A mold used to form a product using the injection molding process. A standard injection mold is made of a stationary or injection side containing one or more cavities and a moving or ejection side.
The resin, or raw material for injection moulding, is usually in pellet form and is melted by heat and shearing forces shortly before being injected into the mould. The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut off and recycled. Some moulds are designed such that it is automatically stripped from the part through the action of the mould.
The quality of the molded part depends on the quality of the mould, the care taken during the moulding process, and upon details of the design of the part itself. It is essential that the molten resin is at just the right pressure and temperature so that it flows easily to all parts of the mould. The parts of the injection mold must also come together extremely precisely, otherwise small leakages of molten plastic can form, a phenomenon known as flash. When filling a new or unfamiliar mould for the first time, where shot size for that particular mould is unknown, a technician should reduce the nozzle pressure so that the mould fills, but does not flash. Then, using that now-known shot volume, pressure can be raised without fear of damaging the mould. Sometimes factors such as venting, temperature, and resin moisture content, can affect the formation of flash as well.
Injection Mold Material
Traditionally, molds have been very expensive to manufacture therefore they were usually only used in mass production where thousands of parts are being produced. Molds are typically constructed from hardened steel or aluminium. The choice of material to build a mold is primarily one of economics. Steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made in the mold before wearing out. Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for moulding hundreds or even tens of parts.
Types of injection molds
There are two main types of injection molds: cold runner mold (two plate and three plate designs) and hot runner mold – the more common of the runnerless molds. The significant difference is the presence of sprue and runner with every molded part in the cold runner type. This extra molded component must be separated from the desired molded part.
Cold runner mold
Developed to provide for injection of thermoset material either directly into the cavity or through a small sub-runner and gate into the cavity.
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. 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.
Three plate mold
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.
Hot Runner 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.
Externally heated hot runners
Externally heated hot runner channels have the lowest pressure drop of any runner system (because there is no heater obstructing flow and all the plastic is molten), and they are better for color changes none of the plastic in the runner system freezes.
Internally heated hot runners
Internally heated runner systems require higher molding pressures, and color changes are very difficult. There are many places for material to hang up and degrade, so thermally sensitive materials should not be used.
insulated hot runners
A special type of hot runner system is an insulated runner. An insulated runner is not heated; the runner channels are extremely thick and stay molten during constant cycling.
Requirements for 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.
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