Vacuum Forming Small Parts - RC Groups
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Nov 28, 2012, 11:09 AM
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Vacuum Forming Small Parts

I've had several people ask me to share how to scratch build plugs and then vacuum form them - on the cheap (and I do mean cheap!). This thread will show how I do it and what materials I use.

this is one of those things that I couldn't give up on until I had a good way to vacufrom at home and without hugely expensive equipment and materials.

The back story:
Years ago - I wont tell you how many - I was in high school and I took a shop class. We had a vacuform table with a heating element on one side and a vacuum surface on the other. In between there was a frame that held a sheet of plastic. You lock the plastic into the frame and then flip over to the heat and back to the vacuum when it softened. I walked around for years remember how that worked and when I started building light RC I longed for that table. I searched the web and found it but its not cheap and the materials come is very large sizes and are also not cheap.

So what could be cheaper and easier to get than a wood box and some plastic plates?
Last edited by mnewman; Nov 28, 2012 at 03:01 PM.
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Nov 28, 2012, 11:17 AM
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Several years ago I got a bug for learning how to vacuum form very light weight parts. I tried several techniques from stretching plastic around a former using a heat gun to the potential purchase of a purpose built table. Then just before I hit submit on the order one of the experiments worked and worked very well. The technique makes it possible to create almost any part for pennies and the supplies are readily available. After several requests to share how I'm doing it, here is a how-to.

Its been a while since I've posted anything to this site. We had a baby girl who is now turning 5 and I'm just starting to get back to building with enough time to post.
Nov 28, 2012, 11:18 AM
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Nov 28, 2012, 11:26 AM
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So lets start with the concept.

I want a small 2.5 inch diameter engine for the front of a micro scale Waco UPF 7. At the scale I'm planning I want detail but I'm not concerned with being too accurate so a close match is good enough.

I did some research and dug up a front view of the engine I want to clone and pulled the image into a graphics software so I could scale it to the right size. This gave me a base to work from when building the plug.

Glue the image to a piece of aircraft plywood and trim the wood to shape. Then start fabricating the wood parts on top the image to produce the look of the engine.

tip* Remember to avoid under cuts that will keep the plug in the mold when the plastic forms around it.
Last edited by mnewman; Nov 28, 2012 at 03:05 PM.
Nov 28, 2012, 11:31 AM
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To get the look of cylinders I drilled a hole in a small block of wood and then ran a bolt through it so I could spin it with my Dremel tool. If you look closely at the cylinder halves above you can see the hole where the bolt passed through. This allows the block to be sanded to the correct diameter (like using a small lathe). The ridges for heat vanes are made by delicately holding a needle file against the spinning wood until a small ridge is cut.

Once I have the cylinder shaped I use a razor saw to cut it in half and then glue both halves to the drawing.

Be careful here...

Since I plan to use this plug for both the front and back of the engine I had to be extremely careful to make sure the diameter of each cylinder is as close to exact as possible and that they are lined up on the drawing closely so the two halves will match when they're formed. I use a set of calipers to check the diameter often.

Oh how I wish 3D printers would come down in price faster. It would be so much cooler if I could just print the engine...
Last edited by mnewman; Nov 28, 2012 at 03:06 PM.
Nov 28, 2012, 12:22 PM
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Cylinder Heads

Once I have the engine halves as plugs the next step are the other parts I'll need for detail. This image shows the plug for the cylinder heads. Each of these are placed on the ends of the cylinders for a more finished look. In the next set of images you'll see some of the other parts like the exhaust ring and the push rods.
Nov 28, 2012, 12:24 PM
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Push Rods

These are going to be push rods that will be placed on the front of the engine. I could have sculpted these into the design of the engine plug but that would mean too many undercuts making it very difficult to release the plug from the formed plastic.
Nov 28, 2012, 12:27 PM
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While I'm created the plugs for each part note that they are each placed on a print out of the scaled engine. Sometimes its easier to build them if I cut out the sections like the image above. Building the push rods in circle would have been way too hard. This is really about trial and error and being prepared to glue your fingers together more than once.

If I didnt mention it earlier I use CA glue because of the time it takes to set small parts.

Also if you can get away with it there are lots of little details you can by using pre-shaped tooth picks or other found objects.
Nov 28, 2012, 12:29 PM
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The Vac Form Box

The box is nothing more than wood box with a circle of little holes in the top and a hole that fits the diameter of our household vacuum hose. To make the holes uniform I used PowerPoint to create a line of dots nine inches wide and then copied it and rotated it until had the circle. The 9 inch diameter is to accommodate the size of the plastic plates I'll talk about next.
Nov 28, 2012, 12:34 PM
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This is the good part...

After about a hundred trials with different cheap plates I found these from Party City. Check out the link and note the range of colors. This means less paint if I can find the right color to start, or at least a good base color. I plan to paint the engine flat black and then scratch it away to show the silver underneath.
Nov 28, 2012, 12:43 PM
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Placing The Plugs

This gets a little tricky...

Place the plugs on the box with enough room to let the plastic form around completely. Think of it like trying to pull fabric around them without wrinkles. You may have to waste a few plates until you get the spacing right.

The diameter of the circle should be smaller than the diameter of the bottom fo the plate.

So here's how it works.

The rim of the plate is raised and it gives you something to hold onto when your are heating the plate. I use our home electric cook top with a larger burner on high. You could use a hot plate or portable electric burner.

Place the plugs.

Turn on the vacuum so its already pulling air through the top of the box. When the plate is ready you wont have time to turn it on so it should be running while you heat the plate.

Hold the plate by the rim and move in circles over the heat. The bottom of the plate will start to soften and sag. Then it will start to tighten back up. It's ready!

Quickly move from the heat and slap the plate over the plugs. Try to aim well so the center of the plate is lined up and you get a good seal as you press it over the circle.

This happens very quickly and you will likely screw up a lot of plates getting used to it. But hey - they're really cheap and for 12 bucks you get 50 tries to get it right.
Nov 28, 2012, 12:46 PM
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The Outcome

This image shows a plate after the vac form and before "sanding out" the parts. I'll describe that next.

Notice how the holes in the box make dimples in the plate and the parts are formed nicely.

tip* I use the dimple sometimes to simulate rivets or bolts. Waste not - want not.
Nov 28, 2012, 12:48 PM
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Sanding out the parts.

The plastic is thin but strong. but you can easily cut through it with a sharp knife or tear and edge. To avoid that I rough cut the parts and then put them on a piece of sand paper on my table. then I slide the part in circles until them come free from their backing. Once in a while hold the part up to the light and you'll see that darker spots where you need to sand more.
Nov 28, 2012, 12:50 PM
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The first rough prototype

Once I think the plugs are ready I usually build a quick prototype like the one in this image. This way I can work out the bugs. Sometimes I will have to modify a plug to get better alignment or I find a problem with how deep the plug is which means the plastic will be too thin.
Nov 28, 2012, 12:52 PM
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If you look close you'll see some tearing where I was moving too fast with the knife. You also see some places where I need to drill some holes in the plug to get the vacuum to pull the plastic deeper. Look closely at the photo of the cylinder heads and you'll see where I drilled several holes. This lets the vacuum pull through a plug and not just around it.

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