How to solve the problem of rough surface of extruded WPC door panels?
When producing wood-plastic WPC door panels, the surface flatness is very important for the quality of the WPC door panels. However, many wood-plastic door factories are helpless in the face of the problem that the surface of the extruded WPC door panels is rough and dull during production. Today, the Sure engineer team provides a specific analysis and solution for this problem.
The extruded wood-plastic WPC door panel is rough and lusterless, which may be due to problems in raw material formula, process parameters, equipment status or cooling and molding.
The following are the specific cause analysis and solutions:
· The ratio of resin to filler is unbalanced
· If the content of wood powder/fiber filler is too high (such as more than 60%), the resin is not enough to wrap the filler particles, resulting in an uneven surface.
· The melting index (MI) of resin is too low, and the fluidity is poor, so it cannot fill the filler gap evenly.
· Improper addition of additives
· Insufficient lubricant: insufficient addition of internal lubricants (such as stearic acid, paraffin), high melt viscosity, poor plasticization of materials, rough surface.
· Lack of gloss agent: no acrylic ester, polyethylene wax and other gloss enhancers are added, or the amount is insufficient (usually 0.2-0.5 parts).
· Excessive stabilizer: too much heat stabilizer (such as lead salt, calcium and zinc) may be precipitated to the surface, affecting gloss.
· Insufficient pretreatment of packing material
· The moisture content of wood powder is too high (>5%), and water vapor is produced during extrusion, resulting in bubbles or spots on the surface; if coupling treatment (such as silane or titanate) is not carried out, the interface between filler and resin is poorly combined, and it is easy to fall off.
· Improper temperature control
· Low temperature: the resin is not fully melted, the mixture with filler is uneven, and granular protrusions appear on the surface.
· High temperature: resin degradation or filler carbonization, gray surface, no luster (reference: PVC base recommended 160-190℃, PE/PP base 180-220℃).
· Screw speed and pressure are abnormal
· If the screw speed is too fast (such as>60r/min), the shear action is strong, resulting in local overheating or degradation of the material; if the screw speed is too slow, the plasticization is insufficient and the surface is rough.
· If the mold pressure is too low (such as <10MPa), the melt is not fully filled and the surface lacks compacted luster.
· The screw is worn or the design is not reasonable
· The screw and screw gap is too large (>3mm), the retention time of material is long, and the plasticization is not uniform; the compression ratio is insufficient (such as <2:1), resulting in poor density of melt.
· Mold runner defect
· The inner wall of the flow channel is rough and has dead corners, so that the material is retained and carbonized; the design of the die outlet is unreasonable (such as the straight section is too short), so that the melt expands seriously from the die and the surface is not smooth.
· The filter is clogged
· If the mesh number of the filter is too high (such as>80) or the use time is too long, the material flow resistance is large, resulting in pressure fluctuations and surface defects.
· The cooling rate is too fast or too slow
· If the cooling water temperature is too low (such as <10℃), the melt will be solidified quickly and the surface will not be fully leveled; if the water temperature is too high (such as>30℃), the setting will be insufficient and the surface will be easily scratched or rough.
· The design of the fixed sleeve is unreasonable
· The spacing between the fixed sleeve and the die is too large, so that the melt droops and deforms; the distribution of internal cooling water channels is not uniform, resulting in inconsistent local cooling.
· The traction speed does not match the extrusion speed
· If the traction speed is too fast, the door plate will be stretched and thinned, and the surface will appear orange peel; if it is too slow, the accumulation will lead to rough surface.
· Adjust the ratio of resin to filler
· Reduce the wood powder content to 50%-55%, increase the resin dosage (such as PE/PP base from 30% to 35%), improve the coating; select high MI resin (such as MI=8-12g/10min PE), improve the fluidity.
· Reasonable addition of additives
· Lubricant: Increase the amount of internal lubricant (such as stearic acid from 0.3 parts to 0.5 parts), or add compound lubricant (such as EBS ethylene butyl stearate, 0.2-0.4 parts) to reduce melt viscosity.
· Gloss agent: Add polyethylene wax (0.3-0.5 parts) or acrylic gloss agent (0.2 parts) to improve surface gloss.
· Coupling agent: wood powder is pre-treated with silane coupling agent (such as KH-550,0.5-1.0 parts) to enhance the interfacial bonding force and reduce the shedding of filler.
· Control the moisture content and fineness of packing
· The wood powder is dried to the moisture content <3%, and the coarse particles are removed by sieving (it is recommended that the particle size <80).
· Optimize the temperature setting
· The "stepwise heating" mode is adopted, such as PVC four-stage temperature setting of 165℃ (feed section) →175℃ (compression section) →185℃ (melt section) →180℃ (mold section), to ensure that the resin is fully plasticized and not degraded.
· If the surface is rough due to insufficient plasticization, the temperature of the melting section can be increased by 5-10℃; if it is degraded due to overheating, the temperature should be lowered and the residence time shortened.
· Match the screw speed with the traction speed
· The screw speed is controlled at 40-50r/min, and the ratio of traction speed to extrusion speed is 1.1-1.3:1. The thickness uniformity of the door plate is adjusted by observation.
· Increase the mold pressure
· Replace the higher mesh filter (such as 60 mesh to 80 mesh), or reduce the die gap, so that the melt is fully compacted under high pressure (target pressure 12-15MPa).
· Check the wear of screw and barrel
· Measure the gap between the screw and the barrel, if it exceeds 3mm, replace the screw or barrel to ensure that the compression ratio is 2.5-3:1 to improve the plasticization effect.
· Polishing mold runner and die
· The inner wall of the mold runner is polished with a mirror (roughness Ra<0.8μm) to eliminate dead corners; the straight section of the die is extended to 10-15 times the pipe diameter to reduce the expansion away from the die.
· Clean the filter
· Replace the filter screen regularly (it is recommended to check it every 4 hours) and remove the retained carbonized material.
· Optimize cooling water temperature and flow rate
· The temperature of the fixed sleeve water is controlled at 15-25℃, and the flow rate is 5-8L/min. Segmented cooling (the water temperature in the front section is slightly higher, and the water temperature in the rear section is lower) is adopted to avoid sudden cooling resulting in uneven surface stress.
· Adjust the position of the shaped sleeve and the traction tension
· The spacing between the molding sleeve and the die is shortened to 5-10mm to ensure timely molding of the melt; the traction machine applies a slight tension (about 5-10N) to avoid stretching deformation.
· Add surface treatment process
· Immediately after extrusion, a thin layer of paraffin emulsion or polyurethane clear varnish is applied through polishing roller or coating device to improve the gloss and smoothness of the surface.
1. Establish a database of recipes and processes
· Record the best temperature, pressure, traction speed and other parameters corresponding to different raw material ratios for quick adjustment.
2. Maintain equipment regularly
· Clean extruder barrel and mold every week, check temperature control system and cooling water circuit every month to ensure stable operation.
3. First piece inspection and process monitoring
· After starting the machine, take the first piece and test it with a roughness meter (target Ra<3.2μm) and gloss meter (target 60° gloss>30GU), and adjust the parameters in time.
Through the above measures, the surface quality of wood-plastic door panels can be effectively improved to achieve smooth and lustrous appearance standards.
