Nozzle cold runners

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... 

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... 

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. ... 

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. 

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. 

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. 

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... 

Function. Central nozzles and manifold nozzles differ only in that the former are designed to interact directly with the injection machine nozzle. Both a central and a manifold nozzle may be in the form of a hot sprue bushing leaving a small sprue and intended mainly for gating to a cold runner. 

Injection nozzles for gating on to a cold runner do not have to meet such high requirements and so cheaper nozzles of simplified design with a rod-type sprue, also known as hot sprue bushings, may be used for this purpose (Figure 4.15a). 

Figures 4.15c-g shows how nozzles are used in cold runner gating. Nozzle 4.15(c) is a point-gate nozzle, while nozzles (d)-(g) are of the sprue-gate type. The sprue length may be reduced by shortening the nozzle, but it should be remembered that a longer sprue helps in nozzle control.

Figure 4.15 Open nozzles for injection into cold runner...

According to recent information [6], use of this sort of minimised hood has enabled hydraulic elbows to be moulded from PVC in a two-cavity mould with a nozzle made of molybdenum and injection in a cold runner. 

In this mould there are eight HR nozzles which are balanced by varying nozzle channel diameters in the 11-18 mm range, thus giving identical pressure drops at the ends of nozzles A-D. Cold runners and the moulding item zones corresponding to them are balanced in the form of separate systems. 

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. 

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. 

Support elements and device drives in customary standard machine versions may be employed for the installation of a silicone cylinder module. A drive for a hydraulic needle sealing nozzle must also be provided. With the use of cold runner nozzles with needle sealing or with a cold runner head, the needle sealing nozzle on the machine can be omitted. Figure 7.9 depicts a LSR cylinder equipment kit. 

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. 

The simple cold runner nozzle with hydraulic needle sealing system, as shown in Figure 7.11 makes possible either direct or indirect injection of silicone parts. Advantages in the application of this system are good thermal separation between the cold nozzle and the heated mould and the flat gate location on the moulded part. 

Simple and sturdy construction of the exchangeable needles and nozzles Optimal thermal separation between hot and cold modules Uniform force application to all nozzles through the central hydraulic cylinder Reduced mould costs, since installation with different moulds is possible Easy matching of new moulds to the same cold runner head Cycle-time savings... 

The deeisive problem with the applieation of an open eold ruimer system exists in thermal separation around the gate zone. It has proven to be veiy difficult to inject a moulded part directly with a cold runner nozzle. The vulcanisation process is not limited to just the part, but also continues in the gate zone of the eold runner nozzle. This vuleanised plug is fixed in the part at the next injeetion as an... 

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