M&M wrote:Hmmm not seeing how that rig would be good for long hold times. Looks like all the injection pressure is generated with screw rotation, which causes lots of heat and shear if the melt is not allowed to exit the barrel as it is extruded. Further how much melt pressure can you get from a screw turning and not damage the melt? A few thousand psi max?
Not knocking the work put into the machine but gee does that really make sense?
Pressure depends on the machine and material, but blow out plugs or rupture disks may be sourced at ratings from 1,500 to 50,000 PSI.
I've seen extruders blow off heads and split barrels, so generating pressure may not be an issue. Isn't there a machine on the market that uses both extrusion AND traditional screw injection to get larger shot sizes?
I've always been amazed that Injection molding works at all, considering that the proper melting and plasticating of a melt depends so much on proper screw design and L/D ratio. Given that the effective L/D ratio is shortened as the typical molding machine moves plastic forward as it retracts, it’s a wonder that a proper melt is ever achieved.
Of course, “squirting” a less than perfect melt through a very small channel will increase sheer to the point that it hides any flaws in the original melt as processed from the screw
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