Thermoforming is a plastic manufacturing process that transforms thermoplastic sheets into specific shapes using molds. The process is straightforward, involving two key steps: heating and forming. Various materials, such as polystyrene, cellulose acetate butyrate, ABS, acrylic, PVC, polypropylene, and polyethylene, can be used. The process yields high surface finish quality, dependent on the mold’s surface condition.

Process Description

Thermoforming involves heating a flat thermoplastic sheet until it becomes pliable, then shaping it using vacuum, pressure, or mechanical force. The process, also known as vacuum thermoforming, pressure thermoforming, blow molding, or mechanical thermoforming, consists of heating (via radiant heaters mounted equidistantly) and forming. The plastic sheet, secured by clamps over a mold cavity, softens under heat and is drawn into the mold by vacuum or pressure through holes at the mold’s base. Once cooled, the formed part is removed.

Applications of Thermoforming

Thermoforming is used to produce a wide range of products, including:

– Open plastic panels and containers

– Food containers and packaging

– Refrigerator door liners

– Plastic signs and displays

– Household appliances (e.g., sinks, bathtubs, shower panels)

– Automotive parts, particularly interiors

– Small boat hulls

– Contoured skylights

– Electrical enclosures

– Braille text pages

– Equipment cabinets  

Materials

Materials suitable for thermoforming include:

– Polystyrene

– Cellulose acetate butyrate

– Cellulose acetate

– ABS

– Acrylic

– PVC

– Polypropylene

– Polyethylene

Design Guidelines

To optimize thermoforming outcomes, consider:

– Maximum sheet thickness: ~3 mm; minimum varies by material (0.05–0.5 mm).

– Minimum cross-section: 25 mm²; maximum dimensions: 7.5 m × 2.5 m.

– Draft angle: ≥1°.

– Tolerance range: 0.025–2 mm, depending on the mold.

– Thickness tolerance: ~20%.

– Corner radius: Greater than product thickness.

– Limitations: Cannot produce large surface areas with holes; features like lettering, ribs, or bosses are costly.

– Shape complexity: Moderate.

Process Variations

Thermoforming has several variations to suit different products and materials:

1. Vacuum Thermoforming

A vacuum is created inside the mold to shape the plastic sheet. The pressure limit is ~1 atm. Air is removed via a suction pump connected to holes at the mold’s base.

2. Pressure Thermoforming (Blow Molding)

Air pressure (3–4 atm) deforms the plastic sheet, similar to glass blow molding. A hole at the mold’s top removes residual air, with vents at the base.

3. Male and Female Molds  

Male molds (convex): Plastic deforms over the mold, with inner surface dimensions matching the mold. Requires vacuum.

Female molds (concave): Plastic deforms into the mold, with outer surface dimensions matching the mold. Compatible with any pressure type.

4. Mechanical Thermoforming

Uses both male and female molds to press the heated plastic sheet without vacuum or pressure, shaping it by direct mold compression.

Economic Considerations

Production rate: 60–360 parts/hour, suitable for high-volume production.

Lead time: Measured in days.

Automation: Fully automatable for high output.

Tooling: Multiple molds per machine increase efficiency; tooling costs vary by complexity.

Machine costs: Simple machines are low to moderate; automated systems are expensive.

Labor costs: Low to moderate, depending on product and volume.

Advantages of Thermoforming

– Excellent surface finish, dependent on mold quality.

– No parting lines, unlike many plastic manufacturing processes.

– Minimal post-processing required.

– Supports various materials, ideal for industrial packaging.

Disadvantages of Thermoforming

– Sheet plastic is more expensive than raw pellets.

– Excessive thinning at sharp corners.

– Multiple parameters (temperature, vacuum pressure, clamping force) require precise control.