The die system is a key forming unit of extrusion blow molding machines, which undertakes the functions of uniformly distributing the molten plastic delivered by the extrusion system, shaping it into a parison (or film bubble) with a specific cross-sectional shape and uniform thickness, and stably conveying it to the subsequent clamping and blowing process. Its structural precision, flow channel design and operating stability directly determine the surface quality, dimensional accuracy and mechanical properties of the final product. Due to the long-term contact with high-temperature and high-pressure melt, frequent switching of production recipes, and improper daily maintenance, the die system is prone to various faults. This document focuses on common faults of the die system, analyzes the causes in depth based on fault phenomena, and proposes corresponding solutions and preventive maintenance measures.

1. Uneven Discharge at Die Exit

(1) Fault Phenomena

The thickness of the extruded parison (or film bubble) is uneven in the circumferential direction; the wall thickness deviation of the product (such as hollow containers, films) exceeds the allowable range; the film bubble swings irregularly during the blowing process, and the opening is difficult or uneven; the parison has obvious “ribbing” or “wrinkling” phenomena at the die exit.

(2) Fault Causes

• Uneven temperature distribution in the die: Partial damage or insufficient power of the die heating coil, resulting in low temperature in local areas of the flow channel; malfunction of the temperature control sensor (thermocouple) installed on the die, leading to inaccurate temperature detection and uncontrollable local overheating or undercooling; uneven heat conduction caused by carbonization and scaling on the inner wall of the die flow channel.
• Die flow channel blockage or wear: Long-term use leads to accumulation of carbonized materials, decomposed impurities and raw material residues in the die flow channel, especially at the corner of the flow channel and the die lip, which hinders the uniform flow of the melt; the inner wall of the flow channel is worn due to long-term scouring of the melt, changing the flow field distribution and causing uneven velocity distribution of the melt at the exit.
• Mandrel deviation or die lip deformation: The mandrel (core mold) is offset due to loose fixing bolts or wear of the positioning mechanism, resulting in uneven gap between the mandrel and the die lip in the circumferential direction; the die lip is deformed due to long-term high-temperature working conditions or improper disassembly and assembly, and the gap cannot be kept uniform.
• Unstable melt performance: Large fluctuations in the melt flow index (MFI) of the raw material lead to uneven viscosity of the melt entering the die; the melt contains undispersed impurities or bubbles, which affect the flow uniformity at the die exit; excessive shear rate caused by too high screw speed leads to unstable melt flow.

(3) Solutions

• Calibrate the die temperature system: Use an infrared thermometer to detect the actual temperature of each heating zone of the die, and compare it with the instrument display value to calibrate; replace the damaged heating coil and faulty thermocouple, and ensure that the temperature control instrument works normally; regularly clean the carbonized materials on the inner wall of the die to ensure uniform heat conduction.
• Clean and maintain the die flow channel: Shut down the extrusion blow molding machine for cooling, disassemble the die according to the operation manual, and use professional tools (such as copper scrapers, high-temperature resistant cleaning agents) to thoroughly clean the flow channel and die lip; check the wear of the inner wall of the flow channel, and repair or replace the die parts if the wear exceeds the limit; polish the inner wall of the flow channel to ensure smoothness.
• Correct the mandrel and die lip: Check the fixing bolts of the mandrel, tighten them if loose, and replace the worn positioning parts; use a feeler gauge to measure the gap between the die lip in the circumferential direction, and adjust the adjusting bolts of the die lip to ensure uniform gap; replace the deformed die lip parts, and pay attention to avoiding collision and deformation during disassembly and assembly.
• Stabilize the melt performance: Strictly control the quality of raw materials, ensure the stability of the melt flow index, and homogenize different batches of raw materials before use; check the extrusion system, adjust the screw speed and temperature to avoid excessive shear and melt degradation; ensure that the raw materials are fully dried to remove moisture and avoid bubbles in the melt.

2. Melt Fracture at Die Exit

(1) Fault Phenomena

The surface of the parison (or film) extruded from the die is rough, with obvious “sharkskin” or “orange peel” patterns; severe cases show wavy, twisted or even broken parison; the film bubble is easy to rupture during the blowing process, and the product surface has obvious flow marks and defects.

(2) Fault Causes

• Excessive shear rate: The die lip gap is too small, and the melt flow rate is too high when passing through the narrow gap, resulting in excessive shear stress exceeding the bearing capacity of the melt; the screw speed is too high, leading to excessive shear of the melt in the extrusion system, and the unstable melt enters the die and is further sheared and damaged.
• Improper die temperature setting: The die temperature is too low, the melt viscosity is too high, the fluidity is poor, and the melt cannot flow smoothly at the die exit, resulting in flow fracture; the temperature difference between different regions of the die is too large, leading to uneven viscosity of the melt and local shear concentration.
• Poor design of die flow channel: The flow channel has sharp corners, sudden changes in cross-section or uneven transition, which causes local eddy current and shear concentration of the melt; the surface of the flow channel is rough, increasing the friction resistance between the melt and the wall, and triggering melt fracture.
• Raw material problems: The molecular weight distribution of the raw material is too wide, and the low-molecular-weight components are easy to decompose under shear, leading to unstable flow; the raw material contains impurities or incompatible additives, which destroy the uniformity of the melt and cause flow defects.

(3) Solutions

• Adjust process parameters to reduce shear rate: Appropriately increase the die lip gap to reduce the flow velocity and shear stress of the melt at the exit; reduce the screw speed and adjust the extrusion amount to make the melt flow in a stable laminar flow state; if necessary, reduce the back pressure of the extrusion system to avoid excessive shear of the melt.
• Optimize the die temperature setting: Appropriately increase the die temperature to reduce the melt viscosity and improve fluidity; ensure that the temperature of each heating zone of the die is uniform, and eliminate the local low-temperature zone by adjusting the heating power.
• Improve the die flow channel structure: For dies with unreasonable flow channel design, modify the flow channel to ensure smooth transition of the cross-section and eliminate sharp corners; polish the inner wall of the flow channel to reduce surface roughness and reduce friction resistance.
• Optimize raw material selection and treatment: Replace raw materials with narrow molecular weight distribution to improve the shear stability of the melt; remove impurities in the raw materials through screening and filtration; check the compatibility of additives, and avoid adding incompatible components.

3. Die Blockage

(1) Fault Phenomena

The extrusion amount of the parison decreases significantly, and the die pressure rises sharply; the die exit discharge is intermittent or even interrupted; the product surface has obvious black spots, carbonized particles and other defects; the heating power of the die is significantly increased, but the temperature is difficult to maintain.

(2) Fault Causes

• Melt degradation and carbonization: The die temperature is set too high, or the local temperature is overheated, leading to long-term residence and degradation of the melt in the die flow channel, and the decomposed products are carbonized and accumulated; the screw speed is too low, the melt residence time in the extrusion system and die is too long, resulting in degradation and carbonization.
• Impurity accumulation: The raw materials contain a large amount of impurities, or the recycled materials are not properly treated, and the impurities are filtered out by the filter screen and accumulate at the die entrance; the filter screen is damaged, and large particles of impurities enter the die flow channel and are stuck.
• Improper shutdown and maintenance: The extrusion blow molding machine is shut down without purging the die, and the residual melt in the flow channel cools and solidifies, which is not cleaned in time before the next startup, leading to blockage; the die is not properly maintained after long-term use, and the carbonized materials and residues accumulate continuously.
• Low melt fluidity: The melt flow index of the raw material is too low, or the die temperature is too low, resulting in poor melt fluidity, slow flow in the die, and easy accumulation and blockage.

(3) Solutions

• Emergency treatment for blockage: Immediately shut down the extrusion blow molding machine and reduce the die temperature to avoid further carbonization of the melt; disassemble the die according to the operation procedures, and use professional tools to thoroughly clean the blocked parts, carbonized materials and residues; for stubborn blockages, use high-temperature resistant cleaning agents or bake at a suitable temperature (avoid overheating) to remove the blockages.
• Adjust process parameters to avoid melt degradation: Reduce the die temperature to the reasonable range of the raw material; adjust the screw speed and extrusion amount to shorten the melt residence time in the die; check the heating system of the die to eliminate local overheating.
• Strengthen raw material and filter screen management: Strictly control the quality of raw materials, remove impurities through screening and drying; regularly replace the filter screen of the extrusion system, and check the integrity of the filter screen to prevent impurities from entering the die; reasonably control the proportion of recycled materials and ensure that the recycled materials are clean.
• Standardize shutdown and maintenance procedures: Before shutdown, use clean raw materials to purge the die to remove the residual melt in the flow channel; after shutdown, clean the die in time and apply anti-rust oil to the relevant parts; establish a regular die maintenance system and thoroughly clean the die at fixed intervals.

4. Die Leakage

(1) Fault Phenomena

Molten plastic overflows from the connection parts of the die (such as the connection between the die body and the adapter, the connection between the die lip and the die body); the leaked melt cools and solidifies, adhering to the surface of the die and the parison, causing scratches on the product surface; in severe cases, the leakage is large, affecting the normal discharge of the die and causing production interruption.

(2) Fault Causes

• Loose sealing parts: The sealing ring (gasket) at the die connection is aged, deformed or damaged due to long-term high-temperature and high-pressure environment; the fixing bolts of the die connection are loose due to thermal expansion and contraction during the heating and cooling process.
• Uneven mating surface: The mating surface of the die connection parts is uneven due to improper processing or long-term wear; there are carbonized materials, residues and other impurities on the mating surface, which affect the sealing effect.
• Excessive die pressure: The filter screen is blocked or the die lip gap is too small, leading to a sharp rise in die pressure, which exceeds the bearing capacity of the sealing parts and causes leakage; the extrusion amount is too large, and the melt flow rate exceeds the design capacity of the die, resulting in excessive internal pressure.
• Improper die assembly: The die parts are not assembled in place during disassembly and maintenance, or the assembly sequence is wrong, leading to poor sealing; the torque of the fixing bolts is uneven during assembly, resulting in uneven stress on the sealing surface.

(3) Solutions

• Handle leakage and replace sealing parts: Immediately shut down the extrusion blow molding machine and cool down, clean the leaked melt and solidified materials on the die surface; disassemble the leaking connection parts, check the sealing ring, and replace the aged, deformed or damaged sealing ring with high-temperature and high-pressure resistant products (such as fluorine rubber sealing rings).
• Clean and repair the mating surface: Use a grinding tool to polish the uneven mating surface to ensure flatness and smoothness; thoroughly clean the mating surface to remove carbonized materials, residues and other impurities that affect the sealing effect.
• Reduce die pressure: Replace the blocked filter screen to reduce the back pressure of the extrusion system; appropriately increase the die lip gap to reduce the flow resistance of the melt in the die; adjust the extrusion amount to make the die pressure within the design range.
• Standardize die assembly operations: Assemble the die parts in strict accordance with the operation manual and assembly sequence to ensure that each part is in place; use a torque wrench to tighten the fixing bolts evenly to ensure uniform stress on the sealing surface; after assembly, conduct a pressure test to check the sealing effect before formal production.

5. Product Surface Defects Caused by Die (Scratches, Scorch Marks, Bubbles)

(1) Fault Phenomena

The product surface has obvious linear scratches or concave-convex marks; there are local yellowing, blackening and scorch marks on the product; small bubbles are distributed on the product surface or inside, which cannot be eliminated by subsequent cooling and shaping.

(2) Fault Causes

• Scratches: There are burrs, scratches or adhered solidified materials on the die lip; the mandrel surface is rough or has scratches, and the melt is scratched when passing through; foreign objects (such as metal debris, hard plastic particles) adhere to the die exit.
• Scorch marks: Local overheating of the die leads to carbonization of the melt; carbonized materials accumulate in the die flow channel and are brought out by the melt to adhere to the product surface; the die temperature is too high, and the melt residence time is too long, resulting in degradation and carbonization.
• Bubbles: The raw materials are not fully dried, and the moisture evaporates into water vapor when passing through the high-temperature die; the melt decomposes to generate gas due to overheating, and the gas cannot be discharged in time and remains in the product; there is air trapped in the die flow channel, which is mixed into the melt during the flow process.

(3) Solutions

• Eliminate scratches: Polish the die lip and mandrel surface with fine sandpaper to remove burrs and scratches; clean the die exit to remove adhered solidified materials and foreign objects; check the raw materials and filter system to prevent hard impurities from entering the die.
• Remove scorch marks: Adjust the die temperature to eliminate local overheating; thoroughly clean the die flow channel to remove accumulated carbonized materials; adjust the screw speed and extrusion amount to shorten the melt residence time and avoid melt degradation.
• Eliminate bubbles: Strengthen the drying of raw materials, ensure that the moisture content meets the process requirements; check the die temperature, avoid excessive temperature leading to melt decomposition; purge the die before startup to remove the air trapped in the flow channel; check the exhaust system of the extrusion system to ensure that the gas generated during plasticization is discharged smoothly.

6. Preventive Maintenance Suggestions for Die System

1. Regular cleaning and inspection: Establish a regular die cleaning system (the cycle is determined according to the production volume and raw material characteristics), and thoroughly clean the flow channel, die lip and other parts; regularly check the wear, deformation and sealing of die parts, and replace damaged parts in time.

2. Standardize temperature control: Regularly calibrate the die temperature control instrument and thermocouple to ensure accurate temperature detection; avoid setting the die temperature too high, and strictly control the temperature within the range suitable for the raw material.

3. Strict raw material management: Ensure the cleanliness and stability of raw materials, remove impurities through multiple filtration; dry the raw materials in accordance with the process requirements to avoid moisture-induced faults.

4. Standardize assembly and disassembly operations: Operators must be professionally trained before disassembling and assembling the die, and strictly follow the operation manual to avoid damage to parts due to improper operation; use special tools for assembly and disassembly to ensure uniform tightening torque of bolts.

5. Record operating data: Regularly record the die pressure, temperature, product quality and other data during production, analyze the operating status of the die through data changes, and predict potential faults in advance.