Mold filling jetting

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... 

K. Oda, J. L. White and E. S. Clark, Jetting phenomena in injection mold filling mold filling. Polymer Engineering Science, 16, 585-592 (1976). 

Figure 15.23 Jetting in injection-mold filling. (From Ref. 30)...

When the mixture of monomers is cast in an open mold, the air bubbles formed at the jet nozzle in the mold usually have enough time to leave the material due to the low viscosity of the reactive liquid. When more viscous oligomers and prepolymers are used, particularly in the case of low-lifetime reactive mixtures, it may be necessary to use some simple procedures, for example, filling through a pipe immersed into the mold, to prevent the formation of air bubbles in the product. 

It was established experimentally that a jet-type of flow appears when the characteristic size of the stream is less than the minimal gap between the walls of the mold.277 This proves that the cause is related to the ratio between the size of the injection nozzle and the coefficient of swelling of the elastic liquid moving inside the mold rather than to the axial momentum of the moving liquid. This conclusion is also supported by evidence that highly filled polymers, which are less elastic than pure melts, form jet-like patterns at lower flow speeds. 

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. 

KINEL 5504 Is 65% filled with quarter-inch glass fibers, exhibits the highest mechanical properties of any KINEL (49,500 psi flexural strength and 3.25 MSI flexural m ulus at room temperature), and is generally compression molded. Current commercial end-uses include jet engine parts typical of these are the blocker doors used for retro-thrust in the Rolls-Royce RB.211 engines of the Lockheed L-1011. 

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. 

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