Vacuum Venting System Design and Application Key Points for Injection Molds
2026-05-28 09:43:05
Injection Molds
Vacuum venting system is an essential auxiliary structure for high-precision injection molds. During injection molding, air trapped in cavities and volatile gas decomposed from plastic melts will cause product burning, bubbles, short shots, poor weld line strength and surface blemishes. Traditional natural venting relying on gaps between parting lines, ejector pins and inserts has limited efficiency, and can hardly meet the production requirements of thin-walled parts, deep-cavity products and parts made of glass-filled materials. By extracting internal gas with negative pressure, the vacuum venting system effectively solves venting problems, improves molding quality and extends mold service life.
1. Overall Structural Design of Vacuum Venting SystemA complete vacuum venting system consists of vacuum generator, negative pressure pipelines, sealing components, vent grooves and control units. The matching performance of each component determines the operational stability. The vacuum generator is the core component for producing negative pressure. For ordinary plastic parts, the negative pressure is set from -0.06MPa to -0.08MPa. For high-speed thin-wall molding and thermally decomposable materials, the value shall be increased to -0.08MPa to -0.1MPa to avoid residual gas. Excessively high negative pressure will suck molten plastic into pipelines and cause blockage.
Pipelines shall be arranged in short and straight routes with fewer bends. The inner diameter of branch pipelines shall be no less than 6mm for small single-cavity molds, while the main pipeline of multi-cavity large molds shall be over 10mm. Sealing structure is the foundation of stable negative pressure. High-temperature rubber sealing strips are installed on parting lines. Excessive fit and sealant are applied on vent inserts. Precision clearance fit is adopted for moving parts such as ejector pins to prevent air leakage.
Different from traditional vent grooves, the depth of vacuum vent grooves is strictly controlled. The depth ranges from 0.015mm to 0.025mm for general plastics, and reduces to 0.01mm to 0.015mm for low-viscosity and glass-reinforced materials, which balances smooth gas discharge and flash prevention. Vent grooves are arranged at gas accumulation areas and connected to external air extraction ports.
2. Layout and Design Specifications of Vent GroovesGas flows and gathers following the melt filling path, which is the basic principle for vent groove layout. Annular vent grooves are set at the end of main runners and sub-runners to discharge air in advance. Blind corners, rib roots and areas with uneven wall thickness, as the last filling positions, are key venting areas with independent concentrated vent structures. For deep tubular parts with slender cores, internal venting holes are drilled inside cores to solve venting difficulties caused by limited external space.
Symmetrical layout is adopted for multi-cavity molds, and each cavity is equipped with independent vent grooves and branch pipelines to ensure uniform negative pressure. Vent grooves are prohibited on appearance and assembly surfaces. All grooves converge to the gas collecting tank on the outer side of the mold and then connect to vacuum pipelines. For transparent and optical products, hidden vent structures are adopted to guarantee surface finish.
3. Control and Linkage Design of Vacuum SystemThe vacuum system operates in linkage with injection actions instead of continuous running. Under normal molding conditions, vacuum starts synchronously with injection and stops immediately when injection is completed, preventing plastic blockage in pipelines during packing stage. For thick-wall large parts, the air extraction time can be extended to the early packing stage. For high-speed thin-wall molding, the vacuum shall be activated 0.1 to 0.3 seconds in advance.
Pressure detection and overload protection devices are equipped to monitor negative pressure in real time. The system will alarm and stop automatically once air leakage or blockage occurs. When multiple molds share one vacuum unit, independent solenoid valves are configured for separate control. Meanwhile, vacuum parameters shall be adjusted according to mold temperature to adapt to different molding conditions.
4. On-site Application, Maintenance and TroubleshootingMold trial run is the key stage to verify venting performance. Operate the vacuum system alone to check the stability of negative pressure. If the pressure drops rapidly, air leakage points shall be inspected and repaired one by one. If products are burnt, add supplementary vent structures. If flashes appear, reduce the depth of vent grooves or lower negative pressure. For materials releasing corrosive gas such as PVC and POM, anti-corrosion materials are used for pipelines and generators.
Daily maintenance is required to ensure long-term stable operation. Check pipeline aging and detachment every day, and clean carbon deposits and plastic residues in vent grooves every week. Common faults include insufficient negative pressure, groove blockage and material shortage. Insufficient pressure is mainly caused by air leakage and pipeline blockage. Blockage results from over-deep grooves or excessive negative pressure. Improper air extraction speed will disturb melt flow and lead to filling defects.
Vacuum venting system is an important supporting structure for sophisticated injection molds. Reasonable design and standardized application can eliminate venting-related defects, shorten molding cycle and realize stable mass production of complex high-precision plastic parts.
