Vacuum-forming Plastic Parts

Vacuum-forming Plastic Parts by Rob Caso As seen in "Scale" in the Fall 2012 and Winter 2013 issues of Park Pilot

The canopy for Rob Caso’s de Havilland Mosquito bomber was formed using the techniques that he describes for you in this article.

Vacuum-forming is the scratch-builders’ way to make custom canopies, cowlings, wheel pants and other parts that are difficult to fabricate with flat pieces of wood or plastic. Vacuform-formed parts offer advantages when clearance is an issue. Vacuum-forming is easier than you might think and can be done using readily available materials and equipment you can build. Equipment The following equipment is needed to make vacuum-formed parts: • Vacuum source • Vacuum-forming table • Heat source • Aluminum racks to mount the sheet plastic • Clips to hold the plastic to the rack • Support stand I built a table with an inexpensive vacuum motor from a repair shop, incorporating a switch and an AC outlet for my toaster oven. I commandeered an old toaster oven for the heat source but note that its size will limit the size of the parts that can be made. The vacuum-form table houses a perforated box mounted just above its surface. The vacuum motor is inside this box. Build the table slightly larger than your aluminum rack — the dimensions of which will be determined by the size of your toaster oven. The box is made from 3/8-inch plywood with a 1/4-inch Plexiglas top drilled with 3/32-inch holes. I screwed the box together and sealed the joints with silicone. If you are using an external vacuum source, such as a shop vacuum, cut a hole in the side of the box for the vacuum’s hose, using some rubber radiator hose to provide a seal. Apply some dense, sticky-backed weather stripping to the box’s surface, using the outer dimensions of the aluminum rack. This provides a nice seal during the forming process. To save plastic, I have a number of racks in different sizes, some of which are made of plywood. For these, I made removable cardboard frames to block off the area outside of the forming surface that would be open to air outside the perimeter of the rack. Don’t forget to add the seals for these. The racks are made from a single piece of aluminum right-angle stock, cut at the corners with the ends riveted. Use clips from an office supply store to hold the plastic on the racks during the heating process. Finally, you need a support stand inside the oven for the racks. This stand will provide clearance to allow the heated plastic to sag. Making Plugs A “plug” is nothing more than a male mold over which the hot plastic is formed under a vacuum. It must be able to withstand heat, and be strong enough to withstand compression during the forming process without deforming or crushing.

The vacuum-forming process starts with making the “plugs.” Plugs are the forms over which the plastic parts will be vacuum-formed.

A drop tank for Rob’s P-47 is shown here in sequence. The wood plug on the left, then the Water Putty plugs, the vacuum-formed parts on the right, and the finished tank.

The material you use for the plug is also dependent upon the type of part you are making and how many you plan to make. Solid balsa is a convenient material, although it will compress slightly and leave grain marks that will be transmitted to the surface during forming. This is okay for opaque parts that can be sanded, but clear plastic is more finicky, and balsa will not yield enough clarity. Balsa is also only good for roughly four to five formings, after which it may begin to fray and crack. I use a plaster-like product called Water Putty (WP) for plugs when I am making multiple parts, or for those that require a higher degree of clarity, like a canopy. A WP plug is slightly more work to make, but the parts that it produces are excellent.

Durham’s Water Putty is a plaster-like material that Rob recommends for forming clear parts.

I start with a balsa plug, over which I vacuum-form some .030-inch polystyrene plastic. I then sand and wax the inside of this forming, which now becomes a female mold, and pour in some WP. Since WP is somewhat dense, you may have to add some fiberglass cloth and epoxy to the outside of the mold to prevent it from distorting when you pour in the WP. After it sets up, I take out the hardened WP and fine-sand it. I also epoxy it to a 1/8-inch Lite-Ply base to keep the corners rom chipping. I have also successfully made “quicky” plugs for small runs using hard, open-cell foam, polyester resin and 2-ounce or 4-ounce fiberglass. You can make one of these quicky plugs in roughly 15 minutes. The plug’s outside dimension must be tolerenced, which means taking into account how the finished part will be used. For example, if you want a vacuum-formed cowling that lies flush with the fuselage, the plug must be made slightly smaller to allow for the thickness of the finished forming, usually approximately .020 inches less.

Here are several finished Water Putty plugs.

Complex plugs require holes to be drilled around their perimeter, allowing the vacuum to pull the plastic all the way around the plug.

Here are the vacuformed exhaust ports for Rob’s scratch-built Hawker Hurricane.

All plugs should be given a plywood base to prevent chipping. Again, note the air holes.

Here’s Rob’s dedicated vacuform table setup.

This closeup shows Rob’s vacuum-forming unit with the vacuum motor hiding inside.

An aluminum frame accepts simple clips to hold the plastic while it heats. The stand elevates the hot plastic inside the oven.

Different-size frames may be made — along with cardboard block offs — for smaller runs.

The plug should not have any sides that have greater than an 85-degree angle, and positively none that are greater than 90 degrees. There are three reasons for this: the hot plastic forms webs when pulled over plugs having vertical sides with corners; the plastic gets thin on such surfaces, which is only okay if the thin side is not going on your model (adjoining surfaces may also thin out); and if the plug has an undercut, you may not be able to get the part off of the plug. Plugs also should be made 1/4-inch to 1/2-inch higher than the finished part. This allows for complete forming and eliminates any radius around the bottom of the finished part. The plastic that rounds off at the base of the forming may then be cut off, yielding a complete and distortion-free “good” part. One simple way to achieve this is to make an accurate plug and add a piece of 1/8-inch plywood to the bottom of the plug. Finish it by sanding the plywood part flush with the sides of the plug.

Both the upper and front window areas were constructed using the techniques in this article.

Another good procedure is to elevate the finished plug slightly off the forming table with strips of 1/32-inch balsa so that the vacuum completely pulls the plastic around the edges of the plug. For complex plugs, another important step to remember is drilling small holes through the plug, near and around any irregular shapes on the plug and at its base. Doing this ensures that the plastic will completely form to the plug. Finally, for plugs that “drop off” on one side, such as the back of a canopy, a sloping transition section should be added to that area to allow the plastic an even descent to the vacuum table. This feature will negate webbing, as well as thinning of the plastic at the corners.

Rob has a love-hate relationship with his Dauntless plug. It’s a nice part when formed, but tough to remove from the plug.

Rob made a fiberglass mold for the sides of his P-47’s cowl. The part for the front was made from a vacuum-formed piece, into which he laid up glass cloth and epoxy.

Materials for Parts I use .030-inch white, high-impact polystyrene (HIPPS) and clear .030-inch PET-G (the material that soda bottles are made from) to make my parts. Both materials are easy to get from any industrial plastics supplier. Some shops will even cut it to the size of your forming racks for a nominal charge, otherwise you will probably have to buy large sheets and cut them yourself. Note that clear PET-G comes with a protective plastic skin on each side. This plastic skin must be removed prior to forming. Pulling Parts Have your oven, vacuum and vacuum box in close proximity and conveniently arranged. This will minimize the panic as you pull the hot plastic out of the oven and place it on the box. The plug should be centered on the box and be dust-free. Clip the plastic to the rack using three clips on the long side and one or two on the short. With the oven preheated to 350 degrees, place the rack on the stand in the oven, and allow the plastic to heat up and sag approximately 1-1/2 inches. Turn on the vacuum, briskly take the hot plastic out of the oven and snuggle it down over the plug, making sure that it comes firmly in contact with the rubber seal around the perimeter of the box. When this happens, the vacuum takes over and pulls the plastic tightly around the plug. Turn off the vacuum and allow a minute for everything to cool off, then pull the plug out of the part. Different plugs require different levels of sag in the plastic, and this takes a little practice to get right. The key is to estimate the volume of the sag in the hot plastic so that it is roughly the same as the volume of the plug. Again, experimentation and experience are the keys. Certain plugs are difficult to remove from the vacuum-formed plastic part. Vacuum-forming the cowl for my Dauntless always results in a wrestling match because of the relatively deep pull, and there is no corner to grab it once the plug is deep in the forming. I have used cooking spray on the plug with some success, although I avoid silicone sprays if the part is to be painted later. Silicone is difficult to remove completely, and will cause fisheye in your paint job. After two or three pulls, a plug will break in. Its surfaces will become polished in the process, and will gradually get easier to extricate from the forming. Generally, however, a properly made plug will pop from the newly vacuum-formed piece with little effort.

These canopy plugs feature a transition section (denoted by “X”) to help the plastic properly form. This transition section is then cut off.

Here is another shot of Rob’s canopy plugs, this time a side view to show off those all-important transition steps.

Rather than using a transition for the Bulldog canopy, Rob simply made a second canopy plug and glued the two back to back. This technique lets two canopies be made with a single pull.

Safety Be smart when working with hot mounting frames and plastic — a pair of cotton work gloves will help protect your hands. Never leave plastic unattended in an oven, and always unplug the oven when not in use. Final thoughts If no commercial part is available to complete that special model, vacuum-forming is a viable option, even for the casual modeler. It’s a true feeling of accomplishment when you make your own complex parts for a model and, in the end, much of this can be easy to do. References and Resources: Do it Yourself Vacuum Forming by Doug Walsh is published by Vacuum Form Piedmont Plastics, Incorporated ( It’s the source for HIPPS and PET-G.

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You can make your plugs that have straight sides in three pieces. The nose cowl for example. Get three pieces of wood for the mold. Two for the two ends of the mold and the third for a thin wedge between the two. The idea is to put the wedge between the parts and be able to remove it when the mold is formed to make it easy to remove the cores. Stack the three pieces together and saw a slot the the back of them all at the same time. Make another piece that would be a key to fit in the slot to align the three pieces accurately. Make it a snug fit. Shape the mold with it all put together. Sand and finish the parts. When the part is made remove the key and wedge first and the other parts will have more room to be removed. The better the fit of the pieces the less you will be able to see the edges in the finished part.

Had you considered using air pressure to blow the part off the plug?
Try directing a blast of air back through the vacuum holes and/or around the base.
Good luck!
AMA 1013013 / MAAC 86683

Excellent information!

Well done and informative, thank you all..............Dan

How about a dvd on this subject with in-depth how-to's on building the Vacuum Forming Machine and the type of plastic needed to match the project. With alot of the Hobby Industry disappearing including "kits", many of us "old guys" who love to build now must now "scratch-build" our aircraft. Sometimes the right canopy or other parts are not available. Just a thought. Nick.

Hi Nick! That is a great suggestion. I have passed it along to our editor and video team.

great article

interesting and very illustrative article.

Excellent article!

Great article

Excellent tutorial

I have started dusting both balsa and water putty plugs with baby powder to act as a release agent. Rub the powder into a balsa plug to make it more slippery and to fill any imperfections. This method also gives a tiny bit of additional clearance between the plug and the hot plastic, allowing the plastic to be removed more easily. Don't do this with clear parts!

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