Injection Molding
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Unstable Screw Retraction in Block Copolymer Polypropylene '2079851475616104452
Problem Phenomenon
During the plasticizing retraction process, the screw jumps, the retraction speed is uneven, or the retraction is not in place, and the pre-plasticizing position fluctuates greatly.
Unstable screw retraction causes inconsistent injection amounts each time, directly affecting product weight and dimensional stability.
Cause Analysis
（1）The back pressure is set too high or unstable, and the plasticizing resistance is large
（2）The barrel temperature is uneven, and the low melting section temperature causes plasticizing difficulty
（3）The screw is severely worn, and the plasticizing efficiency decreases
（4）The raw material particle size is uneven or contains too much powder, and the discharge is uneven
（5）The screw rotation speed and back pressure are improperly matched
Solution
（1）Appropriately reduce the back pressure to 3-8MPa and ensure the back pressure is stable
（2）Check the temperature of each barrel section to ensure the melting section temperature is at a reasonable value within the 200-270°C range
（3）Check the screw wear; if the flight gap exceeds 0.2mm, the screw needs to be replaced
（4）Screen the raw material and control the powder ratio to no more than 5%
（5）Adjust the matching relationship between the screw rotation speed and the back pressure, with a rotation speed of 40-100rpm
Key Takeaway
Unstable screw retraction is caused by large plasticizing resistance or equipment wear, and the back pressure, temperature, and screw condition need to be optimized.
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