Low-VOC (Volatile Organic Compound) injection molding materials are widely used in automotive interiors, home appliance housings, medical devices, and food-contact components, where odor and environmental compliance are critical. The core objective of processing these materials is to suppress thermal degradation, reduce the release of small molecular compounds, and control residual odors, while ensuring product appearance, dimensional accuracy, and mechanical performance. Compared to conventional materials, low-VOC formulations impose stricter requirements on raw material preparation, temperature control, shear rate, equipment cleaning, and post-treatment; inadequate control at any stage can lead to excessive VOC emissions, odor rebound, or performance degradation. This article outlines the key processing requirements and control points for low-VOC injection molding materials.
Ⅰ. Raw Material Selection and Pre-Treatment: Locking in Low-VOC Performance at the SourceRaw material quality is the primary prerequisite for low-VOC processing. Strict selection and standardized pre-treatment are essential to prevent impurities, moisture, or residual volatiles from entering production. Priority should be given to low-VOC-specific modified materials (e.g., automotive-grade low-odor PP, eco-friendly ABS, odor-free TPE/TPU), whose VOC emissions are 60-80% lower than conventional grades, with authoritative test reports (e.g., VOC content <50 μg/g, odor grade ≤3.5). The use of regrind must be controlled; multiple melt cycles concentrate small-molecule impurities, so regrind blending ratios should be ≤30%, with prior deodorization and purification. Raw materials must be stored in sealed, dry conditions to avoid moisture absorption or dust adsorption, which can lead to hydrolytic degradation and odor generation during processing.
Pre-treatment focuses on drying and pre-devolatilization. Moisture causes hydrolytic degradation at high temperatures, releasing aldehydes and ketones. Drying parameters must be precisely matched to the material: ABS/PC at 80-100°C for 4-6 hours, PC regrind at 100-120°C for 6 hours, and PA6/66 at 80-90°C for 4 hours, ensuring moisture content between 0.02% and 0.05%. For high-specification products, a low-temperature pre-bake deodorization step (70-80°C for 2-3 hours) can be added to remove easily volatile residual compounds in advance, further reducing post-molding VOC release risk.
Ⅱ. Injection Temperature Control: Low-Temperature Melting to Prevent Thermal DegradationTemperature is the most critical factor affecting VOC release. Low-VOC materials require a low-temperature molding, zone-controlled temperature approach, minimizing barrel, nozzle, and mold temperatures while maintaining melt flowability to avoid thermal degradation and the formation of VOCs such as styrene, formaldehyde, and alkanes.
Barrel temperatures should use a front-high, rear-low gradient setup to reduce melt residence time in the high-temperature zone: for low-VOC PP, rear zone 180-190°C, middle zone 195-205°C, front zone 200-210°C, nozzle 205-215°C; for low-VOC ABS, rear zone 190-200°C, middle zone 210-220°C, front zone 225-235°C, nozzle 230-235°C; for TPE/TPU, rear zone 170-180°C, middle zone 185-195°C, front zone 195-205°C, nozzle 200-210°C. Excessive temperatures must be avoided; when PP exceeds 210°C or ABS exceeds 240°C, degradation accelerates sharply, doubling VOC emissions and reducing mechanical performance.
Mold temperatures should be moderately reduced and uniformly controlled; excessively high mold temperatures aggravate small-molecule volatilization on the cavity surface, causing surface fogging and residual odor. Typically controlled at 40-60°C: 50-60°C for automotive interior PP parts, 45-55°C for ABS, and 35-45°C for TPE, with temperature differences between core and cavity ≤2°C to avoid uneven internal stress and late-stage odor release.
Ⅲ. Plasticizing and Injection Parameters: Low Shear, Short Residence to Minimize Small-Molecule FormationLow-VOC materials are highly sensitive to shear heat and melt residence time. High shear causes molecular chain scission, producing low-molecular-weight volatiles; prolonged residence accelerates thermal degradation. Therefore, screw speed, backpressure, injection speed, and holding pressure must be optimized to achieve low shear, fast plasticizing, and short residence.
Screw speed should be controlled at medium-low (30-60 rpm) to avoid excessive shear heat from high-speed rotation; backpressure set at medium (5-8 bar) to ensure uniform melt mixing and degassing without forcing small-molecule impurities into the melt, which would cause slow leaching later. Melt residence time in the barrel must be strictly controlled at 3-5 minutes, achieved through rational production batch planning and reduced idle heating time; if the machine stops for more than 5 minutes, the barrel must be purged to avoid degradation of residual material. On restart, clean low-VOC material should be used to flush the barrel.
Injection speed should use multi-stage medium-low injection: slow (20-30 mm/s) in the first stage to fill the gate and avoid shear heat from jetting; medium (40-60 mm/s) in the second stage to fill the cavity, reducing melt friction heating. Holding pressure should not be too high (50-70% of injection pressure), with holding time 1.5-2.0 times the filling time to ensure part density without excessive internal stress leading to later VOC release and deformation. Cooling time should be sufficient (20-40 seconds depending on wall thickness) to ensure full cooling and minimize residual internal stress, suppressing small-molecule migration.
Ⅳ. Equipment and Mold Control: Cleaning and Sealing to Prevent Secondary ContaminationThe cleanliness and sealing of equipment and molds directly affect low-VOC product quality; residual old material, carbon deposits, oil, or leaked volatiles can cause excessive VOC and abnormal odors.
Injection molding machines require regular deep cleaning: the barrel and screw should be cleaned weekly with a dedicated low-VOC purging compound (e.g., PMMA clear material, eco-friendly purging masterbatch) to remove residual carbonized material and deposits; monthly screw disassembly for thorough cleaning of thread gaps is recommended to eliminate stubborn impurities that would continuously degrade and release VOCs. Hoppers should use double-layer negative pressure hoods to prevent raw material dust volatilization and external impurities from entering; vents should be fitted with closed filters to collect organic gas emissions and prevent fugitive releases.
Molds must have smooth vents, tight sealing, and clean surfaces. Vent grooves should be 0.02-0.05 mm deep and 5-10 mm wide, positioned at the last fill location, gate sides, and wall thickness transitions to discharge volatiles and air trapped in the cavity, preventing burn marks and residual odor. Parting lines and guide pins must be cleaned regularly to remove oil and iron filings, ensuring tight mold closing to prevent flash and volatile leakage. Mold surfaces should have mirror polish or low roughness (Ra ≤0.8 μm) to reduce small-molecule adsorption, facilitating demolding without residual odor. High-VOC solvents such as acetone or thinner must not be used for mold cleaning; water-based solvent-free cleaners are preferred, with thorough drying before mold closing.
Ⅴ. Auxiliary Material Selection and Post-TreatmentMisuse of auxiliaries is a major cause of excessive VOC in low-VOC products. High-VOC auxiliaries must be comprehensively replaced, and standardized post-treatment used to further reduce residual volatiles.
Release agents should prefer water-based or solvent-free types with VOC content ≤5%, replacing traditional oil-based release agents (≥60% VOC), reducing auxiliary VOC emissions by over 85%; micro-atomization during application avoids overspray leading to residual oil in the cavity, causing part fogging, oil spots, and odor. Lubricants, stabilizers, and other additives should be low-volatility, odor-free eco-friendly grades, avoiding sulfur or chlorine-containing components that release irritating gases at high temperatures.
Post-molding deodorization is required to eliminate residual small-molecule volatiles. Common processes include low-temperature baking (70-80°C for 2-4 hours) to accelerate release of residual VOCs; air curing (24-72 hours at room temperature in a ventilated environment) to allow natural odor dissipation; adsorption deodorization (adding 5% zeolite or molecular sieve adsorbents to capture aldehydes and ketones, reducing VOC by 30%). Post-treated parts must be hermetically packaged in odor-free PE bags or cartons to avoid re-adsorption of odors, and stored in dry, ventilated conditions to prevent odor rebound due to moisture.
ConclusionLow-VOC injection molding is a systematic process centered on source control, low-temperature/low-shear processing, equipment and mold cleaning, and post-treatment reinforcement. Every step must be strictly controlled to eliminate potential VOC release triggers. Compared to conventional injection molding, low-VOC processing requires greater attention to detail and standardized operations. By optimizing process parameters, standardizing operating procedures, and strengthening quality testing, products can consistently meet low-VOC, odor-free environmental requirements while maintaining appearance, dimensional, and mechanical performance, satisfying stringent demands in automotive, home appliance, and medical industries, while reducing process emissions to achieve both environmental and quality improvements.
