Burn marks can be defined as small dark brown or black discolorations on the surface of a molded part, usually found at the end of the material flow path or in blind pockets.
Possible Causes & Remedies:
◇ EXCESSIVE INJECTION SPEED OR PRESSURE
Remedies: Reducing the injection speed or pressure will allow enough time for the gases or trapped air to escape through normal vent paths.
◇ EXCESSIVE BACK PRESSURE
Remedies: Use minimum back pressure. All materials will benefit from approximately 50-psi back pressure, but some require up to 300 psi. The material supplier is the best source of information regarding proper back pressure settings for a specific material. When adjusting back pressure use increments of 10 psi.
◇ HIGH SCREW ROTATION SPEED
Remedies: Adjust the screw rotation speed. An average speed should be approximately 100 rpm. But, specific materials require specific rotation speeds. Consult the material supplier for the proper speed for a specific resin. When adjusting up or down, do so in increments of 10 rpm.
◇ IMPROPER COMPRESSION RATIO OF SCREW
Remedies: The material supplier can recommend the proper compression ratio for a specific resin. A general-purpose screw can usually be used to provide adequate compression but specific conditions may require a screw that is specially designed for a given material.
◇ FAULTY TEMPERATURE CONTROLLERS AND HEATER BANDS
Remedies: Inspect and calibrate temperature controllers at least every 3 months. Inspect and replace damaged heater bands as necessary. In addition, look for broken or crimped wires, poor insulation, rusted areas on the heater bands, and poor electrical connections.
◇ EXCESSIVE BARREL TEMPERATURES
Remedies: Establish proper barrel temperatures and profile. The material supplier will provide accurate barrel temperature requirements. The profile should have the barrel temperatures increase progressively from rear to front.
◇ IMPROPER SPRUE BUSHING-TO-NOZZLE SIZING
Remedies: Using bluing dye or thick paper, press the nozzle against the sprue bushing, and check the impression of the openings of each. They should be close to the same and not be off center. Replace the nozzle tip or the sprue bushing if they do not match. Re-center the heating cylinder to the mold if they are off center.
◇ 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 approximately 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. Blind pockets can be vented using flush core pins or fake ejector pins and grinding a flat down the entire length of the pins.
◇ UNDERSIZED GATES
Remedies: Size the gate according to the material supplier's recommendations. The gate should be as thin as possible to minimize cycle time, but as thick as necessary to reduce the tendency to degrade material. Gates should be installed in removable inserts so they can easily be altered or replaced.
◇ EXCESSIVE USE OF REGRIND
Remedies: Use 100% regrind, or, if mixing with virgin, limit the amount of regrind to 15% by weight. It may be necessary to use no regrind at all, especially in some medical and electronic products.
◇ INCONSISTENT PROCESS CYCLE
Remedies: If 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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