Solutions to Common Production Problems for Rigid PVC Profiles and Pipes

Date:2023-3-20 Author:hcscrew

HUACHEN screw provides expert solutions to common production problems for rigid PVC profiles and pipes. Our extensive knowledge and experience in PVC extrusion screw barrel ensure that your operations run smoothly and efficiently. Contact us today to learn more about our solutions!

1. Yellowish colour of extruded products

The yellowing of profiles or pipes is caused by the decomposition of PVC, which decomposes by dechlorination to form a conjugated polyolefin series, which is yellow in colour. In most cases, PVC decomposes as a result of its formulation, process or equipment.

Solutions to Common Production Problems for Rigid PVC Profiles and Pipes — HUACHEN screw barrel factory

a. Equipment problems (sudden yellowing).

(1) Check whether the thermocouples in each area of the barrel are working normally and whether the fans in each region are running normally. If there is a problem with the thermocouple or fan, it will cause local overheating of the screw barrel resulting in yellowing of the product.

(2) Check whether the screw’s heat-conducting oil is sufficient, whether the oil pump is working properly and whether the oil pipe is blocked. If the oil circuit is not blocked, the frictional heat of the screw cannot be effectively discharged. This will lead to overheating of the screw resulting in material decomposition and yellowing.

(3) Check and adjust the conical twin screw gap or replace the screw. In the case of serious screw wear, the screw and barrel clearance becomes large, the screw on the material pushing ability becomes poor. This will lead to material in the barrel back flow, so that the material in the barrel of the internal heat time will become longer, resulting in yellowing. (This situation is generally manifested in the use of the screw for a longer period of time, a slight increase in the feeding ratio, feeding mouth on the material bubble equipment)

(4) Check the mixing device, whether the mixing time is correct, whether the cooling water circulation is normal, and whether the cooling water temperature is acceptable. If the hot mixing time is too long or the cold mixing cannot cool the material to a normal temperature, the material will consume the stabilizer in the formula during mixing or storage. In the use of stabilizers, effectiveness will be diminished, causing the production process to be sluggish, which will result in yellowing of the product.

b. Process problems

Generally, after the yellowing begins, the extrusion process of PVC by heating and screw and barrel shear, can result in decomposition of the PVC.

(1) Lowering the barrel temperature can improve the yellowing phenomenon.

(2) Reduce the speed of the main machine within the appropriate range and improve the feed ratio, which can reduce shear and make the material in the screw groove more uniformly mixed and the stabiliser fully effective, thus reducing yellowing.

(3) Improper start-up methods also cause yellowing of products. Re-opening the machine may solve the problem.

c. The formula problem

The yellowing of the product generally leads to the replacement of additives or manufacturers. From the following aspects, according to the actual situation to adjust the formula.

(1) Increase the content of stabilizers.

(2) Adjust the balance between internal and external lubricants.

(3) Solve the problem of yellowing by adding pigments or increasing the ratio of fluorescent whitening agents.

2. Air bubbles appear on the product cross-section and the inner wall is not smooth.

During processing, moisture and small molecules evaporate, causing air bubbles to appear on the cross-section of the product. The following solutions are generally available.

(1) Ensure that the raw material is dry and free of moisture.

(2) Check whether the vacuum pump is faulty and whether the vacuum pipeline is blocked.

(3) Increase the ratio of stabilizer or lubrication content, depending on the specific situation.

(4) Adjust the temperature of the first two zones, reduce the third and fourth zones, and reduce the speed of the host.

3. The surface of the product is rough, with protrusions or bubbles.

(1) Too much moisture in the material, re-stir the material to dry.

(2) The temperature of the three or four zones is too high for plasticisation, which is not conducive to the removal of water and small molecules by vacuum, so lower the temperature of the three or four zones appropriately.

(3) If the melt strength does not meet the required standard, reduce the extrusion speed or increase the ACR ratio.

4. Poor glossiness of the product.

(1) The material is not plasticized well. Increase the processing temperature and host speed appropriately.

(2) The temperature of the mouth die is too high or too low, the inner wall of the mouth die is not smooth, adjust according to the actual situation.

(3) There are precipitates on the mould, the mould is clogged by air channels, etc., clear and unclog in time.

(4) Insufficient outer slip in the formula. Increase the outer slip appropriately, but too much may delay plasticisation and block the mouth mould.

(5) The formulation of ultramarine and fluorescent whitening agent is a good match.

(6) Use good quality stabilizers and calcium powder.

5. Products with yellow or black lines on the surface

(1) The black line is mostly due to the decomposition of the material caused by local friction between the extruder screw and the barrel overheating. Usually because the axial clearance between the screw and the screw is too small resulting in poor coaxiality between the screw and the barrel or uneven radial clearance. Check and adjust the screw fit.

(2) The yellow line indicates the mouth mould of the dead end or the core part of the dead end. Check the mouth mould and the core.

The above two problems can also be resolved by adjusting the lubrication and stabilizer.

6. The surface of the product has black spots

(1) There are impurities in the material. Check whether there are impurities in the raw material.

(2) There is stagnant material in the vacuum port. Check and clean up.

(3) Check if a long-lasting material is adhering to the hot mixer’s inner wall.

About the twin screw barrel Factory: HUACHEN

HUACHEN Screw has been a leading conical twin screw barrel manufacturer in Zhoushan Jintang since 1993. As one of the leading companies in designing and processing parallel and conical twin screw extrusion screw barrel, HUACHEN Screw can provide its customers with a wide range of services, including drawing rebuilding, design, modification, and customized screw barrels. Our company has also successfully exported and supplied screw barrels to customers worldwide.

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