One of the most common shortcomings in plastic injection molding is spreading. In this column, I’ll help identify different forms of spreading, as well as common (and less common) causes. I will also provide various solutions for removing spreader from your process.
The most common reasons for distraction are moisture, shear, and heat. In almost every scenario, calculating which of these three factors is causing spread helps define the best approach to correcting the condition. Each of these categories offer tell-tale signs that can help identify the type of spreading that is present. By understanding each condition, we as mold makers can make process changes to remove spread from a process.
humidity:: The most common cause of spreading is moisture. One of the most important indicators of moisture in the process is that the spread does not visually appear in the same place every time, or in some cases the entire spread is spread over the entire part.
Another visual check for moisture would be the flushing puddle itself. If the puddle is foamy and / or covered with bubbles, moisture can cause the splay condition.
By holding material from the dryer in your hand, you can check that the dryer is working and that the material is hot. However, it does not check whether the material is dry. A moisture analyzer should be used to check the specific moisture content. It is also important to note that some materials can become overdried. Refer to the material manufacturer’s recommendations for an ideal moisture level.
Once the cause of the spreading has been identified as moisture, the cause of the high moisture content needs to be verified. Here is a list of the reasons why hydration might be present:
• • Improper drying. The material must be dried for the time and / or temperature specified by the material manufacturer. Let the material dry longer at the right temperature. Let the first 25 pounds off the dryer cone and try again.
• • The dryer is not working properly. Feel the supply and exhaust air lines for the dryer. The supply line should be hot and the return line warm. In addition, it is advisable to install manual temperature indicators with probes in the hoses where the air enters and exits the dryer. If the leaving water temperature does not match the set point, or if the return temperature is more than 20 ° F below the set point, there may be a problem with the dryer performance.
• • The material throughput is not correct. An example of this would be using a 100 lb dryer with a material consumption of 60 lb / hr. The material passes through the dryer in 1.5 hours. If the normal drying time is 2-4 hours, the material will not receive an acceptable amount of drying. It’s also important to note that most dryers have the material running from the center. For this reason, extra time should be added to ensure that the material draining from the center has enough time to dry. Based on a drying time of 4 hours, a throughput of 5 hours would be sufficient for drying. A dryer size of 300 lb or larger should be used.
• • The material between the filler neck and the dryer supply has sat too long. Material in these areas can absorb moisture in as little as an hour. At startup, drain 25 lb of material from the loading system at the drain. This ensures that the loaded material was removed in a dry state.
• • Mold licks. A water connection is leaking or there is an internal water leak in the mold. Examine the shape for signs of a water leak and correct the condition.
Heat: Too much heat can look similar to excess moisture. The spread condition may completely cover a part or appear inconsistent in different areas of the part. Look for signs of stickiness or burning. Another sign can be the smell of overheated material. Here are common causes of overheating:
• • The running temperatures are too high. One of the key ways to identify this condition is by melting temperature. Make sure the melting temperature is within the window set by the material manufacturer. If you are developing a process at this point, the melting temperature should first be set at the bottom of the melting window and then gradually increased until the parts are in acceptable condition.
• • The back pressure is too high. It is important to note that while back pressure is key to mixing the material, overuse can overheat the material. It can also lead to a decrease in the molecular chain and even material degradation. The back pressure should be adjusted with these variables in mind.
• • The dwell time of the barrel is too long. The time the material spends in the cylinder can cause material degradation. One countermeasure is to lower the barrel temperature in the feed zone to reduce the residence time effect. Another thing to consider is screw recovery time. The screw should finish restoring 1.5 to 2 seconds before the end of the cool down period. If a process requires a long cool down period, the screw turning delay can be used to complete the recovery at the desired time.
Scissors: In general, the shear expansion is repeatable and occurs in the same location on a part. The best way to determine the root cause is to determine where within the flow front the shear is occurring. Splay events at the beginning, in the middle and at the end of the filling tell different stories. Listed here are common causes and methods for shear spreading to correct the condition.
There are several reasons a processor might notice the start of filling. These can be:
• • Decompression after turning. The screw is sucked back too far, creating air in front of the material. Reduce the suction position and / or speed to reduce air pollution. In general, the decompression should be set to 0.1-0.4 inches, using only the speed required to achieve the desired position.
• • Deviation of the nozzle / mold temperature. A sharp drop in temperature when material from the nozzle enters the mold can cause the start of filling to spread. Increase the mold temperature or decrease the nozzle temperature to compensate for the condition.
• • Goal size. Smaller gates may require a slower filling start speed.
• Clogged nozzle tip / hot runner waste. In situations where the spreading condition is new to a validated process, ensure that the injection and / or peak pressure have not changed. The tip should be inspected for burrs caused by repeated impacts on the sprue bushing. The runner himself can sometimes provide insight into the ridge conditions.
Game conditions that recur during mid-fill are also a clear indication of shear conditions. Slower injection speeds in the affected filling area often correct the situation. In addition, areas within the tool that are affected should be inspected for burrs. Shape details that hinder material flow can also become a source of shear.
Splay, which occurs repeatedly at the end of the filling, is another shear-spread identifier. It is a sign of overheating at the gate or waste at the end of the filling. The end of filling shear can often be corrected by slowing down the filling speed at the end of the filling. It is also important to note that the “waxing” of the material (becomes more viscous) a shear condition can arise at the end of the filling. Faster filling at the beginning and in the middle of the flow front may be required to correct for the shear at the end of the filling.
Understanding the effects of moisture, heat, and shear under processing conditions is your best defense against spreading. First, investigate what type of spreading is occurring. This helps identify which sources need to be assessed as the root cause. Once the root cause is identified, it will be easier to make the necessary adjustments to correct the condition.
ABOUT THE AUTHOR: Garrett MacKenzie is the owner / editor of plastik411.com and a consultant / trainer for the plastic injection molding industry. He has made his procedural expertise available to many top companies such as Glock, Honda, Johnson Controls and Rubbermaid and currently works for a company that supplies automotive products to Yenfeng, Faurecia and other top automotive suppliers. He can be contacted at [email protected]