The polymer flows from the nozzle to the mold which is coupled to the nozzle by a sprue bushing. In a mold with multiple cavities, the melt flows to each cavity by runners and is fed to the cavity through a gate. The gate is simply a restriction in the flow path just ahead of the mold cavity and serves to direct the flow of the melt into the cavity and to limit back flow. In a cold runner mold, the melt in the runners is allowed to cool and solidify. The runners are later separated from the part and may be reground and recycled into the feed. In hot runner molds, the runners are heated, therefore the molds are much more expensive to build but regrind costs are minimized. Melting of plastic resin. Injection of melt into the mold. Cooling of the mold. Removing the part.
Two-Plate Mold. The conventional two-plate mold consists of two halves fastened to the two platens of the molding machine’s clamping unit. When the clamping unit is opened, the two mold halves open, as shown in (b). The most obvious feature of the mold is the cavity, which is usually formed by removing metal from the mating surfaces of the two halves. Molds can contain a single cavity or multiple cavities to produce more than one part in a single shot. The figure shows a mold with two cavities. The parting surfaces (or parting line in a cross-sectional view of the mold) is where the mold opens to remove the part(s).
In addition to the cavity, there are other features of the mold that serve indispensable functions during the molding cycle. A mold must have a distribution channel through which the polymer melt flows from the nozzle of the injection barrel into the mold cavity. The distribution channel consists of (1) a sprue, which leads from the nozzle into the mold; (2) runners, which lead from the sprue to the cavity (or cavities); and (3) gates that constrict the flow of plastic into the cavity. There are one or more gates for each cavity in the mold.
An ejection system is needed to eject the molded part from the cavity at the end of the molding cycle. Ejector pins built into the moving half of the mold usually accomplish this function. The cavity is divided between the two mold halves in such a way that the natural shrinkage of the molding causes the part to stick to the moving half. When the mold opens, the ejector pins push the part out of the mold cavity.
A cooling system is required for the mold. This consists of an external pump connected to passageways in the mold, through which water is circulated to remove heat from the hot plastic. Air must be evacuated from the mold cavity as the polymer rushes in. Much of the air passes through the small ejector pin clearances in the mold. In addition, narrow air vents are often machined into the parting surface; only about 0.03 mm (0.001 in.) deep and 12 to 25 mm (0.5 to 1.0 in.) wide, these channels permit air to escape to the outside but are too small for the viscous polymer melt to flow through.
Other Mold Types. The two-plate mold is the most common mold in injection molding. An alternative is a three-plate mold. There are advantages to this mold design. First, the flow of molten plastic is through a gate located at the base of the cup-shaped part, rather than at the side. This allows more even distribution of melt into the sides of the cup. In the side gate design in the two-plate the plastic must flow around the core and join on the opposite side, possibly creating a weakness at the weld line. Second, the three-plate mold allows more automatic operation of the molding machine. As the mold opens, it divides into three plates with two openings between them. This forces disconnection of runner and parts, which drop by gravity (with possible assistance from blown air or a robotic arm) into different containers beneath the mold.
The sprue and runner in a conventional two-plate or three-plate mold represent waste material. In many instances they can be ground and reused; however, in some cases the product must be made of “virgin” plastic (that which has not been previously molded). The hot-runner mold eliminates the solidification of the sprue and runner by locating heaters around the corresponding runner channels. While the plastic in the mold cavity solidifies, the material in the sprue and runner channels remains molten, ready to be injected into the cavity in the next cycle.