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.
 

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