Vacuum forming, is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mould by applying vacuum between the mold surface and the sheet.Normally, draft angle must be present in the design on the mould (a recommended minimum of 3°), otherwise release of the formed plastic and the mould is very difficult.
AP Hollings are a company with 50 years experience of vacuum forming and we are able to provide a one stop solution for your vacuum forming of plastics, and plastic fabrication requirements. We offer a comprehensive design service and in conjunction with our in house tooling facilities and our CNC machine shop, provide a very cost effective solution for all of your prototyping or production requirements. Working with all types of plastics we can advise you on materials and tooling methods to help you make the best choice and route to manufacture. In conjunction with vacuum forming and plastic fabrication services we are also able to provide added value through sub-assembly and painting. Normally, draft angle must be present in the vacuum form tool design, we recommended minimum of 2º otherwise release of the vacuum formed plastic part from the tool is very difficult.
This article is intended to give an insight into the vacuum forming process.
A vacuum forming machine consists of a single or double heating element, a frame for holding the plastic, a platen to hold the tool or pattern used to make the shape of the part you wish to produce and a vacuum tank. The heaters are usually at the back of the machine and the clamping frame and platen at the front. The platen sits within the frame and can be moved up and down where as the clamping frame is in a fixed position for holding the plastic during the moulding process. Some machines are manually operated and others are semi automatic or fully automatic.
The object of each type of machine is the same in that they all heat plastic materials until soft enough to mould over a tool, pattern or mould, under vacuum. When the plastic is under vacuum, the suction achieved is around 28 HG about 1% less than what might be considered as ultimate vacuum. The tool can be made using various materials providing that they are strong enough not to collapse when under vacuum.
The surface of the tool will reach around 60 to 80 degree’s when used in production. Different plastics require different temperatures to become soft enough to mould. Materials such as High Impact Polystyrene and ABS plastics will soften at around 160/170 degree’s and Polycarbonates at around 180/190 degree’s. The design of the tool is important to, you need to build into your design a draft or taper down the sides of two degree’s or more and add radii’s to all sharp corners.
(The bigger the better for ease of forming) There is a rough formula for moulding recesses or pockets that might be needed in the tool. The width and length of the recess should be twice that of the depth of the recess to allow the material to be drawn down into the pocket without the material becoming so thin that it splits when under vacuum. Materials such as High Impact Polystyrene and ABS plastics will soften at around 160/170 degree’s and Polycarbonates at around 180/190 degree’s. Once the tool has been designed and made with the points as shown above in mind you are now ready for moulding. The tool is fastened to a board which is in turn fastened to the platen. The platen is lowered and a plastic sheet is placed onto the frame and clamped in position.
The heater or heaters are brought over the plastic and left in position until the plastic becomes soft and pliable. The heaters are then withdrawn and the tool brought up into the plastic and the vacuum switched on to suck the plastic around the mould and remove all of the air between the tool and the plastic thus creating a replication of the surface of the tool. Whilst the vacuum is on, the sheet is cooled using compressed air until it becomes solid and retains its shape.
Once this has been done, the tool and platen can be dropped down and the plastic removed from the clamping frame ready for trimming. You can mould as many parts as needed providing that the tool has been designed and made using the right materials for the quantities required. Materials such as High Impact Polystyrene and ABS plastics will soften at around 160/170 degree’s and Polycarbonates at around 180/190 degree’s. Information about materials and their specifications and uses can be found on various web sites.
Vacuum Part Finishing
All trimming and router fixturing are designed and made in house, this includes suction cup designs, for use to complete our customers parts. Paint finishes can be applied, to achieve colours not available is sheet form. We also undertake full assembly of vacuum formed parts together with client free issue parts and hardware sourced by us. The assembly fixture used are designed and built in house. These concepts are also available as a purchased service and assembly aids and checking fixtures can be supplied on request.