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-... 

One approach to overcome the compound waste issue has been the development of the cold pot system, which has the same result as the cold runner ... 

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... 

What advantages are there to using a hot runner mold rather than one that has cold runners ... 

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. 

The principal types are two-plate, three-plate, and stack molds. Others include the family mold that has multiple cavities of different shapes in one mold.3, 324 A further distinction concerns the feed system that can be either the cold or hot type. These classifications overlap. Three-plate molds will usually have a cold runner feed system, and a stack mold will have a hot runner system. Two-plate molds can have either feed system. 

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... 

Injection moulding Mater-Bi can be injection moulded using normal injection presses, with cold runners or hot chamber injection systems. The maximum injection temperature is less than 200 °C. Novamont claims that about 10% of the scraps can be reused in injection moulding, which is about the same as traditional plastics. Mater-Bi can be coloured using the Mater-Bi-based, biodegradable masterbatches. 

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],... 

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. 

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... 

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 ... 

However, if the upper mold plate and upper part of the middle section are maintained at temperatures well below what would initiate crosslinking, the residual cake can be integrated in the following cycle (cold runner transfer mold). 

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. ... 

Mo niiistrated here are cold runner two-plate, cold runner... 

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 ... 

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). 

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. 

Unfortunately, solidified TS runners, flash, and defective parts cannot be recycled as they are with TPs. Some operations can use this material as a filler after it has been granulated but in practically all molding plants using TS, the material is lost and has to be considered in the part cost. One should consider using cold runners with TSs. Materials must be of a type that will permit their use for example, the time cycle should permit material to remain in the fluid state. As different hardeners, accelerators, and so on, can be used, it is practical to use cold runners. They have limited use in TM but are used more widely in IM. 

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. 

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. 

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. 

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