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Two-Stage Injection Molding

Author:Haifly Machinery Date:2010-4-25 23:37:16
My favorite part about working on R&D projects is that they tend to challenge you to think outside the box, try new things, and learn about the latest technologies.  One of our recent development projects involved injection molding a long, thin-walled tube (picture a miniature drinking straw) with a wall thickness that shrinks down to .0035” over its nearly half-inch length.  By comparison, that’s roughly the same thickness as a human hair.  Even after running dozens of Moldflow® studies for gating locations and flow analysis, the only thing we were confident of was that it was going to be a challenge to fill the parts out completely.
After struggling on our first sampling, the instinct was to look for higher flowing materials to help make the distance more manageable.  We started with a PE material with a Melt Flow Rating (MFR) around 50 g/10min and then moved on to a similar material with a MFR of 110.  We were expecting to see a noticeable improvement in the 110, but what we found was no appreciable difference on the fill.  It was determined that this was primarily due to leakage at the check ring / non-return valve, common to all traditional, reciprocating screw injection machines.
This brought us to one of the more interesting suggestions on the project.  We decided to sample the tool in one of Sodick-Plustech’s (SPT) micro injection machines.  This machine piqued our interests initially because of its two-stage (plunger-style) injection approach, but as we found is well-suited for this type of application for several reasons.
Like a traditional, reciprocating screw machine, Sodick’s two-stage injection technology (shown here) utilizes a small screw to melt and convey material.  But unlike traditional machines the screw is not responsible for injecting plastic into the mold (or any high speed lateral movements).  It feeds a second chamber, which is metered precisely, and then injected into the mold via a high-speed piston.  The feed screw shuts off after material is loaded into the chamber, which eliminates back-flow without the use of a check ring / non-return valve.