Vacuum forming is an effective process for shaping plastics. A vacuum forming machine heats a sheet of plastic, stretches it over a mold, then pulls it into shape with the help of a vacuum pump. This simplified version of thermoforming is less expensive and easier to use than conventional thermoforming techniques. However, vacuum forming machines have a few problems. They're only suited to some types of production and do not work well for very large batches.
Shallow Part Depth
Vacuum forming machines can only produce relatively shallow parts, according to Plastic Ingenuity. Forming deep parts requires pneumatic or mechanical stretching of the plastic sheet before it contacts the mold surface. This extra step can be difficult, costly and inconvenient.
Cost
Unlike other thermoforming processes, vacuum forming machines require an extruded plastic sheet. Conventional thermoforming uses a resin base in pellet or powder form. This base is easier and less costly to transport. The plastic sheets needed for vacuum forming require an extra step, which adds cost and time to the vacuum forming process.
Waste
Since vacuum forming requires sheets of plastic, rather than powder or pellet base, it also wastes more material. Each vacuum-formed part must be cut away from the surrounding material, leaving a quantity of unformed plastic. While this material can be ground up and remelted into additional plastic sheets, it is often considered waste. Many manufacturers who use vacuum forming must design molds carefully in order to use as much of the plastic sheet as possible.
Batch Size
Vacuum forming is not suitable for large batch applications. Intertrade Industries suggests this process for applications with a low to medium product quantity where mold costs need to remain low. Other processes, such as thermoforming or injection molding, may produce more consistent results with higher efficiency than vacuum forming.
Low Detail
Vacuum forming is best for parts with relatively low detail and low relief. Vacuum forming machines cannot easily produce plastic parts with very strong details because the vacuum pressure is not high enough to pull the plastic sheet into a complex mold. Manufacturers wanting to produce a more complex part may need to turn to other plastic forming processes.