Short shots

In the low pressure process, a short shot of a resia containing a blowiag ageat is forced iato the mold where the expandable material is aUowed to expand to fiU the mold under pressures of 690—4140 kPa (100—600 psi). This process produces stmctural foam products with a characteristic surface swid pattern produced by the coUapse of ceUs on the surface of molded articles. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

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... 

Short shot can be avoided by proper mold design and control of polymer melt conditions—namely, temperature and injection pressure. This relationship is shown in Figure 7.76. Within the area bounded by the four curves, the specific polymer is moldable in the specific cavity. If the pressure and/or temperature are too low, short shot will result. If the temperature is too high, thermal degradation of the polymer can occur. If the temperature is too low, the polymer will not be molten. If the pressure is too high or the polymer is too fluid, the melt can flow into the gaps of the mold, creating thin webs of polymer attached to the molded article in an undesirable part... 

In mold cavities with inserts and nonuniform thickness distribution, the flow pattern is more complicated. This problem was investigated by Krueger and Tadmor (6) using a thin, rectangular 1.5 x 6-in cavities like the one shown in Fig. 13.5. Inserts of various shapes and sizes could be placed in different locations in the mold, creating either obstacles to the flow or regions of different thickness. PS was injected, and the position and shape of the advancing front could be traced by a series of short shots, as shown in Fig. 13.8. 

Figure 13.8(a) shows the trace of the advancing front recorded from the experimental molded pieces in Fig. 13.8(b). The solid lines are the experimentally measure advancing front lines obtained from the short shots, the simulated results discussed later are marked by the x signs, and the broken curve shows the weld lines clearly visible in the finished products. 

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.]...

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. 

They also demonstrated that minimum cycle times are obtained at asrvvalues between 2 and 2.5. Boundary temperatures are found at asrv 2 for "short shot" and at asrv 4.5 for flashing (Fig. 24.6). 

Sherwood number, 56, 59 Shielding constant, 368 Shift factor, 443 Shish kebab structure, 705, 731 Shore D hardness, 831 Shore hardness testers, 838 Short-range order, 25 Short shots, 804 line region, 807 Short-term behaviour, 820 Short-time stiffness, 25 Side chain effects, 184 Significance of the shift factor, 450 Silky lustre, 876 Simha/Boyer rule, 100... 

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. 

MPX functionality is arranged into three modules, the first of which is the Setup Xpert, a module that allows users to perform a variety of injection-velocity- and pressure-phase-related setup routines to fix certain defects, such as short shots, flash, burn marks, sink marks, etc. The objective of Setup Xpert is to achieve one good molded part with no defects. The basic process is that a user molds a part, then provides feedback to the MPX system regarding molded part quality. The MPX system then processes this feedback along with data being collected from the machine and (if necessary) determines a process change that will improve the result. 

Using the coinjection procedure, a solid melt is injected to form the solid, smooth skin against the cavity surface. Simultaneously a second short shot melt with blowing agent is injected to form the foamed core. With a full second shot, the mold can incorporate pins or a mold that opens similar to high-pressure foam molding. 

Low pressure or short shot conventional IM are the most commonly used because they are easy and simple to operate. Suited for economical production particularly of large, complex, 3-D parts. Usual cavity pressure is 200 to 500 psi (1.4 to 3.5 MPa). An accumulator can be... 

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