Calculating Direct Machine Downtime Penalties from Moisture Induced Part Defects
Calculate downtime penalties by summing unabsorbed press hourly burden, loaded labor, scrapped resin mass, purge cycles, and hot runner clearing expenses.
Barrel Drool
Meaning
Injection moulding processing defects include the uncontrolled leakage of molten resin from the nozzle tip during plasticizing or carriage retraction. Thermal degradation or low melt viscosity allows barrel drool to exit the nozzle face before the mold closes. Excess resin accumulates on the mold face, creating part contamination or nozzle stringing during production.
This condition stops applying when shut-off nozzles physically block the orifice during screw rotation.
Thermal Origin
Temperature settings along heating zones govern melt viscosity. When heating bands deliver heat beyond target processing windows, resin thins excessively. Overheating accelerates melt flow within the metering zone.
Lower viscosity allows gravity and residual pressure to push material past the tip orifice.
Decompression Dynamic
Screw retraction after plasticizing creates a vacuum buffer inside the barrel tip to prevent material leakage. Inadequate suck-back distance leaves residual hydraulic pressure inside the melt stream, forcing fluid forward. Excessive suck-back draws atmospheric oxygen into the hot melt, causing oxidation specks or moisture blisters in the next shot.
Setting precise suck-back parameters balances internal pressure without introducing ambient air into the barrel cavity.
Defect Propagation
Unchecked material leakage causes physical contamination on clamping surfaces and introduces cold slugs into downstream parts. Residual polymer resting on the parting line causes mold damage under clamp tonnage, resulting in costly tool repair. Part quality suffers as semi-solidified material enters the cavity, creating surface blemishes or structural knit line weaknesses.
Scrap rates increase when automatic operations halt due to stringing triggering safety eye alarms.