Mold halves

Similar to matched metal stamping except that a metal mold half is replaced by a block of solid rubber. Processing material cannot flow to the extent that it can with matched-metal forming. However, more uniform pressure is exerted on the material charge due to the rubber pad. 

With backpressure the process is performed in conventional injection molding machines (IMMs) (Chapter 4). The cover stock is inserted and located in an open mold. A shear edge mold permits draw-in of the cover stock during the closing cycle to avoid wrinkles and damage by stretching of the fabric. Molds require special attention. They generally use a hot runner system with its shut-off nozzle(s). All mold elements such as ejector, core pulls, and slides have to be on the injection side mold half. 

The surface temperature of each mold half should be held within + 4 F. At the high temperatures at which nylon RIM tools are operated, this requirement necessitates the use of more coring than is sometimes used for urethane tools. In steel tools. 75 inch diameter circulating oil core holes, spaced at 2.5 Inches on centers and located roughly 2-inches from the cavity surface have worked quite well. An average temperature difference of about 5°F can be... 

B Sheet formed by simultaneous motion of two mold halves, a - Male mold half, b - Preheated, clamped sheet, c - Female mold half d - Applied force. 

Present day developments include Lotus for production of car bodies, the female mold half is first coated with a gelcoat and dry reinforcement (mainly continuous filament mat and some woven roving) and PUR foam formers are placed on it. The foam inserts have previously been wrapped in continuous filament mat to create a stiff torsion box on each side. The inner male mold, with an airtight peripheral seal, is then placed over the loaded mold and bolted in place and resin is then drawn evenly into the mold by vacuum. 

For contact molding, only one mold-half is needed, which can be either male or female, depending on which face is required to be smooth (Figure 5.74). Almost any material of sufficient rigidity can be used such as those reviewed for making the mold with the most common method being glass fiber-reinforced TS polyester. 

Replacement of upper mold half — PP integral skin foam still a long way from satisfying current standards... 

The clamping unit consists of one fixed platen and one movable platen. One mold half each is installed on one of the platens. 

The opposite applies to the forees being transmitted. When the mold is wide open, it is only necessary to transmit enough force to displace the movable mold half. However, when the mold is elosed, it is necessary to transmit a very large amount of force in order to hold the mold shut in opposition to the injeetion pressure. Both eonditions ean be fiilfilled quite effectively with the toggle lever. 

The most common special feature of the mold is the quick change volume control insert. Rigid volume control is necessary for certain products, such as dairy containers. Here HDPE is used (for many excellent reasons), and the container slowly shrinks and changes in size for many hours after molding. Because of production volume control requirements, some dairies must fill containers molded half an hour before fill and then switch to filling containers molded several days previously. Volume-control inserts that displace the difference in volume between the two types are added to the mold, usually as a disc in the side wall, to ensure that the volume and fill levels are the same in both containers at the time of filling. The device works because HDPE shrinkage is reduced virtually to zero for the life of the container when filled with milk or juice and stored at cold temperatures. 

In positive type mold makes, the force in the upper mold half is allowed to enter the cavity in the lower half with a tight fit between the two components, thus providing direct and positive pressure on the compound in the mold cavity as well... 

Individual valve for each cavity mold half... 

Mating a matching mold half to the other half containing jute and clamping tightly ... 

When back-flow valves are used they must be inspected, and serviced, regularly as they play an important role within the injection unit. These comments apply particularly to high speed machines where screw cushion may not be used and extreme accuracy is demanded. Shut off nozzles are not widely used nowadays, due to material leakage and degradation taking place within the nozzle assembly. Guidance will be given on the types of nozzles used for each material. No matter what nozzle is used, the correct nozzle contact force should be used as, if it is too much, then the fixed mold half may be pushed off the platen. 

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