Cold runner molding

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

Typically, the selection of gate location and the decision between cold runner molds (two-plate or three-plate molds) or hot runner molds are important to the product design when considering the mold cost and the plastic flow within the mold. Today, about 90% of all molds built are still two-plate molds, which are considerably cheaper than three-plate or hot runner molds. For many reasons, in most cases, hot runner molds are better, but these questions must always be asked ... 

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. 

Full round runners are preferred where typical diameters of 6 to 8mm (0.236 to 0.315in) (for component wall thicknesses of up to 4mm (0.157in)) and 8 to 12mm (0.315 to 0.472in) (for component wall thicknesses of greater than 4mm (0.157in)) are recommended. Z type, cold slug well pullers are preferred for cold runner molds. 

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. 

Although runners should deliver melt to each cavity at the same time, they may be naturally balanced or artificially balanced. In naturally balanced runners the distance from sprue to all gates is the same, whereas artificially balanced runners have the same pressure drop from sprue to all gates. Cold runner molds also incorporate cold slug wells at each turn in the melt flow. These collect the cold melt from the sprue plug (melt frozen at the nozzle), thereby preventing this melt from entering the cavities. 

Figure 11-30. Gate breaking and clipping are low-cost removal methods when the part design permits locating a recessed gate in a hidden area, as in this handle. This type of problem is eliminated by using hot runner molds for TPs and cold runner molds for TSs.

The most sophisticated type of gate is valvegating. It is used in both hot and cold runner molding. The principle of valve-gating is that the opening and closing of the gate is controlled by mechanical means such as hydraulic or pneumatic... 

Examples of molding circuits and automation circuits Mold Desi0n and Maintenance for Dia fnostics (20 hour Module) Design considerations Mold design basics Cavity and core construction Heat transfer considerations Cold runner molds Hot runner systems Freeing mechanism/part ejection Mold maintenance... 

Beaumont, J. P., Revolutionizing Runner Design in Hot and Cold Runner Molds, SPE-ANTEC, May 2001. 

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. 

Initially, runs were performed at injection velocities ranging fi om 150 nnn/sec (76.0 ccm/sec) down to 25 mm/sec (12.7 ccm/sec) using a standard cold runner mold that made ASTM test specimens. The barrel temperatures followed the 295°C profile provided in Table 1. During injection the screw traveled 50 mm (25.3 ccm) and the transfer position was adjusted slightly to maintain a 23 mm cushion and acceptable parts. It was determined by color tracing that it took three cycles for plastic to travel fi-om the feed zone into the forming shot. 

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