Solidifying runner

Molds with different operating actions include sliders, unscrewing devices, and knockout systems to eject products as well as solidified runners at the proper time. These basic operations in turn require all kinds of interactions such as fill-time and hold pressure (Figure 22). 

In this type of insulated runner mold design, the runner plate and cavity plate are bolted together and are only opened to remove the solidified runner after breaks in the molding cycle or to remove fireeze-ups. 

If the injection molding compound solidifies in the feed channel, the sprue has to be demolded, either in combination with the molded part or already separated from the molded part, which can be done using a mechanism inside the mold. All of the solidified runners need a correspondingly higher injection volume on the machine side. 

In practice there are a number of other factors to be taken into account. For example, the above analysis assumes that this plastic is Newtonian, ie that it has a constant viscosity, r). In reality the plastic melt is non-Newtonian so that the viscosity will change with the different shear rates in each of the three runner sections analysed. In addition, the melt flow into the mould will not be isothermal - the plastic melt immediately in contact with the mould will solidify. This will continuously reduce the effective runner cross-section for the melt coming along behind. The effects of non-Newtonian and non-isothermal behaviour are dealt with in Chapter 5. 

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... 

Hot Runners. When the molten plastic is pumped into the water-cooled mold, the cooling system solidifies both the plastic product in the mold cavities and also the plastic material in the runners. Later the solid runners must be separated, reground, and reused. This is an extra burden on the process. An alternative is to avoid cooling the runners, and actually keep them hot, so that the molten polymer in them remains ready for the next shot into the mold. 

The term IM is an oversimplified description of a quite complicated process that is controllable within specified limits. Melted or plasticized plastic material is injected by force into a mold cavity (Figure 4.1). The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels or runners which direct the flow of the melted plastic to the individual cavities (Chapter 17). The process is one of the most economical methods for mass production of simple to complex products. Three basic operations exist. They are the only operations in which the mechanical and thermal inputs of the injection equipment must be coordinated with the fundamental behavior properties of the plastic being processed. These three operations also are the prime determinants of the productivity of the process since manufacturing speed will depend on how fast we can heat the plastic to molding temperature, how fast we can inject it, and how long it takes to cool (or solidify) the product in the mold. 

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... 

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... 

In injection moulding processors often use multi-cavity moulds to make several products in one cycle. The runners required to supply material to each cavity contain a considerable amount of material which, if the product itself is small, may be of the same mass as the parts themselves. This is evidently quite uneconomic. Hot rurmer systems solve this problem by keeping the material in the runners from solidifying between moulding cycles. 

The use of thermoplastics material requires that the mould be maintained at a constant temperature - usually around 75 °C to 95 °C - to cool and solidify the material within the mould before the moulding can be ejected. This is achieved by circulating water through the mould and makes the process much faster than compression moulding. Although material is used in the sprue and runners, material wastage is low since it can be reused. 

The mould is now ready for pouring. When the metal has solidified, the mould is broken up to release the casting. The runner and riser are broken off, and the rough edges are removed by fettling (Le. hand grinding). 

The process starts with the production of an expendable wax pattern of the shape required which is then coated with a refractory material to produce the mould, which is allowed to dry. The mould is heated, melting the wax, which is allowed to run out and so produce a cavity. Further heat is applied to fire the mould before pouring the casting metal to fill the cavity left by the melted wax. When the molten metal has solidified, the refractory shell is broken away to release the casting. The casting is cut away from any runners and is dressed or fettled and finished as required. 

The runner is a channel in the die through which the molten metal fills the die cavity. The term is also applied to the metal that solidifies in this channel. The gate is that part of the die through which the metal enters the die cavity from the runner. 

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. 

With TS closed molds (TM, IM, etc.), the complete mold can be at the maximum heat, which cures and solidifies both the runner and the molded parts. With TSs, this runner is called a hot runner. If the mold is designed so that the maximum heat surrounds only the cavity and the lower heat (similar to the lower pot heat) surrounds the runner, that is called a cold... 

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