Plastic Injection Troubleshooting
Identifying and correcting splay defects
Plastics processors are responsible for making process corrections when defects affect their scrap rate. One of the most common defects in plastic injection is splay. Splay adjustments can be a simple fix, or several hours of babysitting a press and head scratching. This article will help to identify various forms of splay, as well as common (and less common) causes. It will also provide various solutions for removing splay from your process.
First, let’s consider the most common reasons for splay. These would be
In nearly every scenario, calculating which of these three are leading to splay events will help to define which approach will best correct the condition. Each of these categories offers telltale signs that help to identify which type of splay is present. It is through understanding each condition that we as molders make process changes to remove splay from a process.
The most common cause of splay would be moisture. 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.
Another visual check for moisture would be the purge puddle itself. If the puddle is foamy and/ or riddled with bubbles, moisture might 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.
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
• Material was not dried for the time and/ or temperature provided by the material manufacturer. Allow the material more time to dry at the correct temperature. Drain the first 25 pounds from the dryer cone, and retry start up.
• 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 w/ probes installed into the hoses where the air enters and exits the dryer. If the supply temperature does not match setpoint, or if the return temp is >20F 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 pounds per hour. The material passes through the dryer in 1.5 hours respectively. If the normal dry time is 2 to 4 hours, 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 should be added to assure that material draining from the center has had sufficient time to dry. Based on 4 hours dry time, 5 hours throughput would be sufficient for drying. A dryer size of 300 pounds or higher should be used.
• Material between the feed throat and dryer supply has sat too long and taken on moisture. Material in these areas can take on moisture in as little as an hour. At start up, drain 25 pounds material from the loading system at the feed throat drain. This will assure that material loaded has been retrieved in a dry state.
• 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 conditions are another common cause of splay. Too much heat can have a similar appearance to moisture splay. The splay condition can completely cover a part. It can also appear inconsistently in various areas on the part. Look for signs of stickiness or burning. Another sign can be the smell of overheated material. Here is a list of common causes for overheating.
• Barrel temperatures are too high. One of the key methods of 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.
• Back pressure is too high. It is important to note that while back pressure is key to material mixing, overuse can overheat the material. It can also lead to molecular chain reduction, and even material degradation. Back pressure should be set with these variables in mind.
• Barrel throughput is too long. The amount of time that material spends in the barrel can lead to material degradation. One method of reducing throughput heat is to lower the barrel temperature in the feed zone to reduce the throughput effect. Another thing to take into consideration is screw recovery time. The screw should finish recovery 1.5 to 2 seconds prior to the end of cooling time. When a process requires a lon cooling time, screw rotate delay can be used to finish recovery at the desired time.
Shear is the last category of splay root causes. Generally, shear splay is seen to be repeatable at 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 is a list of common shear splay causes, and methods for correcting the condition.
• Beginning of fill. There are a several reasons a processor might note beginning 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 suck back position and/ or speed to reduce exposure to air. Generally, decompression should be set at .1 to .4, 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 mold from nozzle can lead to beginning of fill splay. Increase mold temp or decrease nozzle temp 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 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.
Middle of Fill: 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 conditions. Mold details that obstruct the flow of material can also become a shear source.
End of Fill: Splay that occurs repeatedly at the end of fill is another shear splay identifier. It is a sign of overheating 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 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 are your best form of defense against splay events. First, investigate what type of splay is being observed. This will help to identify what sources must be evaluated as a root cause. With the root identified, it becomes simpler to make the necessary adjustments to correct the condition.