Molded plastic, ejection cycle times

The mold is cooled, usually by water, to solidify the plastic. When the plastic object has been cooled sufficiently to maintain its shape, the mold is opened, along the mold parting line, and the object is ejected. The cycle time in injection molding depends on the size of the molded part, as well as on the molding conditions and on the thermal and mechanical characteristics of the plastic being molded. In some case, the cycle time can be as short as two seconds. 

Two other significant costs in the manufacture of micromolded parts are material costs and operational costs. The material cost associated with micromolding is piimaiily the cost of the plastic from which each part is made. Therefore, the excess volume of the part should be mininiized so as to use the minimum amount of material. The operational cost is directly related to the cycle time for each part. In typical micromolding runs, 70-80 % of the total machine cycle time is spent waiting for the molten thermoplastic to cool and solidify in the mold before part ejection. 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

Fig. 19.35. Schematic of injection molding operation. A. When the mold starts to close, the screw has just finished charging the front end of the cylinder. B. With the mold closed, the heating cylinder then moves toward the sprue bushing and is ready to commence the injection cycle. C. The hydraulic cylinder then forces the screw forward under the injection head and fills the cavity. D. Cooling and charging cycle the mold is filled and, after the holding time, the screw starts to rotate and charge the front end of the cylinder. E. The screw is still charging the front end of the cylinder, and the cylinder starts to move away from the sprue bushing. F. The finished cycle the mold opens and the molded part is ejected with the sprue bushing. Du Bois, J. H., and John, F. W., Plastics, Reinhold, 1967 by permission of Van Nostrand Reinhoid Co.)...

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