Thermoplastic foams produced by means of a chemical expanding agent have been employed since the early days of injection molding. Why? Shrink marks are avoided while shrinkage and distortion are reduced when using foam. Further refinement of the foam process has made it possible to make lighter parts – and this reduction in weight will certainly be of interest to the auto parts industry. Another advantage is in the improved capacity to resist sound and heat. Also, reduced viscosity means lower injection pressure and this is why lower pressure foam processing is used for in-mold decoration. Reduced viscosity can also be advantageous when adapted to the melting temperature because the cooling time (and therefore the cycle time, too) can be lessened.
The Process
Today chemical expanding agents (the gas required for the foam is created as a by-product during plasticizing) are in competition with the foam process, which uses economical, environmentally friendly and atmospheric gases like carbon dioxide and nitrogens then inserted into the melt. With the MuCell process from Trexel the gas is applied during the plasticizing process whereas Optifoam from Sulzer applies gas during the injection process through a special machine nozzle. Although physical expanding agents generally produce better grade foam than chemical expanding agents the costs involved with physical agents (machinery and process management) are considerably higher.
The Technology
A foam process which uses chemical-expanding agents may sometimes need a shut-off nozzle and a special screw-positioning control - but this expenditure is far less than that required by physical expanding agents. A special machine nozzle and a gas-dosing unit are needed for the Optifoam process and the MuCell process requires a special plasticizing unit and gas-dosing station. In addition to this, a license agreement with Trexel is needed in order to use the MuCell process. It is relatively easy to equip machines that are already in use so that chemical expanding agents and the Optifoam process can be employed. However, the cost of equipping machinery for the MuCell process can be very high.
Various foam processes is available for all of our machines (MAXIMA, ELEKTRA and K-TEC). Our applications
team can help you select the most suitable process and machine and will
be happy to answer questions about retrofitting installed machines at
your facility.
Strong Together
A disadvantage of foam-processed mold is its less attractive appearance – it may have a marbled or smeared look. For this very reason Ferromatik Milacron has combined the foam process with the monosandwich process. We have successfully overcome this problem and the non-foam-processed skin mold creates an attractive surface which is, for example, acceptable for the visible parts of an automobile.
