Retractable structure of injection mold is used to solve problem of undercut demoulding inside or on the side of plastic part. Its design directly affects mold life and quality of plastic part. Following are classification, movement mode and design points of retractable structure.

 

1. Lifter
Movement mode: Driven by ejection force of ejector plate, it slides along fixed inclined groove or inclined guide column to achieve synchronization of ejection and lateral contraction.
Often linked with ejector plate, plastic part is ejected first when demoulding, and inclined ejector is laterally disengaged from undercut.
Applicable scenarios: Small internal undercuts (such as buckles, grooves), compact mold design.

2. Slider

Driven by inclined guide column: When opening mold, inclined guide column drives slider to slide laterally along guide rail.
Spring reset: After demoulding, spring resets, and positioning pin is required to ensure accuracy.
Movement mode

 

Innovative design: For multi-buckle indentation structures (such as 14 inner circumference undercuts), a T-slot-connected pull block can be used to drive slider, and movement accuracy can be ensured by wire cutting.

 

Applicable scenarios: Structures with large side undercuts, deep cavity demolding, or large strokes (such as threads, side holes).

3. Angled lifter

Movement mode: Parting surface is designed as an inclined surface. When mold plate is separated, angled lifter is forced to slide along inclined surface to complete ejection and lateral contraction simultaneously.
Case reference: Active angled lifter cooperates with driven angled lifter to realize master-slave motion control through mandrel, which is suitable for inner undercut feature of the whole circle.
Applicable scenarios: Undercut is located near parting surface and requires synchronous ejection and lateral demolding.

4. Hydraulic/pneumatic core pulling

Movement mode: Independent cylinder/cylinder drive, core pulling action is independent of mold opening sequence, and timing and stroke can be flexibly controlled.
Applicable scenarios: Deep holes, long-distance core pulling (such as tubular plastic parts), and complex undercuts in multiple directions.

 

5. Rack and pinion/connecting rod mechanism

Movement mode: Non-linear motion trajectory is achieved through gear rack meshing or connecting rod transmission.
Innovative application: For inner circumference undercut, T-slot connection drive between pull block and slider can be designed, multi-directional movement can be achieved in combination with independent guide grooves.

 

1. Stroke calculation
Retracted stroke must be greater than undercut depth (usually +2~3mm safety margin).
Angle of top/slider needs to balance lateral force and friction (recommended 5°-12°).

 

As shown in figure, it is obvious that retraction distance of active inclined slider 1 and driven inclined slider 3 is different, and relationship between them depends on bevel angle α:
S2=S1xsin(a)
For form of splitting core into 3+3, a=30°, above formula becomes:
S2-S1/2 That is, retraction distance of driven inclined slider 3 is half of active inclined slider. Therefore, when calculating retraction distance, driven inclined slider 3 must be demolded as basis for calculation.

2. Guide and positioning

Slider needs to be equipped with wear-resistant guide rails, and lifter needs to be designed with guide grooves or rollers.
Reset mechanism uses spring + hard limit dual positioning to avoid mold interference.

3. Strength and processing

Contact area between lifter and slider head needs to be quenched (HRC50+).
Wire cutting is recommended for complex structures such as T-slots to reduce processing costs and ensure accuracy.

4. Dynamic verification

Use tools such as UG motion simulation to simulate motion trajectory and verify that there is no interference.
For coordinated action of multiple sliders (such as 14 buckle positions), guide groove needs to be designed independently

Simple undercut: Prioritize lifter (low cost, easy maintenance).
Multiple buckle position retraction: Use slider + T-slot drive structure (such as 14 inner circle undercut cases).
High precision requirements: Inclined guide column slider with nitrided guide rail (better stability).
Through reasonable selection and optimized design (such as modular block mold core, standardized parts), complexity and cost of mold can be reduced and yield rate can be improved.