The present invention generally relates to the molding of a multi-layered molded product having a primary molded product and one or more layers of plastic material laminated or coated on the primary molded product. More particularly, the present invention relates to an in-mold coating apparatus for the manufacture of the multi-layered molded product.
In the field of automobile industry, the use of plastic material as a material for component parts of an automobile body structure such as, for example, bonnets, inner door panels and fenders, has now become popular not only because it contributes to the reduction in weight of an automobile as a whole, but also because some plastics have a physical property superior to metal.
However, it is generally experienced that as compared with the plastics containing no fiber reinforcement the fiber-reinforced plastics when plasticized generally tends to exhibit a poor fluidity and, accordingly, when a sheet-like or plate-like product is manufactured from fiber-reinforced plastics by the use of a plastics molding technique, the molded product is susceptible to the formation of pinholes. Where the sheet-like or plate-like molded product is subsequently painted, or otherwise formed with a surfacing coating, and the applied paint or coating is then dried by the irradiation of, for example, infrared rays of light, the presence of the pinholes necessarily deteriorates the surface finish of the final product because some or all of the pinholes expand to rupture under the influence of heat of elevated temperature.
In view of the foregoing, a practice is generally employed to form or laminate a coating layer on the surface of the sheet-like or plate-like molded product to cover up the indentations present on the surface of the molded product by the use of an in-mold coating technique.
The formation of the coating layer on the molded product according to the in-mold coating technique is carried out by separating mold halves from each other a slight distance corresponding to the desired thickness of the subsequently formed coating layer after the formation of the molded product within the molding cavity, and then injecting a plasticized material for the coating layer into the space which is defined between the molded product and a molding surface of one of the mold halves as a result of the separation of the mold halves through the slight distance. In order for the final molded product, comprised of the primary molded product having the coating layer, to have a uniform thickness substantially all over the entire area thereof, the molded product should be firmly retained by the other of the mold halves at the time of separation of the mold halves from each other.
However, it is well known that, when the mold halves are separated from each other regardless of the purpose for which they are separated, that is, whetherever they are separated for opening the mold assembly or whetherever they are separated for the injection of the coating material, a negative pressure is induced within the molding cavity before the latter becomes communicated with the atmosphere, which negative pressure acts to drag the molded product to lift off from the molding surface of said other of the mold halves.
According to the prior art exemplified by the Japanese Patent Publication No. 58-1672, published Jan. 12, 1983, there is disclosed a mold assembly comprising movable and fixed molds, a knock-out rod slidably and drivingly extending through the fixed mold for ejecting the final molded product, a plurality of retaining rods slidably and drivingly extending through the movable mold, and an injection cylinder for injecting a plasticized coating material into the molding cavity through a passage defined in the movable mold. The retaining rods are driven relative to the movable mold, when the latter is moved away from the fixed mold a distance substantially equal to the desired thickness of the eventually formed coating layer on the primary molded product, to allow their tips adjacent the fixed mold to urge the primary molded product against the fixed mold thereby avoiding any possible lift-off of the primary molded product from the fixed mold. This patent publication describes the formation of the primary molded product by injecting a plasticized material into the molding cavity from another injection cylinder separate of that for the injection of the coating material.
It is pointed out that, where the primary molded product is formed by injecting the plasticized material into the molding cavity before the formation of the coating layer thereon, the actual injection of the coating material must be suspended until the primary molded product solidifies completely, or otherwise markings of the retaining rods will be formed on the surface of the primary molded product. It is also pointed out that, if the primary molded product is pliable or still soft, local portions of the primary molded product other than that depressed by the retaining rods will be susceptible to the localized lift-off from the fixed mold under the influence of the negative pressure induced within the molding cavity as the movable mold moves away from the fixed mold.