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#138 PET Plastic

Category : Molding materials
January25, 2013

PET is the abbreviation for polyethylene terephthalate plastic resin.

PET has a melting point near 270°C, while the molding temperature is 270 to 280°C, and hence its injection molding is done at a relatively high temperature. One of the features of this plastic is its fluidity. Although its fluidity is good above the melting point, the flow suddenly becomes difficult when the solidification starts. In other words, there is a very close relationship between the temperature of the plastic and the fluidity. Temperature management of the mold and temperature management of the plastic are mandatory items in the case of PET.
In most cases, hot runners are used in the injection molding of PET, and in addition, valve gates are selected in order to make certain the management of the fluidity by mechanically opening and closing the gate.

Since the fluidity is good, it is necessary to give considerations to the mold clearance management and to mold deformations due to the filling pressure. In particular, in the design of the manifold of the hot runner, the key points will be prevention of deformation and management of the thermal expansion margin.

Further, since PET reacts sensitively to water content, it is necessary to carry out thorough preliminary drying of the molding material pellets and also to make the management level high. This plastic carries out hydrolysis when it reacts with water. Therefore, the material pre-drying equipment is mandatorily required in the injection molding of PET.

In the process of solidifying from the fluid state, the size of the crystals of PET becomes larger and the crystals can easily become spherocrystals. When this happens the molded articles will not be transparent but will become turbid white in color. Consequently, if transparent molded items are to be prepared, it is necessary to cool rapidly in order to suppress the growth of spherocrystals. Therefore, the molds should be provided with a system for rapid cooling. It is necessary to maintain the mold temperature between 5 and 15°C, for which a chiller is used. Since it is necessary to manage the region of change of plastic temperature inside the mold in the temperature gradient from 270 to 280°C to 5 to 15°C, it can be understood how difficult the injection molding of PET is.

At present, the injection molding technology of PET plastic has advanced and reached a mature stage, and the mold technology of super multiple shot molds (100 to 200 shots) is being used in the world in the mass production mode.

Although the raw material of PET plastic is a petrochemical material, at present bio-PET plastic is being developed. This makes the raw material to be produced from vegetable origin, and it is possible to produce a plastic with completely the same composition as the petrochemical based PET plastic. Supply of such material on a commercial basis has already started.

The locations supplying the raw materials for PET plastic is predicted to be not oil producing countries but is predicted to shift gradually towards agricultural products producing countries.