BLACK SPECKS

BlSpeck

Black specks can be defined as smalldark particles or spotson the surface of an opaque injectionmolding part or within a transparentpart.

MACHINE

EXCESSIVE RESIDENCE TIME INBARREL

Explanation: Under the best conditions,a shot size should represent 50% of thecapacity of the injection cylinder (barrel).This will result in processing thematerial for one cycle while preparingthe material for the next cycle. Thus, amold requiring a four-ounce shot shouldbe run in a machine that has a barrelwith an eight-ounce capacity. The morematerial left in the barrel between shots,the greater the likelihood of thermaldegradation. This degradation is whatcauses the black specks.

Solution: Strive for a 50% shot-to-barrelratio. This is ideal but can go as low as20%, if the material is not too heatsensitive (like polypropylene) and up to80% if the material is extremely heat

sensitive (like PVC). It is not a goodidea to empty the barrel for every shotbecause more time will be required tobring the next mass of material up toproper heat and degradation may occur.

TRAPPED MATERIAL

Explanation: If any molten resin istrapped along the flow path (mostnotably in the heating cylinder), it willstay there until it degrades. When thishappens, the degraded material becomescarbonized, then chars and becomesbrittle. At that point, it will flake awayfrom the area of entrapment and enterthe melt stream appearing as blackspecks or streaks.

Solution: Inspect the barrel liner, nozzle,non-return valve, and check ring fornicks, cracks, rough surfaces, peeledplating or stuck resin. Then, stone andpolish as required, replace any damagedmechanisms, and inspect the main andsecondary runners, as well as the spruebushing, for nicks, rough surfaces orsharp corners. Round off sharp cornersand radius corners where possible tominimize material trapping and shearpoints.

CONTAMINATION IN INJECTIONBARREL

Explanation: Any type of contaminationin the injection barrel may be the causeof streaks, spots, and specks. It may bein the form of dust particles that fellfrom the ceiling into an open hopper,pellets from other materials, residualresin from an improper changeover, oreven pieces of food that accidentally fell

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into a container of material ready to beplaced in the hopper.

Solution: To remove this type ofcontamination it may be necessary toincrease the temperature of the injectionbarrel and, using a purging material witha wide melt range, purge thecontaminate(s) from the system.

UNCONTROLLED HEATER BANDSOR THERMOCOUPLES

Explanation: Improperly sized or looseheater bands or thermocouples can causelocalized degradation of the material byexposing it to extreme heat. They maybe calling for more heat than normal dueto malfunction or improper sizing. Evena heater band that is not working can bethe cause of such overheating. Thereason is that adjacent heater bands mustincrease heat to compensate for thenonworking band.

Solution: Check each heat zone toensure that all heater bands are workingproperly, are properly controlled,properly sized, and are tight against thebarrel. A conductive sealant should beused to ensure full contact with thebarrel. Be sure to replace bands with theproper size, voltage, and wattagerequirements as stated in the machinemanual.

DAMAGED BARREL OR SCREW

Explanation: A cracked injectioncylinder or pitted screw is a cause ofmaterial hang-up and degradation.Eventually this degraded material breaks

loose and enters the melt stream,appearing as specks or streaks.

Solution: Inspect the injection unit forcracks and nicks in the walls. Sometimesdamaged cylinder walls can be weldedbut it is usually more effective to replacethe cylinder liner. Pitted screws must bewelded, ground and replated, or replacedwith new stock.

OIL LEAKS

Explanation: Hydraulic components orfittings that are in the proximity of theinjection cylinder may leak. Thisleakage may get into raw materialstorage containers and find its way intothe material hopper. The oil will burn atthe temperatures needed for molding andwill degrade and char. This degradedmaterial is a source for streaks andspecks.

Solution: Eliminate all hydraulic leaksas soon as possible after they occur.

MOLD

SPRUE BUSHING IS NICKED,ROUGH, OR NOT SEATING

Explanation: A damaged sprue maycause material to stick and be held inresidence at elevated temperature until itdegrades and decomposes. At that point,it will break loose and enter the meltstream as streaks or specks.

Solution: Inspect the internal surfaces ofthe sprue bushing. Remove any nicks orother imperfections. The tapered holeshould be highly polished. Check thesprue bushing-to-nozzle seal with thin

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paper or bluing ink to ensure that thenozzle is centered to the bushing andthat the hole and radius dimensions arecompatible for the nozzle and thebushing.

IMPROPER VENTING

Explanation: Air is trapped in a closedmold and incoming molten plastic willcompress this air until it auto-ignites.This burns the surrounding plastic andresults in charred material in the form ofspots and specks.

Solution: Vent the mold by grinding thin(0.0005”-0.002”) pathways on theshutoff area of the cavity blocks. Ventsshould take up a minimum of 30% of theperimeter of the molded part. Vent therunner, too. Any air that is trapped in therunner will be pushed into the part.

CONTAMINATION FROMLUBRICANTS

Explanation: Excessive use of moldrelease will clog vents. The trapped aircannot be evacuated and burns. Also,grease that is used for lubricating cams,slides, ejector pins, etc., can seep intothe mold cavity and contaminate themolded part.

Solution: The remedy is to keep themold as clean as possible and clean thevents if they become clogged. A whiteash will be present if the vents areclogged. Also, make every effort toeliminate the use of external moldreleases.

MOLD TOO SMALL FOR MACHINESIZE

Explanation: If the mold is placed in toolarge of a machine, the chances are thatthe heating cylinder of that machine willbe large enough to result in extensiveresidence time of the raw material in theheated cylinder. This will result indegraded material that will be injectedinto the mold causing streaks andspecks.

Solution: Place all molds in properlysized machinery. A rule-of-thumb statesthat the machine should inject between20% and 80% of its capacity every shot.

MATERIAL

CONTAMINATED RAWMATERIAL

Explanation: The most common causesof black specks and streaks are moldingcompound contaminants. Suchcontamination is usually the result ofdirty regrind, improperly cleanedhoppers or granulators, open oruncovered material containers, and poorquality virgin material supplied by themanufacturer.

Solution: This type of contamination canbe minimized by dealing with highquality, reputable suppliers and by usinggood housekeeping practices. Properlytrained material handlers will also helpreduce contamination.

OPERATOR

INCONSISTENT PROCESS CYCLE

Explanation: It is possible that themachine operator is the cause of delayedor inconsistent cycles. This will result in

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excessive residence time of the materialin the injection barrel. If such acondition exists, heat sensitive materialswill degrade, resulting in black specks orstreaks.

Solution: If at all possible, run themachine on automatic cycle, using theoperator only to interrupt the cycle if anemergency occurs. Use a robot if an“operator” is really necessary. And,instruct all employees on the importanceof maintaining consistent cycles.

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