What is residual melt cushion for thermoplastic processing?
Cushion is used to ASSURE colete material mold cavity complete FILL 2 PACK is achieved. Precise molding uses a “FIXED” VOLUME shot size of a PLUNGER or screw pot like the SODICK system, where an ASSURED excess melt VOLUME is injected to make up for SHRINKAGE.
Melt cushion is added the Screw Inject STROKE because the shut-off ring has a “FLOAT” distance and the added CUSHION ASSURES a final PEAK PACK AMOUNT. Both Barrel ID and screw shut-off clearance change with WEAR.
Cushion should be a minimum amount that is injected and the next cycle, after screw return.
END of screw return should be a few seconds before mold opening and RPM determined by the material “Heat” sensitivity.
Generally, 3-7 mm should be correct in most cases. You should keep in mind, that melt cushion value should be stable cycle to cycle within 0,1-0,2mm (depends on product precision requirements), and cushion value must be bigger than decompression after dossing – otherwise you lose control over injectable volume.