In injection mold design, lifter mechanism is core structure for resolving issues related to undercuts, side recesses, and complex curved surfaces in plastic parts. It combines ejection and side core-pulling functions, and its design rationality directly determines quality of plastic part, mold life, and production efficiency. Compared to slider mechanisms, lifter mechanisms are more compact and lower in cost, and are widely used in molds with undercuts. However, strength, stability, coordination must be considered, design pitfalls must be avoided. This article systematically analyzes its design key points and practical techniques from aspects of design fundamentals, core parameters, and structural design, providing a reference for practitioners.

 

1.1 Core Design Purpose
Core function of lifter mechanism is to convert vertical ejection force during mold opening into lateral displacement through tilt angle, achieving "ejection and lateral movement simultaneously," smoothly detaching from undercuts of plastic part, ensuring smooth demolding without damage, while simultaneously ensuring stable mold production, balancing efficiency and precision.
1.2 Applicable Scenarios and Design Prerequisites
Lifter mechanism is suitable for: plastic parts with internal/external undercuts that cannot be formed using a moving mold slider; narrow mold space; shallow undercuts (≤10mm) and numerous undercuts requiring a simplified ejection mechanism.
Design Prerequisites: Clearly define dimensions of mold template, mold core, etc.; determine key parameters of plastic part undercuts; confirm stroke and power of ejection system, avoiding interference risks with ejector pins, water channels, etc.
1.3 Basic Mechanism Components
Standard lifter mechanism consists of five core components that work together to ensure smooth movement and structural stability, as follows:
Lifter Body: Divided into lifter head and lifter rod. Lifter head contacts plastic part and transmits ejection force, while lifter rod connects to slide block and is core force-bearing component.
Guide Block: Includes integrated guide blocks and split guide blocks, constraining trajectory of lifter and preventing wobbling and jamming; (commonly bronze guide blocks).

 

Slide Block: Installed on ejector pin base plate, it fixes and supports lifter rod, limits direction of movement, and ensures smooth movement of lifter.
Reset Mechanism: Ensures precise reset of lifter after ejection, preventing mold closing collisions; commonly uses spring reset (in conjunction with a reset rod for high reliability).
Limit Mechanism: Limits maximum ejection stroke of lifter, consisting of a limit block and adjusting shims, allowing for flexible adjustment to prevent over-ejection.

Core parameters of lifter mechanism are tilt angle, ejection stroke, and strength parameters, which need to be accurately calculated based on plastic part structure and mold space.
2.1 Tilting Angle Design
Tilt angle (θ) is a core parameter, affecting demolding effect and lifespan of mechanism. Required values are as follows:
Standard value is 3°~12°, with 5°~8° recommended to balance lateral displacement and mechanism stability.
Limitations: Minimum ≥3° (to avoid excessive stroke and insufficient lateral force); Maximum ≤12° (to prevent excessive lateral force and mechanism wear and jamming).
Calculation method: θ = arctan(lateral displacement S / ejection stroke H), where S must be greater than undercut depth + 1.5~3mm safety margin, H is determined by ejector plate stroke and part height.
Special scenarios: Deep undercuts use a small angle and long stroke, while shallow undercuts use a large angle and short stroke. Motion simulation is required after design to avoid interference.
2.2 Ejection stroke and lateral displacement design
Ejection stroke (H) and lateral displacement (S) must be matched to ensure complete undercut disengagement. Key points are as follows:
Lateral displacement: S = undercut depth + shrinkage + 1.5~3mm safety margin, to compensate for machining and assembly errors.
Ejection stroke: H = S/tanθ, and not less than part height + 5~10mm, to ensure complete demolding.
Stroke verification: Ensure ejector plate stroke is greater than lifter stroke to avoid mechanism jamming.
2.3 Strength and Thickness Parameter Design
Lifter bears lateral and axial forces, requiring reasonable thickness design and strength verification. Key points are as follows:
Thickness Design: 6mm for length < 100mm, 8mm for ≥ 100mm, and 10mm or reinforced with ribs for > 150mm.
Strength Verification: Bending strength verification is required to ensure adequate head contact area. For slender lifters, instability must be considered and guide length increased.
Root Treatment: Machining a radius of 0.5~2mm is necessary to avoid stress concentration leading to fracture, to facilitate machining and assembly.

Lifter structure design must balance smoothness, sealing reliability, and ease of assembly, focusing on head, guide, reset, and clearance structures.
3.1 Lifter Head Design
Head directly contacts plastic part. Design considerations are as follows:
Reverse sloping angles are strictly prohibited to avoid jamming during demolding and damage to plastic part.
Prioritize horizontal sealing; for vertical sealing, strictly control gap to prevent flash. Set a 2°~3° slope on glue-covered surface for easy demolding.
Polish head to a mirror finish (roughness ≥ #1200). Apply 0.03~0.05mm of glue to top surface to form a "scraper" to prevent slippage.
Head contour should fit undercut shape, with precise fit at ribs and sufficient clearance.
3.2 Guiding and Lubrication Structure Design
Guiding and lubrication ensure lifespan of mechanism. Key points are as follows:
Guide slide is made of wear-resistant material such as 40Cr (HRC45~50), with a clearance of 0.02~0.03mm on each side. Guide length is ≥ 1.5 times stroke of Lifter.
Guide slide is machined with oil grooves (0.5mm deep, 2mm wide, 10~15mm spacing), filled with high-temperature grease; high-speed molds can be equipped with automatic lubrication.
Combined Lifters can be equipped with copper alloy + graphite guide bushings to reduce wear and facilitate maintenance.
3.3 Reset and Limit Structure Design
Reset and limit mechanisms ensure safe operation. Key points are as follows:
Spring reset is the most widely used, but spring force must be greater than reset resistance; slingshot reset is suitable for space-constrained scenarios; forced reset is suitable for high-precision molds.
Limit blocks are made of wear-resistant materials such as SKD11 and equipped with adjusting shims, allowing for flexible stroke adjustment to prevent over-ejection.
Multiple lifters require a linkage mechanism to ensure synchronized action and prevent plastic part deformation or mechanism jamming.
3.4 Clearance and Interference Inspection
To avoid interference between lifter and other structures, key points are as follows:
Head and product rib should have a single-sided clearance ≥0.5mm. Ejector pins and water channels on motion trajectory must have clearance or delayed ejection.
Use CAD or Moldflow to simulate motion trajectory and comprehensively check for interference with ejector pins, mold cores, etc.

Material properties and machining/assembly precision determine lifespan of mechanism and must strictly adhere to relevant specifications.
4.1 Material Selection
Materials for each component must balance wear resistance, toughness, and strength, specifically as follows:
Slanted ejector body: H13, SKD-61, and other hot-work die steels are preferred, nitrided (HRC48~52); high-precision dies can use powder metallurgy materials such as DAC55.
Guide slider: 40Cr, bronze; limit block: SKD11, and other high-strength wear-resistant materials.
Spring: High-temperature resistant type; reset rod: 45# steel or Cr12 (quenched).
4.2 Machining Requirements
Lifter and guide slider are ground together. Straightness and tilt angle tolerance of lifter rod are ±0.01mm.
Materials require quenching and tempering. Lifter surface is nitrided to improve wear resistance and toughness.
Head is polished to a mirror finish (Ra≤0.8μm), the oil groove is flat, and root rounded corners are machined properly.
4.3 Assembly Points
Thoroughly clean components before assembly to remove oil, metal shavings, and other contaminants to prevent jamming.
After assembly, manually check movement of lifter and adjust clearance to be uniform.
Apply high-temperature grease to mating surfaces, ensure proper spring compression, and guarantee accurate reset.
Perform initial mold trial at low speed, observe demolding effect, and adjust parameters as needed.

For common lifter problems, implement corresponding solutions based on root cause analysis to ensure stable operation of mechanism.

6.1 Design summary
Core of lifter design is "balance," following principle of "analysis first, calculation second, design third": analyze space between plastic part and mold, calculate core parameters, optimize structure and avoid interference, strictly control material and processing quality.
Key points: "Three Dos and Three Don'ts": Do control angle to ≤12°, leave a safety margin, and perform interference simulation; don't neglect root strength, heat treatment, or force use of external undercut scenarios.
6.2 Optimization Trends
Lifter design is showing trends towards intelligence, modularity, and high precision, specifically as follows:
Intelligent Simulation: Combining AI and CAE to optimize parameters, reduce trial molding, improve efficiency and accuracy.
Modular Design: Developing a standard component library to shorten cycle and reduce costs.
New Materials and Processes: 3D printing integrated inclined ejectors using high-performance powder metallurgy materials to extend lifespan.

Design of lifter mechanisms in injection molds is a systematic task that balances theoretical calculations and practical experience. Core principle is to adhere to principles of "precision, stability, and efficiency." In actual design, it is necessary to first analyze undercut of plastic part and mold space to determine type of lifter; then scientifically calculate core parameters such as tilt angle and stroke to balance strength and flexibility; subsequently optimize key structures and perform interference checks, while strictly controlling material selection, processing, and assembly quality. Only by accurately controlling every detail, integrating theory with practice can we design an lifter mechanism that meets quality requirements of plastic parts and ensures stable operation of mold, thus providing support for efficient injection molding production, reducing maintenance and failure costs.