Injection molding cold shot

Two-shot injection molding n. Confusingly, this term has been used in the literature for two processes that are distinctly different. One is described under double-shot molding. In the other process, one first injects a metered amount of one material into a single-cavity mold. As this material just begins to chill against the cold mold surfaces, a second material is injected. This fills the interior and forces the first material outward to the cavity surfaces. The second polymer, usually a reclaimed material forms the interior of the finished article, while the virgin material first injected forms the outer shell and surface of the article. 

Since a hot runner produces considerable thermoset scrap, it may be replaced by a cold runner that keeps the molten resin at 66 to 99°C, still usable for the next shot. Another innovation is injection-compression molding, where the mold is left about 1/4 in open during injection and then clamped shut to finish the cycle by compression this reduces flow orientation and improves impact strength. 

MOLD CAVITY DID NOT COMPLETELY FILL WITH PLASTIC This is called a short shot and there are a couple of possible reasons for it occurring. When the hot plastic hits the cold surface of the mold cavity it solidifies very quickly. If you do not inject the plastic into the mold fast enough or if you stopped pulling the injection lever before the mold cavity was full, the charge would solidify prematurely. 

Short shot Partial fill of part Low injection speed, low dosage stroke, cold mold... 

To obtain a short cycle time, the hot polymer is injected into a relatively cold mold. As a result, the polymer will already be starting to soHdify on the wall of the mold while it is shll being injected. This will cause an additional resistance to the flow. In extreme cases, the resistance to the flow may become so great that the mold will not fiU completely. This is described as a short shot . 

Initially, runs were performed at injection velocities ranging fi om 150 nnn/sec (76.0 ccm/sec) down to 25 mm/sec (12.7 ccm/sec) using a standard cold runner mold that made ASTM test specimens. The barrel temperatures followed the 295°C profile provided in Table 1. During injection the screw traveled 50 mm (25.3 ccm) and the transfer position was adjusted slightly to maintain a 23 mm cushion and acceptable parts. It was determined by color tracing that it took three cycles for plastic to travel fi-om the feed zone into the forming shot. 

---