Meaning
Manufacturing methods that use heat and pressure to shape flat plastic sheets into three dimensional forms are widely used in thin wall packaging. In the industrial setting, sheet thermoforming begins with a roll or sheet of extruded plastic that is fed into a heating station. Once the material reaches its glass transition or melting range, it is moved over a mould where vacuum or air pressure forces it into the cavity.
This process is highly efficient for producing large volumes of lightweight containers and trays.
Thermal Softening
Precise control of the heating cycle is required to ensure the sheet is pliable enough for forming without sagging or burning. Infrared heaters are typically arranged in zones to provide a uniform temperature across the entire surface. If the edges of the sheet are cooler than the centre, the material will stretch unevenly, leading to thin spots in the final part.
The window of processability depends on the thermal properties of the specific resin being used.
Pressure Application
Vacuum suction pulls the softened sheet against the mould surface to capture the fine details of the tool. In heavy gauge applications, positive air pressure is often added to the opposite side of the sheet to improve the definition of sharp corners and textures. This force must be applied rapidly before the plastic cools and hardens against the metal mould.
The speed of the forming stroke influences the final wall thickness distribution.
Draw Constraint
Stretching a flat sheet into a deep cavity naturally results in the thinning of the material as the surface area increases. The draw ratio, defined as the depth of the part compared to its width, determines the feasibility of a particular design. Parts with high draw ratios may require the use of a plug assist to pre-stretch the material before the vacuum is applied.
This mechanical aid helps to distribute the plastic more evenly and prevents the formation of weak areas. Careful optimization of the plug geometry is necessary to avoid marring the surface of the sheet.
