How to Solve the Material Scorching Problem During the Extrusion Process of WPC Doors?

How to Solve the Material Scorching Problem During the Extrusion Process of WPC Doors?

Wood-plastic doors have gained widespread adoption in the home decoration and interior renovation industry in recent years due to their significant advantages such as environmental friendliness, durability, moisture and corrosion resistance, and aesthetic appeal, with market demand continuing to grow. However, during actual production, many manufacturers frequently encounter the phenomenon of material scorching during the extrusion process—where materials undergo excessive thermal decomposition or coking under high temperature and pressure. This not only causes frequent production line interruptions and reduced efficiency but also severely compromises the physical properties and visual quality of the final products, leading to lower product pass rates. Addressing this common technical challenge, Yongte Company's professional technical team has developed comprehensive solutions through extensive practice and research. Relevant manufacturers are advised to adopt the following systematic approaches to effectively prevent and resolve material scorching issues during the wood-plastic door extrusion process.

The extrusion scorching in wood-plastic doors (exhibiting localized blackening, discoloration, or granular charred residues) primarily results from the combined effects of four factors: localized overheating, melt retention, excessive shear stress, and unstable formulation. Prioritizing improvements in five key areas—temperature control, lubrication, wood powder quality, mold design, and screw mechanism—is most effective for rapid resolution.

1、Core reason of WPC door material scorching in extrusion

· Temperature failure: Excessive temperature (>180°C) in the barrel, die head, or die cavity; elevated shear heat; localized hot spots, leading to PVC decomposition and wood powder coking.

· Melt retention: accumulation in mold dead zones, material buildup at the convergence core, screw wear/design flaws, or prolonged retention and degradation of old material.

· Formula imbalance: excessively fine wood powder / high moisture content, insufficient lubrication, lack of stabilizers, excessive foaming agents, resulting in a sharp increase in viscosity and resistance.

· Improper process conditions: excessive rotational speed, unstable back pressure, feed fluctuation, insufficient cooling, cumulative shear heat, and significant pressure fluctuations.

2、Solution step by step

1. Precise Temperature Control (Top Priority)

Typical processing temperatures for PVC wood-plastic composites: barrel 155–170°C, head 165–175°C, die 170–175°C; exceeding 180°C is strictly prohibited. Wood powder becomes prone to char at temperatures above 170°C, and PVC decomposes at temperatures exceeding 180°C.

Segmented Gradient:

Cooling procedure: During slurry preparation, first reduce the temperature by 5–10°C while simultaneously decreasing the screw speed (12–18 rpm) to minimize shear heat generation.

Temperature measurement and calibration: Measure the melt temperature using a contact-type thermometer to avoid discrepancies between the displayed reading and the actual value; inspect the heating coil/thermocouple for damage or localized overheating.

2. Formula optimization (to reduce viscosity, stabilize the material, and prevent carbonization)

· Key aspects of flour powder control:

The moisture content should be ≤3% (dried at 80–100°C for 2–4 hours); higher moisture content may lead to excessive foaming and localized overheating.

The particle size ranges from 80 to 120 mesh; particles finer than 150 mesh exhibit excessive adsorption of additives, significantly increased viscosity, and a tendency to coking, whereas those too coarse demonstrate poor plasticizing properties.

The filler content ranges from 50% to 55%; values exceeding 60% result in severely poor fluidity, significant resistance, and a substantial increase in scorching risks.

· Lubrication system (shear reduction, anti-retention):

Internal sliding: Stearic acid (0.3–0.5 parts) + EBS (0.2–0.4 parts), which reduces melt viscosity and minimizes shear heat generation.

O Outer coating: 0.3–0.5 parts PE wax to improve demolding performance and prevent material accumulation on the mold walls.

O Strictly avoid insufficient lubrication; otherwise, frictional heat will surge dramatically and cause localized burning.

· Stability and foaming:

Stabilizer: 3.5–4.5 parts of calcium-zinc stabilizer to prevent high-temperature decomposition of PVC; reduce the proportion of recycled material (<20%), as recycled material is prone to degradation.

foaming agent: 0.3–0.5 parts of AC foaming agent and NC foaming agent; excessive dosage may increase foaming resistance and cause local retention of scorching.

3. Clean the mold and runner system (eliminate dead zones and prevent material accumulation).

Form removal and cleaning: All scorching residues must be removed from the mold cavity, flow core, and diverter cone, including any accumulated material or carbon deposits in dead zones. Use a copper brush with a specialized mold cleaner to avoid scratching the mold walls.

Flow Channel Optimization:

Eliminate right angles and dead corners; ensure smooth fillet transitions in flow paths (R ≥ 3 mm) to prevent stagnation.

The lip clearance of the die mold should be uniform; excessively small clearance results in high resistance and localized overheating, while excessive clearance may lead to deformation.

·Temperature balance of the mold: The temperature deviation across all regions of the die shall be ≤±2°C; localized high-temperature areas may cause material scorching; inspect for any partial damage to the heating ring.

4. Screw and process adjustments (reducing shear stress, stabilizing extrusion)

· Screw Parameters:

Operating speed: 12–18 r/min. Excessively high speeds may cause shear-induced thermal explosion and melt degradation; excessively low speeds result in poor plasticity and pressure fluctuations.

Back pressure: 0.8–1.5 MPa – ensures stable melt flow and prevents local retention; excessively high pressure leads to significant resistance and overheating.

Screw condition: Check for wear or material accumulation; severe wear may lead to retention zones or carbonization. Clean the screw regularly (once every 7–15 days).

Stable feeding:

Use a forced feeder to prevent wood powder bridging and material interruption; restarting after a material interruption may lead to caking.

The hopper should be dry to prevent moisture absorption, clumping, and uneven feeding.

5. Mixing and drying (basic prerequisites)

· Mixing process: High-speed mixing (1000–1500 r/min) → Heating to 90–100°C → Low-speed cooling below 40°C before discharge; uneven mixing may lead to localized insufficient additive content or scorching formation.

· Powder drying: 80–100°C for 2–4 hours, with moisture content ≤3%; high moisture content may lead to foam formation instability and localized overheating.

3、Quick Investigation Process (completed in 10 minutes on-site)

1. Check material distribution: Die area → high mold temperature/die material accumulation; Head/confluence core → high temperature/retention; Barrel → high rotational speed/screw wear.

2. Measuring melt temperature:>180°C → Immediately cool down and reduce rotational speed.

3. Camphor powder: Moisture content> 3% / excessively fine → dry and replace with coarse powder.

4. Clearing the mold: Remove material accumulation in dead corners → Disassemble the mold for cleaning and perform corner rounding.

5. Oil lubrication adjustment: High viscosity and elevated discharge resistance → Add internal slip agent (stearic acid/EBS).

4、Preventive Measures (Long-term Stability)

· Daily tasks: Measure temperature (melting point/mold temperature), inspect feed materials, and examine product surfaces.

· Weekly: Clean the screw, clean the mold opening, and inspect the heating ring/thermocouple.

· Monthly: Calibrate temperature controls; analyze wood powder moisture content and particle size; optimize formulations.