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T5 · Processing / MachineMould & Die DesignGATE 2027 Syllabus (XE5)Shop-Floor Standard
Injection Mould Cavity Shrinkage Dimension Sizing Formula
Calculates the required machined mould cavity dimension (Dc) from the desired cold molded plastic part dimension (Dp) and polymer linear shrinkage rate (Sl).
Mathematical Canonical Relation
Plain Formula: Dc = Dp * (1 + Sl)
Live Engineering Parameter Solver
Input your production or test values below for instant, verified output.
Verified Engine
Dp_val
Range: 5 – 500 mm
Sl_pct
Range: 0.2 – 4.5 %
Calculated Engineering Result:
Dc = 51.000 mm
Engineering Verification:Mould steel cavity is oversized by +1.000 mm to compensate for resin cooling contraction.
When & Where to Apply
Use during CNC machining and EDM electrode sizing of injection mould cavities and cores to ensure final molded parts meet drawing tolerances.
Common Engineering Pitfalls
- Ignoring differential shrinkage between flow direction and transverse direction in fiber-filled resins
- Neglecting post-molding crystallization shrinkage in semi-crystalline POM, PA, and PBT
Physical Assumptions & Validity Limits
- Uniform isotropic shrinkage across part section
- Standard ambient measurement at 23°C / 50% RH after 24-48 hours relaxation
- Optimum pack and hold pressure applied during moulding
Variable Symbols, SI Units & Dimensional Specifications
| Symbol | Variable Meaning | SI Unit | Typical Industrial Range |
|---|---|---|---|
| D_c | Machined mould cavity dimension | mm | — |
| D_p | Required nominal part drawing dimension | mm | 50.00 |
| S_l | Linear shrinkage rate fraction (e.g., 2% = 0.020) | fraction (mm/mm) | 0.020 |
Worked Industrial Numerical Example: 50.00 mm POM Acetal Gear Diameter Cavity Sizing
Problem Statement:
A precision acetal (POM homopolymer) spur gear requires finished outside diameter Dp = 50.00 mm. Acetal linear shrinkage is 2.0% (Sl = 0.020). Determine the CNC cavity machining diameter.
Step-by-Step Numerical Solution:
- Apply shrinkage equation: Dc = Dp * (1 + Sl)
- Substitute values: Dc = 50.00 * (1 + 0.020)
- Compute cavity dimension: Dc = 50.00 * 1.020 = 51.00 mm
Final Answer: Dc = 51.00 mm
💡 Engineering Interpretation: The CNC toolpath must cut the mould cavity to exactly 51.00 mm (+0.02/-0.00 mm) so that after cooling and post-mold shrinkage, the gear contracts to the nominal 50.00 mm drawing dimension.
Authoritative Technical Reference & Source Provenance
Injection Molds for Beginners by Rainer Dangel (Hanser, 2020), 2nd Edition, Page 82-88.
Connected Curriculum Lesson
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