Twin sheet thermoforming is the process of pressure forming or vacuum forming two sheets of plastic. Essentially simultaneously, with a separate mold on the top and on the bottom. After the plastic is molded, the plastic remains in the molds and while still at forming temperature, the two molds are brought together under high pressures, which causes the two sheets to weld together wherever a weld is dictated. While being pressed together, a pin is pushed through one of the sheets and air is blown in which causes the sheets to be pressure formed, if the air were trapped within, the panel would collapse. Finished products are hollow and have molded features on both sides, they are also attractive, structurally rigid and detailed.
Parts can be made of dissimilar materials with twin sheet thermoforming. Twin sheet processes create a 3-dimensional part with formed features on both sides. Parts are able to use a thick sheet on the outside for stability and use a thinner sheet on the inside for rigidity. When considering thickness, it is smart to make sure the difference in depth between the bottom and top sheets be 0.0625 of an inch or less. A variance greater than that number would make the twin-sheets at risk of being out of sync. Since the two sheets can be dissimilar, twin-sheet forming allows for a two-color product option and custom coloring. To form two sheets instead of one, both sheets must be precise and properly aligned. Complete understanding and control of conditions are required, as well as the appropriate vacuum and cooling devices.
Advantages of twin sheet thermoforming include:
• Ability to add internal reinforcements such as rigid foam components
• Twin sheet forming has increased structural stability
• Can have molded in attachments
• Allows for air and wiring ducts
• Can accommodate multiple textures with variable wall sizes
• Ability to control both sides of the part
• Lower tooling cost than competitive processes
• No secondary bonding required
• Improved aesthetics caused by tool contact on both sides
• Formable in multiple materials and sizes
Different types of twin sheet machinery may be required for specific applications. This can be dictated by:
• Material gauge
• Material type
• Material heating to cooling ratio
• Material hot melt strength
• Bonding properties
• Material shrink
• Part size
• Part design
• Amount of pressure required
Twin sheet thermoforming materials can include, thermoplastic polyolefins (TPO) which has high impact resistance, acrylonitrile butadiene styrene (ABS) has good impact and chemical resistant properties, high density polyethylene sheets (HDPE) has high impact strength and is used for industrial products, high impact polystyrene (HIPS) is lightweight and inexpensive, acrylic carries a great shatter resistance along with weather resistance.