Moulding of filled synthetic plastics mould compositions
Cured thermoset plastics artefacts are made directly from a non-homogeneous blend of a filler and a solid particulate thermosetting material, by use of a screw injection plastic mold machine which is operated to cause the non-homogeneous blend to plasticize or flux so that a homogeneous intimate mixture of the filler and the thermosetting material is formed in the barrel of the machine. Thus, the plastic mould sequence includes the provision or formation of the non-homogeneous blend in the barrel.
This invention concerns improvements in or relating to the plastic mould of thermosetting plastic mould compositions. An essential feature of the manufacture of thermosetting plastic mould compositions has been the pre-mastication of the thermosetting resinous material with the filler so that the latter becomes fully compounded with, impregnated by, and coated with, the resinous material. This may be effected under either dry conditions where the resinous material is essentially solid and is melted or fluxed during the mastication processing, or in the presence of a solvent or other liquid medium optionally followed by drying. The resultant plastic mould compound is subsequently converted, in a separate stage, into cured artefacts by a plastic mould or extrusion process.
The objects of this prior method are to facilitate handling of the glass fibres, to ensure adequate dispersion of the fibres in the resin material, and to maintain fibre length so that no significant loss of properties occurs. To meet these objects, a special method of and apparatus for, mixing the fibres with the resin material are proposed. The dough-like material fed directly from the mixing apparatus to the injection device is, essentially, a homogeneous intimate mixture of the fibres in the resin material, and, as is well known in the art, the compounding action of the injection device must be minimized to prevent fibre degradation. We have discovered that injection plastic mould machines and extruders can be used to effect final compounding of rough blends of fillers in thermosetting resin materials and to produce cured artefacts directly from such materials without previously forming a homogeneous intimate dispersion of the filler in the resin and without using specially designed mixing apparatus.
In a homogeneous dispersion, the filler (or reinforcing material) particles are intimately mixed with and impregnated or coated with the resin; in a non-homogeneous blend of the type used in our process, the filler is not coated or impregnated to any great extent by the resin, but becomes intimately compounded therewith only in the injection or extruding machine.
The thermosetting substance preferably is a thermosetting resin which is partly condensed but essentially solid at ambient temperatures in the absence of solubilizing quantities of water or other solvent. If a liquid plasticizer, such as water or monocresylglyceryl ether, or other liquid components is used, it should be present in such an amount that the thermosetting material is not liquid, nor (preferably) visibly wet. Preferably, not more than about 10% by weight (of the weight of the thermosetting substance) will be water or other solvent. Any volatile liquid used initially in the non-homogeneous blend, is preferably vented from the barrel during compounding. Since the resins used, or produced in the barrel, are of the condensation type, water or other volatile substance may be evolved during any condensation which may occur under the action of heat, as the filler and thermosetting material are progressed through the barrel. Thus the filler preferably is capable of absorbing the volatile material, or the barrel may be vented.
The more fully but not completely condensed material which issues from the downstream end of the barrel is moulded immediately by direc injection into a mould (or by the so-called injectioncompression technique) or by passing it directly into a heated extrusion head, and is thereby cured.
By virtue of the present invention, it is now possible for the plastic mould composition manufacturer to produce only unfilled or partially filled synthetic thermosetting materials for supply to trade moulders who themselves, without much effort or capital expenditure, can roughly mix the thermosetting material with the desired amount and type of filler to produce the non-homogeneous blend referred to above.
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