BLACK SPECKS

Black specks can be defined as small dark

particles or spots

on the surface of an opaque part or within a

transparent part.

MACHINE

EXCESSIVE RESIDENCE TIME IN BARREL

Explanation: Under the best conditions, a shot

size should represent 50% of the capacity of the

injection cylinder (barrel). This will result in

processing the material for one cycle while

preparing the material for the next cycle. Thus, a

mold requiring a four-ounce shot should be run in

a machine that has a barrel with an eight-ounce

capacity. The more material left in the barrel

between shots, the greater the likelihood of

thermal degradation. This degradation is what

causes the black specks.

Solution: Strive for a 50% shot-to-barrel ratio.

This is ideal but can go as low as 20%, if the

material is not too heat sensitive (like

polypropylene) and up to 80% if the material is

extremely heat sensitive (like PVC). It is not a

good idea to empty the barrel for every shot

because more time will be required to bring the

next mass of material up to proper heat and

degradation may occur.

TRAPPED MATERIAL

Explanation: If any molten resin is trapped along

the flow path (most notably in the heating

cylinder), it will stay there until it degrades. When

this happens, the degraded material becomes

carbonized, then chars and becomes brittle. At

that point, it will flake away from the area of

entrapment and enter the melt stream appearing

as black specks or streaks.

Solution: Inspect the barrel liner, nozzle,

non-return valve, and check ring for nicks, cracks,

rough surfaces, peeled plating or stuck resin.

Then, stone and polish as required, replace any

damaged mechanisms, and inspect the main and

secondary runners, as well as the sprue bushing,

for nicks, rough surfaces or sharp corners. Round

off sharp corners and radius corners where

possible to minimize material trapping and shear

points.

CONTAMINATION IN INJECTION BARREL

Explanation: Any type of contamination in the

injection barrel may be the cause of streaks,

spots, and specks. It may be in the form of dust

particles that fell from the ceiling into an open

hopper, pellets from other materials, residual

resin from an improper changeover, or even

pieces of food that accidentally fell into a

container of material ready to be placed in the

hopper.

Solution: To remove this type of contamination it

may be necessary to increase the temperature of

the injection barrel and, using a purging material

with a wide melt range, purge the contaminate(s)

from the system.

UNCONTROLLED HEATER BANDS OR

THERMOCOUPLES

Explanation: Improperly sized or loose heater

bands or thermocouples can cause localized

degradation of the material by exposing it to

extreme heat. They may be calling for more heat

than normal due to malfunction or improper sizing.

Even a heater band that is not working can be the

cause of such overheating. The reason is that

adjacent heater bands must increase heat to

compensate for the nonworking band.

Solution: Check each heat zone to ensure that

all heater bands are working properly, are

properly controlled, properly sized, and are tight

against the barrel. A conductive sealant should

be used to ensure full contact with the barrel. Be

sure to replace bands with the proper size,

voltage, and wattage requirements as stated in

the machine manual.

DAMAGED BARREL OR SCREW

Explanation: A cracked injection cylinder or

pitted screw is a cause of material 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 for cracks and

nicks in the walls. Sometimes damaged cylinder

walls can be welded but it is usually more

effective to replace the cylinder liner. Pitted

screws must be welded, ground and replated, or

replaced with new stock.

OIL LEAKS Explanation: Hydraulic components or fittings

that are in the proximity of the injection cylinder

may leak. This leakage may get into raw material

storage containers and find its way into the

material hopper. The oil will burn at the

temperatures needed for molding and will

degrade and char. This degraded material is a

source for streaks and specks.

Solution: Eliminate all hydraulic leaks as soon as

possible after they occur.

MOLD

SPRUE BUSHING IS NICKED, ROUGH, OR

NOT SEATING

Explanation: A damaged sprue may cause

material to stick and be held in residence at

elevated temperature until it degrades and

decomposes. At that point, it will break loose and

enter the melt stream as streaks or specks.

Solution: Inspect the internal surfaces of the

sprue bushing. Remove any nicks or other

imperfections. The tapered hole should be highly

polished. Check the sprue bushing-to-nozzle seal

with thin paper or bluing ink to ensure that the

nozzle is centered to the bushing and that the

hole and radius dimensions are compatible for

the nozzle and the bushing.

IMPROPER VENTING

Explanation: Air is trapped in a closed mold and

incoming molten plastic will compress this air until

it auto-ignites. This burns the surrounding plastic

and results in charred material in the form of

spots and specks.

Solution: Vent the mold by grinding thin

(0.0005-0.002) pathways on the shutoff area of

the cavity blocks. Vents should take up a

minimum of 30% of the perimeter of the molded

part. Vent the runner, too. Any air that is trapped

in the runner will be pushed into the part.

CONTAMINATION FROM LUBRICANTS

Explanation: Excessive use of mold release will

clog vents. The trapped air cannot be evacuated

and burns. Also, grease that is used for

lubricating cams, slides, ejector pins, etc., can

seep into the mold cavity and contaminate the

molded part.

Solution: The remedy is to keep the mold as

clean as possible and clean the vents if they

become clogged. A white ash will be present if

the vents are clogged. Also, make every effort to

eliminate the use of external mold releases.

MOLD TOO SMALL FOR MACHINE SIZE

Explanation: If the mold is placed in too large of

a machine, the chances are that the heating

cylinder of that machine will be large enough to

result in extensive residence time of the raw

material in the heated cylinder. This will result in

degraded material that will be injected into the

mold causing streaks and specks.

Solution: Place all molds in properly sized

machinery. A rule-of-thumb states that the

machine should inject between 20% and 80% of

its capacity every shot.

MATERIAL

CONTAMINATED RAW MATERIAL

Explanation: The most common causes of black

specks and streaks are molding compound

contaminants. Such contamination is usually the

result of dirty regrind, improperly cleaned hoppers

or granulators, open or uncovered material

containers, and poor quality virgin material

supplied by the manufacturer.

Solution: This type of contamination can be

minimized by dealing with high quality, reputable

suppliers and by using good housekeeping

practices. Properly trained material handlers will

also help reduce contamination.

OPERATOR

INCONSISTENT PROCESS CYCLE

Explanation: It is possible that the machine

operator is the cause of delayed or inconsistent

cycles. This will result in excessive residence

time of the material in the injection barrel. If such

a condition exists, heat sensitive materials will

degrade, resulting in black specks or streaks.

Solution: If at all possible, run the machine on

automatic cycle, using the operator only to

interrupt the cycle if an emergency occurs. Use a

robot if an operator is really necessary. And,

instruct all employees on the importance of

maintaining consistent cycles.