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.''
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.
Excessive Screw Rotation Speed
Explanation: Screw rotation is utilized to transfer material through the barrel from rear to front. Also, it is used to impart frictional heat to the resin, which helps to melt it. If the rotation is too fast, some of the material may overheat and degrade due to increased frictional shear. As it is injected into the mold, the degraded material pushed to the surface appears as splay.
Solution: 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
Explanation: The nozzle, being the final transfer point between the heated barrel and the mold, is a critical area and must be scrutinized whenever splay patterns occur. If the nozzle is too hot, too small, or obstructed, the molten material may become degraded as it travels through the area and will surface in the mold cavity as splay. This is due to frictional shear heat generated at the point of obstruction or overheating.
Solution: 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
Explanation: Ideally, a shot size should be between 20% and 80% of the barrel capacity, with 50% being just right. The lower figure can be used for non-heat-sensitive materials, while the higher number should be used for very heat-sensitive materials. Using such a ratio will ensure that the residence time is not too great. If it is, the material will thermally degrade in the barrel and may appear as splay in the molded part.
Solution: 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.
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: When switching materials on a specific press, there is a tendency to start molding with the new material without properly purging all of the older material out of the barrel. The older material may not mix with the newer material due to temperature differences or chemical incompatibility and a smearing effect may appear on the surface of the molded parts. This smearing is also considered to be splay.
Solution: 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.
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: If the mold base is cracked it is possible that water from cooling lines can enter the cavity. This water gets molded into the part and turns to steam as the molten plastic captures it. The steam will be sprayed out across the cavity surface as splay.
Solution: 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.
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.
HOME DEFECTS RESOURCES BOOKSTORE SHOWCASE SEMINARS CONTACT US
Copyright Texas Plastic Technologies
Worldwide rights reserved