Compression molding is a process of molding in which a feeding material is placed into an open, heated mold cavity. The mold is then closed with a top plug and compressed with large hydraulic presses in order to have the material contact all areas of the mold. The charge cures in the heated mold.
There are two forms of compression molding, namely SMC (sheet molding compound) and BMC (bulk molding compound), which differ in the feedstock material form. For SMCs, it is usually in the form of 5 mm thick sheets (material feedstock placed between polyethylene film carriers), while for BMCs it is in thicker, 20–50 mm, pelletized form. In the compression molding process, the reinforcement is normally included in the feed material.
Resins, fillers and additives are automatically weighted and mixed together to form the mother paste. In a second mixer, more fillers, colour pigments and chopped glass fibre are automatically weighted and mixed together within a computer-controlled process to ensure a perfect impregnation or wet out of the glass fibre.
The resulting mix is then sent to an extruder, which compacts the BMC material and packs it into bags. At the end of the process, after BMC bags pass through a metal detector to prevent material from having any metallic foreign object, BMC bags are packed into boxes.
BMC must mature before it reaches optimal conditions for moulding. Typically, the required maturation time is between 2 and 4 days, depending on the specific formulation. Even after the material has matured, it continues to thicken at a speed dependant on the storage temperature. Therefore, there is an optimal utilisation period of typically, between 1 and 6 months, dependant mainly on the specific formulation and storage temperature, which is recommended to be between 15 and 25ºC. Higher storage temperatures can significantly reduce the usability time of BMC.
Resins, fillers and additives are automatically weighted and mixed together. The colour pigments and thickeners are measured and mixed continuously with the mother paste in a computer-controlled process forming the impregnation paste. The impregnation paste is immediately transferred to the glass fibre impregnation line.
The impregnation paste is transferred to a doctor box where it is deposited onto a moving carrier film. Simultaneously the glass fibre is chopped to a pre-determined length and dropped onto the layer of paste. A second film is coated with a layer of paste and then used to cover the fibre cuttings creating a “sandwich”.
The resulting sheet is then sent through a series of compaction rollers to ensure a perfect impregnation or wet out of the chopped fibre. At the end of the impregnation line, the sheet can be rolled up using rolls, MegaRolls or folded into boxes.
SMC must mature before it reaches optimal conditions for molding. The resin paste changes from a liquid state during manufacturing to a semi-solid state at its optimal condition period. This change in viscosity is achieved through the introduction of the thickening agent during manufacturing. Typically, the required maturation time is between 2 and 4 days, depending on the specific formulation. Even after the material has matured, it continues to thicken at a speed dependent on the storage temperature. Therefore, there is an optimal utilization period of typically, between 1 and 6 months, dependent mainly on the specific formulation and storage temperature, which is recommended to be between 5ºC and 25ºC. Higher storage temperatures can significantly reduce the usability time of SMC.
Literature
https://astar.es/index.php/en/products/bmc/
https://en.wikipedia.org/wiki/Transfer_molding
Journal of Mechanical Engineering and Sciences Volume 12, Issue 1, pp. 3479-3493, March 2018
Durability and Life Prediction in Biocomposites, Fibre-Reinforced Composites and Hybrid Composites Woodhead Publishing Series in Composites Science and Engineering 2019, Pages 27-62
Design and Manufacture of Plastic Components for Multifunctionality Structural Composites, Injection Molding, and 3D Printing 2016, Pages 53-101
Figures
Figure 1: https://astar.es/index.php/en/products/bmc/
Figure 2: https://astar.es/index.php/en/products/bmc/
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