In injection molding production, especially when dealing with plastic products with uneven wall thickness, complex structures, high appearance requirements, precise dimensions, traditional single-stage injection molding, due to fixed parameters and uncontrollable melt flow, is prone to various product defects, leading to low yield rates and low production efficiency. Multi-stage injection molding, as a core process function of modern injection molding machines, breaks down injection process into multiple stages, independently setting parameters such as injection speed, pressure, and position for each stage. It precisely controls the entire process of melt filling and holding pressure within mold cavity, becoming a key process for solving injection molding problems and improving product quality. This article will deeply analyze core role of multi-stage injection molding, as well as its principles and practical methods for specifically solving various product defects, providing practical reference for machine setup and quality control in injection molding workshops.

Summary: Multi-stage injection optimizes flow, filling, and holding pressure through "segmented control," effectively improving product quality, reducing defects, and lowering defect rate.

Multi-stage injection molding refers to dividing the entire injection process into 2-5 or even more stages during injection molding process, based on screw position, injection time, or mold cavity pressure as switching nodes. Each stage independently controls injection speed and pressure, achieving precise, segmented control of melt flow.
Its core principle is adapting to flow pattern of melt within mold cavity: After melt enters mold cavity from gate, flow path, resistance, and cooling rate constantly change. Single-stage injection cannot cover the entire process, while multi-stage injection can achieve "fast at the beginning, stable in the middle, slow at the end, and precise pressure holding," allowing melt to fill cavity smoothly, uniformly, and without defects, while reducing internal stress, compensating for cooling shrinkage, and avoiding molding defects from source.

(I) Precise Control of Melt Flow, Adapting to Complex Product Structures: For complex products with irregular shapes, multiple cavities, abrupt changes in wall thickness, or inserts, resistance in different areas of mold cavity varies greatly. Multi-stage injection allows for parameter switching at key locations such as runner filling, thin-walled sections, thick-walled sections, and cavity ends. For example, low-speed, stable flow is possible in thin-walled areas, while high-speed filling occurs in thick-walled areas, preventing melt flow imbalance and ensuring complete filling of complex structures without dead zones or material shortages.
(II) Eliminating Product Appearance Defects and Improving Surface Quality: Single-stage injection is prone to problems such as melt turbulence, air entrapment, and shear overheating, leading to defects such as silver streaks, blemishes, weld lines, scorching on product surface. Multi-stage injection, through segmented speed adjustment, allows for rapid passage through gate in initial stage to avoid cold material, uniform filling in the middle stage to reduce melt impact, and deceleration and venting in final stage, resulting in smooth, layered melt filling and significantly improving product surface finish to meet high-gloss and matte finish requirements.
(III) Reducing Internal Stress and Warping Deformation: Single-stage injection, with its high pressure and high-speed single-pass filling, generates significant internal stress in mold cavity, making the product prone to warping, deformation, and cracking after demolding. Multi-stage injection molding employs slow filling and segmented holding pressure, allowing melt to cool uniformly and maintain regular molecular orientation, significantly reducing internal stress and ensuring product dimensional stability. It is particularly suitable for precision plastic parts, long strips, and flat products.
(IV) Optimizing Holding Pressure and Solving Shrinkage and Sinking Issues: During cooling, injection-molded products experience volume shrinkage. Single-stage holding pressure is limited and cannot effectively compensate for shrinkage in thick-walled areas, ribs, and bosses, easily leading to shrinkage and sink marks. Multi-stage injection molding, combined with multi-stage holding pressure, allows switching between appropriate holding pressure and speed at the end of filling, specifically compensating for shrinkage in these areas, ensuring a smooth product surface free of sink marks and cavities.
(V) Enhancing Production Stability and Expanding Process Window: Multi-stage injection molding allows for flexible adjustment of parameters in each stage, adapting to rheological properties of different raw materials (such as PC, ABS, PA, PP, etc.). It also reduces stringent requirements for mold venting and gate design, simplifies machine setup, improves stability of different batches, reduces scrap rates, and increases production efficiency.
(VI) Protecting Molds and Equipment, Extending Service Life: By reducing speed and pressure in final stage, high-speed, high-pressure impact of melt on cavity, core, inserts, and other precision parts of mold is avoided, preventing mold deformation and wear. Simultaneously, it reduces equipment overload operation, lowers screw and barrel wear, extends service life of mold and injection molding machine, and reduces production costs.

(I) Short Injection (Insufficient Filling)
Causes: Slow single-stage injection speed, insufficient pressure, melt cooling and solidification during flow, or high resistance in runner and gate leading to delayed filling.
Multi-Stage Injection Solution:
First Stage (Large Screw Stroke, Runner Filling Stage): High speed and high pressure, quickly propelling melt through runner and gate to avoid cold material blockage;
Second Stage (Cavity Filling Stage): Medium speed and medium pressure, smoothly filling cavity body;
Third Stage (Cavity End): Low speed and low pressure, ensuring melt fills dead corners and avoiding material shortages due to insufficient venting caused by excessive speed.
(II) Flash (Overfill, Burr)
Causes: Excessive injection speed and pressure at the end of injection phase, causing melt to break through mold parting line, insert gaps, or overfilling thick-walled areas.
Multi-stage Injection Solution: Fill cavity to 90% at a normal speed in initial stage, then significantly reduce speed and pressure at the end to prevent overfilling.
For thick-walled areas, reduce pressure and speed before filling thick-walled area to avoid excessive melt extrusion and flash. Simultaneously, use appropriate holding pressure to prevent overflow.
(III) Weld Lines
Causes: Insufficient temperature and uneven speed when two melt streams merge, resulting in incomplete fusion and obvious weld lines, affecting appearance and strength.
Multi-stage Injection Solution: Increase injection speed before melt fusion to raise melt flow temperature and enhance fusion. Slightly reduce speed during fusion to allow melt to adhere fully, reducing weld lines. Simultaneously, increase mold temperature to further weaken weld lines, bringing them to an acceptable appearance standard.
(IV) Air Bubbles, Trapped Air, and Scorching
Causes: Air is trapped during high-speed melt filling, resulting in poor cavity venting. Unable to escape, gas is compressed and heated, leading to air bubbles, internal pores, or surface scorching.
Multi-stage Injection Solution: In initial stage of filling, rapidly advance melt. When cavity is approximately 80% full, immediately slow down to reduce melt flow rate and allow sufficient time for gas to escape. At the end, use low speed and low pressure to prevent gas compression and scorching, especially for deep cavities, blind holes, and poorly vented areas. Segmented deceleration is core method for solving trapped air.
(V) Shrinkage and Depression
Causes: Thick walls, ribs, roots of product cool and shrink rapidly, resulting in insufficient melt compensation and surface depressions.
Multi-stage Injection Solution: Switch to a holding pressure stage at the end of filling, using multi-stage holding pressure. First, use high pressure to compensate for shrinkage, then gradually reduce pressure, continuously replenishing shrinking areas with melt. For thick-walled areas, extend holding time of corresponding stage to ensure melt fully compensates for shrinkage, eliminates depressions, and ensures a smooth product surface.
(VI) Warpage and Deformation
Causes: Excessive internal stress during melt filling, uneven cooling, and uneven molecular orientation lead to deformation due to stress release after demolding.
Multi-stage Injection Solution: Use uniform and slow filling throughout process to avoid internal stress caused by high-speed impact; Control filling speed in stages to ensure uniform cooling in areas with different wall thicknesses; Use multi-stage holding pressure, gradually decreasing pressure from high to low to reduce residual internal stress; For long strips and flat products, use low-speed filling throughout to ensure uniform melt flow and completely solve warpage and deformation problem.
(VII) Silver Threads and Spots
Causes: Excessive melt shear speed, resulting in shear overheating, or raw material containing water or entrapped air, forming surface silver threads and watermarks.
Multi-stage Injection Solution: Reduce initial injection speed to reduce melt shear force and avoid shear overheating degradation; Ensure uniform and stable filling process to prevent air entrapment; Ensure raw material is thoroughly dried to eliminate silver thread and spot defects from both process and raw material perspectives.

Stage Division Principles: Simple products (flat plates, regular small parts) should have 2-3 stages; complex products (uneven wall thickness, irregular shapes, precision parts) should have 3-5 stages. Switching point should be based on screw position, closely matching mold cavity filling node.
Speed and Pressure Control: Follow principle of "faster speed past gate in initial stage, stable filling of cavity in the middle stage, slower discharge and shrinkage compensation in final stage." Speed should decrease from high to low, pressure should match speed to avoid high-pressure, high-speed overlap.
Material Compatibility: For materials with poor flowability (PC, PMMA), appropriately increase speed in initial stage; for shear-sensitive materials (ABS, acrylic), control speed throughout to reduce shear; for glass fiber reinforced materials, maintain a stable speed in the middle stage to avoid glass fiber exposure.
Coordinated Adjustment: Multi-stage injection requires coordinated adjustment of barrel temperature, mold temperature, holding time, and cooling time. Optimizing a single parameter cannot achieve the best results.

Multi-stage injection molding breaks through limitations of traditional single-stage injection molding. Through segmented and refined control, it achieves precise control of melt flow. It not only systematically solves common product defects such as short shots, flash, weld lines, shrinkage, and warpage, but also improves product dimensional accuracy, surface quality, and production stability. It is an indispensable core process in modern injection molding production for complex, high-requirement, and precision products.
For injection molding machine operators and production managers, mastering principles and adjustment techniques of multi-stage injection molding can significantly reduce scrap rates, increase production efficiency, lower production costs, and easily handle various challenging injection molding problems. It is a key process for improving workshop production levels and product competitiveness.