Shallow Part DepthVacuum 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. If you have ever seen a hard boiled egg sucked magically into a bottle, you have seen a thermodynamic vacuum. Essentially, the vacuum is formed when the contents of a container is heated.
The gasses or liquids in the container expand and some are forced out of the container. In the case of the magic egg, a glass bottle has several lit matches dropped through the bottles neck. When the egg, or some other type of lid is placed on the container, it prevents air from returning and creating equilibrium. If the lid is soft, like a hard boiled egg, it will change shape and the air pressure outside the bottle will push it through the opening. If the ontainer is sturdy, like a tin can, it will retain its shape. But tin cans, which are usually vacuum sealed, will make a small sighing sound when they are opened. This is the sound of the air rushing into the can when the seal is broken.
WasteSince 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.
This is one of the only ways, outside of a lab, to create a true vacuum. The only tool needed is the lowly suction cup. While it seems simple, the suction cup is actually a very useful and complex object. By pushing the air out from between two objects, air pressure holds the suction cup to the surface. The shape of the suction cup helps to determine the strength of the seal. A wider surface area or a deeper cup changes the strength of the seal and determines how much weight the suction cup will hold. However, since suction cups require air pressure, they don't work in outer space, where there is little to no air pressure.
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.
Cost Unlike other thermoforming processes, vacuum forming machine 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.