Thermoforming - Vacuum Forming

What it's all about

In vacuum forming, or thermoforming, extruded plastic sheet is heated and formed over a cast aluminum mold to produce a part. Thermoforming is one of the most versatile and economical processes available for manufacturing returnable packaging and many other products.

Thermoforming is a method of manufacturing plastic parts by preheating a flat sheet of plastic, then bringing it into contact with a mold whose shape it takes.
Any process of forming thermoplastic sheet which consists of heating the sheet and forcing it into a surface mold. Also known as pan forming. This can be done by vacuum, pressure, and/or direct mechanical force. The process offers close tolerances, tight specifications, and sharp detail. With the proper finishing techniques, thermoformed parts can be comparable to injection molded pieces in quality, while costing much less.

Thermoforming is used primarily in low-cost applications for the simple embossing of thin plastic sheet and cannot be used to form as rigid shapes as vacuum forming is capable of. Thermoformed plastics are invariably thermoplastics because thermosetting plastics assume their final shape through heat and so can not be molded with this process.

The plastics that are used in thermoforming include: acrylic, low density polyethylene (LDPE) and crystalline polyester (CPET), semi-crystalline polymers, including high density polyethylene, polypropylene and polyethylene.

Thermoplastic rubber is also a unique thermoplastic material which can also be processed using thermoforming techniques.
Two key factors to be considered in the thermoforming process are:
1. the sheet quality
2. the material melt strength during forming

The strengths of thermoforming are as follows:
. Parts have low internal stresses therefore good physical properties to work with.
. Lead time and mold changes are relatively short and easy to perform.
. Large part don't increase costs.

Thermoformed parts can be undecorated, printed or hot-stamped.

Core extrusion quality will affect the part in many ways. The control that the extruder maintains from each run is important. Changes in extrusion speed, direction, temperature, and gauge will modify the amount of stress that the sheet has when it is delivered to the thermoformer. Differences in that stress will change the rate at which the part molds thus changing the dimensions.

Molds must be temperature controlled with internal cooling channels to allow for consistent mold temperature. Aluminum is the material of choice because it has very high coefficient of thermal conductivity that allows consistent cooling cycle times through the entire production run of components.

Understanding Thermoforming The primer introduces thermoforming in which plastic articles are shaped from softened sheet to manufacturers interested in the process. It discusses machinery, methods of heating sheet, sheet stretching and cooling, trimming, molds, and quality control. No formal technical background is required.

Thermoforming: A Practical Guide Based on training courses on thermoforming principles and processes, this book provides an introduction to the area of plastics manufacturing. It presents information on thermoforming-related subjects, ranging from materials' properties and processing data, to machines, tools and troubleshooting. Chapters address issues like thermoplastics, basic principles and terms, thermoforming procedures, heating of thermoplastics forming materials, heat-balancing of thermoforming tools, demolding, finishing thermoformed articles, pitfalls, and thermoforming tools.