Runnerless mold

These materials have been successfully molded in all types of mold, for example, 2-plate, 3-plate, insulated runner, stack and hot runner, en using the hot manifold type of runnerless mold, the runner channel must be streamlined so that there are no dead spots (nil flow areas) which are likely to cause degradation accurate temperature control of the manifold is essential. To maintain the sparkling clarity of this material, use vacuum smelted steel as this can take a very high polish and is non-porous. 

Runnerless molds are highly suitable for cellulosic materials however, the types of components that are used for runnerless type molds (for example toothbrush handles etc.) require to be produced in many different colors. One of the main problems associated with runnerless type molds is the amount of material that is wasted during a color change and therefore the selection of the specific, runnerless design is very important with respect to overall production costs. To overcome this problem, the use of an insulated, hot tip, runnerless-type design should be considered for cellulosics. 

Mineral-insulated cable heaters are often coiled and used on hot sprue bush-ing/runnerless molding nozzles. The coil heaters are capable of high operating temperatures, up to 815°C. 

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. 

Another gate valve is the spear system in which the probe, using electronic controls, alternately melts and freezes the gate as required by the cycle. Both techniques, and other similar ones, are particularly suitable for use in runnerless molding. 

Multicavity tools can be built to make over 100 parts per cycle. These are complex molds that can cost hundreds of thousands of dollars. The cold runner systems in multicavity tools can be extensive, wasting plastic. The runner has to be separated from the part, handled, discarded, or reground for reuse if possible. An alternative to cold runners and a trend in the molding industry are hot runners or runnerless molds. Hot runners serve the same purpose as cold runners, ie. 

The machine must be capable of consistent repetitive action. All molded parts, gates, and runners must be ejected automatically. There is usually some method of separating runners and gates from molded parts when runnerless molding is not used (i.e., wiper mechanisms, air blast, etc.). Some systems weigh the part after ejection and stop the machine or sound an alarm on off-weight parts. 

Figure 7-6. Runnerless Mold in Which No Additional Heat Is Supplied to Runner (Wide diameter insulated runner allows the outer circumference of the melt to solidify against runner walls while the central core remains molten in preparation for the next shot.f...

Figure 7-7. Heated Probes Added at Gate of Runnerless Mold Which Also Uses an Insulated Runner (Referred to as runnerless since no scrap runner is produced with part) ...

Ernest J. Csaszar, Runnerless Molding Without Hangups , SPE Journal 28 (February 1972). 

Runnerless Mold in Which No Additional Heat Is Supplied to Runner. 7-6... 

Heated Probes Added at Gate of Runnerless Mold, Which Also... 

If cold runner molds are used in preference to runnerless molds, for economic reasons, then a hot sprue bushing should be incorporated in the mold so as to reduce the material content wi in the feed system and allow faster cycle times to be achieved. 

PP is readily use for runnerless molds due to its wide processing range and good thermal stability. The comments on runnerless molds for this material, are very similar to that given for HDPE. 

PP-EPDM is readily use for runnerless molds due to its wide processing range and good thermal stability. 

The insulated runner type of runnerless mold and certain types of hot sprue bushings can be used for UPVC, however careful consideration needs to be given to the relative thermal stability of the molding compound (i.e. the type of stabilizer system used) and the anticipated cycle time at which the mold is expected to run at before deciding which approach to opt for. 

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