Injection molding jetting

The so-called RIM-process (reactive injection molding) is a current realization of the reactive molding process. The heart of the process is the shock mixing of the reactive ingredients, which is achieved by collision of two jets injected at a pressure of 10 - 20 MPa. The reactive mixture is injected into the mold in a laminar flow regime the pressure at this stage does not exceed 0.1- 0.4 MPa.259 The practical development of this method relies on automatic control systems and modem high quality equipment. 

The rapid blending of two steams that is achieved by impinging jet technology, as developed for reaction injection molding by Edwards (1984) was adapted for crystaUization by Midler et al. (1994) and further developed by others [examples Mahajan and Kirwan (1996),... 

The injection molding machine should be capable to deliver a shot size of 50% or less of the injection stroke. Typically, the machine should be able to inject its rated capacity in less than 2 seconds, except for high viscosity resins or when warping or jetting must be prevented by slower injection rate. Depending on the grade, the cylinder temperature setting should be between 190 and 220°C. 

Technically speaking, flow casting would be a more accurate term than injection molding, since the mold is filled across a melt front and not, as in metallic diecasting (better injection molding), by a jet of liquid. 

Two-impinging-jets are also used in reaction-injection-molding processing equipment to provide good micromixing for viscous fluids [3-6]. It is one of the techniques used to reduce the timescale of micro-mixing in precipitators [6]. 

Injection molding machines Resin should be dried < 0.02 %. Do not use resin that has been out of the dryer for > 20 min. Cold molds are difficult to fill and require higher injection pressures. Hot molds, generally, give better finish and less molded-in stress. Melt temperature is very sensitive to very small changes in rpm or back pressure despite sensor or controller set point. Measure with hand held pyrometer or laser sensor. Need slower fill rate for spme-gated parts to prevent blush, splash or jetting. If the walls are > 5 mm, then slow fill helps reduce sinks and voids. 

It has been long realized that there are two regimes of injection mold filling. These involve a slowly expanding front or jetting into a mold. Oda et aL [Ol] have shown that jetting into a mold is the result of the polymer or compound having a low extrudate swell. The often complex occurrence of this swell is associated with the melt temperature and its variations in runners. Isayev and his coworkers [13, 14, 16] have extensively studied pressure... 

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