Cold runner sprue system

The part taken from the mold is, in most cases, a finished product ready to be packed and shipped or ready to be used as a part of an assembled unit. In contrast to metal forming, there is very little if any wasted material in injection molding. For cold runner TP systems most runners and sprues are reground and reused. By using hot runner molds, the sprue and runner systems remain in a melted state in the mold and become part of the next finished part (Chapter 17). The hot runners can be thought of as an extension of the plasticizing chamber. 

One should consider including a safety factor of 10 to 20 percent to ensure sufficient clamping force, particularly when not familiar with the operation and/or material. Then the clamping force would be 290 to 317 tons in the cavity. However, because of partial hardening of plastic as it flows through the relatively restrictive sprue and runners in a cold-runner TP system (or a hot-runner TS system), the actual pressure in the cavity is less than the applied plunger pressure (Fig. 2-1). 

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 or subsequent cycles. Consequently, there is little or no feed system waste with a hot runner system. Effectively, a hot runner system moves the interface between the machine plasticizing system and the mold to a point at or near the cavities. In a cold runner system, the interface is at the outside surface of the mold, at a point between the machine nozzle and the sprue bush. 

Cold runner feed systems include three principal components sprue, runner, and gate. The sprue is a tapered bore in line with the axis of the injection unit, that conducts the melt to the parting line of the mold. The runner is a channel cut in a parting face of the mold to conduct melt from the sprue to a point very close to the cavity. The gate is a relatively small and short channel that connects the runner to the cavity. The gate is the entry point of the melt into the molding cavity. 

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... 

Hot or cold runner systems can be used. For cold runners, the sprue should contain a cold-slug well to receive the cold melt emerging first from the nozzle of the extruder. The cold well is typically the largest diameter of the sprue with a depth of 1.5 times this diameter. Runner size 6.3 to 9.5 mm diameter. ... 

With cold-runner systems for TPs (or hot-runner systems for TSs), the projected areas include runners and sprues. For hot-runner TPs (cold-runner TSs), runners and sprue are not included. As an example, if the projected area is 132 sq. in., and a pressure of 4,000 psi is required, the clamp force is ... 

When designing molds for HIPS, and GPPS, the principles are very similar but these materials do tend to flow more easily. This results in smaller runner systems, e.g. 2.5 to 5.0mm (0.098 to 0.197in) in diameter, and thinner wall sectioned components. It can also withstand being flexed without cracking. Due to the flexible nature of HIPS, submarine type gates and reverse tapered sprue pullers are widely used for cold runner molds which are intended to cycle automatically. Small undercuts can be jumped or bumped off during ejection of the component. 

Once the interlocks are satisfied, the mold closes. Then the injection unit occasionally moves slightly forward to coimter a motion called sprue breakaway. With cold runner systems, the injection rmit is moved slightly away from the sprue bushing to facilitate sprue removal and/or prevent nozzle freeze-off. Thus, injection unit must be moved forward to provide intimate contact between the nozzle and sprue bushing during injection. Sprue breakaway is not used with hot runner systems. 

Molds are also classified by their runner systems. In a cold-runner mold, the sprue and runner solidify and are ejected with the part. Insulated runners have much larger diameters than standard runners. As a result, the outer part of the runner solidifies while the center remains fluid. This reduces the scrap generated during molding and facilitates the changing of materials or colors. Insulated-runner molds are more difficult to operate because the gate tends to freeze off. With hot-runner molds, the runner is always in the melt state. 

Direct gating to a cold runner in a multi-cavity mould. In this design, the CR is only partially replaced by an HR system. A simple open HR nozzle eliminates the sprue (Figure 1.3e), thus shortening the reqnired mould opening path and making it easier to separate the sprue protrusion from the moulding on the transporter belt. This method is used when the production volume or product shape does not jnstify the use of HR nozzles for each cavity. 

Example. In a typical 16-cavity mould (Figure 2.2) the quantity of waste for three types of gating will be compared. In layout (a), with cold runners throughout, the volume of sprue is some 25 cm in a type (b) mould with a simple HR system with four nozzles and... 

The application of this type of standard specification makes possible manufacturing of moulds with a minimum injection by the precise separation of the cold sprue from the hot cavity. The sprue markings are minimal thanks to the pneumatic needle sealing system. Cycle times can be reduced because of the high temperatures of the cavity. As an ARBURG standard for example, cold runner systems are offered with up to four gate points. 

If cold runner molds are used in preference to runnerless molds, for economic reasons, then a hot sprue bushing should be incorporated in the mold so as to reduce the material content wi in the feed system and allow faster cycle times to be achieved. 

Regranulation of the feed system of a cold runner type mold may be accomplished by using a work handling device or sprue grabber, so that when the feed system, when removed from the mold, is fed directly into the regranulator. Use of this technique ensures that minimal contamination occurs during the regranulation operation. 

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. 

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