Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work – To degas or prevent gas entrapment
Patent
1985-09-10
1988-12-20
Thurlow, Jeffery
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
To degas or prevent gas entrapment
26432814, 26432817, 425203, 425205, 425587, B29C 4548, B29C 4550, B29C 4563
Patent
active
047924241
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of injection moulding PVC products from powdered raw material, while employing an injection moulding machine having an injection moulding unit of the cascade type having upper and lower screw extruders wherein the lower extruder has a reciprocating screw.
Injection moulding of PVC products from powdered raw material is known. This technology of manufacturing is particularly applied for producing tube fittings.
The PVC powder is thereby supplied from a hopper to the filling opening of the injection unit of an injection moulding machine. The rotating screw takes the powder and transports it forwardly in the direction of the injection nozzle. Under the influence of frictional forces caused by the rotating screw and heat which is supplied externally, the powder is plasticized and homogenized into a molten mass of a certain temperature. During rotation the screw moves backward until the amount of molten material required for moulding the product has been obtained in the space between the screw and the injection nozzle. Thereupon the rotary movement of the screw is stopped and, as soon as the injection mould is ready for it, the molten material is injected into the mould by axial movement of the screw towards the injection nozzle.
It is known to apply, with injection moulding apparatus, additional prejellifying apparatus, comprising a single screw transport device, whereby the adjustment of material supply occurs by adjustment of the speed of rotation of the transport screw. The degree of prejellifying, however, depends amongst other things, on this speed of rotation, by which this solution cannot be applied for PVC powder as base material. Certain problems for the greater part relate to the fact that PVC powder contains quite an amount of air and removing the gas from the material in the screw is very difficult. By an insufficient removal of gas, inclusions of gas and air in the molten material will be produced during plasticizing and consequently also in the injection-moulded product. Moreover, the material is homogenized less well because of the inclusions of air, so that locally the temperature of the molten mass will be increased too far and decomposition of the PVC will occur as a consequence of the small stability of PVC at high temperatures.
The consequence of these problems, sketched here only very summarily, is that the quality of products moulded by injection from PVC powder, is often open to criticism, both as regards the mechanical properties and as regards the appearance.
The object of the invention, therefore, is to attain an improvement of the quality for the art of injection moulding of PVC products from powder.
To this end, the method according to the invention is characterized in that the powdered raw material is subjected to a prejellifying treatment during forced transportation, in which it is heated and brought to agglomerate in a relatively short time, and subsequently in this condition is supplied to the upper injection extruder unit.
In this manner the material is densified on micro-scale, whilst on macro-scale, a coarse material structure is obtained. The raw material is heated until the outside is sticky whilst nevertheless it remains porous both in the interior of the particles and at the surface thereof.
Removal of the gas from the material in the screw is improved hereby, so that inclusions of air into the molten mass and into the injection-moulded final product, will be reduced and consequently an improvement of the quality of the final product is attained.
Generally speaking the removal of air from the raw materials has a great influence on the homogeneity and quality both of the molten mass and of the final product.
When a vacuum is applied at the location between the end of the prejellifying and the filling opening of the plasticizing screw, an absolute pressure is applied of less than 50 kPa, preferably, however, an absolute pressure of less than 20 kPa.
In this manner not only a removal of humidity from the raw material is realized, but also a
REFERENCES:
patent: 3911069 (1975-10-01), Fujita et al.
patent: 3913796 (1975-10-01), Aoki
Tentoni Leo B.
Theibault A. Robert
Thurlow Jeffery
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