MIMF

For the past couple months, I've been obsessed with making my own plastic pickup covers through a vacuum thermoforming process. This is, of course, something we see everyday in "blister" packaging and various plastic objects. But it was a post on the Music Electronics Forum by John H that got me interested in making my own covers. And those of you who know me, you know I can't pass up a DIY opportunity!

The basic bits and pieces of a plastic thermoforming setup are: heat source, vacuum table, a mold of the thing you want to make, and thermoformable plastic. I used 1/16" black ABS. The plastic is heated to is thermoforming temperature (around 300deg F), you place it on the vacuum table over the mold, turn on the vacuum, and it sucks down over the mold. Now, as I found out, this sounds a lot simpler than it is, and there has been A LOT of manufacturing R&D over the decades and information out there that can give you fairly predictable outcomes. I, of course, jumped right in and learned some things the hard way.

Here is my heat source. It's a Porter Cable heat gun and an "oven" that I made. My idea was to place the plastic in this aluminum frame over the "oven" to heat it. Seemed like a good idea, but I was not getting even and sufficient heating. Here are some first failed attempts with a dummy mold. One even turned out pretty well, after I blasted it with more heat until the edges sucked all the way down.

After some advice from folks on the MEF, I reconfigured the heat gun and oven to keep more hot air in the box. Unfortunately, while it did make for more consistent heat, now the air was blowing unevenly on the plastic, creating hot/thin spots. I reconfigured once again and it now looks like this. This arrangement worked the best, and I made a change to the table, as well, to seal the edge better. I still wasn't getting good draw-down on the plastic, though. These attempts are now using the actual pickup mold for the pickups I'm going to build.

While watching some youtubes of folks doing vacuum forming at home, I noticed some guys actually pressing the plastic down with gloved fingers to get edges pulled down completely. This seemed like a bad idea, as I was sure that it would create this spots or ugly deformities, but the plastic is already cooled enough that it's not a problem. I tried pushing the edges with a piece of wood, and it turned out pretty well. To improve upon that idea, I made a ring, just slightly bigger than the base of the pickup mold, to press the last 1/4" to 3/8" of plastic down evenly around the edges. This worked so well, that I busted out four more identical covers! This whole adventure didn't end up costing much. The heat gun was $35. The quarter sheet of ABS was about $15. The plywood and hardboard used for the vacuum table, oven, and molds were odds and ends I had around the shop. The aluminum 'L' stock for the plastic holding frames was about $15.

Not shown are some of the bigger failures, including blistering and tearing of the plastic, due to overheating. If you're interested in giving this a try, read up on some of the info out there. Even plastic heating is a very important aspect, and there are some very specific recommendations for mold height vs. sheet area.

If I was going to do this again, I'd build a bigger table, maybe 12x12. There is a LOT of waste, but if the plastic area is too small, you run the risk of stretching/thinning the plastic or not getting the detail you want. I'd also look into heating with a heat lamp instead of blown air.

Fire away with any critique or questions! Thanks for looking!

Nice work!!

Now you can form fit guitar cases.

Cool project Jason,

The placement of the heat gun makes sense. Hot air will rise, so with this setup you will fill up your oven from above and the hot air will be "pushed" downwards evenly.
Simple but effective.

Keep it comin'!

Steve Senseney wrote:Nice work!!

Now you can form fit guitar cases.

I think I'll need a bigger heat gun!

Hans Bezemer wrote:Cool project Jason,

The placement of the heat gun makes sense. Hot air will rise, so with this setup you will fill up your oven from above and the hot air will be "pushed" downwards evenly.
Simple but effective.

Keep it comin'!

The oven space is small enough that it gets plenty hot, pretty quick. The upward blowing oven arrangement probably would have worked fine if I closed off the bottom a little, but the plastic was cooling too much as I moved the frame over to the vacuum table. One of the participants on the Music Electronics Forum, a manufacturing engineer by trade, made the suggestion to put the oven over the plastic as it sits on the table, so the table gets warm, too, and the vacuum can be pulled immediately.

Success ended up being more about the directional flow of the forced air. This heat gun has a low and high fan setting, like a blow-dryer, and I set it on high to keep the temperature up (thermostat set at about 75%) and on the plastic. Even on a 7"x7" square surface, if the air was moving a little faster in one area over another, that spot would get too hot and thin or tear when the vacuum was applied (see picture). I made a small baffle/interrupter thing that is suspended in the center of the oven to disperse the air a little more randomly, but I still found one side getting a bit too hot (you can see this in some of the last pictures, on the right side of the plastic). I could probably change the temperature setting, and slow down the fan, but this would require a longer "cooking" time: these attempts had a timer set for 8 minutes. For what I'm doing, I think this is going to work just fine, and I feel confident that I could build other pickup molds and get similar results.

By the way, I realize that my pictures focused a lot on the product and not the tools. The vacuum table is pretty simple - ply box with a 3/4" hardboard table - and uses the shop vac for vacuum. (I do have a vacuum clamp/press system, but this process needs some super-fast, big-volume suck.) When I upgraded the table, I put a lip around the edge to seal the plastic, and drywall sanding sheet on the table to improve air flow around the mold. These ideas I got from some youtube videos. The oven is just 1/4" hardboard, with a couple layers of aluminum foil on the inside (held with foil tape). The sides get pretty warm, but not enough to burst into flames.

I think a heat lamp would be a better heat source.

On the vacuum side, I'd try to use both the shop vac and your vacuum press system. Plumb it so that each system can be opened and closed via valves, then use the shop vac to pull the initial vacuum then quickly switch to the vacuum press system to pull the final, stronger vacuum, while keeping the heat lamp on it.

Just some thoughts...

Mario Proulx wrote:I think a heat lamp would be a better heat source.

On the vacuum side, I'd try to use both the shop vac and your vacuum press system. Plumb it so that each system can be opened and closed via valves, then use the shop vac to pull the initial vacuum then quickly switch to the vacuum press system to pull the final, stronger vacuum, while keeping the heat lamp on it.

Just some thoughts...

Using the vacuum press for the secondary suck is a good idea. The shop vac is working pretty hard, basically pulling against nothing, and the vacuum press is made for that.

Yes, I think a lamp might be a better heat source. It would be easy to wire a ceramic outlet onto that lid.

Actually the Vac motor's job gets easier as the vacuum increases which is why the motor speeds up. Most shop vacs are not designed to have limited airflow. They are so cheaply manufactured that any drop in airflow will cause them to overheat very quickly. Your best bet would be to add a small tank to your vacuum pump to "store up" vacuum. I found a 20 gal steel tank from a portable compressor for $20. Any leaks will probably undo whatever storage you have. I'd just get by with what you have now, it seems to work and you aren't going into full production mode just yet.

A heat lamp would be a lot quieter too. That's gotta be good.

What Michael said.

I also still think using your vac. press system will be way better, and you'll likely not have to press down to "assist" the forming.

Some of the local makers who use vacuum have rigged up tanks using PVC pipe lengths and fittings. The ones I've seen use small diameter pipe; 2" or so, in fairly short lengths, and gang them to get capacity.

I went looking through the old Archives to find something else, and found this instead.
http://www.mimf.com/library/Vacuum_form ... -2008.html

Geez, if you hang around this place long enough, you pretty much cover every topic! Down towards the bottom is a post by a guy named Scott, who is/was a professional thermoformer. One of his last comments is that this particular style of vacuum table I've built is something of a simplification and devolution of the process, and that one of the problems DIY folks have is overcompensating with heat to make up for insufficient vacuum. Boom.

Ah, here's what I was looking for: didn't have to go all the way back into the Archives...
http://www.mimf.com/phpbb/viewtopic.php ... son+vacuum

Like I said, I have a vacuum press system, but I'd need to do some reconfiguring of my little table, or just build a new one, to make it work for thermoforming. Depending on how often I need to crank out some covers, and how crisp and accurate I want the quality, this is something I will consider.

An alternative to thermoforming would be to mold something. I just got some two-part silicone rubber molding compound from Micro-Mark. It cures cold in about four hours, and doesn't stick to much of anything except itself. They show how to make two-part molds, which can then be used to cast quite detailed parts from resin, which they also sell. You could easily make a master of what you want from just about anything, and duplicate it quite a number of times. The silicone is even heat resistant enough to work with some low melting temperature metals. Jewelers use pretty much the same stuff to make wax duplicates for investment casting by the 'lost wax' method.

We have a long and detailed archived discussion on molding, too. David Robinson's photo-essay on molding your own plastic pickup covers and other small parts [Pictures] It was great, don't miss checking it out.

Happy New Year!

The silicone mold compound I got from MicroMark cures in four hours, and gives a longer working time than the stuff he talked about. They also have a stronger 10:1 mold compound that has a 45 minute pot life. I have not tried their casting resins as yet: I'm actually making the mold for electroforming. They sell a nearly clear resin and colors to mix in, as well as a couple of white resins. The instructions I got with the mold compound indicate that it might stick to epoxy, but in that case you can use a release agent.

Jim, I remember that discussion - good stuff!

Alan, what is it you're making with the molding?

One of my students is restoring a circa 1780 English guittar (yes, that's how its spelled). It's a ten-string cittern, with interesting tuners: there's one on my web site. This one was used for parts in restoring another, and now he wants to restore this one as well to use in playing duets. The rose in it was used for the other instrument. It's an open work casting, depicting (I guess) a shepherd and his lass, that was probably made from master that was hammered up from thin sheet. It's beyond my skill to duplicate it as a carving and get the style right, so we're going to try to make a mold using the original as a master, and electroform a copy. We're just at the beginning of the process, with a lot to learn. As far as I'm concerned, that's one of the best things about my job.