Key Notes for PVC Injection Molding
PVC injection molding requires strict control over thermal stability, fluidity, venting, mold and equipment compatibility, as well as process parameters and safety protection. The goal is to prevent decomposition, scorching, sticking, and product defects.
Raw Material Preparation and Drying
PVC has low hygroscopicity, but recycled or damp materials may contain trace moisture. This moisture can cause bubbles, silver streaks, and surface pitting in products. Dry the materials at 60–80℃ for 2–4 hours, with the maximum temperature not exceeding 85℃ to avoid premature decomposition. When mixing new and recycled materials, the proportion of recycled materials should not exceed 30%. Recycled materials must be sieved to remove impurities and agglomerates, preventing feed channel blockages and poor melt fluidity.
Core Process Parameter Control
Temperature Management
Temperature control is critical to prevent PVC decomposition, which releases hydrogen chloride (HCl) that corrodes equipment and contaminates products. The barrel temperature should be set at 160–180℃, with the rear section at 150–160℃, middle section at 170–180℃, and front section at 160–170℃. Flexible PVC can be 5–10℃ lower, while rigid PVC should not exceed 185℃. The nozzle temperature is 170–180℃, slightly lower than the barrel front section to prevent drooling. The mold temperature is 20–40℃ for flexible PVC and 40–60℃ for rigid PVC. Low mold temperatures shorten the molding cycle, avoid product yellowing and deformation, and reduce melt residence time in the cavity to lower decomposition risks.
Pressure and Speed Settings
The injection pressure ranges from 80–120MPa for flexible PVC and 100–140MPa for rigid PVC. Adjust the pressure based on product wall thickness and runner length; increase pressure appropriately for thick-walled or long-runner products to ensure full cavity filling. Use medium-low injection speed to avoid shear overheating, local scorching, melt turbulence, and air entrapment. The holding pressure is 40–60MPa for 5–15 seconds, preventing product shrinkage and sink marks. Release pressure promptly after holding to avoid internal stress concentration.
Screw Speed and Back Pressure
Set the screw speed at 30–60r/min. Low speed reduces shear heat, prevents melt overheating and decomposition, and ensures uniform plasticization. The back pressure is 3–8MPa; moderate back pressure removes gas from the melt and improves plasticization quality. Excessively high back pressure causes melt overheating and decomposition, while insufficient back pressure leads to uneven plasticization and low product density.
Mold Design and Adaptation Requirements
Runner and Gate Design
Use circular or trapezoidal runners with a surface finish of Ra≤0.8μm to reduce melt flow resistance and avoid residence and decomposition. The runner diameter is 4–8mm; avoid overly thin runners that cause shear overheating. Prioritize side gates or submarine gates with appropriate dimensions: 2–4mm wide and 0.8–1.5mm deep. This prevents shear overheating and scorching at the gate and facilitates gate removal without damaging products.
Venting and Cooling System
Add vent grooves (0.02–0.04mm deep, 5–10mm wide) at the last filling positions and cavity dead corners. These grooves discharge gas and HCl generated by decomposition, preventing bubbles, scorching, and short shots. The mold must be equipped with uniform cooling channels to ensure cooling efficiency, shorten the molding cycle, and avoid product deformation and warping caused by uneven cooling. The cooling water temperature is controlled at 20–30℃, matching the mold temperature.
Mold Material and Surface Treatment
Choose corrosion-resistant and highly polishable mold steels such as S136 and NAK80 to avoid HCl corrosion and ensure product surface finish. Polish the cavity and core surfaces to Ra≤0.4μm, reducing melt adhesion, lowering sticking risks, and facilitating demolding.
Equipment Adaptation and Maintenance
Equipment Requirements
Select general-purpose injection molding machines with a screw length-diameter ratio of 18–22:1. Use gradual, low-shear screws to reduce melt shear heat. The barrel and screw should be chrome-plated or made of corrosion-resistant alloys to prevent HCl corrosion. Adopt self-locking nozzles to prevent melt drooling and decomposition, with nozzle aperture matching the gate size to avoid flow marks.
Equipment Maintenance
Before production, clean the barrel with PE or PP to remove residual impurities, then transition to PVC material to prevent decomposition caused by other residual raw materials. During production, regularly clean residual melt in the barrel, nozzle, and runners to avoid long-term residence and decomposition. If production stops, empty the PVC material in the barrel promptly or lower the barrel temperature below 100℃ to prevent decomposition from prolonged high-temperature storage. After production, thoroughly clean the barrel and screw with PE material to remove PVC residues, avoiding decomposition during subsequent production.
Safety Protection and Environmental Protection
Safe Operation
PVC decomposition releases HCl gas, so activate the workshop ventilation system. Operators must wear gas masks and protective gloves to avoid inhaling harmful gases or getting burned by molten material. Do not force injection when the barrel temperature is insufficient to prevent equipment overload due to unplasticized melt. Avoid direct contact with scorched or decomposed products to prevent skin and respiratory irritation from residual HCl.
Environmental Treatment
Purify waste gas generated by decomposition through waste gas treatment systems (e.g., alkaline solution absorption) before discharge to avoid environmental pollution. Classify and recycle waste PVC materials and scraps; do not mix them with other raw materials. These can be dried again for use in low-requirement products to reduce waste.
Common Defects and Solutions
Yellowing and scorching: Lower barrel and nozzle temperatures, reduce injection speed and screw speed, improve mold venting, and shorten melt residence time in the cavity.
Bubbles and silver streaks: Enhance raw material drying, increase back pressure, optimize vent grooves, and avoid air entrapment in the melt.
Sticking and difficult demolding: Increase mold temperature, optimize draft angle (≥1.5°), polish the cavity surface, and use PVC-specific release agents to avoid affecting product surfaces.
Shrinkage and sink marks: Increase holding pressure and time, extend cooling time, optimize gate positions, and ensure sufficient melt replenishment.
