Plastic Mold Strength Calculation

Plastic mold strength calculation mainly checks whether the stress, deformation, and stiffness of key parts such as cavities, cores, templates, guide pins, and ejector pins meet safety requirements under injection pressure, clamping force, and holding pressure, so as to avoid cracking, deformation, flash, and ejection failures. Below is a complete explanation from the aspects of calculation basis, step-by-step calculation of core parts, safety factor, verification process, and practical simplified methods, combining theory with on-site application.

I. Calculation Basis: Core Parameters and Premises

1. Key Load Parameters (Must Be Determined First)

Cavity pressure P: The pressure exerted by the melt on the inner wall of the cavity during injection, which is the core load for strength calculation. Routine values: 25–40 MPa for PP/PE, 30–50 MPa for ABS/PS, 40–70 MPa for PA/PC/POM, and 50–80 MPa for high glass fiber reinforcement (30%+). Take the upper limit for precision parts and the lower limit for ordinary parts; it can also be accurately obtained by mold flow analysis (Moldflow).

Clamping force F_clamp: F_clamp = P × total projected area of the cavity (including runner and gate), unit kN, used to check the stiffness of templates, guide pins, and mold bases.

Material mechanical parameters: Common mold steels include 718H, S136, NAK80, and P20. Yield strength σs: about 800–900 MPa for 718H, 1000–1200 MPa for S136, and 550–650 MPa for P20. Elastic modulus E = 2.06×10⁵ MPa, Poisson’s ratio μ = 0.3.

Safety factor n: The mold is subjected to alternating loads and temperature stress. The safety factor for strength n ≥ 1.5–2.5, and for stiffness n ≥ 1.2–1.5. Take the upper limit (2.0–2.5) for cavities and cores, and the lower limit (1.5–2.0) for templates and guide pins.

2. Basic Assumptions

Mold parts are made of homogeneous, isotropic, and linear elastic materials, and deformation is within the elastic range.

The load is uniformly distributed, ignoring temperature stress, residual stress, and local stress concentration (finite element correction is required for complex structures).

The cavity is simplified to a rectangle/circle for calculation, and the equivalent size is taken for irregular cavities.

II. Step-by-Step Calculation of Strength and Stiffness of Core Parts (Most Commonly Used)

1. Cavity Side Wall Strength (Most Critical to Prevent Mold Expansion and Cracking)

It is divided into circular cavity and rectangular cavity. Priority is given to checking the maximum tensile stress and maximum deformation (deformation ≤ 0.02–0.05 mm to avoid flash and dimensional deviation).

(1) Circular Cavity (Cylindrical, Inner Diameter D, Wall Thickness t, Height h)

Circumferential tensile stress (hoop stress, maximum): σθ = P×D/(2t)

Strength condition: σθ ≤ σs/n → Minimum wall thickness t ≥ P×D/(2×σs/n)

Radial deformation: ΔD = P×D²×(1−μ/2)/(2×E×t)

Stiffness condition: ΔD ≤ allowable deformation (e.g., 0.03 mm)

(2) Rectangular Cavity (Length L, Width W, Wall Thickness t, Height h, h≥t, Treated as a Plate Under Uniform Load)

Maximum bending stress of the long side wall: σ = 6×P×L²/(h×t²)

Maximum bending stress of the short side wall: σ = 6×P×W²/(h×t²)

Maximum deformation: δ = 0.142×P×L⁴/(E×h×t³) (midpoint of the long side)

Strength/stiffness condition: σ ≤ σs/n, δ ≤ allowable deformation

2. Core Strength and Stiffness (Prevent Bending, Fracture, and Deformation)

Most cores are cantilever beams/simply supported beams, subject to melt lateral pressure, and need to check bending stress and deflection.

Core diameter d, length L, subject to uniform pressure P, equivalent load F = P×d×L

Maximum bending stress: σ = 32×F×L/(π×d³) = 32×P×d×L²/(π×d³) = 32×P×L²/(π×d²)

Maximum deflection (cantilever): δ = 4×P×L⁴/(E×d⁴)

Strength condition: σ ≤ σs/n; Stiffness condition: δ ≤ 0.02–0.05 mm (≤0.01 mm for precision parts)

Slender core (L/d > 5): Must add support, splicing, beryllium copper cooling, or use finite element detailed calculation.

3. Strength of Moving/Fixed Templates (Prevent Template Bending and Fracture)

The template is treated as a simply supported beam/bidirectional plate under clamping force and cavity pressure, and needs to check bending stress and deflection.

Template thickness h, span L (guide pin spacing), width B, total load F = F_clamp

Maximum bending stress: σ = 3×F×L/(2×B×h²)

Maximum deflection: δ = F×L³/(4×E×B×h³)

Strength condition: σ ≤ σs/n; Stiffness condition: δ ≤ 0.05–0.1 mm (avoid flash due to gap on the parting surface)

4. Strength of Guide Pins and Guide Bushings (Prevent Bending and Shearing)

Guide pins are subject to lateral force and bending load. Diameter d, length L, maximum bending stress σ = 32×F×L/(π×d³), σ ≤ σs/n

Guide bushing fit clearance: H7/g6 to avoid jamming. The number of guide pins is ≥4, evenly distributed.

5. Strength of Ejector Pins and Ejector Sleeves (Prevent Bending and Fracture)

Ejector pin diameter d, length L, ejection force F_eject (≤5–8 kN per piece), bending stress σ = 32×F_eject×L/(π×d³), σ ≤ σs/n

Ejector pins are evenly spaced to avoid excessive local load. Slender ejector pins should be added with guide sleeves.

III. Irregular/Complex Cavities: Simplification + Finite Element (CAE)

Routine formulas are only applicable to regular shapes. For irregular, multi-cavity, spliced, deep-cavity, and thick-rib molds, the following methods must be used:

Equivalent simplification: Simplify the irregular cavity into the closest rectangle/circle, take the maximum force size for calculation, and the result is conservative.

Finite element analysis (ANSYS/Moldflow): Establish a 3D model, apply real cavity pressure and temperature loads, calculate stress distribution, maximum stress, and deformation cloud map, accurately check dangerous sections (fillets, corners, splicing positions), and modify local wall thickness/structure.

Local reinforcement: Add fillets (R≥3 mm), thicken, and add reinforcing ribs/support columns in dangerous areas (sharp corners, thin edges, insert roots) to reduce stress concentration.

IV. Complete Verification Process (On-Site Practical Steps)

Determine cavity pressure P, projected area, clamping force, mold steel σs, E, and safety factor n.

Calculate the tensile stress and deformation of the cavity side wall (circular/rectangular), and check strength + stiffness.

Calculate the bending stress and deflection of the core, and check strength + stiffness.

Calculate the stress/deformation of templates, guide pins, and ejector pins, and check them one by one.

If not satisfied, thicken the wall thickness, increase the diameter, shorten the cantilever, add support, or replace with high-strength steel.

Verify complex structures with mold flow + finite element, and finally determine the size.

V. Practical Simplified Quick Calculation (On-Site Rapid Estimation)

Minimum wall thickness of circular cavity: t ≈ 0.08×D (P=50 MPa, σs=800 MPa, n=2)

Wall thickness of rectangular cavity: t ≈ 0.1×L (long side length L, P=50 MPa)

Core diameter: d ≥ 0.1×L (core length L, cantilever)

Template thickness: h ≥ 0.02×L (guide pin spacing L, conventional clamping force)