Vacuum forming

(The following article was originally published in SNOAR NEWS around 1985, including the original bitmapped MacPaint illustrations)

How to Build and Use Your Own Vacuum-Forming Device       By George Gassaway

Occasionally a model requires a special part or set of parts that are not commercially available, and is not practical to make light enough with conventional scratchbuilding methods. In some cases, the best answer would be to vacuum form parts if you had the means to do so. Now you can, with a crude homemade forming device that can be built in one night for as little as $20, depending on materials. It does require two appliances most people have, an oven and a vacuum cleaner. Due to the use of an oven for heating the plastic and the risk of fire if the oven is left unattended, this is recommended only for adult modelers or for use under adult supervision.

Building the Vacuum Box: The Vacuum Box's purpose is to serve as a good interface between the plastic held in the holder frame and the vacuum from the vacuum cleaner. Therefore, the vacuum box can be just about any size and can be built from many different materials as long as certain criteria are met. I made the vacuum box from fiberboard, which is fairly cheap but is not easy to cut. Plastic is a good material to make most of the box from, as long as the top of the box is not plastic. The top needs to be some sort of heat resistant sturdy material that will not sag from the combined effects of the hot plastic in the frame and the vacuum itself. The vacuum box needs to have some supports under the top surface to keep the top surface from sagging while forming, the supports not interfering with the vacuum to any of the holes in the top of the box. The box must be airtight, except for the holes in the top surface and the side connection to the vacuum hose (the side connection can simply be a short piece of BT-55 or other tube that fits the hose). The top of the box can be made of pre-drilled perfboard from an electronics store (easy but expensive), or fiberboard/hardboard/plywood with holes you drill yourself (cheap but takes time). Your biggest decision will be in choosing what size to make the vacuum box, as it will determine the maximum size plastic you will be able to form. I chose to make a vacuum box two inches deep with top dimensions of 12" by 18" so that it could handle the largest plastic/frame combination that could fit into the oven, although so far the largest size frame used has been one 13" by 12" to hold a 12" by 11" sheet of plastic.

Building the Holder Frame: The frame holds the plastic during heating and forming. There are different materials to use for the frame. Metal would be best for heat resistance, and modelers with the equipment and skills to do metal working may want to make the frame from metal. The frames can be made from 1/4" hardboard (such as Masonite) and cut out, or they can be constructed from materials such as spruce. I have made frames out of 1/8" thick spruce, each top and bottom frame using eight pieces (four atop four others, with the joints overlapping). An upper frame and lower frame is required, the plastic will be held between them. Holes will need to be drilled in equally spaced locations for the bolts and nuts which hold the frames together. It is best to tape the frames together so that the holes will be aligned properly, and make a mark on each frame to indicate how to align them once they have been apart.

 

The best bolts to use are 6-32 with wing nuts to hold the frames together; the wing nuts make installing and removing the plastic much easier. Use cyanoacrylate type glue to permanently bond most of the bolts to the bottom frame, upside down with the threads sticking up. Do not glue bolts in place along one side however, so that they can be temporarily slipped out of place. This will allow you to more easily slide out leftover scrap plastic and slide in a new sheet from the side the bolts are removed from (this way, you don't have to remove and replace all those wingnuts every time you form a part). Since the heads of the bolts will protrude down from the bottom frame, add some strips of spruce along the bottom frame so that the bottom frame will meet flush with the top of the vacuum box for a fairly airtight seal.

You will most likely find a need to make several different size frames to hold different sizes of plastic, depending on how large or how small the part to be formed is. Plastic sheets 4"x 4" can handle small nose cones (<1" dia and <2.5" long) and 6"x 6" plastic sheets for larger cones up to egg capsule halves. Some frame sizes will be dependent on the source for the plastic. Evergreen Scale Models plastic sheets are commonly 6" x 10.5", so a special size frame is needed to hold Evergreen plastic (for economy, it is good to make a smaller frame to hold a half sheet too, which is 6"x 5.25"). Once the size of the plastic sheet has been determined, plan out the size of the frame so that the bolts of the frame will be just outside the edges of the plastic sheet. For example, for a 6"x 6" sheet of plastic, the frame should be about 7"x 7" with 1/2" frame overlap beyond all edges of the plastic. There should also be about 1/2" of the frame gripping the plastic sheet. This would mean the frame would measure 7"x 7" on the outside and 5"x 5" on the inside, the frame itself having a one inch "wall thickness" all around. See figure #2.

 

Other items: A holder needs to be made to hold the plastic and frame above the oven rack. This can be any heatproof material that can hold the frame 3-4" off of the rack. Sheet metal can be bent into a large "U" shape to hold the frame.

The vacuum box has holes all over the top, but you may want to use a frame smaller than the area with the holes. To effectively cut off a vacuum leak through those holes outside of the frame area, cut out a mask from poster paper. The rectangular hole in the mask should be about 1-2" smaller than the frame inside dimensions, to allow a little leeway if the frame is not centered exactly during forming. The outside mask dimensions should be about the same as the top of the box, or at minimum it should be enough to cover the outermost holes in the vacuum box top. The mask can be secured by tape along the edges. Make enough masks to handle the different frame sizes.

Molds: Molds can be made from several materials, such as balsa, hardwood, metal, resin, and plastic (most any material which can adequately withstand the heat and vacuum). It is even possible to use plastic model parts as molds as long as the hollow plastic is filled inside with something such as plaster and the part formed over it is not from very thick plastic (thick plastic will tend to transfer some of its heat to the plastic mold so much as to disfigure it or cause it to sag). Other hollow molds should likewise be filled, as the vacuum force can cause the mold to sag or collapse.

The mold should have a flat bottom and not have any undercut. No undercut means that the sides of the mold must not taper back inwards once past the maximum width, otherwise the part cannot be removed from the mold. This is not a problem with most nose cone shapes since at worst the sides become parallel. For a nose cone mold, make it a bit longer than where it will be trimmed for the shoulder. There will be a limitation as to how tall a nose cone can be and still have the plastic draw down properly around the mold. Four to five inches will be about the maximum height the plastic will draw with this homemade set-up, depending on the exact shape being formed (a wide cone like an Apollo capsule or SRB nose will draw well, while a long 3:1 type ogive may be difficult). Such high draw parts will also need large sheets of plastic, such as an 11"x 12" sheet. Experience will give you a better idea what shapes and sizes are likely to form properly or not.

Most modelers would tend to use molds made from wood, typically balsa or some hardwood. Balsa is easiest for modelers to turn, but is not as robust as turning one form hardwood ( remember, the mold isn't going to fly, so it doesn't matter what it weighs. . . ). The mold needs to have a good heatproof finish. The best finish is to use finishing resin. If you use resin, pay attention to the warnings about eye protection from the hardener, avoiding skin contact, and its use in a well ventilated place. A dope, primer, or similar type finish is no good, as the hot plastic will cause it to bubble. Besides the resin providing a smooth finish which the formed part will duplicate, it will be easier to remove the part from a smooth mold than a rough mold. Still, some non-tapered shapes may be difficult to remove from the mold. This sticking can be reduced by dusting the mold with talcum powder as a lubricant. Do not use any mold release waxes or liquid type lubricants, as they will tend to bubble from the heat.

Plastics: There are many different plastics which can be used for vacuum forming, but use only those that are recommended for such use. Such acceptable plastics are styrene, A.B.S., butyrate, vinyl, and acetate. There are others, check with a local plastics supply house for the best type for your purpose for a reasonable price. Plastic sheets can be bought in large sizes up to 4'x 8', at a cost per square foot tremendously cheaper than the hobby store variety. However, sometimes the only way to get a certain thickness is to get a few small sheets of hobby plastic as plastic dealers do not have all thicknesses. I found a sign shop which uses and sells plastic sheets offering 2'x 3' styrene .06 plastic for $2.50, with lower prices for thinner plastic. It isn't a great grade of styrene, but it works for most parts made from it. So, you can shop around for a good price. The thickness of plastic will mostly be determined by just what size the part to be formed is and what tradeoffs in strength, stiffness, and mass you can live with. Most big noses will be strong enough with .06 plastic, an egg capsule made from .06" plastic is plenty strong for anything short a high speed crash. Some BT-50 or smaller noses may be better if made from .04 plastic. Special non-nose cone parts which need to reproduce mold detail or have minimum weight could use very thin plastic .01-.02" thick if the strength is adequate for the application.

Forming: (safety note- Due to the fumes released by the hot plastic, set up a fan and open a window to help ventilation). Choose the plastic to use and mount it in the holder frame. Turn on the oven and set it for about 250-300 degrees for styrene (other plastics may take less heat or more heat). As the oven is heating up, set up the vacuum box and connect it to the vacuum hose. The vacuum box should be as close as possible to the oven, as the plastic loses heat within seconds. Place the mold in the center of the window of the mask. In some cases, it may be necessary to make a small pedestal to hold the mold about 1/4-1/2" above the vacuum box so that the plastic will form completely around the lower portions of the mold. Dust the mold with some talcum powder, which will act to help the formed part slip out of the mold.

When the oven has heated up, place the frame holder on the oven rack and place the frame with the plastic on the holder. The plastic must be watched continuously to see when it is hot and soft enough for forming. Some thin plastic can be ready in less than 60 seconds, thicker plastic can take 2-3 minutes. Styrene tends to sag a good deal as it becomes hot and soft enough to form, so you can watch for the sag. Other plastics may have different tendencies, and become taut and rubbery. It may be necessary to open the oven door to poke a corner with a dowel to determine how soft the plastic is.

When the magic moment arrives, turn on the vacuum, open the oven door, and grab the frame (use some sort of hand protection; gloves or potholder). Quickly remove the frame and lay it over the mold, with the frame centered over the mold and mask window. Once the frame bottom seats flush with the mask window, the plastic will immediately form over the mold. If it does not form well, it is likely that either the plastic was not hot enough or the vacuum cleaner is not powerful enough (shop vacuums are best, but most regular vacuums should be able to do the job). If the plastic was not hot enough and the plastic is still intact, it may be possible to put the plastic back in and reheat for another try. When the part has formed, turn off the vacuum and let the plastic cool for about a minute or so before trying to remove the mold.

It is preferable to use an assembly line approach so that the most parts can be formed in the shortest period of time. Use a modeling knife to score the plastic near the inner edges of the frame. Do this before the plastic completely cools, as it will score more easily. Once the plastic and frame are cool enough to take off of the box, pop the mold out (if it will come out easily), and with the aid of a modeling knife cut and rip the plastic from the frame edges along the scored lines. Once done, put the formed plastic portion aside and loosen the wingnuts of the frame. It should be possible to remove the remaining "crust" of plastic from the frame by pulling it from the inside. Remove the removable bolts along one side of the frame, slide in a new sheet of plastic, replace the bolts, tighten the wingnuts, and you're ready to start forming another part. Certain long parts with near-parallel sides will not easily pop out, so this quick method will be slowed down by the effort needed to remove the mold. Some parts may turn out to be impossible to remove, so cut them carefully off of the mold and try again.

A special note about using metal molds. They should be preheated in the oven , then carefully taken out (use gloves or mitts!) and placed on the vacuum box before heating up the plastic in the oven (I must admit I do not know just how hot it must be, but it does not have to be close to the oven temperature. It may not need to be any warmer than 150 degrees, which may be enough to prevent forming problems while also not being too dangerous, no use heating the mold beyond what is needed). The reason is that if a metal mold is used cold, the hot plastic tends to cause air bubbles to expand and create blisters on the surface of the plastic. This tends to happen with metal molds due to their acting like heat sinks, while other types of mold, especially wooden ones, do not tend to have this problem. The metal mold does not have to be reheated in the oven every time you form a part, as it will remain warm or hot for a period of time before cooling too much, and will absorb an extra blast of heat every time a hot sheet of plastic is vac-formed over it. Another reason for warming the mold, a cold metal mold tends to cool and shrink the plastic so quickly it can be very difficult to remove the mold from the vac-formed part. By contrast, heated molds are among the easiest to remove formed parts from, as long as the part is not removed too early for it to be distorted.

More often than not some nose mold shapes do not tend to be removed easily form the vac-formed parts. This is especially true of nose shapes or other parts which have some parallel or near-parallel sides (by contrast conical shapes are the easiest to remove). The best technique developed for removing reluctant nose molds is to not try to pull straight back out, but to twist and pull. To be able to grasp the mold to remove it, most of my molds have holes drilled into the base of the molds so that 1/8" rods can be inserted into them after forming, the rods used as handles for twisting and pulling the mold out of the vac-forming. However, the locations of the holes for the rods must be carefully located so as not to cause wooden molds to split, especially fragile balsa molds)

Once a part has been formed over a mold, it is possible to "clone" the mold by casting one using the vac-forming as a female mold. Simply mount the whole vac-formed sheet (before removing the part from the scrap portion of the plastic sheet) upside down and pour in the casting liquid (polyester resin is preferable) which when hardened will create a duplicate of the original mold. Due to shrinkage, the resulting new mold will be slightly smaller, however, which may or may not be a problem depending on its use (not of much concern for egg capsules, very important for precise-fitting with the outside diameter of a body tube).

Trimming - For nose cones which were formed on a mold turned on a drill or lathe face plate, mount the mold again on the drill or lathe and put the part on the mold, with some of the excess plastic taped to the mold to hold the part in place. Carefully touch a modeling knife to the nose cone as it spins on the mold to score the plastic nose where it should be trimmed. This ensures the nose shoulder will be straight when completely trimmed.

 

Final Notes: Vacuum forming can do some parts well, and others poorly or not at all. It's great for some Sport Scale models but difficult for precision scale parts due to the problems of controlling the final thickness of the finished part, thus the external dimensions of parts made with male molds in this way (and with this crude type of vacuum forming). It takes some time to learn how to do small noses such as BT-20 or smaller. Wayne Hendricks has leanred how ot do thme fairly well, and have made an art of forming ultralight 30mm noses for FAI models, to the extent that all US Team members use "Wayne's cones" for their 30mm Parachute and Streamer models.

Here are some examples of what sort of parts have been made for certain models: A Sport Scale Delta main core nose cone, with 9 nose cones and 9 nozzles formed for the Castor strap-ons (models with multiple parts like this are very good vac-form candidates). It has been used to make dozens of custom shaped egg capsules for Eggloft altitude and Eggloft duration models.

It has also been used to make special parts for models of the Space Shuttle. Originally the parts were orbiter main engines and OMS pods (weighing about 5 grams total at 1/72 scale and formed from .015-.02 plastic). With the all-up shuttle model the parts list expanded to SRB noses and aft skirts, ET nose and aft dome, and the orbiter nose section. The orbiter nose was formed by using half shell molds from a plastic model kit. The orbiter nose will not form in one piece, so it was formed in two halves the same as the plastic kit and glued together with a reinforcement strip inside of the seam. The most significant forming was the ET nose, which is 4.6" diameter and about 7" tall, weighing 30 grams. Originally it was formed in two half-shells. More recently it was successfully formed in one piece but one side had to be carefully split to allow removing it from the mold.

More recently our team has used vac-forming to fabricate parts on the 1/39.5 scale Little Joe II's we have been flying. Some parts are obvious choices, such as the large BPC cone (or Capsule on earlier Little Joe II's without BPC's), Escape rocket skirt, and Escape rocket nose. All of the nozzles were also vac-formed, the 4 escape rocket nozzles, 16 RCS thruster nozzles, and the 4 Algol & 2 Recruit motor nozzles. Non-round shapes were also formed, the fin root fairings as well as the elevon actuator fairings on the fins.

Other potential uses for special models would be to create shells for unusual fuselage shapes such as the shuttle orbiter, ME-163, Natter, and lifting body models. It should even be possible to duplicate the old Centuri Saturn model wrap-arounds by pouring polyester resin in an unused wrap-around to create a male mold, then form over the new resin mold with thin plastic. For that matter, you can make up your own corrugation molds for vehicles such as the Ariane and Shuttle ET intertank. Imagination may be the only real limitation.

I hope this has taken some of the mystery out of vacuum forming. It really isn't all that difficult, and the results are usually well worth the effort. Be careful, and have fun making your own custom parts.