On-Line Solutions To Injection Molding Problems
Barrel Temperature Too High
Explanation: If the barrel temperature is too high, the resin absorbs an excessive amount of heat and will decompose and/or carbonize. The charred molecules that are created will not bond with the surrounding material and will float to the surface during injection. This results in a spray of charred particles on the surface of the molded part, which are usually fanned out in direction emanating from the gate location.
Solution: Decreasing the barrel temperature allows the plastic to stay molten without burning or charring. The molecules bond together as desired and splay is eliminated.
Explanation: During the plasticizing process, there are volatiles that are released from the melting plastic. If not properly removed, these gases will join to form pockets that are pushed into the melt stream and carried into the cavity where they show up on the molded part as splay.
Solution: 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.
Explanation: Gates that are too small will cause excessive restriction to the flow of the molten plastic as it passes through. This restriction may cause enough shear heat to thermally degrade the plastic as it fills the cavity. The degraded material gets sprayed across the surface of the molded part as splay.
Solution: 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
Explanation: On new molds, a machining burr may have been left on the parting line. Or, on older molds the parting line surface may have started to peen over. In either case, an obstruction forms that will interfere with the material flow going through the runners and/or gates and cause an overheating condition due to excessive shearing action. This causes some of the material to be thermally degraded and it is molded into the part as splay.
Solution: 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.
Explanation: Excessive moisture does not belong in the molding compound. Moisture turns to steam when heated in the injection unit, and these steam pockets interfere with molecular bonding of the plastic. This causes splay, which is a visual defect, but also creates a weak part due to brittleness.
Solution: 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
Explanation: The machine operator may be the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, some of the material will overheat and degrade. It may end up being carried into the cavity and sprayed across the molded part surface as splay.
Solution: 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.
Splay can be defined as a ``splash-like'' appearance (or spray pattern) on the surface of a molded part. Splay is sometimes called ``silver-streaking.''
Some common causes and solutions are listed below.
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NOTE: For more detailed information on the causes and solutions of this defect, you can find it in our BOOK, or ONLINE SEMINAR.