By: Timothy Huang, 2/12/2018

Vacuum Forming is the process of heating up a thermoplastic until it is pliable. The plastic sheet is then placed over a mold and a vacuum is applied between the sheet and the mold surface, forming the sheet into the mold. Afterwards, the plastic mold is ejected from the mold surface. Compared with other methods of forming such as injection molding or thermoforming, vacuum forming is a cheaper alternative. The vacuum forming operates using low forming pressures, therefore tooling for the machines are relatively cheaper. And since vacuum forming uses low pressure, less expensive material can be used to create the molds.

Vacuum Forming Gif

The first conception of the idea of vacuum forming dates back to the 1940's. Vacuum formed plastics were first used as a marketing tool to create display covers for articles. It wasn't until 1964 when the vacuum forming machine was perfected and patented. The concept of using a vacuum to remove air was borrowed from other casting processes that required the removal of excess air to prevent weaknesses in the molds. in 1974, an improved process of vacuum forming was developed, similar to how a vacuum forming machine would work today. The molding apparatus consisted of a heater, a molding bed, a vacuum system and a means of separating the mold from the plastic.

Examples of vacuum forming for low volume manufacturing applications include: electronics, packaging, automotive, industrial, prototypes, and displays. Vacuum formed products are often preferred to other methods of molding due to its low cost of production, since the process uses rather inexpensive material to form the molds. And since the process is relatively simple and efficient, production time is also shorter in comparison with other processes. For higher volume molds, such as yogurt containers and disposable cups, more advance machinery is used. 

Since the nature of vacuum forming requires that all air be removed from between the mold and the plastic sheet, the tolerances of +/-0.10" and +/-0.001" for each additional inch for male aluminum machined molds and +/-0.15" and +/-.0015" for female molds. Higher tolerances can also be achieved with CNC trimming, as vacuum formed molds often require trimming after the initial forming. 

Since vacuum forming is rather simplistic in nature, there are limitations to the complexity in which the molds can be created. Vacuum forming is a sheet based molding process, so the thickness of the material must be uniform throughout the mold. Therefor any complex molds that contain complex geometries and varying wall thicknesses cannot be created using the vacuum forming process. It is also ideal for the geometry design to be able to be formed in a single draw direction. The depth of draw is also limited as the material stretches to conform to the mold, and if the draw is too deep, the mold can get too thin. 

1. The thermoplastic sheet is clamped to the forming surface/frame.
2. The plastic is heated until pliable, usually under an aluminum reflector plate, to ensure uniform heating.
3. Once the material has been heated to forming temperatures, it can be pre-stretched to ensure uniform wall thickness.
4. When the material has been heated and pre-stretched, a vacuum can then be applied to help form the mold. 
5. After the vacuum is applied and the plastic is formed, it must be cooled to ensure that it retains its mold shape. On some machines, there are fans to aid in speeding up this cooling process. 
6. Once the part is formed and removed from the vacuum forming machine, the excess material is trimmed.

Vacuum forming machines can range anywhere from $1600-$6000 for smaller, lower volume production models. Larger volume production volume and complex machines can go for $5000-$50000. Tooling for vacuum formed molding is relatively cheaper than its alternatives such as injection molding and has a quicker turnaround rate, making it a more economical choice.