Trapezoid runners

To minimize pressure and heat losses, the runners should be round and as short as possible. Another preferred choice is trapezoid runners that are easy to machine and provide the same amount of resistance to flow as round runners. They should blend smoothly into the gates, without abrupt cross-sectional changes. The runner s length should be reduced and its diameter increased when the thickness of the molded part increases. A runner diameter of at least 6 mm is required for a 10 mm thick part. Thicker parts require runners that are 50%-100% larger in diameter than the thickness of the part. As in injection molding, pressure drop and material loss are de-... 

The channels through which the plasticized material enters the gate areas of the mold cavities are called runners. Normally, runners are either hill round or trapezoidal in cross section. Round cross section offers the least resistance to the flow of material but requires a duplicate machining operation in the mold, since both plates must be cut at the parting line. In three-plate mold designs, however, trapezoidal runners are preferred, since sliding movements are required across the parting-Hne ruimer face. 

The basic parts of the mold are illustrated in Fig. 5.69. The sprue receives the polymer melt from the nozzle and delivers it to the runners. The runners are as short as possible and provide minimum pressure and temperature drop from the sprue to the cavity. There are four basic types round, half round, trapezoid, and modified trapezoid. Round runners are the most efficient type, but are the most expensive because they must be machined into both plates of the mold. Since half-round runners produce the highest pressure drops and greatest cooling of the melt, they are seldom recommended. Trapezoidal runners are a compromise between round and half round, and modified trapezoidal runners merely have rounded bottoms. These runners are machined into only one mold plate, but reduce the pressure and temperature losses observed with half-round runners. Runner sizes are determined from the part-wall thickness. The diameter of the runner connecting directly to the cavity is typically equal to the part thickness, and each upstream branch is increased in diameter as " ... 

The runners are as short as possible and provide minimum pressure and temperature drop from the sprue to the cavity. There are four basic types round, half-round, trapezoid, and modified trapezoid. Round runners are the most efficient types but are the most expensive because they must be machined into both plates of the mold. Because half-round nmners produce the highest pressure drops and greatest cooling of the melt, they are seldom reconunended. Trapezoidal runners are a compromise between round and half-round, and modified trapezoidal runners... 

Spme and runners should be as small as possible to avoid rework of the polymeric material. The entrance diameter of the sprue should be larger in diameter than the nozzle diameter. Full round and trapezoidal runners have been used successfully to mold polymeric materials. Runner layout should be balanced and generously radius for smooth and uniform melt flow. Holdup spots can cause resin degradation, color changes, and property loss, especially in the flame-retardant and toughened grades. 

The runner or runner system is designed to allow for both rapid mold filling and minimum pressure loss. Various types of cross sections are used. Preferred cross sections are circular (full round) or trapezoidal since they both have minimum surface-to-volume ratios, which minimize heat losses and pressure drops. Figure 8-11 shows the relation between the dimensions of a trapezoidal runner. 

Full-Round Trapezoidal Modified Half-Round Rectangular Runner Runner Trapezoidal Runner Runner Runner... 

Ideally the gate should be positioned in line with the central melt flow. A full round will meet this requirement. The trapezoidal runners are not as satisfactory in this respect since the gate cannot normally be positioned in line with the central flow stream. 

For multi-plate moulds, however, positive ejection of the runner system is not practical. Therefore, the basic trapezoidal runner is always specified, the runner channel being machined into the fixed half from which it is pulled as the mould opens. In this way the runner is free to fall imder gravity between the mould plates. 

The cross-sectional shape of the runners is not critical provided that the cross-sectional area of the runner is not substantially greater than the equivalent round section. Trapezoidal or modified trapezoidal sections have a great deal to offer in terms of the control possible for the position of the runner when the mould opens, when they stay with the greatest surface area in contact with the mould. Flat section runners generate high-pressure differentials and engender the risk of scorch. 

The preferred types of runner system are full round or trapezoidal. However, do not make the runners too small (i.e. not less than 4 to 5mm (0.157 to 0.197in) in diameter) as this can create the need for high melt temperatures. The types of gates used for ASA are similar to those used for ABS. TTie size of the gate must be large enough to allow the application of hold-on pressure for the correct screw forward time typical gate depths of 0.6 to 0.7t (where t is the wall thickness of the component) are recommended. 

Runner n. In an injection or transfer mold, the feed channel, often branched to serve multiple cavities, and usually of semicircular or trapezoidal cross-section, that coimects the sprue with the cavity gates. The term is also used for the plastic piece formed in this channel. 

Spiral-flow test n. A method of evaluating the molding flow of a resin to injection or transfer molding in which the melt is injected into a spiral runner of constant trapezoidal cross-section with numbered and subdivided centimeter (or inches) marked along the runner. The mold is filled from a sprue at the center of the spiral and pressure is maintained until flow stops, the number just aft of the molded-spiral tip giving the flow distance. The spiral-flow test has been widely used since it was introduced in the early 1950s but has been standardized in USA only for thermosetting resins, in ASTM D 3123. 

The runner system is one of the most influential factors on the injection molding process and the properties of the injected parts. They are full round, half round, and trapezoidal, and, in general, full round runner are preferable. Runners are to be recycled hence they should be as short as possible and it is preferable to restrict the length and diameter to minimize the amount of material in the molding process. 

The runner system of a transfer mould refers to the series of channels that allow the distribution of the moulding compound to the separate chases and eventually the individual cavities. The runners emanating from the plunger/cull area which feed each individual chase of the mould are known as the primary runners and those that distribute the material from the primary runners to each individual cavity are known as secondary runners. A typical primary runner would be hemispherical or trapezoidal in shape with a width and a depth of 3-5 mm. The secondary runners are generally around 25-35% smaller in area than the primary runners. Runners are usually machined into the ejection side of the mould and are often equipped with their own ejector pins. Whenever possible, the runners are kept short to enhance the material flow and reduce the transfer time required to fill the cavities. 

Tool design generally follows conventional requirements for gates and runners. DSM recommends trapezoidal gates or, for wall thickness more than 3 to 5 mm, fuU sprue... 

Runners are produced in a variety of cross-sectional configurations (Fig. 7.13), but not all of them perform equally well. The best shape for the runner itself is a full-round section, cut in both halves of the mold. This is the most efficient form for melt flow without premature cooling. There are some instances when it is desirable to cut the runner only in one half of the mold, either to reduce the machining cost or where it is mechanically necessary over moving splits. In this case, the preferred runner sections are trapezoidal or modified trapezoidal. The half-round runner provides only a restricted flow channel combined with a large surface area for cooling and consequently is not recommended. 

The resulting values for equivalent hydraulic diameters (Fig. 7.14) clearly demonstrate the superiority of the full-round design for runners cut in both halves of the mold and the modified trapezoidal design for runners cut in one half of the mold. 

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. 

These are normally used when multi-cavities are involved and semi- or fully-automatic working is required. This type of mould, as its name suggests, has an extra plate (see Figure 4.4). This plate usually continues the gate on one of its sides with the complete runner system, preferably trapezoidal. The opposite side of the plate carries part of the mould form (usually the female part). 

As can be seen, the roimd and modified trapezoidal types of runners are the two most satisfactory designs, whereas the ratios exhibited by the semi-eireular and trapezoidal systems make their use less desirable. 

The main objection to the fully round runner is that the runner is formed from two semi-eireular channels cut into each half of the mould. It is essential that these channels are aeeurately matehed to prevent an imdesirable and inefficient ruimer system being developed. Because of extra machining etc., a fully roimd runner adds to the cost of the monld, but only marginally. A modified trapezoidal design is preferred if the runner is to be machined in only one half of the mould. 

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