Draft Angle Design: Getting the Taper Right for Smooth Part Ejection
Draft Angle Design: Getting the Taper Right for Smooth Part Ejection
Draft angle seems like a minor detail in mold design, but it causes more ejection problems than almost any other factor. I have debugged many molds where the parts stick in the cavity or core, and in nearly every case the root cause traces back to insufficient draft. The draft angle is the taper applied to the vertical walls of a molded part that allows it to release from the mold steel after the material solidifies and shrinks.
The amount of draft required depends on the material being molded. Semi-crystalline materials like polypropylene, nylon, and acetal shrink more than amorphous materials and therefore need less draft on the core side because the part shrinks onto the core and pulls away from the cavity. But the same shrinkage that helps core release hurts cavity release. I use a minimum of one degree per side for polypropylene on the core and half a degree for the cavity.
Texture complicates draft angle selection dramatically. A textured surface creates undercuts at the microscopic level. The deeper the texture, the more draft is needed to release the part. The rule of thumb is to add one additional degree of draft for every 0.025 millimeters of texture depth. If the print calls for a standard SPI-C3 texture with a depth of approximately 0.05 millimeters, the draft angle should be increased by two degrees.
Draft direction matters just as much as the angle itself. The designer must determine which side of each vertical wall is the cavity side and which is the core side. Complex parts with multiple planes of draw require careful analysis. A wall that appears vertical may actually be on both the cavity and core sides if the parting line runs across its face. In such cases, the wall must be drafted in two opposite directions.
Internal features like ribs, bosses, and snap-fits need generous draft angles because they are surrounded by steel on multiple sides. I use a minimum of one and a half degrees per side for ribs and bosses, and I prefer two degrees whenever the part geometry allows. Insufficient draft on internal features causes the part to hang up on the core during ejection, bending or breaking the feature.
Deep cores require staged draft angles. A single draft angle on a deep core can make the base of the core significantly larger than the tip, creating a heavy wall section. I use a step draft approach for cores deeper than fifty millimeters. The first twenty-five millimeters get one degree of draft, and the remaining depth gets half a degree. The transition between the two angles should be blended with a radius.
Checking draft angles before cutting steel is one of the most valuable steps in the mold design process. Modern CAD software can analyze draft across the entire part surface and highlight areas where draft is insufficient. I run this analysis on every new part model before approving the design. The analysis takes minutes but can save weeks of downstream troubleshooting.
One practical tip is to polish the draft surfaces in the line of draw. Pushing the steel in the direction of part ejection leaves microscopic scratches that act as tiny runners guiding the part off the core. Polishing perpendicular to the draw direction creates cross-hatch marks that increase friction. This small detail can make the difference between smooth ejection and a stuck part. For more on multi-cavity tooling where draft consistency across all cavities is critical, visit http://www.vhptooling.com/multi-cavities-molds/.