Runner Systems

Even though a molder has received a mold intended to meet part performance requirements at the lowest cost, it may not be cost-efficient because of a poor runner design. One can simulate the gate and runner design on commercial CAD programs such as Moldflow before the runner system is built. Once it is built, very limited rework can be done. 

Nedox electroless nickel Solution treatment followed by TFE impregnation used on copper and ferrous alloys 

TFE ceramic Spray and bake application used for all die materials that can withstand 250°C bake 

Tungsten silicide Solution treatment used on steel and ferrous alloys 

A general, though not absolute, rule is if the number of cavities (N) is such that 

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. 

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. 

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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. 

Assuming that the mean effective pressure also applies to the runner system, then... 

Hence total clamp force for 4 cavities and 1 runner system is given by... 

In this case the runner system will be almost totally in the shadow of the projected area of the cavities and so they can be ignored. 

The rubber compound is fed into the machine and the screw loads a known volume of rubber into the cylinder. When the cylinder is completely filled, the screw then acts as a ram to force the rubber compound into the runner system and the mold. 

Over the years the trend has been toward an increasing use of the more expensive elastomers. Concern has been expressed about the cost of waste compound when using a multicavity mold and injection molding, primarily due to the amount of elastomeric compound in the runner system. There are three approaches to overcome the problem ... 

The use of small injection molding machines with a single cavity mold. This approach removes the need for a runner system and, when using a dia-... 

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-... 

Figure 24 Seal/flash cap/runner system configuration from four cavity mold.

As reviewed in other chapters, different plastics have different melt and flow characteristics. What is used in a mold design for a specific material may thus require a completely different type of mold for another material. These two materials might, for instance, be of the same plastic but use different proportions of additives and reinforcements. This situation is no different than that of other materials like steel, ceramics, and aluminum. Each material will require its own cavity shapes and possibly have its own runner system. 

Molds are used in many plastic processes with many of the molds having common assembly parts (Fig. 8-11). Many molds, particularly for injection molding, have been preengineered as standardized products that can be used to include cavities, different runner systems, cooling lines, unscrewing mechanisms, etc. (Table 3-17). 

In the reinforced RIM (RRIM) process a dry reinforcement preform is placed in a closed mold. Next a reactive plastic system is mixed under high pressure in a specially designed mixing head. Upon mixing, the reacting liquid flows at low pressure through a runner system to fill the mold cavity, impregnating the reinforcement in the process. Once the mold cavity is filled, the plastic quickly completes its reaction. The complete cycle time required to produce a molded thick product can be as little as one minute. 

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. 

In commercial use are a wide variety of machines which transfer fixed volumes of compound to clamped moulds, by the operation of a ram in a cylinder fed with pre-heated slugs. These machines are capable of high speed semi-automatic cycling and are a great advance over the use of loose transfer moulds in conventional presses. The components which are produced are to a large degree flash-free, it only being necessary, in the majority of cases, to remove the injection feed and runner system. 

The injection-molding press was producing a part and runner system that had a mass of 2.15 kg. The mass was plasticated using a 120 mm diameter, 8L/D screw. The screw used for the process had a barrier melting section that extended to the end of the screw, as shown by the specifications in Table 11.9. That is, the screw did not have a metering channel. Instead, the last sections of the barrier section were required to produce the pressure that was needed to flow the resin through the nonreturn valve and into the front of the screw. The specific rotational flow rate for the screw for the IRPS resin was calculated at 9.3 kg/(h-rpm) based on the depth of the channel at the end of the transition section. The screw was built with an extremely low compression ratio and compression rate of 1.5 and 0.0013, respectively. For IRPS resins and other PS resins, screws with low compression ratios and compression rates tend to operate partially filled. The compression ratio and compression rate for the screw are preferred to be around 3.0 and 0.0035, respectively. The flight radii on the screw were extremely small at about 0.2 times the channel depth. For IRPS resin, the ratio of the radii to the channel depth should be about 1. 

As discussed in Section 11.12.2, several root causes exist for black streak contamination in molded parts. These root causes include degradation of the resin in the screw channels and nonreturn valve or runner system, degraded material entering with the resin feedstock, and poorly dispersed pigments in the color masterbatch. To determine if the color concentrate masterbatch was the source of the black streaks, the color concentrate was removed from the feedstock and only natural... 

The modified screw was placed back into the injection-molding press and evaluated for performance. The barrel temperatures were maintained at 245, 255, 260, and 260 °C for the feed zone through the last barrel zone, respectively. This temperature setting was lower than that used for the original screw. The screw was rotated at a speed of 235 rpm, and the back pressure was set so that the pressure at the tip was 10 MPa. The 0.244 kg part and runner system was plasticated in 4.2 s for a specific rate of 0.89 kg/(h-rpm). All parts produced were completely free of the splay defect. The modifications were able to eliminate the bubbles and the unmelted material. 

The function of the runner system is to transmit the molten polymer to the cavities with a minimum of material and pressure drop. Therefore, the runner length must be... 

Figure 3.49 Schematic of different runner system arrangements.

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