Plastic Mold Lifter Structure Design

The angled lifter (oblique ejector) is the most common mechanism for internal undercuts in plastic molds. Its design directly affects mold reliability, product quality, and service life. This article introduces industry-standard design parameters, structural requirements, materials, and precautions for stable and durable lifters.

I. Structure and Working PrincipleA lifter consists of a head, rod, guide block, sliding seat, and reset mechanism. During ejection, the lifter moves vertically and horizontally at the same time to release the undercut. During clamping, the cavity surface pushes the lifter back to position. Lifters are suitable for undercuts between 0.5 and 5 mm.

II. Industry-Standard Lifter AngleRecommended angle: 3°–8°Minimum angle: ≥3°Maximum angle: ≤12°Most commonly used: 5°Core formula:Core-pulling distance = undercut depth + 0.5–1 mm safety margin

Ejection stroke = core-pulling distance / tan(angle)

III. Size and Strength Design

Minimum lifter head width: ≥8 mmMinimum head thickness: ≥5 mmLifter rod thickness ≥ 1/2 of the headFor lifters longer than 120 mm, use double rods or support structuresClearance between lifter and mold insert: 0.5–1 mm per side

IV. Fit Clearance and GuidanceLifter and guide block clearance: 0.015–0.03 mm per sideLifter and mold insert clearance: 0.01–0.02 mm per sideGuide length ≥ 1.5 times ejection strokeLong or high-speed molds use insert-type guide blocks for easy replacement.

V. Anti-Rotation and Sliding SeatSingle lifters must use anti-rotation structures such as flats or keyways. Sliding seats use T-slots or dovetail grooves with 0.01–0.02 mm clearance. Use at least M6 screws with lock washers.

VI. Material and Heat TreatmentStandard lifters: SKD61, HRC48–52, nitridedHigh-wear lifters: H13, HRC50–54Guide blocks: 40Cr, HRC45–50

All mating surfaces must be ground to Ra0.8 or better.

VII. Reset, Limit and Anti-Sticking

Use forced reset pins to avoid spring failure. Install limit blocks to control ejection position. Add textures or small ejector pins on the lifter head to prevent sticking. Ensure full reset before closing.

VIII. Common Design TaboosDo not use angles below 3° or above 12°.Do not design overly thin heads.Do not ignore clearance, anti-rotation, or support.Ensure full clearance in moving areas.

SummaryLifter design must follow industry standards: angle 3°–8°, clearance 0.01–0.03 mm, material SKD61, hardness HRC48–52. Simulate motion to avoid interference. Standardized design ensures long service life, stable production, and high-quality parts.