Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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One of the most common defects in plastic injection molding is splay. In this column I will help to identify various forms of splay, as well as common (and less common) causes. I will also provide various solutions for removing splay from your process. The most common reasons for splay are moisture, shear and heat. In nearly every scenario, calculating which of these three is causing splay will help to define the best approach to correct the condition. Each of these categories offers telltale signs that help to identify the type of splay that's present. Through understanding each condition, we as molders can make process changes to remove splay from a process. Moisture: The most common cause of splay would be moisture. One of the key identifiers of moisture in the process is that visu- ally the splay does not occur in the same place every time, or in some cases the splay is all over the part. Another visual check for moisture would be the purge puddle itself. If the puddle is foamy and/ or riddled with bubbles, moisture could be causing the splay condition. Holding material in your hand from the dryer can verify that the dryer is working, and material is hot. It does not, however, verify that the material is dry. A moisture analyzer should be used to verify specific moisture content. It is also important to note that some materials can be over-dried. Refer to the material manufacturer's recommendations for ideal moisture level. Identifying and Correcting Splay Once the cause for the splay has been identified as moisture, the cause for high moisture content must be verified. Here is a list of reasons moisture splay might be present: • Improper drying. Material must be dried for the time and/or tem- perature provided by the material manufacturer. Allow the mate- rial more time to dry at the correct temperature. Drain the first 25 lb from the dryer cone, and retry startup. • The dryer is not functioning properly. Feel the supply and return air lines for the dryer. The supply line should be hot, and the return line warm. In addition, it is good practice to have manual temperature indicators with probes installed in the hoses where the air enters and exits the dryer. If the supply temperature does not match setpoint, or if the return temp is more than 20° F lower than setpoint, there may be a problem with dryer performance. • Material throughput is not correct. An example of this would be using a 100-lb dryer with material use of 60 lb/hr. The mate- rial passes through the dryer in 1.5 hr. If the normal dry time is 2 to 4 hr, the material is not receiving an acceptable amount of drying. It is also important to note that with most dryers, the material funnels from the center. Because of this, extra time Understanding the effects of moisture, heat and shear in processing conditions is your best form of defense against splay events. Splay adjustments can be a simple fix, or can require several hours of babysitting a press and head scratching. Learn to find the root cause. Get more insights on Injection Molding from our expert authors: Learn more at KNOW HOW INJECTION MOLDING Splay is a defect that can manifest itself in different ways on the surface of a molded part and can be caused by different factors. 24 NOVEMBER 2018 Plastics Technology K now How By Garrett MacKenzie INJECTION MOLDING

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