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Analysis of normal wear fracture and abnormal fracture of PVC screw#plastic extrusion#screw

July 19,2023

1. Screw processing and materials

● Selection of barrel and screw material positioning
Barrel: Cr12MoV (SKD61 alloy steel), 38CrMoAlA
Screw: 38CrMoAlA

● Effect of heat treatment (introduction of several heat treatment processes) on barrel, screw and production process
The barrel is quenched and tempered first, the purpose is to uniform the internal structure of the material and improve the metallographic grade of the material. Part of the heat treatment of the two-stage alloy steel adopts the overall quenching treatment. , the hardness can reach HRC60 after quenching

The final heat treatment of the screw is surface nitriding, the nitriding depth can reach 0.5~0.7mm, and the nitriding depth of large companies can reach more than 0.6mm
● Barrel, screw bimetal processing method and introduction of hard alloy, metal processing method generally adopts two kinds of forging (large parts) and casting (small parts)
● Service life

2.Introduction to the structural characteristics of several types of barrel and screw

1.The difference between conventional, low filler and high filler structure
Geometric shape and pairing relationship of screw edge and screw groove
Less content: the gap between the screw flight and the screw flight should be larger
High content: the gap between the screw flight and the screw flight should be small, the purpose is to increase the shear force and increase theplasticization functionof each regio.
The screw is generally divided into: feeding section, compression section, blocking section, mixing section, exhaust section, metering sectionFeed section: In general, the conveying section has a large flight gap and fast conveying, otherwise, it is small. It also depends on the formula and ingredientsCompression, blocking material, mixing: the main influence is the quality of plasticizing ability, if there is a blocking material before compression, it can increase plasticization, if there is a blocking material, it is not suitable for high calcium, mixing and mixing Compression set together is generally only suitable for those with low calcium.Exhaust section: You can observe the plasticizing ability of the material from the vacuum port, and there are granular, powdery, and ribbon shapes.
Metering section: If the gap between the screw ribs is large, the extrusion will be fast, otherwise, it will be small.

2.Introduction to the structural features of low filler and high filler: the feed section, compression and mixing of the low filler screw are connected together. Groove, add 2 rounds of retaining material, so as to increase plasticization, the exhaust section and metering section at the back are a little larger than the gap between the screw edges; the high-fill screw structure consists of feeding section, retaining material, compression, retaining material, mixing, discharge The gas section, metering section, and the gap between the feed section and the metering section are small.

3. Introduction of screw and barrel constant temperature control structure
Internal circulation: automatic circulation of distilled water
External circulation: The temperature control of the screw can be set to control the temperature of the material, and the screw can be protected during the start-up process and the production process to stabilize production. If it is not used, the product will change color in terms of the processing technology of the profile
Barrel temperature control: There are two types of air cooling and oil cooling. Relatively speaking, the temperature control accuracy of oil cooling is greater than that of air cooling, but the oil circuit is easy to block during use, and the failure rate is higher than that of air cooling.

3.The effect of screw structure characteristics on the processing and mechanical properties of pipe products

1. Less than 30 parts, less than 80 parts, and less than 120 parts, the formula of fillerPVC pipe productsrequires screw structure
More than 120 parts: The gap between the metering sections should be small. If the gap between the metering sections is large, the compression will not be enough. The number of slots will determine the plasticizing effect when mixing. The screw flight gap between the feed and metering section is reduced; below 30 parts: the feeding mixing section and the compression section are connected together, and the others are the same, and the screw flight gap in the metering section is relatively large.

2. The variable ratio of low filler and high filler screw structure affects the processing technology and mechanical properties of products
Different formulations require different screws, how much lubricant is used, if there is too much lubricant, the plasticizing effect is not good, and the flow performance is obviously improved.

3. High filler production formula has an impact on the life of barrel, screw, material, heat treatment, etc.
The high-filling production formula has a great impact on the wear of the barrel and screw, and the service life is not long. If it is more than 100 parts, it will generally start to wear within 5 to 6 months. There are quality problems such as streaks and black lines. Most of the wear of the barrel occurs above the vacuum port and the metering section. Except for the feeding section, the screw has different degrees of wear.
The barrel and screw metering section are made of alloy, and the wear condition is relatively better than that of ordinary structures, but the service life will not exceed two years.

4.The gap between the gear distribution box of the extruder and the screw and barrel; the relationship between advantages and disadvantages

● 55/110: The serial movement gap is about 1.5mm, the unilateral gap is 0.75mm, and the front end is 0.25mm
● 65/132: The serial movement gap is about 2mm, the unilateral gap is 1mm, and the front end is 0.25mm
● 68/146.5: The serial movement gap is about 2.5mm, the unilateral gap is 1.25mm, and the front end is 0.30mm
● 80/156: The serial movement gap is about 3mm, the unilateral gap is 1.5mm, and the front end is 0.35mm
● 80/173: The serial movement gap is about 3mm, the unilateral gap is 1.5mm, and the front end is 0.35mm
● 92/188: The serial movement gap is about 2.8mm, the unilateral gap is 1.4mm, and the front end is 0.4mm

5.The relationship between barrel screw pairing clearance and product processing plasticization

Increased gap: increased back flow, poor plasticization, increased screw wear, and shortened service life.

6.PVC screw fracture is divided into normal wear fracture and abnormal fracture analysis

1. The screw breaks normally:
The screw is a vulnerable product during the extrusion process. The life of the screw varies with the amount of calcium powder added to the product. Generally, the life of the screw with 100 parts of calcium and below is 1-2 years, and the life of the screw with 100-200 parts of calcium is About 1 year, the life of the screw with 200-300 parts of calcium is 6-8 months, and the life of the screw with more than 300 parts of calcium is 3 months. Of course, the choice of calcium powder is also very important. Light calcium can obviously prolong the life of the screw than heavy calcium. If the screw is severely worn, it is normal to break!

2. Abnormal fracture of the screw:
1. The bearing of the distribution box is damaged and the screw is broken.
Cause analysis: The bearing is aging or has hidden quality problems.
Solution: replace the bearing.
Ways to prevent it: check the distribution box irregularly, especially if there is any abnormality in the sound.
2. The heating temperature is not enough to cause the screw to break.
Cause analysis: There is a problem with the line or the heating coil is damaged
Solution: check the circuit or replace the heating coil.
Ways to eliminate it: carefully check the temperature of each area before starting the machine, and check the temperature of each area regularly during the production process.
3. Material interruption in the production process leads to screw breakage.
Reason analysis: the bridge of raw materials in the hopper or the loading personnel did not add materials in time
Solution: hopper plus stirring device.
Elimination method: regularly check the raw materials in the spring feeding or screw feeding hopper, and add an alarm device in the lower hopper.
4. There is a problem with the formula, which leads to the breakage of the screw.
Cause analysis: The ingredients made a mistake and caused the formula to go wrong.
The solution: Mix ingredients carefully according to the recipe sheet.
Ways to prevent it: Check the current of the host machine irregularly, and carefully observe the machine’s operating status when changing the formula.
5. There is foreign matter in the raw material that causes the screw to break.
Cause Analysis: There are foreign objects in the raw material that cause the screw to be stuck
Solution: Put a strong magnet at the feeding port.
Elimination method: irregularly clean up the impurities on the strong magnetic surface.

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