Tunnel gate

Diaphragms or ring gates can be used for molding parts where concentricity is important such as cylindrical parts or parts where weld lines can not be tolerated. Pinpoint gates should not be used unless parts (small) are injected rapidly. Tunnel gating can also be used in such cases. Gate location should be at one of the points listed in Table 6.26. 

Figure T.IO Tunnel gate. (Ref Schwartz, S.S. and Goodwin, S. H., Plastics Materials and Processes, van Nostrand Reinhold, New York, 1982, p. 545)...

Direct side gating replaces cold tunnel gating (Figure 1.3b). Its advantage is that a single nozzle delivers plastic to several cavities, but its application is restricted to certain product shapes. 

If the effort for hotrunner systems are not justifiable, then a tunnel gate (Figure 1.36) which is the most common gate type, is being used. Advantageous is that the automatic separation of the runner from the molded part occurs inside the mold without any additional mechanism. 

Shortly before the cavity the runner of the tunnel gate leads into an angled drilling entering tunnel- like the cavity (Figure 1.36). Therefore, prerequisite for the application is the existence of a side or facing surface, which will accept the gate. A lateral injection will be necessary if the part has not a convenient aperture. 

FIGURE 1.36 Tunnel gate at the end of a sub-distributor, fed by a hotrunner nozzle a, b Runner ejector c restraining cone d runner e cutting edge f molded part g hotrunner nozzle [15]... 

FIGURE 1.37 Tunnel gate positioned in the ejector half of the mold... 

With a curved tunnel gate (also called submarine gate or cashew gate, Figure 1.38) it is possible to position the gate on the back of the molded part where the appearance is as little affected as by the markings of the ejector pins. 

Type of injection Tunnel gate (exchangeable) TT Film gate Fan gate Point gate ... 

If standards are used in areas subjected to wear, the maintenance costs are significantly reduced. In Figure 5.41, sprue bushings and tunnel gate inserts are shown as an example of parts with high stress and associated high wear. They are available from stock, save stocking up in-house, and thereby help reduce costs. 

FiGURE 5.41 Standards for tunnel gate and sprue bushing [1]... 

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