Mold filling short shots

Fig. 13.23 Two-color, PP injection, high-rate mold-filling short shots documenting fountain flow instability. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabilities in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

Fig. 13.30 Four stages of coinjection molding, (a) Short shot of skin polymer melt (shown in dark shade) is injected into the mold, (b) Injection of core polymer melt until cavity is nearly filled, as shown in (c). (d) Skin polymer is injected again, to purge the core polymer away from the sprue. [Reprinted by permission from Design Center, School of Engineering, Santa Clara University, Santa Clara, CA.]...

First the two cavity mold configuration was tested. Four different materials were evaluated and each molded at three different flow rates. The materials were Polycarbontate, ABS, PBT and Polypropylene. All materials were prepared and processed according to the manufacturers recommended conditions. Short shot samples were collected for each of the twelve runs performed on the two cavity mold. The short shot method is based on the 5 Step Process (4) where cavities are filled to about 70% fidl. No pack or hold is used as these are all short shot samples. Short shot samples are used as they provide clearly measurable differences in cavity filling. 

By contrast with ICM, a compression mold design is used where male plug fits into a female cavity rather than the usual flat surface parting line mold halves for IM (Fig. 8-27). The melt is injected into the cavity as a short shot thereby not filling the cavity. The melt in the cavity is literally stress-free it is literally poured into the cavity. Prior to receiving melt, the mold is slightly opened so that a closed cavity exists the male and female parts... 

The most characteristic feature of injection molds is geometrical complexity. In such molds there is a need to predict overall flow pattern, which provides information on the sequence in which different portions of the mold fill, as well as on short shots, weld line location, and orientation distribution. The more complex a mold, the greater this need is. The irregular complexity of the geometry, which forms the boundary conditions of the flow problem, leads naturally to FEMs, which are inherently appropriate for handling complex boundary conditions. 

In extrusion, when the hot melt exits from the die, the compressed gas expands and foams the extrudate. In injection molding, the hot melt is injected into the mold, but the quantity is reduced ( short shot ) so that there is not enough melt to fill the mold the compressed gas expands, and the melt foams and fills the mold. Either way, the foamed melt cools and solidifies, producing a product with a solid skin and a somewhat expanded internal foam structure. It looks like a solid product, but it is lighter in weight and offers a number of advantages. 

Short shot Mold not filled completely Inject pressure too low lack of material... 

Shore hardness See test. Shore hardness, short An imperfection that occurs, such as in a molded part, due to an incomplete fill. In molding, usually it is called short shot. 

Short shot n. In injection molding, failure to fill all cavities of the mold completely, caused by too low melt temperature, too low injection pressure, insufficient plasti-cation time, too constricted gates, too viscous resin, inadequate venting of cavities, etc. Short shots are often made deliberately when testing a new multicavity mold to reveal the pattern of runner flow and the sequence of cavity filling. 

Vents in the mold s cavities and runners permit air in the cavities to leave the mold ahead of the melt front. The absence, poor location, or clogging of vents can produce short shots (incomplete filling of the cavity) accompanied by char marks from burning of the trapped air (commonly referred to as dieseling). Because thin wall molds are filled very rapidly, vacuum is applied to the vents to improve removal of air in the mold. 

The short-shot process starts with the injection of the melt into the cavity (Figure 3). Once the cavity is partly filled with a short-shot to between 50% and 95%, the gas is injected. The gas is best conducted into areas where there are large accumulations of melt. The gas helps to fill the cavity by pushing the melt front further forwards. By this way, hollow cores can be generated in the molded part. The gas is kept under pressure until the molded part becomes dimensionally stable. During this phase, a constant pressure acts over the entire gas chaimel. After the melt is solidified, the gas pressure is removed, either by letting the gas... 

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