A vacuum forming machine is used form plastic sheet into the shape of a mold. Vacuum forming is used extensively in industry to make all kinds of plastic products. For this instructable I'll so you how to make a vacuum form machine and I hope to do more instructables on things you can make with this machine. If you don't know what a vacuum former is and want to know how it works PLEASE READ THIS BEFORE CONTINUING. My machine is not be exactly like the one on the link but the principle is the same.
Step 1: Get the Parts
1. Nichrome wire
2. Hardibacker cement board
3. ceramic standoffs
4. aluminum flashing 14in. wide
5. 1x3in. Wood stud
6. wood screws assorted lengths
7. 8-32 machine screws
8. 14 gauge stranded wire
9. screw terminal block
10. crimp-on connectors
11. 1/4in. thick plywood
1. 4 gang electrical box
2. 4 600watt dimmer switches
3. 4 dials (radio shack)
4. 14 gauge stranded wire
5. wire nuts
6. extension cord with one bare wire end
1. 1/2x3/16in. Balsa Wood
2. 1/4in.Thick Plywood
3. Wood Glue
4. 2x4 Wood stud
1. 1x2 wood stud
2. Two Hinges
3. Two Sash Locks
4. Two 6in. Angle Brackets
1. Jig saw
2. Miter box saw
3. Hack saw
4. Utility knife
5. Wire cutter/stripper
This is the prototype machine I built before I built the real thing. It used one heated in an all aluminum oven. This is an over/under design, the oven over the platen. My machine will be a flip type machine because the oven and platen are at the same level and the frame flips between them.
Step 2: The Oven
Now we need to cut the hadibacker board. Hardibacker board is basically drywall for around showers, it used because it doesn't absorb moisture. Its made with cement so its also a good heat insulator which is why its used in the oven. You can cut the hardibacker by scoring it with a utility knife and then snapping it but that can be hard to do when you need long thin pieces. I would recommend using a jig saw to cut this board. I found out the hard way that you can't use a normal jig saw blade to cut cement board, so you will need to get a masonry blade for your jig saw. When you go to the hardware store to get it, look for one that says it will cut tile, thats what you want. The blade doesn't have teeth, the cutting edge looks like a glue/sand mixture just globed on to the blade. I started with cutting the long thin pieces first, parts VF-03 and VF-04. First I cut them to the 2-1/2in. width and left them a little long. Then I went and fitted the pieces to the frame, so that every piece fit nice and snug. You want to keep the oven as tight as possible so you don't have to worry about holes or gaps for the hot air to escape.
Now you need to make the aluminum inner lining side parts, VF-07 and VF-08. This is aluminum flashing which you can get from any Lowe's or Home Depot. I got a roll that was 14in. wide because the flashing will be used on the bottom panel of the oven which is 11in. wide. Aluminum flashing is very easy to work with and can be cut with normal scissors, I'd recommend a nice sharp pair just to make it easy. Again here cut the pieces to the 2-1/2in. width and a little long, then fit the pieces to the frame. Now screw this all together through the flashing, Hardibacker, and into the wood.
I know the animation is not the best but I think it gives you a really good idea how the oven goes together. This is my first real attempt at using SolidWorks. You might be asking how does the large sheet of Hardibacker, VF-05, attach to the plywood, VF-06. That will be explained in the next two steps.
Step 3: The Electrical Equations
This first equation we need to look at is commonly known as Ohm's law:
V=IR or I=V/R or R=V/I
In this equation V= voltage, I=current, R=resistance. The units for voltage is Volts, for current its Amps, and for resistance its Ohms. For this oven I want to use 120V, so that gets me one part of this equation. Now its time to measure the resistance of the nichrome wire. This can be done with a common multimeter and for each of my heating elements the resistance is about 74 Ohms. The resistance of nichrome varies depending on the diameter of the wire and the length of the wire your using. If your building your own oven with a roll of nichrome wire this where you can make changes to suit your situation.
So lets calculate the current (I), from these two values:
V=120V R=74 ohms
I=V/R I=120/74 I=1.62 A
So if I was to hook up one heating element to the wall, the current through it would be 1.62 Amps. But I want to hook up 8 heating elements not just one. There are two ways to wire the elements, in series or parallel. Each method has its benefits but we are concerned with the resistance of the entire oven. In this way we will consider each element as a single resistor. If I hooked the elements up in series they would all be wired end to end and the total resistance of the system would be the sum of the resistance's of each element. This would mean there would be 592 Ohms of resistance so the current would be:
V=120V R=592 ohms
I=V/R I=120/592 I=0.20 A
You would find that this is not enough amperage heat the wire to the desired temperature. This chart at infraredheater.com shows some common wire gauges and the amperage needed to heat them to a certain temperatures http://www.heatersplus.com/nichrome.htm To get more amps through the wire you need to hook up the elements parallel. This equation shows how to solve for the equivalent resistance in a parallel circuit:
1/Req = 1/R1 + 1/R2 + 1/R3 + ...
In this equation Req is the equivalent resistance, or the overall resistance of a parallel circuit, and R1, R2, R3 etc. are the resistances of each individual resistor. In this case I have eight individual resistors, or heating elements. So using this equation and the individual resistance of 74 Ohms, the equivalent resistance is:
1/Req = 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74
Req = 9.25 ohms
So the current for a parallel circuit would be:
V=120V R=9.25 ohms
I=V/R I=120/9.25 I=12.9 A
Now this is more like the amperage we need to heat the coils up. I could wire the oven in parallel and call it a day but I would like some more control over the temperature and amperage. To do this I decided to use a regular lighting dimmer switch. A dimmer switch is basically a variable resistor, also called a potentiometer. By changing the resistance of the circuit I can control the amperage flowing through it and therefore control the temperature. I got the dimmer switches from Home Depot and they are rated at 600 Watts. A watt is a unit of power so I need some way of relating this to the voltage, amperage, and resistance. This equation does it:
P=VI or P=(I*I)R or P=(V*V)/R
To find the amperage:
P=600 W V=120 V
P=VI 600=120 * I I=5 A
So one of the dimmer switches can handle up to 5 amps at 120 V. The current calculated earlier for the parallel circuit was 12.9 amps, this is to much for a single dimmer switch. So I decided used four dimmer switches, each controlling two heating elements. You can see how it was wired in the pdf.
If you didn't understand some of this stuff that all right, because here is a calculator that can do it for you http://www.imperial-armor.com/ovencalculations.html This does not work in the same way I figured out my oven though. It asks for your final oven requirements and then tells you what you need. I started with what I had and figured out a way to make it work. I think this is way that most people would do it. The calculator also assumes that you are building a purpose built oven with coiled nichrome wire. If you decided to go that route here is a link that can really help with finding supplies, http://www.infraredheaters.com/
Step 4: The Wiring
To make the electrical connections easier to accomplish with the least amount of wire running inside the oven, aluminum flashing was used as a conductor. Pieces where cut in such a way to allow power to come in at four different places and then have a common ground for the entire system. The four small pieces on the sides are the power connections and the large piece in the middle is the ground. This idea short of works like traces on a PCB and was based off that idea.
You might be asking, Why did you use the aluminum? The heating elements emit infrared waves which is basically heat. The infrared reflects off the aluminum and on to the plastic sheet. This type of heating is known radiation. You may have heard of convection ovens which is basically this same set up but a fan is added to move the air around inside oven. This moving hot air helps to heat the items inside the oven quicker. I am considering adding a fan inside the oven later to see if I can get better results.
Step 5: The Platen
Step 6: The Frame
As far as plastic suppliers go, a sign store could be a good local source. Online you could try http://www.mcmaster.com or http://www.professionalplastics.com
Step 7: The Base
Step 8: The First Test
So at least for now I learned a lot about how a vacuum forming machine works and how it could be put together. I might try to change some parts of the machine to make it a little more solid and heat the plastic more evenly. If there is one thing I can tell you about build a vacuum forming machine here it is, when you are testing for the first couple of times have a fire extinguisher ready to go.