Plastic Mold Wall Thickness / Support Calculator

Plastic Mold Wall Thickness & Support Calculator

Support Plate Thickness

Calculates the minimum thickness required for the movable mold support plate under cavity pressure (by stiffness conditions).

Cavity p Support Plate Deflection [δ] Spacer Spacer L (Span) H b = Loading Width (perpendicular to paper)
Cross-section: Cavity pressure p causes deflection δ in support plate between spacers
H = ∛( p · b · L³ / (32 · E · [δ]) )

p = Cavity Pressure, b = Loading Width, L = Span between spacers, E = Elastic Modulus, [δ] = Allowable Deflection.

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Empirical Value: H ≈ (0.12 – 0.15) × L  |  Take the larger value and round up to the nearest 5 mm.

Integral Rectangular Cavity Sidewall

For integral core/cavity rectangular blocks, sidewall thickness is calculated by both strength and stiffness conditions.

Top View Cavity S L Side View Cavity h
Orange area = Sidewall S (Target), L = Long side, h = Cavity depth
By Strength:  S = h · √( p · L² / ( 2 · [σ] · (L² + h²) ) )
By Stiffness:  S = L · ⁴√( C · p / ( E · [δ] ) )    (C ≈ 0.022)

L = Long side of cavity, h = Cavity depth, [σ] = Allowable stress.

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Integral Cylindrical Cavity Sidewall

Integral cylindrical cavities use the thick-walled cylinder formula (Lamé).

Top View Cavity r t p
Orange Ring = Sidewall t, r = Inner radius, p = Uniform radial pressure
By Strength:  t = r · ( √( [σ] / ([σ] − 2p) ) − 1 )
By Stiffness:  t = r · ( ∛( E·[δ]/(r·p) + 1 ) − 1 )

r = Inner Radius of Cavity · t = Wall Thickness.

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Integral Cavity Bottom Plate

Rectangular cavity bottom plate calculated by stiffness (bending of a plate clamped on four sides).

Cross-section Bottom Plate Cavity p T B (Short side)
Pressure p acts on the bottom plate inner surface, causing deflection δ
T = 0.54 · B · ⁴√( p · B / ( E · [δ] ) )

B = Length of short side of bottom plate (shortest projection direction).

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Support Pillar Quantity and Layout

When the support plate is thin, pillars significantly reduce deformation.
Principles: ① Located under high-pressure zones; ② Max spacing ≤ 150 mm; ③ Symmetrical; ④ Height 0.02–0.05 mm shorter than spacers for pre-load.

Side View Cavity p Support Plate Spacer Pre-load Gap 0.02–0.05 mm Movable Base Plate Support Pillar
Side view: Pillars are 0.02–0.05 mm shorter than spacers for pre-load
Quantity n ≈ ⌈ A / (s · s) ⌉    (A = Projection Area, s = Max Spacing)
Reduced Deflection approx:  δ′ = δ / (1 + n · k)    (k ≈ 0.3–0.5 / pillar)
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Top View · Orange dots = Recommended pillar positions (symmetrical distribution)

Reference: Formulas from "Plastic Molding Process and Mold Design". Results are theoretical; engineering application should combine CAE and safety factors (1.1–1.3 recommended).