Apparatus for the injection moulding of unsaturated polyester resin masses

Abstract: The method for the injection moulding of un­ saturated polyester resin masses, possibly reinforced with fibres, comprises the steps of compacting the moulding mass in an accumulation cylinder, generating a subatmospheric pressure in the cylinder and heating the compacted mass at substamospheric pressure by the application of a high-frequency load, and inject­ ing the material into the die. The apparatus for carrying out the method includes an accumulation cylinder (5) provided with an injection piston (10) and constituted by a body of a material transparent to high frequencies, whose walls incorporate the electrode plates (7) of a high-frequency generator.

Description: The present invention relates to an apparatus for the injection moulding of unsaturated polyester resin masses, possibly reinforced with fibres. The transformation of reinforced, unsaturated polyester resin compounds for the production of moulded articles is generally carried out by the introduction of the moulding mass which is almost at ambient temperature, generally between 140° and 180°C. The time necessary for the material to reach the correct polymerisation temperature greatly limits the productivity of the moulding plant and may sometimes have and adverse effect on the homogeneity of the results. In injection moulding in which the material is introduced into the die through a ramming unit, the preheating of the material is particularly problematical since there is a risk that the material will polymerise before injection. FR-A-1 406 685 describes an injection moulding apparatus for thermosetting resins having a pre-heating unit. US-A-2 505 602 describes a moulding apparatus in which the moulding mass is compacted in a cylinder, then the compacted mass is heated by the application of high frequencies and injected into a die. JP-A-52-139 165 describes a transfer mould comprising fluorocarbon resin which is resistant to continuous use and high temperatures. JP-A-60-132 716 describes a method and apparatus for moulding resins provided with a pressure reducing chamber to reduce the air mixed in the molten resin. The object of the present invention is to provide an improved apparatus which enables the productivity to be increased, reduces the moulding times, and at the same time achieves improved results in terms of the homogeneity of the structural characteristics of the injection-moulded article. According to the invention, this object is achieved by means of an injection and ramming apparatus for the injection moulding of unsaturated polyester resin masses, possibly reinforced with fibres, including an accumulation cylinder provided with a piston for injecting the mass into a die, the accumulation cylinder being constituted by a body of moulded plastics material transparent to high frequencies, having walls incorporating electrode plates connected to a high-frequency generator for creating a high-frequency field in the material to heat it to a temperature close to its polymerisation temperature, characterised in that it comprises a metering cylinder provided with a piston for supplying the mass of material to the accumulation cylinder and means for generating a subatmospheric pressure in said accumulation cylinder. Further characteristics and advantages of the apparatus according to the invention will become clear from the detailed description which follows with reference to the appended drawings, in which: figure 1 is a partially-sectioned schematic view of injection-ramming apparatus for carrying out the method according to the invention, and figure 2 is a section taken on the line II-II of Figure 1. With reference to the drawings, an injection and ramming unit comprises a metering-plasticizing cylinder 1 into which a mass M of fibre-reinforced unsaturated polyester resin, typically BMC, is introduced. The metering cylinder is provided with a connector 2 for connecting the inner chamber to a vacuum pump for creating a subatmospheric pressure within the cylinder to evacuate the air contained in the moulding mass. A metering piston 3 is associated with the cylinder and provides for the supply of the material through an elbow duct 4 into an accumulation cylinder 5 through an opening in its side wall. A non-return valve 6 is interposed between the accumulation cylinder and the duct. The moulding mass is preferably kept in the metering cylinder at almost ambient temperature and the material is supplied to the accumulation cylinder with a piston flow if possible, avoiding localised overheating of the moulding mass. The accumulation cylinder 5 is constituted by a tubular body of plastics material which is transparent to radio frequencies, typically PTFE or another suitable material. Electrode plates 7 are incorporated in the cylindrical wall of the accumulation chamber and are connected to a high-frequency generator 9 by a coaxial cable 8. Preferably, there are four electrode plates 7 with arcuate cross-sectional profiles, and these are arranged in facing pairs so as to generate a more or less uniform radio-frequency field within the accumulation cylinder. An injection piston 10 is associated with the accumulation cylinder and provides for the compaction and then the direct injection of the material into the die through an orifice 11 in the accumulation chamber. The accumulation chamber is preferably provided with a connector 12 for its connection to a vacuum pump for creating a subatmospheric pressure in the inner chamber. The material supplied to the accumulation cylinder by the metering cylinder is compacted in the inner chamber, a subatmospheric pressure being generated within the chamber to eliminate from the material air pockets which would create variations in the electrical field produced by the generator 9. The latter is preferably a 27.12 Mhertz radio-frequency generator. The high-frequency field is applied for a time, typically no longer than 15 seconds, until the material is brought to a temperature close to its polymerisation temperature. The injection is then effected directly into the die which is heated to the cross-linking temperature. By virtue of the apparatus according to the invention, the material is treated with the high-frequency load in an atmosphere isolated from the outside, thus avoiding risks represented by the emanation of styrene typically included in the moulding mass. The high-frequency load may also be metered to achieve uniform heating and to avoid the risk of prepolymerisation.

Claims: Injection and ramming apparatus for the injection moulding of unsaturated polyester resin masses, possibly reinforced with fibres, including an accumulation cylinder (5) provided with a piston (10) for injecting the mass into a die, the accumulation cylinder (5) being constituted by a body of moulded plastics material transparent to high frequencies, having walls incorporating electrode plates (7) connected to a high-frequency generator for creating a high-frequency field in the material to heat it to a temperature close to its polymerisation temperature, characterised in that it comprises a metering cylinder (1) provided with a piston (3) for supplying the mass of material to the accumulation cylinder (5) and means (12) for generating a subatmospheric pressure in said accumulation cylinder (5). Apparatus according to Claim 1, characterised in that the accumulation cylinder (5) is constituted by a body of polytetrafluoroethylene (PTFE).

Inventors: DA RE MARIO, DA RE', MARIO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1988
Main classification: B29C45
IPC codes: B29C45