Vacuum Forming

Vacuum forming is a process in which a thermoplastic sheet is heated to the appropriate temperature, stretched around or into a mold/pattern, and conformed to the mold by applying vacuum pressure between the mold surface and the plastic sheet. Because heating of the material to be formed is required, vacuum forming is considered a thermoforming process.

Overview

What is vacuum forming?

Vacuum forming process

Vacuum forming process

Vacuum forming is useful for creating shells of fairly uniform wall thickness. Usually – but not always - it is restricted to forming parts that are relatively shallow in depth. Example objects manufactured by this process are shown below.

blister package

clamshell package

disposable cup lid

decoration

Equipment

The Green Machine

• Location: The plastics room between the foundry and the welding room
• Allowable plastic sheet sizes: 16" x 16" or 2' x 3'

Allowable Plastic Materials

A thickness of 1/8" (0.125") or less is recommended for most materials. Common plastics, in order of preference, are:

1. Polystyrene: high impact grades are best; is usually opaque white; tends to yellow with age; is paintable; solvent bonds well; is cheap.
2. PETG: good toughness and formability; is transparent; looks cool to paint just the inside and leave a very glossy outside surface. Excellent substitute for Lexan (polycarbonate).
3. ABS: has good strength (slightly less tough than Kydex); is opaque, usually black or white in color with texture; machine grade is buff in color with no texture; paintable; solvent bonds well.

Recommended, with reservations:

1. Kydex: PVC-based plastic, good strength and toughness, finishes well, many colors, expensive, special solvent needed to bond, needs approval before use.
2. Acrylic (extruded, not cast): variety of colors, both transparent and opaque, takes longer to heat up, less consistent results, solvent bonds readily, paintable.
3. Polyethylene/polypropylene: Translucent to opaque, waxy, can be used for living hinges. Can become clear when heated but will return to milky appearance on cooling. Heats unevenly. Welds to itself readily, ideal for closed forms. Paint does not stick to these materials. Very difficult to form.

Polystyrene

Kydex

PETG

Mold (called "pattern" or "buck")

Bucks can be any material that does not melt, deform, or outgas toxic fumes at the thermoplastic forming temperature. Common buck materials, in order of preference, are:

1. Modulan: recommended for ease of shaping and sanding; can be created with a good surface finish; is available for purchase in the PRL Model Shop.
   

   REN Shape

2. Medium Density Fiberboard (MDF): is not particleboard; is an inexpensive alternative to modulan; compared to modulan, it doesn't cut as well or hold as fine a level of detail, but is useful for large patterns.
   

   MDF

3. Pink Foam: useful for forms that are organic or otherwise hard to mold out of harder materials. Must be sealed (with something like gesso) and primed (can be anything from auto primer to epoxy to appliance enamel -- anything wet-sandable) before using as a buck. If it's a large form, it's a good idea to mount it on a board of some sort so it doesn't deform with the platform when the vacuum is pulled.

Vacuum Forming Process

Buck/Pattern Preparation

Consider the following questions and tips before making the buck:

Questions

• How will I remove the buck from the plastic part?
• If there are mating parts, how do I accomplish this? Will the mating seam affect the appearance of the product?
• How will I trim the part (including holes and cutouts)?
• Is it acceptable for the plastic part to have radii at the interface where the buck meets the table? In order to minimize this radius, tiny holes can be created to pull air at the horizontal/vertical interface. This radiusing issue can be eliminated altogether by building a 'riser' (or standoff) into the buck and trimming the plastic at the appropriate location. See the figures below.

Tips

• A minimum draft angle of 3 degrees is necessary on all sides of the buck to remove the part after forming.
• The maximum height of a buck is limited to the vertical distance between the table and the frame which is about 10".

Other Rules of Thumb:

• The maximum height of draw for a protrusion is 3 times the width, provided there is enough space between the pattern and the frame.
• The maximum depth of draw for a depression is as deep as it is wide at its narrowest point.
• The minimum size of a plastic/frame: There should be at least as much clearance on all sides of your buck as it is high. For example, to make a 6" cube, a sheet size of 20" x 20" is required: 18" for the part plus 1" extra all around for the frame.
• Depressions and fine details may need extra tiny air holes for pulling the plastic sheet to the buck.
• Sharp corners and tight radii should be avoided, as they may tear the plastic.
• Material tends to bridge across fine detail.
• Any texture in the surface finish of a pattern will be reproduced in the plastic.
• Large flat areas will sag. Give such areas a large upward curve or use details to stiffen them.
• Complex parts can be created by bonding several simple parts with solvent.

Vacuum Forming Process

NOTE: Results from the vacuum forming process are effected by many variables: plastic type, thickness, buck geometry, ambient temperature, vacuum pressure, vacuum pulling speed, etc. Expect at least 3 tries (more for complex geometry) to get the desired results. Purchase material accordingly.

levers and switches on vacuum forming machine

1. Connect the red hose from the vacuum forming equipment to the compressed air from the wall. Be cautious since the compressed air will kick out the hose when connecting/disconnecting.
2. Make sure the perforated platform fits within the 16" x 16" frame by aligning it along markers.
3. Lower the platen, then mount and clamp the thermoplastic on the 16" x 16" frame. Next clamp the 2' x 3' frame using C clamps on all 4 corners.
   

   compressed air hose

   

   platform alignment

   

   clamps

4. Place the buck on top of the perforated platform. Cover the perforations with paper around the buck in order to increase the suction, if necessary.
5. Turn on both top and bottom heaters by flipping on the switches. Both duty cycles should be at 100%. Wait for at least 10 minutes until both heaters stabilize at glowing red. This allows you to repeatably measure the time required to heat up the plastic.
   

   buck on platform

   

   covering perforation

   

   heater switches

6. Slide the frame all the way into the oven till it stops.
7. Make sure that the vacuum lever is "off", and turn on the vacuum switch. The compressor turns on and the vacuum is created. The ideal vacuum pressure is 25 inHg, which typically takes about 2 minutes to achieve. (NOTE: depending on the material and thickness of the plastic, you may have to turn on the vacuum before you heat the plastic.)
8. Wait for the plastic to heat up, soften, and eventually sag. Sag can best be seen from the bottom. The ideal amount of sag depends on various parameters: height, geometry, and complexity of buck, plastic material and thickness, etc. Too little sag (i.e. stiff plastic) results in plastic bridging over details, and too much sag (i.e. very soft plastic) results in webbing and tear. THIS IS WHY MULTIPLE ITERATIONS ARE NECESSARY!
   

   frame inside oven

   

   vacuum lever and switch

   

   sagging of plastic

9. When the plastic is ready, slide the frame towards you until it stops, raise the platen till the plastic seals around the table, and pull the vacuum by turning the vacuum lever to ’on’. This sequence happens very quickly since the plastic will cool down and stiffen fast. If you pull the vacuum lever too fast, webbing will occur.
10. If you have a form follower, use it to stretch the plastic over the buck.
    

    vacuum pulled

    

    webbing

    

    form follower

11. Allow the part to cool for a couple minutes. If it's too hot to handle without gloves, it's too early to lower the platen.
12. Turn off the vacuum pump, close the valve (turn the yellow handle to "off"), and lower the platen. If the pattern is stuck, you can try prying the pattern out, whacking the plastic over a garbage can, or using compressed air. Also, holes for screws can be tapped into the pattern that allow you to pull on the screws and extract the part.
    

    buck stuck on plastic

    

    screws to help remove buck

    

    handles to help remove buck

13. Once the buck is removed from the plastic, cut the plastic around the part and staple the part to a piece of board.
14. Clamp a resawing jig to a bandsaw table. Using the resawing jig as a guide, cut out the part.
    

    vacuum formed plastic

    

    staple part to a board

    

    bandsaw the cut part

15. Smooth the edges with sandpaper if you need to bond other parts.
    
    
    

My part sucks! Causes and potential solutions for less than ideal results

• Problem: Webbing of the plastic.
• Potential Causes: The plastic was too soft (too much sagging, waited too long); pulling the vacuum lever too fast; the buck was too tall relative to the width.
• Potential Solution: Adding a riser the the buck.
• Problem: Plastic not conforming to the shape of the buck.
• Potential Causes: The plastic was too stiff (too little sagging, pulled out too quickly); the vacuum was not strong enough (less than 25 inHg); the buck is too tall.
• Potential Solution: Use a form follower.
• Problem: Buck stuck in plastic
• Potential Causes: There are undercuts; not enough draft angle; surface finish too rough on the side walls
• Potential Solution: Soften plastic with heat gun; add screws into buck; rework buck for next run.