Injection Mold, Parting Line & Draft Angle Vector Prepress Guide
Master mold cavity design, parting line shut-off geometries, polymer shrinkage scaling, EDM rib spark gap allowances, and 2D vector CAD prepress for aluminum and hardened steel tooling.
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1. Why Zero-Draft Geometry Causes Tooling Failure
In plastic injection molding and silicone compression tooling, a part with perfectly vertical ($0^\circ$) walls cannot eject smoothly from the steel mold cavity. As hot molten polymer cools and solidifies under extreme clamping pressure ($300 - 1800\text{ bar}$), it contracts onto the core half and creates severe frictional drag.
Without adequate Draft Angle ($\theta_{\text{draft}}$), the ejector pins will punch through the plastic, produce heavy vertical gouge marks (drag scratches), or cause the part to vacuum-stick in the stationary cavity plate. Even $1^\circ$ of draft creates an immediate $0.017\text{ mm}$ clearance gap per mm of stroke the moment the mold opens.
2. Draft Angle Mathematical Formulas & Depth Growth
Every degree of draft adds lateral thickness from the top of the feature to the parting line base:
Total Wall Thickness Increase = 2 × H × tan(θ_draft)
where H = Depth of cavity / height of rib (mm)
- SPI A-1 to A-3 (Diamond High-Gloss Mirror): Minimum $0.5^\circ - 1.0^\circ$
- SPI B-1 to B-3 (Fine Paper / Stone Polish): Minimum $1.0^\circ - 1.5^\circ$
- SPI C-1 to C-3 (Dry Blast Sand / Glass Bead): Minimum $1.5^\circ - 2.0^\circ$
- VDI 3400 (EDM Spark Finish Ref 24-36): Rule of Thumb: Add $+1.0^\circ$ draft per $0.025\text{ mm}$ of grain texture depth (typically $3.0^\circ - 5.0^\circ$).
3. Thermal Polymer Shrinkage & Mold Cavity Scaling
Toolmakers never cut mold cavities to the nominal part print dimensions. The 2D vector profile must be scaled up by the polymer's volumetric coefficient of thermal expansion (CTE):
- Amorphous Polymers (ABS, PC, Acrylic): Isotropic, low shrinkage ($0.4\% - 0.7\%$). Predictable across flow and transverse directions.
- Semi-Crystalline Polymers (PP, PE, POM Delrin, Nylon): Highly anisotropic, large shrinkage ($1.5\% - 2.8\%$). Molecular chains fold tightly during crystallization.
- Liquid Silicone Rubber (LSR): High thermal shrinkage ($2.5\% - 3.5\%$) requiring strict Flash Shut-Off tolerances ($< 0.005\text{ mm}$).
4. Parting Line Vector Separation & Shut-Off Lands
The parting line is the critical 2D/3D split plane where the Core (A-plate) and Cavity (B-plate) mate under hundreds of tons of hydraulic clamping force:
- Flat Shut-Off Margin: Maintain at least $15 - 25\text{ mm}$ of flat steel-to-steel land surrounding the cavity perimeter to prevent high-pressure resin flash.
- Interlocking Guide Steps: Incorporate $5^\circ - 10^\circ$ tapered side-locks in your CAD layers to align plates before core entry.
5. Standard CAD/CAM DXF Layer Structure for Mold CNC
When preparing 2D cross-sections for CNC high-speed milling or sinker EDM electrode carving:
PART_NOMINAL (Purple):True finished customer part geometry.CAVITY_SHRUNK_DRAFT (Cyan):Scaled and draft-tapered cutting loop for CNC toolpathing.PARTING_LAND (Green):0-flash shut-off sealing boundary.EDM_ELECTRODE_SPARK (Yellow):Offset inward by $-0.05\text{ mm}$ for rough/finish spark gap erosion.
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