Vacuum Thermoforming For Plastic Sheets | Advantages and Disadvantages
Thermoforming is a plastic manufacturing process in which the thermoplastic sheets are formed with the application of heat and pressure in a mold. The thermoplastic sheet is held horizontally over a mold surface and clamped with a holding device. The sheet is heated up to predetermined temperature using a heating element called heater. The thermostat is used to maintain the temperature of the heater. When the temperature becomes substantially high in the mold, the temperature is controlled by adjusting the heater and providing the cooling air. The thermoplastic sheet softens with the application of heat and is pressed into or stretched over the mold surface by application of air pressure or by any other means. The softened sheet conforms to the mold shape and it is held in place until it cools. The mold cavity is opened and the thermoformed part is released. Some of the plastic materials require air cooling in order to make those rigid quickly, because plastic materials have low thermal conductivity. The excess material is then trimmed out from the formed part. Excess material can be reground, mixed with unused plastic, and again reformed into thermoplastic sheets. Thin sheet (up to 1.5 mm) and thick sheet (about 3 mm) can be formed easily.
In this process, the vacuum pressure is used to form the heated thermoplastic sheet into the desired shape. The thermoplastic sheet is placed on the mold surface and fixed with the help of clamping unit. The sheet is heated until it is softens and thereafter vacuum needs to be applied quickly. A surge tank is used to quickly pull the air out between the mold cavity and the sheet. When the vacuum is created, the sheet conforms to the shape of the mold cavity. The formed part is cooled and then ejected from the mold cavity. The schematic of vacuum forming process is shown in figure .
Thermoforming process is used for variety of applications, for example, food packaging, automotive parts, trays, building products and aircraft windscreens. Thick gauge parts are used as cosmetic surfaces on permanent structures such as trucks, medical equipment, material handling equipment, electrical and electronic equipment, spas and shower enclosures, vehicle door and dash panels, refrigerator liners, utility vehicle beds, and plastic pallets. Thin gauge parts are primarily used to package or contain a food item, disposable cups, containers, lids, blisters and clamshells.
Advantages of Vacuum Thermoforming:
- Extremely adaptive to design requirement
- Rapid prototype development
- Low initial setup costs
- Low production costs
Disadvantages of thermoforming:
- Poor surface finish
- Parts may have non-uniform wall thickness.
- All parts need to be trimmed
- Ribs and bosses cannot be molded easily
- Limited number of materials can be used
- Very thick plastic sheets can’t be formed
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