In the PVC extrusion process, common quality problems will affect product performance and appearance. As a professional extruder manufacturer, Yongte Plastic Machinery will explain common problems and solutions in detail from the aspects of appearance, size, physical properties, etc.
Bubbles
Causes: The moisture content of the raw material is too high, and water vaporizes during the processing to form bubbles; the material is poorly plasticized and the gas cannot be fully discharged; the mold exhaust is not smooth, and the gas accumulates in the mold cavity.
Solution: Fully dry the raw material and control the moisture content below 0.3%; optimize the extrusion process parameters, such as appropriately increasing the temperature and adjusting the screw speed to improve the plasticization effect of the material; check the mold exhaust system, clean the exhaust groove or increase the exhaust hole to ensure smooth gas discharge.
Pit
Causes: There are impurities in the raw material, and gaps are formed around the impurities during the extrusion process; the mold surface is rough, and the material is scratched when passing through the mold; uneven cooling leads to inconsistent local shrinkage and pitting.
Solution: Strictly screen and filter the raw materials to remove impurities; polish the mold surface to reduce surface roughness; optimize the cooling system to ensure uniform cooling, and adjust the flow and temperature distribution of the cooling medium.
Scratches
Reason: The product rubs against the equipment parts during traction and cutting; there are sharp protrusions or foreign objects on the surface of the equipment parts.
Solution: Check the traction device, cutting equipment and other parts in contact with the product to ensure that the surface is smooth and free of sharp objects; adjust the position and angle of the equipment parts to keep the product stable during transportation and reduce friction; apply an appropriate amount of lubricant on the surface of the equipment parts to reduce the friction coefficient.
Dents
Reason: Uneven cooling of the product, internal stress concentration, resulting in local shrinkage and dents; uneven product wall thickness, slow cooling of thicker parts, and large shrinkage.
Solution: Optimize the cooling system, such as adjusting the flow and temperature of the cooling medium, and adopting a more uniform cooling method; check the mold design to ensure uniform product wall thickness. For parts with large differences in wall thickness, the mold structure or process parameters can be adjusted to improve.
Color difference
Reason: The colorant is not evenly dispersed and distributed in the material; the feeding speed is unstable, resulting in different colorant content in each batch of products; the temperature fluctuates during the production process, affecting the performance of the colorant.
Solution: Strengthen the raw material mixing process to ensure that the colorant and PVC resin are fully mixed and evenly; adopt a stable feeding method, such as forced feeding, and regularly check the accuracy of the feeding equipment; strictly control the temperature during the extrusion process to ensure that the temperature fluctuation range is within ±5℃, which can be achieved by optimizing the temperature control system.
Dimensional accuracy problem
Diameter and wall thickness deviation
Reason: Mold wear causes changes in mold cavity size; extrusion process parameters are unstable, such as temperature, screw speed, traction speed, etc.; raw material performance fluctuates, affecting the fluidity and molding shrinkage of the material.
Solution: Check the mold regularly and replace the severely worn mold in time; stabilize the extrusion process parameters, and accurately control the temperature, screw speed, traction speed, etc. through the equipment's automated control system; strictly control the quality of the raw materials to ensure that the performance of each batch of raw materials is consistent. If the raw material performance changes, adjust the process parameters in time.
Length deviation
Reason: The traction speed is unstable and does not match the extrusion speed; the cutting equipment is not accurate enough, resulting in inaccurate cutting length.
Solution: Optimize the control system of the traction device to ensure the stability of the traction speed and adjust it in real time according to the extrusion speed; calibrate the cutting equipment regularly to improve the cutting accuracy, and use high-precision cutting tools and positioning devices.
Physical performance problems
Insufficient tensile strength
Reason: Poor quality of raw materials, low degree of resin polymerization or impurities; unreasonable formulation of additives, such as excessive addition of plasticizers, which reduces the strength of the product; improper processing technology, excessive or insufficient plasticization of materials, affecting the internal structure of the product.
Solution: Select raw materials with reliable quality to ensure the degree of polymerization and purity of the resin; optimize the formulation of additives and reasonably control the amount of additives such as plasticizers; adjust the extrusion process parameters to ensure that the material is well plasticized and not excessive, which can be achieved by adjusting parameters such as temperature and screw speed.
Low impact strength
Reason: Insufficient addition of impact modifier or improper variety selection; excessive shearing of the material during processing, which destroys the molecular structure; the cooling speed of the product after molding is too fast, resulting in large internal stress.
Solution: According to product requirements, reasonably increase the amount of impact modifier or replace more suitable varieties; optimize the extrusion process to reduce the shear force of the material during processing, such as adjusting the screw structure and reducing the screw speed; adjust the cooling process, appropriately reduce the cooling rate, reduce the internal stress of the product, and adopt a step-by-step cooling method.
Hardness does not meet the standard
Reason: Insufficient filler addition or inappropriate variety; excessive plasticizer use makes the product soft; processing technology causes abnormal product crystallinity.
Solution: According to the hardness requirements of the product, reasonably adjust the type and amount of filler; control the amount of plasticizer added to avoid excessive use; optimize the processing technology, such as adjusting the temperature, cooling rate, etc., to improve the crystallinity of the product, thereby adjusting the hardness.
