Cycles injection molding

In operation, plastic granules are fed to the machine through the hopper. Upon entrance into the barrel, the screw rotates and moves the granules forward in the screw channels. The granules are forced against the wall of the barrel, and melt 

Zheng et at. Injection Molding, DOI 10.1007/978-3-642-21263-5 l, Springer-Veilag Berlin Heidelberg 2011 

The clamp unit keeps the empty mold closed. The screw moves forward as a ram and forces the melt into the mold cavity. 

When the mould is hlled, the screw is held in the forward position or moves with a small displacement to maintain a holding pressure, during which time the material cools down and shrinks, allowing a little more material to enter the mold to compensate for volumetric shrinkage of the material. The packing phase is also called the holding phase. 

At some stage of packing, the gates have completely frozen. Eventually the cavity pressure is reduced to zero or a low value. The part continues to cool down and to solidify. Meanwhile, the screw starts rotating and moving backward, and the next plasticizing stage takes place. 

3 Cooling Stage This step is where the mold removes heat from the plastic, preparing the parts for ejection from the mold. The parts are generating levels of crystallinity and shrinkage as the SPS resin cools from melt to solid. Due to SPS higher thermal diffusivity values, the resin will cool faster than most semicrystalline materials. 

Poor screw recovery will lengthen cycle time, and cause excessive shear heating of the polymer from increased screw rotation. Polymer feed and conveying in a molding machine is the result of material, equipment, and process condition interactions. As such, all three areas should be involved in a thorough investigation of a poor screw recovery situation. 

Possible corrective actions are as follows (not specifically prioritized). 

5 Ejection Stage The ejection goal is to remove the part from the 

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Polycarbonate can be readily injection molded. Polycarbonates typically require only a short injection molding cycle time, because the polymer flows into the mold easily and solidifies rapidly. We injection mold polycarbonate to produce a wide variety of commercial goods, including compact disks, jewel cases, aircraft windows, kitchen utensils, and clear refrigerators drawers. Polycarbonates are also found in a wide range of disposable medical devices, such as the flow locks on intravenous tubes and the hard, disposable components of dialysis machines. Impact resistant polycarbonate is used to manufacture sports and other safety helmets. Glass fiber reinforced polycarbonate is used in the housings for power tools. 

This article is an overview of the novel technology of self-reinforced LCPs with polyesters, poly(ethylene terephthalate) (PET) and poly(ethylene naphtha-late) (PEN) [10-13, 21, 23], LCP/polyester blends in a polyester matrix form in situ fibrils which improve the mechanical properties. LCPs have an inherently low melt viscosity, and provide LCP/polyester blends that effectively lower the melt viscosity during melt spinning [24], and fast injection-molding cycles. The miscibility between the LCP and polyesters can be controlled by the degree of transesterification [25] in the reactive extrusion step, and fibril formation in LCP-reinforced polyester fibers has been studied. 

Stepto (1997) focused on the injection molding of potato starch including the basis of the process. In addition, the rheological behavior of starch/water melts during the refill part of the injection molding cycle was analyzed quantitatively to give apparent melt viscosities. Finally, the mechanical properties of molded starch materials and the dmg-delivery behavior of starch capsules were also discussed. 

The injection molding cycle is depicting in Figure 7.74. We can begin the cycle at any point we wish, but let us start at the point the screw moves forward and fills the mold with polymer melt. The screw moves forward and fills the mold with melt and maintains the injected melt under pressure, during what is called the hold time. To ensure that polymer does not flow backward, a check valve is attached to the end... 

Figure 7.74 The injection molding cycle. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing, Copyright 1979 by John Wiley Sons, Inc. This material is nsed by permission of John Wiley Sons, Inc.

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

Figure 3.41 presents the sequence of events during the injection molding cycle. The figure shows that the cycle time is dominated by the cooling of the part inside the mold cavity. The total cycle time can be calculated using... 

Sequence of events during an injection molding cycle. 

Figure 3.42 Trace of an injection molding cycle in a pvT diagram.

Figure 3.43 Trace of two different injection molding cycles in a pvT diagram.

Fig. 13.2 The injection molding cycle. [Reprinted by permission from R. C. Donovan, An Experimental Study of Plasticating in a Reciprocating-Screw Injection Molding Machine, Polym. Eng. Set, 11, 353 (1971).]...

The injection molding cycle involves a sequence of events ... 

Figure 4,1 3 Metal injection molding cycle (courtesy of Phillips Plastics)...

When this work began, an important application for TPU elastomers was in automobile exterior body components associated with impact absorbing bumper systems. Cold impact strength, resilience, resistance to heat distortion at paint oven temperatures and rapid injection molding cycles were all required. 

Molten polyesters show low viscosity and small extmdate swell. For these reasons, they have been blended with amorphous polymers to improve the latter s processability and chemical resistance. Elastomers have also been added to polyesters to improve impact resistance. Blends of polyester (either PET or PBT) with polycarbonate, PC, are the most popular (viz., Bay/oF, BCT4201, Calibre , Dialoy P, Ektar MB (with PCTG), Idemitsu SC, MakroblencT, MB4300, NovadoF, Pocan , R2-9000, Sabre , SC 600, Stapron E, Ultra-blencF KR, Valox , Xenoy 1000, etc.). Presence of PC in PET/PC increases crystallization rate of PET, which translates into faster injection molding cycle and lower part distortion upon demolding than those observed for neat PET. 

The article is now removed from the mold, and a new injection molding cycle can begin. 

The situation is somewhat different for elastomers. On the one hand the elasticity required in the application of molded rubber parts makes automatic removal within the injection molding cycle more difficult. On the other hand there are certain complicated article geometries, which it would be impossible to produce profitably by the injection molding method if it were not for the flexibihty of these parts. Examples include bellows and elbows for hoses. It is therefore often impossible to avoid the manual removal of rubber moldings, even in large-scale production. 

Removal is generally performed by automatic ejectors, which are operated by the machine when the mold opens in the parting plane at the end of the injection molding cycle. These ejectors ensure, that the article is separated from the surrounding cavity in spite of imdercuts, adhesive forces, and internal stresses which tend to retain the part within the cavity. 

As a reminder, we will once again explain what happens to the molding compound in the course of the injection molding cycle. 

But there are even more tasks for the mold. It must close tightly during the injection phase so that the melt flows only into the cavity and not into the parting plane. At the same time, however, it must be possible to open the mold so that the molded part can be removed from the cavity at the end of the injection molding cycle. 

The injection molding process consists of individual phases or stages which follow one another, overlapping to some extent, and are continuously repeated. We refer to a process of this type as a cycle and therefore speak of a repeating injection molding cycle. Nowadays, it is customary for the injection molding cycle to proceed automatically from phase to phase. 

Figure 7-1 shows the various phases of an injection molding cycle and their sequence. 

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