Draft Angle Calculator
for Injection Molding

What Is Draft Angle in Injection Molding?

Draft angle is the slight taper applied to vertical walls of an injection molded part that allows the part to release cleanly from the mold during ejection. Without adequate draft, parts can stick to the mold surface, causing ejection marks, scratches, drag lines, or even part breakage.

The draft angle calculator above determines the optimal angle based on three critical factors: material type (which affects shrinkage and surface friction), surface texture (textured surfaces require significantly more draft), and whether you're designing cavity or core side features. For plastic part design assistance, these considerations are fundamental to achieving consistent, high-quality production.

Why Draft Angle Matters for Part Ejection

During the injection molding cycle, plastic shrinks as it cools—typically 0.4% to 3% depending on the material. Semi-crystalline materials like PP and Nylon shrink more than amorphous materials like ABS and PC. This shrinkage causes the part to grip tightly onto core pins and internal features while pulling away from cavity surfaces.

Without sufficient draft angle on core side features, ejection forces increase dramatically. This can lead to bent ejector pins, stressed parts, cosmetic defects, and cycle time increases. Our injection molding consulting services frequently address ejection problems that stem from inadequate draft angles.

How Texture Depth Affects Draft Requirements

Surface textures create microscopic undercuts that mechanically lock the part to the mold surface. The deeper the texture, the more draft angle is required to overcome this interlocking effect. This is why the draft angle calculator includes comprehensive VDI, SPI, and Mold-Tech texture options.

The industry-standard rule is to add 1° of draft for every 0.025mm (0.001") of texture depth. However, this can vary based on material type and specific texture patterns. Leather grains and geometric patterns often require additional draft beyond the standard formula due to their complex surface geometry.

Before finalizing your texture selection, verify that your part geometry can accommodate the required draft angles. Some designs may need to reduce texture depth or eliminate texturing on deep-draw features. Our Moldflow analysis services can simulate ejection forces and identify potential sticking issues before tool construction.

Material-Specific Draft Considerations

Different plastic materials exhibit varying coefficients of friction against polished steel and require different draft angles. Low-friction materials like Nylon (PA) and Acetal (POM) can often release with minimal draft (0.25° – 0.5°), while flexible materials like TPE and TPU grip mold surfaces and typically require 2° or more.

Glass-filled and mineral-filled compounds are particularly challenging. The filler particles create an abrasive surface that increases friction and accelerates mold wear. These materials should always use 1.5° minimum draft, and aggressive textures should be avoided on deep-draw features.

Material shrinkage also influences draft requirements. High-shrinkage materials (PP at 1.5-2.5%) pull away from cavities but grip cores tightly. Low-shrinkage materials (PC at 0.5-0.7%) maintain more consistent surface contact throughout the mold. Use our plastic shrinkage calculator to understand how your material will behave during cooling.

Calculating Draft for Complex Geometries

While the draft angle calculator provides excellent guidance for standard features, complex part geometries require additional consideration. Ribs should have 0.5° – 1° draft per side (total of 1° – 2° included angle). Boss features require draft on both internal and external walls. Snap-fit features often need increased draft to ensure reliable ejection under high-volume production.

Shut-off surfaces where cavity and core steel meet must maintain consistent draft to prevent flash. Any draft angle change on a shut-off surface creates a potential leak point for plastic. This is a common plastic molding defect that can be prevented through proper DFM review.

Where ΔT is the wall thickness change, D is the depth of draw, and θ is the draft angle. For a 50mm deep feature with 1° draft, the wall thickness will decrease by approximately 1.75mm from bottom to top.