Description:

Black spots or brown streaks appear on the surface of the opaque molded parts or within transparent molded parts.

Possible Causes & Remedies:

Machine Cause

◇ EXCESSIVE RESIDENCE TIME IN BARREL

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: Eliminate all hydraulic leaks as soon as possible after they occur.

Mould Cause

◇ SPRUE BUSHING IS NICKED, ROUGH, OR NOT SEATING

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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 Cause

◇ CONTAMINATED RAW MATERIAL

Remedies: This type of contamination can be minimized by dealing withhigh quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.

Operator Cause

◇ INCONSISTENT PROCESS CYCLE

Remedies: 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.

(Source: plastictroubleshooter.com)