What are the key design elements of a plastic injection mold?

The key elements are: cavity and core (forming the part shape), gate (where plastic enters the cavity), runner system (channels feeding plastic from the injection nozzle), ejector pins (pushing the part out after cooling), cooling channels (water lines to control mold temperature), and venting (allowing air to escape). Each element must be designed for the specific resin, part geometry, and production volume.

What is the best gate type for injection mold design?

Gate selection depends on the part: edge gate (simplest, leaves visible scar on part edge), tunnel/submarine gate (self-degating, for automated production), pin/point gate (minimal vestige, for cosmetic parts), fan gate (wide thin gate for flat parts to reduce warpage), and hot runner valve gate (zero waste, for high-volume production). For prototype molds, edge gates are standard; for production, tunnel gates or hot runners.

How much draft angle do injection molded parts need?

Minimum draft angle is 0.5° per side for most resins. Textured surfaces require 1-2° additional draft per 0.025 mm of texture depth. Deep cores or ribs need 2-3° minimum. For undrafted vertical walls, the part will scrape during ejection, causing drag marks and increased cycle time. Adding draft is free in CAD but retrofitting an undrafted mold is expensive.

What materials are injection molds made from?

Production molds use hardened tool steel (P20, H13, S136) for long life (100K to 1M+ shots). Prototype molds use aluminum (7075-T6) for fast machining, lower cost, and adequate life for 1K-10K shots. For high-volume production (1M+ shots), stainless mold steel (S136 ESR) with Rockwell 48-52 HRC provides the best combination of corrosion resistance and wear resistance.

How does CNC machining produce injection mold cavities?

CNC machining cuts the mold cavity in two stages: roughing (removing 80-90% of material with large tools, 3-axis) and finishing (small ball-end mills, 3+2 or 5-axis for complex contours). After machining, EDM (electrical discharge machining) is used for sharp internal corners and deep ribs that cannot be milled. Final polishing is done by hand to achieve the required surface finish (SPI A-1 mirror to SPI D-3 matte).