There are two main types of injection molds: cold runner (two plate and three plate designs) and hot runner – the more common of the runnerless molds. The significant difference is the presence of a 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 Molds
The cold runner mold type is very simple and much cheaper than a hot runner system. The mold requires less maintenance and less skill to set up and operate. Color changes are very easy since all the plastic in the mold is ejected with each cycle.|
The two plate and the three plate designs are the two major types of cold runner molds. The two plate is the simplest type of mold. It has one parting line and hence two sections or halves. The runner system must be located on this parting line and the part must be gated on its perimeter (see figure 1). The runner is ejected together with the part.
Figure 1. Two Plate Mold in Closed (a) and Open (b) Positions
The three plate has two parting lines and therefore three sections. The runner system can be located on one parting line and the part on the other parting line. Three plate designs are used because of their flexibility in gating location. A part can be gated virtually anywhere along its surface. In a two plate design, the part must be suitable for edge gating. With a three plate mold design, the parts must be suitable for center gating. This will allow for uniform part filling without part-weakening weld lines. As is the example shown, the optimum gate position at the center of the cup-shaped product can now be accessed (see figure 2). The runner system can be ejected separately as the mold opens.
Figure 2. Three Plate Mold in Closed (a) and Open (b) Positions
Hot Runner Molds The hot runner mold is essentially a two plate mold with a heated runner system inside one half of the mold. The runner system through the manifold and the drops, conveys the heated plastic to the part (see figure 3). This system allows greater control over melt temperatures and other processing conditions. A hot runner system is considered where production volumes are high and a shorter cycle time is a requirement.
