Cooling runner system

Molds are used in many plastic processes with many of the molds having common assembly parts (Fig. 8-11). Many molds, particularly for injection molding, have been preengineered as standardized products that can be used to include cavities, different runner systems, cooling lines, unscrewing mechanisms, etc. (Table 3-17). 

This is only true for a hot runner system. However, in many injection molding processes the runner system is directly inside the cooled mold, and the flow is not isothermal. 

The feed system is an unwanted by-product of the molding process, so a further requirement is to keep the mass of the feed system at a minimum to reduce the amount of plastic used. This last consideration is a major point of difference between cold and hot runner systems. The cold runner feed system is maintained at the same temperature as the rest of the mold. In other words, it is cold with respect to the melt temperature. The cold runner solidifies along with the molding and is ejected with it as a waste product in every cycle. The hot runner system is maintained at melt temperature as a separate thermal system within the cool mold. Plastic material within the hot runner system remains as a melt throughout the cycle, and is eventually used on the next cycle. Consequently, there is little or no feed system waste with a hot runner system. Effectively, a hot runner system moves the melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3 32> 326-332,490... 

In use are diflbrent runner systems to meet different processing requirements. The more popular are the cold and hot runners. With a TP cold runner, the melt from the sprue to the gate solidifies by the cooling action of the mold as the melt in the cavity soHdifies. A hot runner for TP has the sprue to the gate insulated from the chilled cavity and remains hot so that the melt never cools its next shot starts from the gate rather than the nozzle as in... 

The purpose of an injection mold is to give the shape of the part (cavity), distribute the polymer melt to the cavities through a runner system, cool the part, and eject the part. During the injection molding cycle, the polymer flows from the nozzle on the injection unit through the sprue, then to the runners, which distribute the melt to each of the cavities. The entrance to the cavity is called the gate and is usually small so that the runner system can be easily removed from the part. A typical feed system for injection molding is shown... 

Figure 6-17. Coaxial electrical cable s PS insulation buttons are here being injection molded, using a cold runner system, but it could also use a hot runner one. In this continuous production process, the injection-molding machine (IMM) is on a platform that moves in a rectangular pattern to permit the platens to open and move away from the buttons, as well as to move at the speed of the six-cable copper wire line when the mold is closed and the IMM is injecting the PS. Copper wires are started out with large diameters and are pulled through reduction squeeze rolls to their final thin diameter prior to entering the IMM. The wire-reduction line is to the left of the IMM, with the wire pullers to its left. Automatic devices remove the runners on-line just after they leave the IMM and the additional cooling station fiiat is shown in this view.

Cold Slug n The first material to enter an injection mold, so called because in passing through the sprue orifice it is cooled below the effective molding temperature. In some molds, a small well in the mold opposite the sprue catches the cold slug and thereby prevents it from entering the runner system. 

In contrast to hot runner systems, the material to be processed in the CRS is cooled or specifically tempered. Thus, In CRS, material temperatures of 20 to 80 °C are standard, and the mold Is then heated up to 200 °C. By comparison, the (thermoplastic) material In the hot runner is heated to about 180 to 380 °C and then cooled in the mold. 

High demands on cold runner systems require more reliable temperature control of the cold runner system, especially the nozzles. Water is mainly used as a cooling medium. The advantage is a variable number of cooling circuits to ensure a homogeneous temperature pattern of the cold runner block, including the mounted nozzles. 

Injection molding technology involves the injection of molten plastic into one or several cavities via a hot-runner system (melt-channel distribution system) and rapid cooling of a preform to a low temperature. At this point the freshly manufactured article can be ejected from the cavity. In multilayer technology, more than one plastic resin is injected into the cavity. 

Standard runner systems are machined straight into the mould plates, their temperature therefore being that of the mould temperature, (i.e., usually 20 °C to 120 °C). The material passes through the runner to the cavities which are filled and packed by holding pressure and then the molten material in the runner freezes with the rest of the component during cooling. 

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