Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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should be added to ensure that material draining from the center has had sufficient time to dry. Based on 4-hr dry time, 5-hr throughput would be sufficient for drying. A dryer size of 300 lb or higher should be used. • Material between the feed throat and dryer supply has sat too long. Material in these areas can take on moisture in as little as an hour. At startup, drain 25 lb of material from the loading system at the feed-throat drain. This will ensure that material loaded has been retrieved in a dry state. • Mold leaks. A water fitting is leaking, or an internal water leak is present within the mold. Investigate the mold for signs that a water leak is present, and correct the condition. Heat: Too much heat can have an appearance similar to moisture splay. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. Look for signs of stickiness or burning. Another sign can be the smell of over- heated material. Here are common causes for overheating: • Barrel temperatures are too high. One of the key methods for identifying this condition is melt temperature. Verify that the melt temp. is within the window established by the material manufacturer. If at this stage you are developing a process, melt temperature should first be established at the lower end of the melt window, and then increased gradually until parts are in an acceptable condition. • Backpressure is too high. It is important to note that while backpressure is key to material mixing, overuse can overheat the material. It can also lead to molecular-chain reduction and even material degradation. Backpressure should be set with these variables in mind. • Barrel residence time is too long. The amount of time that mate- rial spends in the barrel can lead to material degradation. One countermethod is to lower the barrel temperature in the feed zone to reduce the residence-time effect. Another thing to take into consideration is screw recovery time. The screw should finish recovery 1.5 to 2 sec prior to the end of cooling time. When a process requires a long cooling time, screw-rotate delay can be used to finish recovery at the desired time. Shear: Generally, shear splay is repeatable, occurring in the same location on a part. The best method for determining root cause is to note where within the flow front the shear is occurring. Beginning-, middle- and end-of-fill splay events tell different stories. Here are common shear-splay causes and methods for correcting the condition. There are a several reasons a processor might note begin- ning-of-fill splay. These might include: • Decompression after rotate. The screw is being sucked back too far, leading to air in front of the material. Reduce the suckback position and/or speed to reduce exposure to air. Generally, decom- pression should be set at 0.1 to 0.4 in., using only the necessary amount of speed it requires to achieve the desired position. • Nozzle/mold temperature variance. A large drop in temperature when material enters the mold from nozzle can lead to beginning- of-fill splay. Increase the mold temperature or decrease the nozzle temperature to offset the condition. • Gate size. Smaller gates might require a slower beginning-of- fill speed. • Nozzle-tip/hot-runner drop obstruction. In situations where the splay condition is new to a validated process, verify that there has not been a change in injection and/or peak pressure. The tip should be inspected for burrs caused by repeated touch-off at the sprue bushing. The runner itself can sometimes offer insight into burring conditions. Splay conditions that recur during middle of fill are also a clear sign of shearing conditions. Slower injection speeds in the affected fill area will often correct the condition. In addition, areas within the tool that are affected by this should be inspected for burring. Mold details that obstruct the flow of material can also become a shear source. Splay that occurs repeatedly at the end of fill is another shear- splay identifier. It is a sign of over- heating at the gate or drop at the end of fill. End-of-fill shear can frequently be corrected by slowing fill speed at end of fill. It is also important to note that "waxing" of material (becoming more viscous) at the end of fill can create a shear condition. Faster fill at the beginning and middle of the flow front might be required to correct end-of-fill shear. Understanding the effects of moisture, heat and shear in processing conditions is your best defense against splay. First, investigate what type of splay is occurring. This will help to identify what sources must be evaluated as a root cause. With the root cause identified, it becomes simpler to make the necessary adjustments to correct the condition. One of the key identifiers of moisture in the process is that visually the splay does not occur in the same place every time, or in some cases the splay is all over the part. PTonline.com 26 NOVEMBER 2018 Plastics Technology I N J E C T I O N M O L D I N G K now How ABOUT THE AUTHOR: Garrett MacKenzie is the owner/editor of plastic411.com, as well as a consultant/trainer to the plastic injection industry. He has provided his process engineering expertise to many top companies, including Glock, Honda, Johnson Controls and Rubbermaid, and currently works for a company that provides automotive products to Yenfeng, Faurecia and other top automotive suppliers. He can be contacted at garrett@plastic411.com.

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