CAD design for injection molding is a different discipline than modeling a part for 3D printing or machining. A model that prints beautifully on an SLA machine can be completely unmoldable — parts stick in the tool, warp as they cool, or show sink marks across every cosmetic surface. After 22+ years of taking products from concept to production at our Glendale studio, we’ve seen how much money a tooling-ready CAD model saves before a mold is ever cut. Here’s what separates a moldable design from an expensive revision cycle.
Why Injection Molding Demands a Different Kind of CAD Model
Injection molding forces molten plastic into a steel or aluminum tool under high pressure, then ejects the cooled part. Every feature in your CAD model has to respect that process: the part must release from the mold, fill evenly, and cool without distorting. Unlike 3D printing, where geometry is nearly free, molding punishes thick sections, undercuts, and vertical walls. And because tooling typically runs from a few thousand dollars for a simple aluminum mold to tens of thousands for multi-cavity steel, mistakes are baked into hardware. Fixing a CAD file costs hours; re-cutting a mold costs weeks and real money.
Draft Angles: The Detail That Makes or Breaks a Mold
Draft is a slight taper added to faces that run parallel to the direction the mold opens. Without it, the part drags against the tool during ejection, scuffing surfaces and sometimes sticking outright.
As a working baseline, most parts need at least 1 degree of draft per side, with 2–3 degrees preferred on textured surfaces since texture grips the tool. Deep ribs and tall bosses need draft too, which means their thickness changes along their height — something the CAD model has to account for from the start. Adding draft late in the design process often shifts mating surfaces and breaks assemblies, which is why we model with draft from the first feature rather than patching it on at the end.
Uniform Wall Thickness: The Rule Behind Warp and Sink
Plastic shrinks as it cools, and thick areas cool slower than thin ones. Uneven walls are the root cause of the two most common molding defects: sink marks (dimples over thick sections) and warp (the part twisting as different regions shrink at different rates).
For most thermoplastics like ABS, polypropylene, and polycarbonate, walls in the 1.5–3 mm range mold predictably. Where a design needs strength, the answer isn’t thicker walls — it’s ribs at roughly 50–60% of the adjoining wall thickness, generous corner radii instead of sharp inside corners, and cored-out bosses around screw holes. These details are exactly the kind of thing we resolve in SolidWorks or Fusion 360 before a mold maker ever quotes the tool.
Undercuts, Parting Lines, and Other Tooling Decisions
Any feature that hooks under another — snap fits, side holes, internal clips — creates an undercut that a simple two-plate mold can’t release. Undercuts are moldable, but they require side actions or lifters that add real cost to the tool. Good CAD design either reorients those features so they pull in the mold-opening direction or makes a deliberate, budget-aware decision to keep them. The same goes for the parting line: deciding early where the two mold halves meet determines where witness lines land and which surfaces stay cosmetic.
Prototype First, Then Cut Steel
The smartest path to a molded part usually runs through a printed one. We regularly 3D print functional prototypes of molding-bound designs so clients can test fit and function before committing to tooling, then apply design-for-manufacturing revisions — draft, uniform walls, ribbing — to the same CAD model. Because our product design team handles both, nothing gets lost between the prototype file and the tooling package your molder receives: native CAD plus STEP files and 2D drawings with tolerances called out.
Get a Tooling-Ready CAD Model in Los Angeles
Whether you’re starting from a sketch, a 3D-printed prototype, or an existing part that needs to move into production, eCadCam can deliver injection-molding-ready CAD from our studio in Glendale. Call 213.489.1173 or visit our product design page to request a quote — we’ll review your part for moldability before you spend a dollar on tooling.