The Vacuum Forming Process
This is the process of moulding extruded plastic sheets heated to a pre-determined temperature. The sheets are heated and then sucked down by applying a vacuum to the underside of the mould, atmospheric air pressure then forces the heated plastic to adopt the shape of the mould.
The Pressure Forming Process
As above but with the addition of applying compressed air to the outer surface of the plastic sheet. More detail can be achieved with this process with the compliments of Mark Hipgrave of MHP Industries Limited, but tooling is more complicated and expensive.
Guidelines for Designing Parts
1) Material Flow and Webbing
When you start to design your part imagine that you are draping a cloth or a sheet of paper over the part. This will give you a good idea of where the problems will be when forming, i.e. showing how the corners will pleat/web during forming.
Tip! Understanding webbing. Take a cigarette packet and try forcing a sheet of paper over it. This will show where webs will develop! Sharp corners are to be avoided as this will hinder the flow of material and maximise the chance of webbing.
Tip! Always use the largest radii or chamfer possible.
These terms relate to the amount of angle on a vertical face to allow the parts to be removed from the tool (imagine how difficult removing a bucket from a sand castle would be with vertical sides).
Good Practice as Picture above
- Allow as much draft as the design will accommodate.
- Male moulds min 3°
- Female moulds min 1°
- Large radii on all corners
Tip! As the plastic cools it will shrink onto a male tool but away from a female tool hence the less draft required on female tools.
3) Draw Ratio/Required material thickness
Draw ratio refers to the height of the part to be formed compared to the footprint of the part (back to the sand castle i.e. Height of sand castle compared to diameter). The taller the part the thinner the material required to cover the whole part.
Tip! Avoid tall narrow areas which cause parts not to release from the mould.
The draw ratio is important to understanding the process with the compliments of Mark Hipgrave of MHP Industries Limited. The 3-dimensional part has a surface area that needs to be covered by a flat two-dimensional sheet. The sheet is heated and formed over or into a mould. It will then stretch to cover the mould and as the sheet stretches, it thins out.
The draw ratio can be calculated but this is only to be used as a guide as every shape will react slightly differently on the flow and distribution of the material.
Draw Ratio Formula
Total surface area of the part A is divided by footprint of the part B.
Draw Ratio A = 200cm2 ÷ 1002 B = 2cm2
Desired thickness = 3mm
Draw Ratio = 2
Material starting thickness required 3 x 2 = 6mm
- Keep the part as shallow as possible. The taller the part the thicker the material required.
- Avoid areas which are narrow and tall as parts will thin too much on forming and split on ejection and cause webbing
Texture can be applied to the parts in 2 ways.
- Use material with texture applied and mould over a male tool (low cost but limited textures are available).
- Use a female aluminium mould which has been textured. This can be expensive with the cost of texturing a mould, 40cm x 40cm x 15cm being in the region of £1000 (best option when using pressure forming as more detail is achieved).
Tip! More taper will be required on textured moulds to avoid the texture scraping on ejection.
5) Pockets and Ribs
Any pockets or space between ribs must conform to a ratio. Depth of pocket or space between ribs cannot be greater than its width and preferably 75% of the pocket depth.
Tip! Two 10mm high ribs must have 10mm spacing between them as minimum.
6) Controlled Surfaces
It is important to consider the controlled surfaces. The controlled surface is always the tool side of the part. This is important for 2 reasons: -
- If 2 parts are to fit together, the only way of ensuring the fit is to have 2 controlled faces meeting each other.
- When trimming on a 5-axis router the parts are usually held on a jig locating the moulding on the controlled face.
Tip! Consider how the part will be trimmed during design as this can make a large saving on the unit cost price.
7) Ribs and Bosses
Ribs can be used to support a flat surface and are created separately and fixed onto the original part. This is the same for internal bosses, which are possible to machine separately and attach with a solvent adhesive on most materials used. Bosses can also be applied with metal inserts for fixing circuit boards etc.
8) Tolerances and Accuracy
Whenever possible allow as large a tolerance as possible.
When using CNC machined aluminium tools use tolerances ± 0.25mm for the first 25mm and ± 0.1 for every 100mm thereafter (Note this is only a guide and will not apply to all materials and designs).
- Large tolerances will help reduce unit costs and speed up deliveries.
- Consider materials to be used as some materials have vastly different contraction rates on forming.
- Be careful with tolerances on PP and HDPE as shrinkage can vary by 2% or more.
Toolmakers Guide to Vacuum Forming
Things to consider
Always allow as much draft as possible. Most of this will be gained by the design of the part, but consider areas which will be disposed of after trimming, as taper up to 45° can be added in these areas.
Tip! Question why a face has no taper! Ask if taper can be added.
2) Over Run
Tools that will be horizontally trimmed must be moulded over depth. This will vary on material and tool design but as a rule of thumb 12mm + material thickness is acceptable.
Tip! Consider the material being used prior to determining the overrun.
Tools can never be made too strong. The forces in Vacuum forming are generally underestimated. Force is 14-7 PSI.
Tip! This can add up over large areas. For example, an area 30cm x 30cm when under vacuum will be nearly 1 Tonne.
Sharp corners are not acceptable to a vacuum former and whenever possible use as large a radius as possible. This applies to both male and female tools.
- No vacuum former will complain that a radius is too large. This will strengthen corners.
- Corners with radii will make for stronger mouldings.
- Square corners create more webbing on the part.
Tools should be finished to a 120 grit in general.
Vertical corners should have finishing lines in the direction of draw.
6) Venting/Shot Blasting
- Consider where to put vents.
- Use small vents on visible areas 0-75mm on vac tools and 0-5mm on pressure form tools.
- Large flat areas may require shot blasting to help air evacuation.
- All pockets, recesses and corners will require venting.
- Consider areas where air may get trapped.
- Vent spacing 25-50mm between centres.
- Tracked or slot venting can be used on the base of tools but consider cooling if a solid aluminium tool.
- Washers/spacers can be used between tool and plate but not if on a water cooled base/plate.
- Tall areas should be vented at the top for ejection.
Tip! A vacuum former will never complain about too many vents.
7) Tool Fixing to a Base Plate
Tools should be fixed down to the base plate firmly.
- Aluminium tools min. fixing 6mm threads.
- Resin tools min. fixing 8mm threads.
- Wood/Proto tools size no 8-10 wood screws.
Tip! Plastic shrinks onto a tool after forming so do not underestimate the forces on ejection trying to pull the tool off the base plate.
8) Tools for PP/HDPE
Wood tools are not suitable for moulding materials as the plastic goes wax like and gets stuck in the grain of the wood during forming.
Aluminium tools require a medium to coarse shot blast to avoid air entrapment on the mould surface.
Tools used for PP and HDPE must be plinthed up by approx. 25mm (this is to stretch the material as being formed). This is necessary due to the nature of the material stretching excessively and wrinkling when heated.
9) Tools for Polycarbonate
Wood, Resin or Aluminium tools can be used for moulding polycarbonate. Aluminium tools will require heaters to be fitted so tools can be run hot at approx. 90 deg C. The temperature must be controlled using a commercially available temperature controller along with a thermocouple fitted to the tool.