Splay Marks can be defined as a “splash-like” appearance (or spray pattern) on the surface of a molded part. Splay is sometimes called "silver streaks".
Possible Causes & Remedies:
◇ BARREL TEMPERATURE TOO HIGH
Remedies: Decreasing the barrel temperature allows the plastic to stay molten without burning or charring. The molecules bond together as desired and splay is eliminated.
◇ EXCESSIVE SCREW ROTATION SPEED
Remedies: Reduce the screw rotation speed. This will allow the material to be properly heated and blended without being degraded. A good starting point for screw rotation on a standard two-inch diameter screw is 120 rpm.
◇ NOZZLE TOO HOT, TOO SMALL, OR OBSTRUCTED
Remedies: Reduce the nozzle temperature 10 degrees F at a time until the splay disappears or the nozzle freezes off whichever comes first. If splay still appears, check for obstructions such as tramp metal or burrs. If splay is still evident, make sure the nozzle opening is large enough for the material being molded. The material supplier can provide this information. And, finally, make sure the nozzle being used is of the proper design for the material being molded. There are many different nozzle designs and some may interfere with proper flow if they are not designed for the material in use.
◇ EXCESSIVE SHOT SIZE
Remedies: Run the mold in a press that is sized to provide the proper ratio of shot-to-barrel size. This will minimize the risk of thermally degrading the material.
◇ TRAPPED VOLATILES
Remedies: Increase the back pressure setting to remove the volatiles. Higher back pressure settings help to disperse the volatiles throughout the melt stream and keep them from joining up to form pockets of trapped gas.
◇ IMPROPER PURGING
Remedies: Purge out the offending material using a minimum of 20 shots. Use a purging compound especially designed to clean barrels, or use scrapped acrylic, which tends to scrub the barrel clean.
◇ SMALL GATES
Remedies: Optimize gate size and shape. The material supplier can provide data on proper sizing and shape, or use a computer finite element analysis program to help make the determination
◇ GATE AND/OR RUNNER OBSTRUCTION
Remedies: Check the parting line and all other areas surrounding the runners and gates. If obstructions exist remove them. Peened edges and burrs can be stoned and polished while cracks and nicks may have to be welded and recut or inserts can be made to replace those areas.
◇ CRACKED MOLD
Remedies: Inspect for cracks, even small cracks, and repair as necessary. If the cracks are in areas that are too critical to weld, it is possible to thread tubing through the cooling channels and let the water travel through the tubing. This will prevent the water from entering the cavity area.
◇ EXCESSIVE MOISTURE
Remedies: Although it is commonly understood that non-hygroscopic material does not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.
◇ INCONSISTENT PROCESS CYCLE
Remedies: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.
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