Vacuum Forming 101

[PTKohlmann]

Buzz Russell asked me about making some windows for his big Cessna 195 at this week's Members Meeting.  While I'm not an expert at vacuum forming I've done it enough to have learned most of the big things not to do, so I gladly agreed to help.  Buzz wanted to make sure that I got something for my efforts.  I decided that turning this small project into a tutorial that others can use would be a fair trade.

Vacuum forming or thermoforming is a simple process that can be used to make lightweight parts.  In Reader's Digest form, the process involves heating a thin sheet of plastic to its softening point, then stretching the plastic over a mold (called a plug) and using vacuum to suck the plastic down tightly to the plug as the plastic cools and hardens.

Sometimes making the tooling can be time consuming but if multiple copies are needed it is often worth the effort.  This is especially true when identical parts are needed on one model, like cowls for a twin.  Vacuum forming is also the best way to make clear parts like canopies.

One key limitation of vac forming is that the plug needs to have some draft so that it can be removed from the finished part.  The process is also limited in how much detail can be replicated.  Below are some examples of parts that I have made for some of my projects with my $5 vacuum former.

As mentioned, I'm not god's gift to vac forming so anyone is welcome to chime in with comments or questions.

Koh

[PTKohlmann]

Just like a hi-fi only sounds as good as its speakers, the parts that you pull will only be as good as the plugs that they are made from.  Plugs can be made from many different materials including balsa, plaster, and MDX.  Most of mine are made from a lite ply frame filled with soft balsa.  A common project is to make a plug by filling an existing part, say a Parkzone T28 cowl, with plaster.  That’s a quick way to make replacement parts.  Just don’t sell them—that’s copyright infringement.

A material that does not work well for plugs is foam.  Foam is easy to work with but it melts when hot plastic hits it.  Guess how I know that;)

Below are the set of plugs that were needed for my 30" early 109 kit.  These are all wood plugs.  The spinner plug was made with a drill press and sanding block.  The nose bowl, chin scoop, and canopy plugs were all made of ply and soft balsa.  The steps used were:

1.    Carve/sand the plug into shape
2.   Fill the plug—I use a skim coat of drywall mud, lightweight spackle works well too
3.   Sand filled plug with 150 grit
4.   Seal plug with water-based polyurethane (WBPU).  I use two or three coats.  Sand lightly between each.
5.   Shoot several coats of automotive primer over the WBPU.  Spraying the first coat a different color (guard coat) is a neat trick to keep from sanding too far. 
6.   Wet sand with 400 grit.  You can go to 800 or so if you want really clear windows but usually 400 is just fine. 

This may sound like a lot of work but most of it is only a minute or two here and there.  I speed it up with a heat gun and can bang this out in an evening.

[PTKohlmann]

In Buzz's case, he provided the plugs for his windows.  They were made from lite ply and shaped so that they just slipped through the fuse openings.  His thought is that when vac-formed, the windows will have a rounded lip that will pop right into place.

I prepped the ply plugs as described above.  Because a left and right handed set is needed, both sides where prepped.

[PTKohlmann]

I tend to jump into projects spontaneously sometimes.  By way of example, one Sunday I realized that I would be needing to vac form the cowl and canopy on my first airplane design or the project would stall.  I’d seen a few homemade vac formers and some industrial ones, too.  So I went out to the garage to see what I could find.

An hour later my vac former was ready.  It is made from an upside down bucket with a plywood table glued to the top (bottom?  It used to be the bottom but now it’s the top.)  A thick bead of silicone sealed the table to the bucket. 

A large hole was drilled into the side of the bucket so that the hose from my shop vac could be jammed into it.  Several large holes were drilled through the table top and the bucket below.  When the vacuum is turned on, it sucks air from above the table and through the bucket.

The plug sits in the middle of the table on a piece of wire screen.  The screen prevents the plug from blocking off the big vacuum holes in the table top.  Most vac formers have a bazillion holes drilled in the table for this purpose but believe me, the screen works just as well and is a lot less work.

Adhesive-backed weatherstripping provides a sealing area around the edge of the table. 

One of the best things about making a bucket-based vac former is that you can store all of your crap in it when you are between jobs.

[PTKohlmann]

The next trick is to stretch out the plastic sheet to be formed.  I used the aluminum extrusions for window screens.  These can be assembled into two rectangular frames.  The plastic sheet is cut to size and then sandwiched between the two frames.  Metal binder clips hold the assembly together.

Plastic is easy to come by.  I order PETG for glazing and high impact styrene for everything else.  You can get it right to your door from US Plastics and others for just a few bucks.  .020" thick is good for smaller parts while .060" is good for those T28 cowls that you are planning to knock off.  Be aware that the thicker you go the less detail you can replicate.

For these materials, I set my oven to 325F.  Once it has preheated, the plastic is loaded in.  It will sag when it heats, so raising the frames up is a good idea.  If the plastic touches the rack it will stick like mad.  Yup, done that.

[PTKohlmann]

Buzz's parts were really easy.  They don't have much volume or detail, so I could knock out the full set of four in a single shot.

The rule of thumb for heating the plastic is that the volume of the sag should match the volume of the plugs.  In this case, 1/4" would be plenty.  I goofed off too long while the first sheet was in and it sagged enough to vac form the head off of my 351C-4V.

Once the second sheet of PET had sagged its 1/4", the shop vac was turned on.  The frame was pulled from the oven, aligned over the weatherstripping seal on the table top, and dropped into place.  The plastic is very rubbery and it pulls down in the blink of an eye, but it cools just as fast.

Extremely important tips:

WAX THE PLUGS!  If you don't the plastic will stick to the plug and it will be destroyed in the ensuing removal process.  I get very angry when this happens.

Remove the filter from your shop vac.  Its suck factor will improve dramatically.

Replace the filter when you are done.  Guess what happens if you don't.  No, I haven't done that one.