Design of Multi-cavity Injection Mold and Balanced Injection Molding Skills
2026-05-27 10:51:57
Injection Molding
Multi-cavity molds are widely used in mass injection molding production, which improve production efficiency and reduce unit cost by molding multiple parts in one cycle. However, increased cavity quantity tends to cause uneven feeding, filling difference, inconsistent product dimension and appearance, and unbalanced clamping force. To realize stable mass production, overall mold structure, gating system, machining & assembly and injection process shall be comprehensively controlled for balanced molding.
Overall Structural Design of Multi-cavity MoldCommon cavity arrangements include linear matrix layout, annular circular layout and H-type branch layout, which shall be selected according to product shape, cavity quantity and injection molding machine parameters. Symmetrical annular layout is preferred for high-precision small parts and molds with a large number of cavities, as all cavities share equal flow path length to realize natural flow balance. Matrix layout is suitable for long strip parts and molds with fewer cavities, which must be fully symmetrical in all directions to avoid different flow path length caused by asymmetric arrangement.
Multi-cavity molds bear large distributed clamping force, so thickened moving plates, fixed plates and backup plates are equipped on heavy-duty standard mold bases to prevent plate deformation, cavity misalignment and flash under long-term high clamping force. Multiple groups of symmetrically arranged guide pins and guide bushes are adopted to enhance mold opening and closing coaxiality and positioning precision. Keep the entire parting surface flat with uniform height to avoid poor sealing and material overflow caused by surface difference. Unify draft angle, polishing grade and cooling layout of all cavities to create identical molding conditions. For molds with more than 16 cavities, additional positioning pins and stop structures are installed to strengthen positioning accuracy and reduce assembly cumulative error.
Balanced Design of Gating SystemRunner balance is the core part, which is divided into natural balance and artificial balance. Natural balance design is prioritized to eliminate flow difference fundamentally. The main runner adopts standard taper structure with high-gloss polished inner wall to reduce melt retention and flow resistance. Sub-runners follow the principles of equal length, equal cross-section and equal turning angle. The cross-section of sub-runners is generally trapezoidal or circular. Circular runners feature low heat dissipation and small resistance for high-viscosity plastics, while trapezoidal runners are easy to process and demold and have wider application. All runner corners adopt arc transition instead of right angles to prevent local shear overheating and disordered flow.
All gates on one mold shall adopt the same type, size and position. Do not mix different gate types. Gate dimensions are determined according to product wall thickness and material properties. Gates are arranged at thick wall areas for smooth filling. If natural balance cannot be realized due to product structure limitations, adopt artificial throttling balance method: slightly reduce gate thickness or length, or machine throttling grooves on sub-runners to limit flow velocity of fast-filling cavities and synchronize filling time of all cavities. Make minor adjustments step by step.
The gating system is designed without dead corners to prevent aging and decomposition of retained material. Consider the convenience of runner sprue removal to adapt to automatic production.
Synchronous Design of Cooling, Venting and Ejection SystemUneven cooling leads to inconsistent shrinkage and deformation of products from different cavities. Symmetrical cooling water channels are arranged for all cavities and cores with identical pipe diameter, distance to molding surface and circuit direction. Independent temperature control is adopted for each zone to control mold temperature deviation within ±2 ℃. Arrange enhanced cooling at slender parts and thick-wall areas to eliminate local hot spots and avoid sink marks, bending and dimensional out-of-tolerance.
Each cavity is equipped with independent venting grooves at parting lines, flow ends and weld line positions. Keep uniform depth, width and length of all venting grooves to prevent burning, bubbles and material shortage in partial cavities. The depth of venting grooves is set according to material characteristics to balance ventilation effect and flash prevention.
The ejection mechanism is arranged symmetrically with unified specification of ejector pins and ejection plates. Keep consistent ejection clearance and stroke to ensure uniform ejection force and avoid scratch, whitening and deformation of partial products. The entire ejection system resets synchronously to maintain stable production cycle.
Accuracy Control of Mold Machining and AssemblyUnified machining technology, cutting tools and parameters are adopted for cavities and runners to guarantee consistent surface roughness and dimensional tolerance. Integral cavity plates or modular inserts are applied for high-cavity molds to reduce assembly error of split structures. Detect concentricity, flatness and runner alignment during assembly to eliminate steps and misalignment at runner joints.
Mark serial numbers for each cavity after mold assembly. Record product status of each cavity during trial molding and make targeted adjustments. Regularly maintain the mold and inspect runner wear, gate deformation and water channel blockage, as uneven wear will break the original flow balance.
Commissioning Skills for Balanced Injection Molding ProcessAfter the mold is well prepared, match precise injection process to realize synchronous molding of all cavities. Conduct short-shot test to observe filling progress of each cavity, and take the slowest-filling cavity as the benchmark for parameter adjustment.
Adopt multi-stage injection speed: low speed for runners and gates to reduce shear difference, constant speed for cavity filling to stabilize flow, and low speed at the final stage to avoid pressure impact. Full high-speed injection is forbidden, which will amplify minor differences of runners and gates and worsen molding imbalance.
Set multi-stage injection pressure, packing pressure and packing time uniformly for the whole mold. Packing stage compensates material shrinkage and stabilizes product dimension. Raise screw back pressure and reduce screw speed to improve material plasticization uniformity and stabilize melt viscosity. Keep stable temperature of barrel, nozzle and cooling water. Limit the proportion of regrind materials to avoid melt performance fluctuation.
Inspect weight, dimension and appearance of products from each cavity regularly during production. When defects occur in partial cavities, troubleshoot mold temperature, water channels and gate blockage first, and then fine-tune local process parameters.
Daily Production MaintenanceClean carbon deposition and residual material on runners and gates regularly to prevent flow channel narrowing and velocity change. Inspect cooling water channels for scale and blockage. Check the movement status of guide pins and ejection components to avoid wear and jamming. Establish a mold maintenance record to mark standard process parameters, defect rules and adjustment schemes for repeated production.
Balanced molding of multi-cavity molds is a systematic work combining structural design, machining accuracy and process commissioning. Based on symmetrical layout, supported by precise machining and assembly, and matched with refined injection parameters, the mold can operate stably to reduce defective rate and give full play to the advantages of high-efficiency mass production.
