How to solve the problem of deformation of WPC door panel?

How to solve the problem of deformation of WPC door panel?

Warping and deformation of wood-plastic WPC door panels is a common quality problem in production. Its causes are complex and usually closely related to formula design, processing technology, equipment performance and cooling and shaping. 

The following is the specific cause analysis and corresponding solution direction suggested by Sure engineer:

First, the formula design is unreasonable

1. The ratio of wood fiber to plastic matrix is unbalanced

· cause:

· If the proportion of wood fiber (filler) is too high (such as more than 60%), the plastic matrix (such as PE, PP, PVC) is not enough to package and bond the fiber, resulting in insufficient rigidity and internal stress concentration.

· Wood fiber has strong water absorption, and some drying is not complete. After water absorption, it is easy to cause deformation in the later stage.

· Solution direction:

· Optimize the ratio of wood powder to plastic, and recommend 40%-55%:45%-60%, and ensure that the amount of compatibilizer (such as maleic anhydride grafted PE) accounts for 3%-5% to enhance the interface bonding.

· The moisture content of wood powder should be controlled below 1%, and it should be fully dried before production (such as 105℃ drying for 2 hours).

2. Excessive plasticizer or lubricant

· cause:

· Excessive addition of plasticizer (such as phthalate in PVC formulation) will reduce the glass transition temperature of resin, reduce the heat resistance of products, and easily soften and deform in high temperature environment.

· If the amount of internal lubricant (such as stearic acid) is too high, it may weaken the friction between the material and the screw, resulting in insufficient plasticization and loose structure.

· Solution direction:

· Reduce the amount of plasticizer, or replace LDPE with high molecular weight resin (such as HDPE) to improve heat resistance.

· The amount of lubricant should be matched with the screw speed and temperature to avoid material slipping or uneven plasticization.

Second, processing technology problems

1. The extrusion and traction speed do not match

· cause:

· The traction speed is much higher than the extrusion speed, which will cause excessive stretching of the product, resulting in the accumulation of internal stress, which will be released after cooling and cause warping.

· Unstable traction speed (such as fluctuation>5%) will cause uneven longitudinal force on the product.

· Solution direction:

· Adjust the ratio of traction speed to extrusion speed, usually the traction speed is 5%-10% faster than the extrusion speed, and monitor it in real time through the tension sensor.

· Ensure that the traction machine track pressure is uniform, and avoid local clamping too tight or too loose.

2. Improper temperature control

· cause:

· The barrel temperature is too high (especially near the die section), and the material is excessively plasticized, resulting in the decrease of melt strength. After extrusion, it will sag and deform due to its own weight.

· The cooling setting temperature is not uniform. If the inlet temperature of the setting mold is too low, the surface of the product will cool quickly and the internal part will shrink slowly to produce stress.

· Solution direction:

· Control the temperature in segments, and the temperature of the compression section is 10-15℃ lower than that of the melting section to avoid overheating; the mold temperature is controlled at 160-180℃ (adjust according to the type of resin).

· The molding die adopts gradient cooling, the water temperature in the front section is controlled at 20-30℃, and the water temperature in the rear section is gradually reduced to 10-15℃ to ensure the consistent cooling rate inside and outside.

3. Screw speed and back pressure are unreasonable

· cause:

· The screw speed is too fast, the shear heat increases, resulting in material degradation. At the same time, the plasticization time is insufficient, the fiber dispersion is uneven, and the structural strength is inconsistent.

· The back pressure is too low, the melt plasticization is not compact, there are bubbles or voids in the interior, and the contraction is uneven after cooling.

· Solution direction:

· Reduce the screw speed (e.g., from 300r/min to 200-250r/min) and extend the residence time of the material in the barrel.

· The back pressure should be appropriately increased (such as increasing the die head resistance) to improve the melt density. Usually, the back pressure is controlled at 5-10MPa.

3. Equipment and mold defects

1. Screw or barrel wear

· cause:

· The gap between the screw and the barrel is too large (such as the new machine gap 0.2-0.4mm, more than 0.8mm), resulting in material retention, uneven plasticization, and inconsistent density of extruded products.

· After the screw thread wear, the shear force decreases, the fiber fails to be fully dispersed, and the bonding force in local areas is weak.

· Solution direction:

· Measure the screw clearance regularly. When the wear is serious, hard chrome plating, tungsten carbide welding or replacement are required (it is recommended to use 38CrMoAlA nitrided steel or double alloy screw).

· Check the screw compression ratio. Wood-plastic production usually uses a gradient screw with a compression ratio of 2.5-3.0, and the thread shape needs to be repaired after wear.

2. Defects in the design of mold flow channel

· cause:

· The cross section of the mold flow channel changes abruptly or there are dead corners, resulting in uneven material flow rate, local excessive shearing or retention, and inconsistent contraction after cooling.

· The centering of the die and the mouth is poor, and the cooling resistance on both sides of the product is different, which causes unilateral shrinkage.

· Solution direction:

· The mold flow channel is optimized to be streamlined, the cross-sectional area change is reduced, and the compression ratio is controlled at 4-6:1; finite element analysis (CAE) is used to simulate the flow field when necessary.

· Adjust the position of the molding die to ensure that it is concentric with the mouth die and avoid eccentric cooling.

4. Insufficient cooling and setting

1. Low or uneven cooling efficiency

· cause:

· The insufficient cooling water of the molding die or the blockage of the water channel leads to the rapid cooling of the surface and slow cooling of the interior of the product, resulting in the "hard shell and soft core" structure, and the core contraction pulls the surface deformation in the later stage.

· If the temperature of the cooling medium (water) is too high (e.g.,>35℃), the heat dissipation capacity decreases, and the product fails to solidify in time.

· Solution direction:

· Increase the cooling water quantity, use spiral or multi-stage cooling molding to ensure that the water flow rate>2m/s; the water temperature is controlled in 10-25℃, and a chiller can be equipped.

· Extend the length of the molding die (e.g., from 1.5m to 2-2.5m), or add a secondary cooling tank after traction (length> 3m).

2. The molding structure is unreasonable

· cause:

· The surface roughness of the molding die is high, and the friction resistance of the material is large, resulting in the failure to release the stress during cooling; or the vacuum degree is insufficient (such as <0.06MPa), and the product does not fit tightly with the molding die.

· Solution direction:

· The inner surface of the mold is polished to Ra≤0.4μm to reduce friction; the vacuum degree is increased to 0.08-0.09MPa to ensure that the product fully contacts the molding die.

V. Follow-up treatment and environmental impact

1. Improper stacking or transportation

· cause:

· The product is stacked without complete cooling, and the bottom plate is compressed by the weight of the upper part to produce plastic deformation; or the force is uneven when stacking (such as single side contact support).

· Solution direction:

· The products are cooled to room temperature (≤40℃) and then stacked on flat pallets, each layer is separated by partitions, and the stacking height is less than 1.5m.

· Avoid bumps during transportation and prevent collisions or squeezes.

2. Environmental temperature and humidity changes

· cause:

· When the finished product is exposed to high temperature (> 60℃) or high humidity (humidity> 80%), the wood fiber swells with moisture or the resin softens, resulting in warping.

· Solution direction:

· The storage environment should be kept dry and ventilated, with temperature less than 40℃ and humidity less than 60%; when necessary, the surface of the finished product should be coated (such as UV paint) to prevent moisture.

Suggestions for troubleshooting and resolution process

1. Priority inspection of process parameters:

· Record the current extrusion temperature, screw speed, traction speed, cooling water temperature, etc., compare with the standard process, and gradually adjust (such as each adjustment of temperature 5℃, traction speed 1m/min).

2. Test formula performance:

· The moisture content of wood powder, melt flow rate (MFR) and product density (target value 1.1-1.3g/cm³) were measured to determine whether the formula was abnormal.

3. Equipment hardware maintenance:

· Measure the screw clearance with a feeler gauge and check the wear of the mold flow channel. Return to the factory for maintenance or replacement if necessary.

4. Simulation of cooling process:

· The surface temperature difference of the product after cooling is detected by infrared thermometer. If it exceeds 5℃, the cooling system should be optimized.