• Introduction to vacuum forming-

Prasanna Gunasekaran

Definition

It is one of the type of the Thermoforming Process in which vacuum is used to get the desired shape.

Vacuum Forming

The process involves heating a plastic sheet until soft and then draping it over a mould. A vacuum is applied sucking the sheet into the mould. The sheet is then ejected from the mould.

Process

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Process in Detail

ClampingThe clamp frame ensures the plastic sheet is held firmly in place during the forming process.

HeatingRadiant heaters are normally used to heat the sheet which has been positioned over the aperture of the vacuum forming machine. For thicker sheet both surfaces may need to be heated and more sophisticated machines allow this. Heaters move into position both above and below the sheet.

Pre-stretchIs used to achieve "even" wall thickness. Air is introduced to blow a small "bubble" and the mould is then raised into the pre-stretched sheet.

VacuumA vacuum is applied, the sheet is drawn into intimate contact with the mould and the mould detail is picked up.

Plug AssistWhere a deep draw is required a top "plug" may be used to push material into the mould during the forming process.

Cooling and ReleaseThe material is allowed to cool. The cooling process may be shortened with blown air or even a fine water spray. The molding may then be released from the mould by introducing a small air pressure.

FinishingAfter molding, any mould finishing may be performed, trimming, cutting, drilling, polishing, decorating etc.

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IMPORTANT CONCERNS IN VACUUM FORMING

A.PART DESIGN

B.TOOLING DESIGN

C.MATERIAL SELECTION

D.PROCESSING TECHNIQUES.

A. PART DESIGN

This is probably the most important consideration in getting a good part that will perform the function you want it to.

Size - is definitely a consideration. If a part is too large for your equipment, that is a problem.

Shape - There are certain shapes that do not lend themselves to vacuum forming because the draw ratio in certain spots and sometimes the entire part become too great. Varying the wall thickness as in injection molding is impractical too.

Draft - Draft is a primary consideration on part design and it depends upon whether the mold is a male or female mold.

5 degrees – male tool 3 degrees – female tool

Undercuts- Should be avoided as far as possible. If required should be given for small dept and at proper location of the mold. It makes the mold design very complicated.

Radii- Generally speaking, the amount of radius is related to the size of the part. It also depends the type of the moldAnd thickness of the sheet.

Texture- Generally textured sheet is used. Regular pattern is generally avoided as it gets distorted after forming. Fine graining is generally used. Increase in grain height will affect the change in thickness.

Part Integrity & Functions- Attention has to be given while designing the part so that it can have strength in built. Mounting points has to be decided so that load will be distributed equally.

Cups Wheel GuardPlastic Trays

Tolerances- Tolerances has to be given on the part depending upon -Type of Mold-Type of material-Type of cooling -Method of trimming

B. TOOL DESIGN

The first thing we have to determine is what type of material are we going to use and what tolerances are expected.

Type of Molds – Male & FemaleDecided depending upon which side of the part is important.

Male Mold –used when inside dimensions have to be Controlled and outside surface is A surface.

Inside dimensions to be controlled

Outside A surface

Outside dimensions to be controlled

Inside A surface

Female Mold –used when outside dimensions have to be Controlled and inner surface is A surface.

Materials for the MoldDepends upon number of parts to be produced, cosmetic use of part, tolerances to be given

WoodWood is the simplest type of material used in vacuum forming. The number one benefit is the cost. It is cheap! It can also be milled or fabricated into a shape pretty easily and it is very readily available. Most patterns to make a thermoforming tool are made from wood.

•Wood is an insulator and cooling the part on a wooden tool will dramatically lengthen the cooling cycle,•The cooling will be uneven depending on the finished thickness throughout the part. This can cause stress in the part and in some serious cases, warping.•Sometimes the grain in the wooden tool will transfer onto the plastic and if the material is thin and the grain in the tool is heavy, it will even transfer through to the cosmetic surface. wood has moisture in it. If you run these molds in production, they will need a good deal of maintenance because they will dry out and crack.•There are a limited number of parts you can pull on a wooden tool.

POLYESTER or FIBERGLASS

This type of tooling is considered permanent tooling for lower volume projects. Whenever, you have a job that requires less than 2000 parts per year, you should consider it as a viable alternative. It too has limitations but it also has advantages. Again, the first advantage is price.

EPOXY TOOLINGThis is also a very popular way to make tooling and it also has some advantages and limitations. It too is considered permanent tooling but the volume of parts you couldexpect to form off of this type of tooling is quite high, that is, in the thousands.

ALUMINUM TEMPERATURE CONTROLLED TOOLING

This type of tooling produces the best and most consistentpart you will get in the thermoforming process. Because the temperature is totally controllable, you can take the part off of the tool each time at the same temperature and expect the shrinkage to be very similar from part to part. A second plus is the cycle time. This is the fastest way you can make a part in thermoforming. However its very costly.

C. PROCESSING

Processing is the third major area that needs to be addressed to assure that acceptable and successful parts are made.

Equipment Function1. Platens - Platens are nothing more than a well-stabilized and securely built

framework to move a mold up or down.2. Clamp frames - As we indicated earlier, you have to have some method

of holding the plastic sheet while you have it in the oven heating up. This is

done with a clamping frame.3. Forming Ovens - A forming oven is nothing more than a chamber with a

heating source.

Acrylonitrile Butadiene StyreneABS

Polyester Copolymer PETG

Polystyrene PS

Polycarbonate PC

Polypropylene PP

Polyethylene (sheet and foamed sheet) PE

Polyvinyl Chloride PVC

Acrylic PMMA

Typical Materials Used for Vacuum forming

Vacuum forming offers several processing advantages over other forming processes. Low forming pressures are used thus enabling comparatively low cost tooling.

Since the process uses low pressures, the moulds can be made of inexpensive materials and mould fabrication time can be reasonably short. Prototype and low quantity requirements of large parts, as well as medium size runs therefore become economical.

More sophisticated machines and moulds are used for continuous automated production of high volume items like yoghurt pots, disposable cups and sandwich packs.

Unlike other thermoplastic forming processes, where powder or resin are the starting point, vacuum forming uses extruded plastic sheet. With vacuum forming a secondary process may be required to trim the formed sheet to arrive at the finished part. The trimmed waste can then be re-ground and recycled

Nature of Use and Limitations

Applications of vacuum formed parts

Typical Products Produced

Baths & Shower Trays Yoghurt Pots

Ski-Boxes Boat Hulls

Machinery Guards Vehicle Door Liners

Refrigerator Liners Sandwich Boxes

Parts of vehicle cabs Exterior Shop Signs

Advantages & Limitations

Advantages – Low tooling cost Less lead-time Clean process as sheets are directly used different thermoplastic materials can be used Good for prototyping Pre-colored sheets can be used.

Limitations – Design flexibility Thickness of the sheet Thickness variation within a part is not possible Difficult to control the A surface Small radii are problems Difficult to maintain the grain structure Only thermoplastic materials can be used

Modern Vacuum forming machine

Heater

Clamps holdingsheet

Dashboard Door Trim

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