Extrusion defects appear due to different obstacles in the polymer extrusion process. Knowing all these obstacles in production is essential. Therefore, we brought this article to introduce the most critical extrusion defects in the polymer extrusion process.
We know that many factories and foundries have appeared due to the advancement of modern civilization. But do all these factories offer similar quality products? Not. Some foundries need help to afford quality manufacturing set-up. As a result, the products from these factories come with different types of polymer extrusion defects. We introduce this article and hope you will learn the fundamentals of extrusion defects and how to identify quality seal strips. Sit tight and read the following sections carefully. Try to understand the possible causes of the problems; ultimately, you will figure out the possible remedies.
Polymer Extrusion Defects
A product defect is a deviation of the product’s characteristics from the target specification by any manufacturing process. Polymer extrusion defects may appear from a single cause or a commutative cause of multiple factors. In the polymer extrusion process, these defects usually come from three primary sources: extrusion mold design, raw materials choice, and manufacturing process.
We have discussed the most critical polymer extrusion defects in the following. In the Seashore rubber factory, our experts focus on the precautionary steps to prevent the following extrusion defects. The high-quality sealing strips we offer are very popular with our customers.
Inaccurate Dimension
Identification of the Defect
- Changes in thickness or uneven thickness.
- Unusual response from the hot extruded polymers during cooling.
- Other dimensional inaccuracies.
Causes
- Improper operation of the machine’s parameters, such as direction and draw ratio between the extruder and the puller, is the primary reason for this defect.
- The heat generated from the barrel dramatically impacts the final product quality. Inappropriate heating will lead to dimensional inaccuracy.
Surging changes the hot molten polymer’s dimension and limits production tolerances. Surging refers to the variation from a cycle behavior of the output generated by the extruder.
Remedies
- In improper operation, the puller and screw speed may be inaccurate. In addition, check the extruder temperature and set it at a typical range suitable for the specific extrudates. These steps will also solve the overheating problems.
- Replacing the screw would be the best decision in case of unusual surging. You could also install a gear pump between the extruders and the extrusion dies. Maintaining the extruder speed at least 10% slower or faster would also solve this problem.
Blow Hole Formation
Identification of the Defect
- Different sizes of holes on the final product.
- Large cavities at the edge of the product.
Causes
- Resin is an essential material in the polymer extrusion process. Contamination of resin generates gas in the molten extrudates. These gas bubbles finally become holes or cavities in the final polymer product.
Remedies
- Clean the extruder and the die.
- Verify the quantity of the resin materials.
- Ensure proper mixing of the raw materials in the hopper. In this case, for better material mixing, you could increase the rpm of the back pressure.
Bubbles and Pits
Identification of the Defect
- A dotted line and different bubble sizes may appear on the profile.
- In addition to the above appearance, different sizes of pits may also appear due to moisture absorption.
Causes
- Trapped air inside the extrudates causes bubbles, pits, and dotted lines.
- Some polymers absorb moisture. In those cases, moisture absorption may create such a defect.
- An inaccurate ratio of the materials during mixing may also lead to this defect.
Remedies
- If it has trapped air problems, you could melt the temperature of the materials. In addition, try your best to avoid the over-speed of the extruder.
- For moisture absorption problems, you can use a vent in the extruder. Besides, the pre-drying method of the raw materials could solve the case.
- Finally, ensure proper mixing of the materials and check the master data if necessary.
Sharkskin Melt Fracture
Identification of the Defect
- Large or small sharkskin melt fractures on the seal strips.
- In severe cases, gross melt fracture may also appear.
Causes
- Increased shear rates in the polymer melts will create such distortion. Specific shear rates of the polymer melt can ensure the smooth surface of the extrudates.
Remedies
- Adjust the shear rate by lowering the value.
- You could also slow the extruder rates.
- Reducing melt viscosity may also solve this issue.
- Finally, you could also verify the die temperature, and if necessary, you can increase it.
Clogging in Extrusion
Identification of the Defect
- There are intense and thick lines on the profile.
- Clogging in extrusion.
Causes
- This defect usually appears due to the jams or clogs inside the extruder nozzle. These obstacles prevent the molten material from freely going through the nozzle. It will result in clogging defects on the seal strips.
Remedies
- Verify the quantity (percentage or ratio) of the resin materials.
- Check for foreign materials or contaminants.
- Increase the rpm of the back pressure for better mixing.
Centering Problems and Chatter Marks
Identification of the Defect
- Chatter marks on the surface of the seal strip.
- The symmetrical problem of the polymer profile.
Causes
- Inappropriate system engineering or inaccurate installation leads to this defect.
Remedies
- Check the alignment of the production line.
- Adjust the extrusion die set.
Sink Mark
Identification of the Defect
- Sink mark on the surface of the seal strips.
Causes
- The sink mark problem is one of the most common rubber extrusion defects. This defect appears due to the indentation opposite the extruder rib. When the molten material cools, it first happens in the flat area. More materials are present at the cross-section. At this moment, shrinkage occurs in those thicker sections, making an indentation opposite the rib.
Remedies
- Changing the part design would solve this problem 100%.
Volatile Defects
Identification of the Defect
- Volatile defects may appear as slays or other forms of defects.
Causes
- This type of problem usually comes from the poor and non-uniform gloss of the material. Gloss is a typical optical feature determing whether the material is transparent or opaque. Improper adjustment of the rollers may lead to poor gloss.
- This problem may also appear due to the non-uniform roller temperature, inaccurate gauge, volatiles, or inappropriate melting temperature.
Remedies
- Set the roller temperature as high as possible. In addition, you should also ensure there is no surging in the system (share rate falls).
- Verify the quantity of the resin materials in the mixture.
- Finally, adjust the extrusion die nozzles.
Weld Lines
Identification of the Defect
- A weld line forms through two polymer fluids. In other words, if one type of material is used, the weld line may also appear due to any obstacles in the cross-section.
Causes
- Weld line forms through two polymer fluids. In other words, if one type of material is used, the weld line may also appear due to any obstacles present in the cross-section.
Remedies
- Increase the barrel temperature and clean the extrusion die to remove foreign materials or contaminants.
- After that, you can increase the pressure on the materials for attachments.
Gels and Plate Out
Identification of the Defect
- Gels and plate-out defects appear as surface defects that look like the fish’s eyes.
Causes
- Gels refer to the high-molecular-weight unmelted polymer that may elongate when passing through the extrusion dies. After coming out from the extrusion die, these gels may return to their original shape or remain in the melted polymers.
- Plate-out is the low-molecular-weight polymer that migrates to the extrusion surface and is deposited to the extrusion die lips.
Remedies
- Verify the proper melting temperature and streamline the extrusion die to prevent stagnation.
- Later, verify the extrusion speed and change the formulation process.
Warpage
Identification of the Defect
- Twisted or deformed seal strips.
Causes
- Non-uniform shrinkage causes internal stress, which ultimately leads to the warpage defect.
Remedies
- Check the puller alignment with the extruder.
- Adjust the melting temperature.
Final Thought
From the above explanation of distinct types of extrusion defects, we have found that these defects come from the improper setting of the technical parameters. Applying the mentioned remedies makes it possible to decrease the extrusion defects in the product. Fortunately, Seashorerubber is one of the most successful companies in the world. We promise the best quality rubbers and plastic seal strips for different applications. We keep the latest technology and maintain the proper manufacturing processes. So, our seal strips and weather stripping have high-quality finishing and a long life.
If you want to know more about our products, contact us through any platform you may like. We are always online, so you can contact us by phone, WeChat, WhatsApp, or email. Besides, our company has a team of expert engineers and customer service.