seal.
Next I began working on the frame to hold the plastic sheets. I glued and screwed together a 1 foot square frame from
1X2s, and added some steel corner braces for extra reinforcement.
I wanted to be able change out sheets quickly and easily. Some home-made vacuum formers I saw on the Internet stapled their plastic to
to their frames. This would be slow, tedious and limit the number of times the frame could be re-used. So I decided to build a hinged door to go on top of
the frame and hold the plastic sheet. This would allow me to quickly change out sheets.
Here the door has been installed on the frame. A piece of thin veneer plywood was used as a spacer between the
frame and the hinges. Hooray! It's done. Now I need some plastic.
I bought a 4X8 foot sheet of 30 mil styrene plastic at Farko Plastics Supply, which just happens to be about a mile from my job.
How convenient. The sheet of plastic only cost $15.
I cut it up the big plastic sheet into 32 1 foot square pieces. I have seen pre-cut sheets for
vacuum forming for sale online at places like Ebay and Amazon, but they were quite a bit more expensive, even before shipping
was figured in. I decided to stick with cutting my own sheets for now. The plastic cuts very easily with either a razor knife
or sturdy scissors.
Here a piece of the styrene plastic has been inserted into the frame. The door is held closed by three short drywall screws. The screws
go right through the thin plastic and help hold it in place. The heads of the screws are countersunk flush with the top of the door. The
frame is now ready to go into the oven.
Here I am all set to start vacuum forming some parts. Early testing showed a couple of problems. Tests molding rounded parts worked ok,
but parts with sharp corners weren't being reproduced well. So I made a few changes. One was the addition of another vacuum cleaner.
The second change was that I drilled more holes in the pegboard. Basically I doubled the number of holes. It seemed like the holes were
too far apart and some areas weren't getting good suction. The third change was the addition of the black felt around the edges of the
pegboard. The felt allows for a good air seal between the bottom of the frame and the pegboard.
Here three wooden mold pins are sitting on the pegboard. These pins are the shapes I need to reproduce many, many times. They are a
funny shape, and difficult to machine by hand. I machined these three with a CNC router my brother and I built. A video of the
machining of one of these pins is below. The idea is that I will make molds of them using vacuum forming, then fill the molds
with plaster to make all the copies I need. The pins have been sprayed with a teflon mold release agent. This is only necessary
because no relief angles were designed into these pins, and the plastic holds really tightly onto them after vacuum forming.
Here the frame has been placed in my kitchen oven under the broiler. Some terra cotta bricks are being used to hold the frame up off
the oven rack and close to the broiler coils. The bricks allow space for the plastic to sag down as it heats up without touching the
rack. The wood frame smokes and chars a little bit, but doesn't seem to be in any danger of catching on fire during the few minutes it
takes to heat up the plastic. It will be hot though, so use an oven mitt to remove it.
The plastic has been under the broiler for a few minutes and is starting to sag down. How much to let is sag is a matter of trial and error,
and depends on what you are molding. Too little sag and you won't get good reproduction. Too much sag makes wrinkles and webbing. You will
have to experiment to find out what works best for you.
The frame with the saggy plastic was removed from the oven and immediately placed over the base unit, with vacuums already running. The plastic
sucked down and stretched over the mold pins nicely. There are some wrinkles and what I am calling webbing between the pieces. Maybe I let the
plastic sag too much and the excess material made the wrinkles. This is not really a big problem in my case. If you need very faithful
reproduction, you could experiment with ways to minimize it.
Here is a view of the bottom side of the plastic still in the frame. You can see the three mold pins embedded in the plastic.
the plastic holds onto them quite tightly, even though they were sprayed with a teflon mold release agent before molding. A hole is drilled
in the bottom of each mold pin. Screwing a screw into this hole allows me to pull the mold pins out of the plastic using the screw as a handle.
Here is a view of the cavities in the plastic after the mold pins were removed. The wrinkles and webbing on the other side of the plastic just make small
creases on this side. They are not really a problem since the plaster I will be molding in these cavities sands very easily, and only a minute or so of
sanding gets rid of the ridges left on the parts.
Here are three plaster copies of the original mold pins. Ridges left on them from the wrinkles and webbing in the plastic
have been sanded away. They are amazingly good reproductions of the original wooden parts. Even the wood grain shows in places.
Now I just need to make about 75 more of them.