TABLE OF CONTENTS Calendered or extruded sheet can be thermoformed to shape by applying heat, using vacuum or pressure forming over a preform or mould, and cooling.
The vacuum forming process is particularly suitable for polyvinyl chloride (PVC)-based formulations, due to good melt extension and strength properties over the softening temperature range.
This process involves heating a flat sheet using radiant electric heat (heating can be from one side or both sides) to a softened state (above the glass transition temperature, T g) followed by the deformation of the sheet over the mould and into the shape required. The deformation occurs due to all the air between the sheet and the mould being removed, creating negative air pressure. The mould may need to be maintained in a particular temperature range to allow slip but also to induce forming. Cooling may be by forced air and/or water mist. The resulting product has variable thickness due to the induced non-uniform biaxial stretch. PVC-based materials are relatively easy to thermoform, due to their highly elastic behaviour over a fairly wide temperature range above T g.
Tooling can be basically of concave shape (female, negative) into which the sheet is drawn or of convex shape (male, positive) over which the sheet is formed. Tools are not subject to high stress and can be made from aluminium or fibre-reinforced epoxy or polyester resin with fine holes or slots through which vacuum is applied.
Small products, e.g., packaging, can be formed using this...
