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VACUUM FORMING - Hardware

 

Introduction

    How Vacuum Forming Works

Materials

    HIPS

    PETG

Vacuum Box

    Frame

    Base

    Perforated Board

Moulds

 

Previous Page -  Vacuum Forming Introduction

Next Page – Project 001

 

 

Introduction

These pages show how to build a vacuum forming box, which uses a domestic gas oven and vacuum cleaner, to make parts for radio controlled aircraft.

How Vacuum Forming Works

The vacuum box consists of a board with an array of holes drilled through most of its area which sits on top of a box which is connected to the vacuum cleaner.

When the vacuum cleaner is on, air is drawn down through the holes. A mould is placed on the perforated board and a frame holding a heated sheet of polystyrene is placed over the mould and lowered onto the perforated board. The low air pressure under the polystyrene sheet causes the ambient air pressure to form the sheet around the mould. The polystyrene quickly cools and retains the shape of the mould. The moulded part is then removed from the frame, trimmed, assembled if necessary and painted if required.

Excellent results can be achieved with some low-tech equipment and a bit of practice.

 

 

Materials

There are two types of material I use for vacuum forming. They are HIPS (High Impact PolyStyrene) and PETG (Polyethylene Teraphthalate Glycol which is a type of Polyester). I much prefer HIPS.

 

HIPS

HIPS comes in sheet sizes 1370 x 760 mm and in a variety of thicknesses. It is supplied in only black or white and both have a shiny side and a matt side. I have successfully used thicknesses from 0.25mm, to 1.5mm.

From these sheets, I can cut 9 sheets sized 390 x 250 mm (full sheet) and 3 sheets 195 x 250 mm (half sheet) with very little wastage. These sizes not only divide the supplied sheet with minimal wastage, they are convenient sizes to fit the inside dimensions of my gas oven while held in a wooden frame.

 

The 0.5mm supplied sheet costs about $6.00 in small quantities so if I make a part from a half sheet, the material cost is about 29 cents!

A minor inconvenience of HIPS is that most types of paint do not stick to it. This problem is easily solved by spraying on a plastics primer first. I use ‘Flexi Prime’ made by HiChem Paint Technologies in Hallam Victoria.

The melting point of HIPS is 240°C.

 

The best glue to use on HIPS is a plastics glue type ‘Contacta Professional’ made by Revell. It is commonly available from hobby shops. It forms a very strong bond between two HIPS parts because it dissolves each surface and dries quickly with the two parts ‘merged’ together. It allows up to about 20 seconds of slide before the parts bond.

 

HIPS is available from Profile Plastics in Bayswater, Melbourne or Mulford Plastics in South Dandenong, Melbourne.

 

PETG

PETG has a complicated name but it is just the stuff that soft drink bottles are made of.

PETG is supplied as a clear sheet sized 2440 x 1220 (8’ x 4’). I have used 0.5mm thickness.

 

The 0.5mm supplied sheet costs about $50.00 so if I make a part from a half sheet, the material cost is about 93 cents.

PETG does sag a lot when it is heated. When you see it sagging below the frame in the oven, it is ready to take out and place on the mould.

A disadvantage of PETG is that it absorbs moisture from the air while it is stored. Before it is used for vacuum forming. It must be warmed up first to remove the moisture. If this is not done, the moisture under the surface creates blisters when it is heated in the oven. Also, it is not as good as HIPS at wrapping around large angle curves without forming unwanted fillets.

 

PETG is also available from Profile Plastics in Bayswater, Melbourne or Mulford Plastics in South Dandenong, Melbourne.

 

 

Vacuum Box

Frame

The frame is made of two parts the same size, clamped together by screws and tightened by wing nuts.

The outside edge dimensions of the frame which holds the polystyrene sheet are determined by the dimensions of the inside of the oven when the frame sits on the top shelf. The frame should slide onto the shelf with about 10mm clearance on each side.

The dimensions of the cut out section in the middle of the frame should be about 15mm inside the dimensions of the polystyrene sheet.

Install ¼” screws with counter sunk heads up through holes in the lower frame. The heads of the screws should be flush with the underneath surface of the frame. Glue all the screws into frame except those along one side so that sheets can be inserted and removed without removing all the wing nuts.

 

Full sheet frame.

 

 

 

Half sheet frame.

 

Dimensions of full sheet and frame.

 

Dimensions of half sheet and frame

Base

The dimensions of the box are determined by the frame which holds the plastic sheet. Make the box with the same outside dimensions as the frame.

The base of the box can be an open top box made of a material that is strong enough to handle the vacuum created by a domestic vacuum cleaner. I use a 1600W vacuum cleaner and it works well.

12mm particle board is strong enough to use.

To test the box, place a board on the box which has a foam seal around the edges of the underneath of the board. The foam tape should be a ‘closed cell’ type so air cannot pass through. Attach and turn on the vacuum cleaner and check that the box does not fail.

Perforated Board

The perforated board is made of 12mm thick particle board whose dimensions are the same as the frame.

 

Drill a grid of 1mm diameter holes through the board over all the area inside the foam tape. The spacing of the holes should be 10mm or less. The closer the spacing, the more holes required but the better the results will be.

Around all the upper and lower edges, place adhesive backed foam tape to form a seal with the base and with the frame when it is placed on the board.

A back board is attached to assist in placing the frame quickly and accurately on the perforated board.

 

 

 

 

Base and top of the vacuum box

 

 

Moulds

Moulds are made from a variety of materials including balsa, plaster and any soft wood that is easy to shape like pine. Note that any imperfections in the mould may show in the surface of the moulded part. Wood grain is an example of this. A smooth surface can be created on wooden moulds with a 1 to 2 mm layer of wood filler which can be filed or sanded smooth.

 

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When making the mould, ensure that the mould extends beyond the edges of the final size of the part. Design the mould so that the part is trimmed along a flat surface instead of on a corner. Internal corners tend to block off vacuum as the polystyrene sheet shrinks around the mould. To overcome this, create a cavity under the base of the mould and drill holes through to the cavity from the internal corner. This will ensure all of the air in the corner is removed and the sheet is drawn all the way into the corner. Where possible, make sides on an angle instead of making vertical sides. This helps when the mould is removed.

 

 

 

 

 

Ensure that when a mould is designed that it can be removed from the formed part after the forming process. If there are overhangs on the mould, removing it may be difficult.

 

A mould can be copied so that two or more moulds can be used with one polystyrene sheet. To do this, first make a part with a single mould.

Copy a mould by pouring plaster in the part and waiting for it to set. Many moulds can be made in this way.

With many identical moulds, many parts can be made with one sheet.

Vacuum Forming Procedure

Caution! It is recommended to use gloves to prevent being burned when removing a frame from the oven. Wooden frames usually do not get very hot but metal screws and wing nuts can!

 

Set up the vacuum box next to the oven with a mould placed in the centre of the perforated board.

Place the polystyrene in the frame and tighten the wing nuts.

Preheat the oven to 230°C for about 10 minutes.

Place the frame in the oven on the top shelf (hottest part of the oven) and start timing.

I use the heating times shown in this timing table.

 

Timing table

Material

Sheet Thickness (mm)

Heating Duration @ 230°C

HIPS, white

0.25

30 sec

HIPS, white

0.375

45 sec

HIPS, white

0.50

1 min

HIPS, white

0.75

1 min 35 sec

HIPS, white

1.0

2 min 10 sec

HIPS, white

1.5

2 min 40 sec

HIPS, black

0.75

1 min

PETG

1.0

1 min 45 sec

These times may vary with different ovens. Experiment to get the best result.

 

Turn on the vacuum 5 to 10 seconds before the heating time is complete.

When the time is up, quickly remove the frame from the oven and place over the mould and lower onto the perforated board.

As soon as the frame is on the board, the polystyrene contracts around the mould.

Leave the vacuum running for 10 to 15 seconds while the polystyrene cools.

Turn off the vacuum and wait for the vacuum motor to stop. Lift off the frame and the mould may still be inside the polystyrene.

Remove the mould and the part is ready for trimming.

 

 

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Next Page – Project 001