Molding cycles

Set up the complete molding cycle to repeatedly meet performance requirements at the lowest cost by interrelating material/machine/ mold controls. 

Between 250 and 450°F (121 and 232°C), plastics used include glass or mineral-filled phenolics, melamines, alkyds, silicones, nylons, polyphenylene oxides, polysulfones, polycarbonates, methylpentenes, fluorocarbons, polypropylenes, and diallyl phthalates. The addition of glass fillers to the thermoplastics can raise the useful temperature range as much as 100°F and at the same time shortens the molding cycle. 

Inserted nameplates Unit cost high Labor cost high Investment moderate Partially limited Restricted Not critical Good durability Longer molding cycles Allows three-dimensional as well as special effects. 

Blistering Demolded too soon Improper catalytic action Extend molding cycle Check resin mix for accurate catalyst content and dispersion... 

Are secondary operations required except to remove sprue No No Yes, e.g., removing material where a window is required (often done within the molding cycle)... 

We can explain the injection molding process with reference to the molding cycle illustrated in Fig. 13.2. The cycle consists of four stages injection, packing, cooling, and ejection. 

An injection mold performs several functions during the molding cycle. In addition to giving the polymer the desired shape, it distributes the molten polymer, cools it, and ejects the product. 

The extrusion blow molding cycle is illustrated in Fig. 14.2. The extrusion component of the cycle is normally continuous. As soon as one length of parison has been captured by the mold, another length starts to form. To allow room for a new length of parison to emerge from the die, the mold moves aside as soon it has captured a parison and the knife has severed it. The mold is rapidly translated to a remote blowing station where inflation takes place. After the product is ejected, the open mold moves back under the die where it surrounds and captures another length of parison. 

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 technique for monitoring the dielectric loss factor is relatively simple. Two metal electrodes are placed opposite each other at critical locations on opposite sides of the mold. When the sheet molding compound (SMC), is placed between the electrodes, a capacitor is formed. The dielectric power loss is monitored continually throughout the molding cycle, as outlined in Section 6.1.2.2. 

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 process provides fast molding cycles, unlimited shelf life for the sheet, large part capability, and design flexibility. The process also allows for scrap materials to be recycled. Trim waste from the molding operation and defective parts can be ground up and recycled into the basic sheet process in controlled amounts. Some of this waste has also been used as input for injection molding. 

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