The Spring for Plastic Injection Mould

Functions of Mold Springs

Mold springs are one of the most widely used elastic components in injection molds. Their main functions include:

• Unloading

• Material Pressing

• Pushing Mechanisms

• Ejecting Molded Parts

Springs are color-coded into five standard categories based on their load capacities, making it easier to identify and select the correct spring for specific applications.

Spring Series Specifications

Common outer diameters of mold springs include:
Φ6, Φ8, Φ10, Φ12, Φ14, Φ16, Φ18, Φ20, Φ22, Φ25, Φ30, Φ35, Φ40, Φ50, etc.

Categories of Mold Springs

Springs are generally categorized by their function or mounting location in the mold:

• RP Springs (Return Plate)

• TR Springs (Top Return / Backup)

• Slider Retaining Springs

• Ejector Return Springs

Spring Hole Design Guidelines

• The spring hole diameter should be 1–2 mm larger than the spring's outer diameter.

• The hole should be 20–30 mm deep into the B plate.

• For RP springs, hole diameter should be:

  Hole Ø = (Ejection Distance × 2) + 10 mm
  Spring length = Depth of spring hole + 10 mm

• When automatic production is needed, TR springs are often added under the RP plate.

• If an angle lifter mechanism without kiss-off is used, do not install springs under RP — this prevents the lifter from pulling the product back into the mold.

Spring Installation Requirements

• Preload (initial compression):

• 5–10 mm for standard pre-action

• >10 mm when return force is prioritized

• Maximum deflection should not exceed 40% of the spring’s free length.

• For export molds to the U.S., springs must not be placed directly on return pins.

• Recommended preload amount: ~10% of the spring’s free length

• Ejector return springs: Use preload amount (not ratio) — typically 10–15 mm

️ Selection Considerations

Mold Base Width & Spring Layout:

• For long, narrow bases (Length ≈ 2 × Width), add 2 extra springs for balance.

• Always use symmetric layouts and prefer larger diameter springs when possible.

Free Length Calculation for Return Pin Springs

Formula:

Free Length (L) = (E + P) / S

Where:

• E = Ejector travel distance (E > product ejection distance + 15~20 mm)

• P = Preload (10–15 mm, depending on return resistance)

• S = Deflection ratio (30–40% of total spring length)

Then, the installed spring length:

L1 = L – Preload

Slider Retaining Spring Design

• Common spring diameters: 10 mm, 13 mm, 16 mm, 20 mm

• Allowable deflection: ¼ to ⅓ of free length

• Usually, two springs are installed per slider

Free Length Calculation:

Free Length (L) = (Slider Stroke + Preload) / Deflection

Where:

• Slider Stroke = Distance the core pulls

• Preload depends on:

• Vertical slider: weight-based preload

• Horizontal slider: preload ≈ 10% of spring length

• Spring elastic coefficient is used to determine required preload force.