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SBS Vacuum Forming

By Roger Inwood

SAFETY

What you are about to attempt involves the use of hot equipment and plastic. DO NOT allow the plastic to ignite as it will give off highly toxic fumes. Please use this technique with caution and use appropriate safety equipment and common sense.

Introduction:

Vacuum-forming is a very handy and simple modeling technique to learn and master as it will allow you to produce shapes and forms not easily made using other techniques, for example the compound curves of front or rear fenders or truck cabs.

Overview:

Basically all that is done is to heat some sheet plastic till it becomes pliable then the sheet plastic is placed over the master pattern on a vacuum-box and vacuum applied which pulls the plastic down and around the master pattern creating your new part.

Equipment Requirements:

a. Vacuum cleaner

b. Vacuum box

c. Sheet holding frame(s)

d. Sheet plastic

e. Master pattern

f. Heat source (oven)

g. Raising blocks

The Equipment

Vacuum Box

First you will need a box, there are commercially available boxes but for around $20-30 you can build one yourself using basic tools with the advantage of a custom size to suit you.

The example shown in the photo measures 300x300x70mm (12x12x3") made from scrap MDF (or any wood laying around). The top plate on the example is perspex with multiple holes drilled for the vacuum (note the dowelling to support the centre). In the base of the box is a piece of tubing to fit the vacuum nozzle. The main requirement is that the internal or external joints are sealed.

Sheet Frames

These are to hold the sheet plastic while it is heating in the oven, the examples shown are MDF with small bolts epoxied to the bottom frame.

The deciding factor on frame sizes will depend on

A: Your oven size

B: What scale you work in, the largest frame I use is 300x300mm (12x12") but the 150x150mm (6x6") frames are used for the majority of forming tasks. If using a smaller frame than the Vacuum Box you will need to seal-off the vacuum holes on the box that are not covered by the frame.NB. the frames need to be made from a non-heat conducting material (wood/alloy) do not try using steel - it doesn't work.

Master Pattern

These can be made from virtually any material you are comfortable working with. Most Master Patterns I use are made from MDF blocks cut, carved and sanded to the required shape. The pattern does need to have a smooth finish with no undercuts.

One thing to remember with vacuum-forming is that you can not get sharp edges (corners).

Sheet Plastic

Before beginning, ensure you have a good supply of sheet plastic. For most vacuum-forming tasks I use .75mm (.030") or for large patterns I will use 1.0mm (.040"). I purchase the large commercial sheets 1500x900mm (5x3') from a local plastics company and is far cheaper than using Evergreen or Plastruct sheet. When buying, let the supplier know what you intend to do with it as some types of plastics are less suitable for home vacuum-forming (ABS Butyrate,Copolyester) the type I normally use is HIP (common styrene). From experience I have found that .75mm (.030") plastic sheet after forming will give a wall thickness of approx .5mm (.020") and 1.0mm (.040") gives a wall thickness of approx .75mm (.030"), this is dependant on the overall size of the intended piece.

The Process

1. Once we have made our Vacuum Box, Sheet Frames and Master Pattern its time to put the theory to the test,

2. Pre-heat the oven to 175 degrees Celsius (347degrees Fahrenheit)

3. While the oven is warming setup the Vacuum Box and Vacuum cleaner as close as safely possible to the oven with the Master Pattern centered on the box

4. Place the sheet plastic (in its frame) in the oven on some raising blocks (fire bricks/green wood) this is to allow for the heating sag. Also place a large tray on the bottom rack, just in case the plastic "gets away" (becomes molten).

5. Watch, watch and watch some more, while the plastic is heating DO NOT wander off or answer the phone, just watch the plastic heating. At first it will draw tight in the frame then slowly it will "relax" and then start to sag, unfortunately this is where practice is the only teacher, but for the 150x150mm (6x6") frames you can expect at least 30-40mm (1-1.5") of sag. This happens slowly so don't panic

6. Quickly (but carefully, the frames do get warm, I use padded gloves) remove the frame from the oven and centered over the Master Pattern place down on the Vacuum Box

7. Turn on the vacuum cleaner and watch amazed as the plastic is drawn down and around your Master Pattern, turn off the vacuum cleaner.

8. Remove the raising blocks from the oven and switch off the oven

9. Allow the plastic and frame to cool a bit more.

10. Now you can remove the plastic from the frame and start trimming. The Master Pattern will be very tight in the plastic so careful trimming is required to remove it without damaging the Master or the new part (Handy Hint - I often place a woodscrew in the base of the Master after vacuum forming and place the screw head in a vise to assist in pulling the formed plastic away from the Master)

11. If the plastic did not pull down completely over the Master you can sometimes place the same sheet frame back in the oven and reheat it and try again. Otherwise its try again with a fresh sheet and try leave it heating a little longer.

Conclusion

No doubt there will be some trial and error involved when first learning this technique, please persevere as the end results are well worth the effort. Now you have no excuse not to build that 1950's engine with its gracious flowing curved fenders, or those stepped rear fenders for the 1920's chain-driven vehicles. Of course the true advantage of Vacuum forming is that once formed the part will retain the new shape and not try to flatten/straighten out as happens when just bending flat plastic sheet into a new shape.

While various books and Internet sites use other variations of the technique ie. different box or frame styles, different temperate ranges etc the method explained here are the results that I have found to work for me (I've used this methodfor 9 years now), working in 1/35th - 1/24th scales. The main thing is to have a go, then adjust and experiment with the technique till it suits you and the parts you want to create.

Best of luck to those that wish to give it a go.

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