Processing

Sheet Moulding Compound

The use of Sheet Moulding Compound (SMC) is a very important production process for the production of GRP parts. SMC is a flat sheet-like compound, produced on a continuous moving belt process.

A typical formulation for a SMC is given below.

As glass fibre reinforcement in most cases cut glass rovings are used. Calcium carbonate (chalk) is the most commonly used filler. Magnesium oxide in the formulation can react with the free acid groups of the resin. This results after a few days in an enormous increase of the viscosity of the SMC formulation. A leather-like sheet is formed, which is almost non-sticky and easy to handle. In this stage the resin is still not polymerised.

In the production of SMC the resin is premixed with all the components of the formulation to a paste. On the SMC machine first the paste is dosed on a polyamide or polyethylene film by means of a doctor blade. In the next stage glass rovings are cut to lengths of 25 – 50 mm and sprinkled on top of the resin layer. Then a second film, on which also a resin paste is applied, is put on top of the glass fibre. The whole package between the two films is then pulled through a compacting section of the machine. In this phase the glass fibre is impregnated and the compound is homogenized. At the end the SMC is rolled and stored during a few days to give the thickening reaction sufficient time. The ready and thickened SMC is stored and sold in the form of rolls.

Depending on the requirements for the finished product a wide variety of formulations is nowadays available. Products are made that possess high weather - or corrosion resistance, excellent surface finish or high mechanical properties.

In the production of car body parts, SMC systems have been developed, that cure without shrinkage. The result is a high quality surface of the parts, a so-called “Class-A” surface. This quality is equivalent to “mirror-like”.

The leather-like consistency of the SMC makes it possible to process it fully automatic in the press operation. Robot–controlled cutting installations cut the SMC into predetermined pieces. The protective polyamide film is removed from the pieces, which are then placed into the steel mould. The mould is closed and at a pressure of 100 bar and a temperature of 150°C the SMC flows into the mould cavities where it cures. The cycle time for an SMC press operation lays -depending on size and complexity of the product- between 30 seconds and 3 minutes.

The modern SMC production installations represent the latest technology. Most of the process parameters like weight per area unit and composition are controlled fully automatic.