Cold runner systems

The use of multicavity injection molds together with a cold runner system. The objective of the cold runner system is to prevent the runner system in the mold from curing during the molding process. The result is that the elastomeric compound held in the run-... 

Hot runner systems are therefore used in the processing of thermoplastics. However, the processing of elastomers involves the use of cold runner systems, which prevent the elastomer compound from cross-linking prematurely in that system (see Figure 4-5). 

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 ... 

Cold runner systems have a similar temperature control requirement to the plasticising and injection unit, since the cold runner acts as an extension to the nozzle, controlling the rubber temperature to preclude any build up of scorched material while it awaits injection into the mould. 

An alternative solution may be to use a cold runner system that feeds each group of cavities directly. Such systems work well with components with cross sections of a few millimetres. 

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. 

Multicavity tools can be built to make over 100 parts per cycle. These are complex molds that can cost hundreds of thousands of dollars. The cold runner systems in multicavity tools can be extensive, wasting plastic. The runner has to be separated from the part, handled, discarded, or reground for reuse if possible. An alternative to cold runners and a trend in the molding industry are hot runners or runnerless molds. Hot runners serve the same purpose as cold runners, ie. 

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.

Analogous to the hotrunner gate of thermoplastic materials, cold runner systems are used for thermosets and elastomers. These systems help to prevent runner losses. 

On the market, cold runner systems (CRS) are available for the processing of siii-cone rubber (LSR) and other elastomers. The processing of thermosets using CRS is still In development stage. 

A distinction is made between machine-and mold-constrained cold runner systems. Standardized cold runner systems contribute significantly to the economic side. The standard elements are shut-off nozzles optionally equipped with chokes that provide enormous economic and production advantages. 

FIGURE 2.72 Cold runner system, 16 cavities (company picture DME)... 

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. 

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. 

Typical injection molding stock temperatures for Vamac compounds at the nozzle should be in the range of 70°C-85°C. The cold runner systems should be set in the same range of 70°C-85°C. Scorch can be a serious problem if the temperature exceeds 100°C-110°C. Mold cavity temperatures are typically 175°C-190°C for injection molding cycle times of 1-3 min. 

Cold runner systems that are ready to use are offered as standard in the marketplace. These fimction especially well with needle sealing nozzles that are controlled hydraulically or pneumatically. In order to allow integration with the process sequence of the injection moulding machine, interfaces were developed for control of this type of cold runner. The machine control unit also monitors the position of the needles. 

Mould-integrated and machme-integrated open and closed cold runner systems (self manufacture)... 

Machine-integrated central cold runner systems with needle shut-off mechanism, standard (ARBURG)... 

As an alternative to the conventional injection system, cold runner systems are coming more and more into use in the processing of liquid silicone. These systems may be compared with the hot runner technology found in thermoplastic processing. 

Figure 7.15 Cold runner system as mould standard specification...

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. 

Cold runner systems are the state-of-the-art in mould technology. Here, mould-integrated or maehine-integrated cold runner systems with needle sealing nozzles are generally used. 

Because shrinkage is so dependent upon the production conditions it is usual to quote a range for each material. The actual value used to size a cavity may need to be obtained by producing a pilot cavity and then, producing components under production conditions. (If a molding is produced by a cold runner system, which is then changed to a hot runner system, then it will be probably found that different shrinkages are obtained. If these... 

Case 1 was analyzed with the two-pin gate cold runner system on the part. Case 2 was analyzed with the single-pin gate cold runner system on the part. The injection molding process eonditions for Cases 1 and 2 are listed in Table 2. 

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