Vacuum forming machines consists of a single or double heating element, frame for holding the plastic, moving platen or table 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, whereas the clamping frame is in a fixed position for holding the plastic over the tool 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. These materials include:- wood & MDF for prototype or low volume, through to resin tooling board or cast resins for mid volume parts, up to aluminium either machined or cast for high volume The latter would depend on depth of feature and thickness of material and hence heat required to soften material prior to forming. All tooling would be vented to vacuum side by small 0,5-1,0mm dia. holes around the tool and at key draw areas and features to pull the plastic to form over the tool.
The surface of the tool will reach around 60 to 80 degree’s when used in production. Various plastics require different temperatures to become soft enough to mould. * Materials such as High Impact Polystyrene and ABS plastics will soften at between 150/190 deg C and Polycarbonates at around 185/205 deg C*. The design of the tool is very important, you need to build into your design a draft or taper down the sides of 2 degree’s or more and add radii to all 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. Once the tool has been designed and made with the all foregoing 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, timing is critical, so checking and recording for repeatable quality forming is essential. The heaters are then withdrawn and the tool brought up into the plastic, the vacuum switched on to suck the plastic around the mould to 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 fans and or 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 to obtain finished part. 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. * Information about mechanical properties of vacuum forming plastic materials and uses can be found on various plastic manufacturers web sites.