Designing Injection Molded Parts in FreeCAD

Designing plastic injection molded components requires balancing functional part geometry with manufacturing constraints like uniform wall thickness, draft angles, and mold separation. In FreeCAD, the optimal workflow utilizes the Part Design Workbench to build a fully parametric model, hollow it out systematically, apply draft, and reinforce it with molded features such as ribs and bosses. Following this structured approach ensures your design is ready for tooling without costly redesigns.

1. Set Up the Part Design Environment

Start inside the Part Design Workbench and create a new Body. Injection molded components should strictly reside in a single active Body to maintain parametric history and avoid broken Boolean operations. Always define critical dimensions—such as nominal wall thickness, shrinkage allowances, and clearance tolerances—using the Spreadsheet Workbench. Referencing spreadsheet parameters in your sketches ensures rapid iteration across the entire part.

2. Model the Primary Outer Shape

Begin by modeling the macro shape of the part using additive features:

  • Create a sketch on a primary plane (XY, XZ, or YZ) and extrude it using the Pad tool.
  • Keep the initial solid feature simple, defining only the exterior envelope.
  • Avoid applying final fillets or fine details at this stage, as downstream operations like shelling and drafting rely on clean geometric faces.

3. Hollow the Part (Uniform Wall Thickness)

Uniform wall thickness is essential to prevent sink marks, internal voids, and uneven warpage during cooling.

  • Select the open face of your solid body where the mold core will penetrate.
  • Use the Thickness tool in the Part Design Workbench.
  • Set the type to Inward and input your nominal wall thickness (typically 1.5 mm to 3.5 mm for common plastics like ABS or Polypropylene).
  • If the Thickness tool fails due to complex geometry, alternative methods include modeling the internal volume via a second sketch and applying the Pocket tool, leaving consistent wall boundaries.

4. Apply Draft Angles to Vertical Walls

All vertical faces perpendicular to the mold parting plane require a draft angle to allow the part to eject without scraping or damaging the tooling.

  • Select the Draft tool in the Part Design Workbench.
  • Choose a neutral plane (usually the mold parting line or the primary base sketch plane).
  • Select all vertical faces parallel to the draw direction.
  • Apply a draft angle of at least 1° to 2° for smooth surfaces, and 3° to 5° for textured or EDM finishes.
  • Best Practice: Apply draft immediately after the initial solid or shelling step so internal ribs and bosses naturally inherit draft orientations.

5. Add Structural Ribs and Gussets

Instead of thickening walls to increase stiffness, use ribs:

  • Rib thickness should be 40% to 60% of the nominal wall thickness to avoid visible sink marks on the opposite cosmetic surface.
  • Create a datum plane or sketch perpendicular to the wall.
  • Draw simple open wire lines where ribs are needed.
  • Use the PartDesign Subshape/Rib tool (or pad a thin closed profile) to project the rib down to the internal walls.
  • Ensure each rib has a 0.5° to 1.5° draft angle applied to its side faces.

6. Model Screw Bosses and Fastening Features

Bosses serve as connection points for self-tapping screws or heat-set inserts:

  • Create a sketch on the inner base face containing concentric circles for the boss outer diameter (OD) and inner core pin (ID).
  • Set the boss wall thickness to approximately 60% of the nominal wall thickness to prevent sink marks.
  • Use the Pad tool to extrude the boss upward.
  • Connect bosses to nearby external walls using small gussets or ribs to prevent breakage during fastener insertion.

7. Apply Fillets and Radii

Sharp internal corners create stress concentrations and inhibit plastic flow during the injection cycle:

  • Apply internal fillets with a radius equal to at least 50% of the nominal wall thickness.
  • Ensure the outer radius equals the inner radius plus the wall thickness to maintain consistent cross-sectional thickness.
  • Apply edge rounds using the Fillet tool at the very end of your feature tree to minimize model recomputation errors.

8. Validate Parting Lines and Export

Before sending the file for mold flow analysis or tooling quotes:

  • Review the model visually along the draw axis to verify there are no unintended undercuts that would require expensive side-actions or lifters.
  • Export the model as a high-precision neutral format (STEP AP214 or STEP AP242) to preserve face metadata, surfaces, and color-coded tolerance definitions for toolmakers.