Cracking of the Inner Wall of Pipes

Fine cracks appear on the coating surface, with crack directions randomly or parallelly arranged, and in severe cases forming a network of cracks. Coating cracking is the result of internal stress exceeding the fracture toughness of the coating—the difference in shrinkage rate between the coating and the substrate generates interfacial stress during curing, an overly thick coating increases the absolute value of stress, and an overly fast curing speed leaves no time for stress relaxation. A cracked coating not only loses its protective and decorative functions, but the cracks also become channels for corrosive media to invade the substrate.

1. Cooling/environmental factors: the mandrel temperature is too low, causing the inner surface melt to cool rapidly without sufficient fusion. 2. Cooling/environmental factors: core mold cooling water seepage. 3. Die design factors: The flow channel geometry, straight section length, and outlet size of the die directly affect melt flow stability and product dimensional accuracy. Unreasonable flow channel design (presence of dead corners, excessively large contraction angle) is an important root of melt fracture, material buildup, and flow marks, and needs to be addressed starting from die structure optimization.

1. Process parameter optimization and adjustment: Increase the core mold temperature to ensure the melt temperature of the inner surface is appropriate. 2. Mold/tooling improvement and maintenance: Check whether the mandrel cooling channel is cracked or leaking. 3. Systematic troubleshooting method: Troubleshooting of extrusion problems follows the order of "temperature → pressure → speed → die → cooling". First check and stabilize the temperature control of each section, then observe head pressure fluctuations to judge the feeding and screen condition, then verify the matching relationship between screw speed and haul-off speed, further check the condition of the die flow channel and sizing sleeve, and finally optimize the cooling water channels and water temperature distribution. Change only one variable at a time to confirm the effect. 4. Preventive management: Establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify countermeasures as PFMEA control points. Establish a regular equipment maintenance system and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

The essence of coating cracking is that the internal stress exceeds the fracture toughness of the coating. The core of the solution is "stress reduction + toughening": control the coating thickness to avoid excessive stress accumulation, adopt mild curing conditions to allow stress relaxation, add toughening agents to improve the coating's fracture toughness, and select a coating system with a shrinkage rate close to that of the substrate.