Injection molding Cold runner

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-... 

Between the injection nozzle on the end of the extruder and the gate that leads into the mold cavity the polymer flows through a channel known as a runner . It is vital that the polymer does not solidify in the runner before the mold is completely filled. We can prevent premature solidification in the runner in one of two ways we can use a large diameter unheated (cold) runner in which the polymer solidifies after the polymer in the mold cavity, or we can use a heated (hot) runner in which the polymer does not solidify. Moldings that are produced... 

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. 

Most molds for injection molding are unique in design, which depends on application, fluoroelastomer compound, and feed system (hot or cold runners). Standard systems are distinguished as two plate, three plate, or stack molds [58],... 

The runner size depends on the material being processed and whether it is a TS cold or hot runner. As reviewed in Chapter 2 and Fig. 2-7, with TPs a hot runner solidifies with the injection molded part. If a cold runner is used (with TP), only the molded parts solidifies there is no runner scrap. 

Since a hot runner produces considerable thermoset scrap, it may be replaced by a cold runner that keeps the molten resin at 66 to 99°C, still usable for the next shot. Another innovation is injection-compression molding, where the mold is left about 1/4 in open during injection and then clamped shut to finish the cycle by compression this reduces flow orientation and improves impact strength. 

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. 

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. 

Runnerless Injection Molding n (1) In molding thermoplastics, a process in which the mnners are insulated fi-om the cavities and kept hot, so that the molded parts are ejected with only small gates attached. See also Hot-Runner Mold. (2) In thermoset molding, the same as Cold-Runner Injection Molding. 

Like thermosets the elastomers will only cross-link in the mold by introducing additional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the... 

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. 

Cold Runners Cold runners are machined into the mold plates therefore, the temperature corresponds with the overall mold temperature. The material in the mnner freezes after injection, and needs to be demolded with the part after each injection molding cycle [8]. The optimum runner cross-sectional shape for SPS is the full-round mnner profile shown in Rgure 14.11. The trapezoidal and modified trapezoidal profiles are common alternatives. 

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. 

Tooling. Tooling is relatively expensive for injection molds. Production molds are normally machined and burned (electro-discharge machined) from tool steels (P20) and must be sfructured to withstand high pressures. Precision is required to incorporate cooling lines, ejector pins, and cavity pressure transducers or temperature monitoring sensors. The use of hot runner manifolds adds further complexity and cost, but may be justified because cold runners and their associated costs are eliminated in manufacturing. 

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. 

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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