Sprue system

One can see from Figure 2.10 that a three-plate mold operation necessitates removal of the ruimer and sprue system, which must be reground, and the material reused. It is possible, however, to eliminate the runner system completely by keeping the material in a fluid state. This mold is called a hot-ruimer mold. The material is kept fluid by the hot-runner manifold, which is heated with electric cartridges. 

Runnerless molds eliminate the runner system entirely (and usually the sprue system as well). The basic idea behind the runnerless mold is that, although sprue and runner channels are included in the mold, the sprue and runners are kept molten and are never formed as items that are ejected. The resin is maintained in the fluid state by various techniques, which include several types of supplementary heaters in the runner plate. 

Split molds Surface finish Sprue-system Positioning accvuracy... 

Figure 7.14 Conventional sprue system, mould-integrated...

The cold ruimer sprue system may be divided into categories. First of all, there are mould-integrated and machine-integrated, open and closed cold ruimer systems, which are primarily self-manufaetured. These systems are not generally available sinee they are utilised in speeial applications by individual process technieians. 

Solution Although this runner system is symmetrical, it is not balanced. If the runner had the same diameter throughout all sections, then the mouldings close to the sprue would fill first and would be over-packed before the outermost... 

This type of mould, also often referred to as a three plate mould, is used when it is desired to have the runner system in a different plane from the parting line of the moulding. This would be the case in a multi-cavity mould where it was desirable to have a central feed to each cavity (see Fig. 4.38). In this type of mould there is automatic degating and the runner system and sprue are ejected separately from the moulding. 

The runners and sprues are necessary in a mould but they are not part of the end-product. Unfortunately, it is not economically viable to discard them so they must be re-ground for subsequent reprocessing. Regrinding is expensive and can introduce contamination into the material so that any system which avoids the accumulation of runners and sprues is attractive. A system has been developed to do this and it is really a logical extension of three plate moulding. In this system, strategically placed heaters and insulation in the mould keep the... 

Figure 25 Schematic diagram of an injection/transfer molding machine [9]. (a) Hydraulic separation unit for upper mold plate, (b) Hydraulic separation unit for middle mold plate, (c) Shuttle system with automatic sprue nipple removal, (d) Brushing unit for cleaning middle mold plate, (e) Hydraulic ejector for automatic ejection.

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

Tube flow is encountered in several polymer processes, such in extrusion dies and sprue and runner systems inside injection molds. When deriving the equations for pressure driven flow in tubes, also known as Hagen-Poiseuille flow, we assume that the flow is steady, fully developed, with no entrance effects and axis-symmetric (see Fig.5.13). 

Inevitably, in multi-cavity injection molds, some of the mold cavity gates are located further than others from the sprue that delivers the melt from the plasticating unit. If the runner system that distributes the melt from the sprue to the individual cavities has a constant... 

The two become equal upon cessation of flow when the mold is full. The difference I — P2 indicates the pressure drop over the sprue and runner system. The pressure drop across the gate is given approximated by P2 — Pj,. We note that just downstream of the gate, the pressure P3 increases with time throughout the filling process (from about 0.4 s to 1.3 s). As Example 13.1 pointed out, such a pressure trace approaches conditions of constant filling rate. This is supported by ram position measurements, which were also retrieved at 0.02-s intervals. We further note that, upon mold filling, when P5 sharply increases, there is also a steep increase in all the pressures except the nozzle pressure, which is then reduced to 5500 psi. 

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 Sesame is designed so that molders can use a smaller runner and sprue, which gives them more control over the amount of plastic and pressure used to form the part itself. A smaller runner and sprue also means less material waste. While screw-and-barrel systems waste as much as 99.7% of the shot material, the Sesame wastes less than 80%. This is particularly important when molding expensive materials like biodegradable plastics, which cost as much as 10 per gram. The Sesame can handle any type of moldable plastic, as well as silicone rubber. Super-small medical parts that have been molded by the machine include ... 

The feed system is the flow melt passage in the mold, between the nozzle of the IMM and the mold cavity (Figure 17.1). This feature has a considerable effect on both the quality and economy of the molding process. The feed system must conduct the plastics melt to the cavity via a sprue, runner and gate at the correct temperature/pressure/time period, must not impose an excessive pressure drop or shear input, and should not result in non-uniform conditions at the cavities of multiimpression molds. 

The primary advantage of these systems is that all material goes into the part, that is, none is lost to scrap in runners and sprues. For molds designed to run millions of cycles, a tremendous cost savings is realized. The main disadvantage is that the molds are more costly to design and build, and they require a separate temperature-control system. 

Celiac disease, now called sprue or gluten enteropathy, is probably due to a genetic flaw that entails immune system dysfunction. The people who get this disease have bowel reactions to the chemical gliaden in the gluten of wheat, barley, rye, and oats, with inflammation and mild-to-severe damage to the small intestinal wall. Symptoms include irritability, abdominal distress and distention, vomiting, diarrhea, terrible gas problems, and weight loss. 

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