For design engineers who are new to designing for plastic injection molding, there can be a knowledge gap in making sure the CAD model of a part design is properly prepared for this manufacturing process.
Injection molding has design rules that differ significantly from machining or 3D printing. Plastic flow, cooling behavior, material shrinkage, and ejection mechanics all influence how a part must be designed. When these rules are overlooked, issues typically surface late in the process, often after tooling has been made or during production. That can mean added cost, timeline delays, or design revisions that could have been avoided.
There are specific design features that must be considered from the start. Draft angle, a slight taper or angle applied to the vertical walls of a molded or cast part, parallel to the direction of the mold’s opening, is one example. Without it, the part may not eject from the mold easily, causing scratches, warping, or breakage. A feature that looks straightforward in CAD, such as an undercut, can make a part unmoldable without expensive tooling mechanisms.
This post covers the most important design considerations for plastic injection molding. For detailed specifications and guidelines on each topic, read our full plastic injection molding design guide.
Wall Thickness
Uniform wall thickness is one of the most fundamental principles in injection mold design. Consistent wall thickness allows molten plastic to fill the mold and cool at an even rate throughout the part. Thick sections cool more slowly than thin ones, which increases the risk of sink marks, internal voids, and warpage.
Where a thicker section is structurally necessary, it can be replaced with ribs. They use less material while maintaining the part’s strength.
The right wall thickness also depends on the material being used. Different resins have different flow characteristics and shrinkage rates, so the appropriate thickness for ABS may not be suitable for polypropylene or polycarbonate. Material selection and wall thickness decisions should be made together with your injection molder for best results. See our available materials page for an overview of common resins and their characteristics.
Draft Angles
Draft is a slight taper added to any surface parallel to the direction in which the mold opens, allowing the part to release cleanly during ejection. Without adequate draft, parts may drag, scuff, or require excessive ejection force, all of which can damage cosmetic surfaces and shorten tool life.
A common starting point is 1° of draft per side. Textured surfaces require additional draft beyond this baseline because the deeper the texture, the more friction there is during ejection, and insufficient draft will cause drag marks.
Draft requirements can also vary by material. Softer or more flexible resins may release at lower draft angles, while rigid or high-friction materials generally require more.
Ribs and Bosses
Ribs are used to increase part stiffness without adding wall thickness, making them the preferred way to build in structural strength. Increasing wall thickness directly raises the risk of sink marks and lengthens cycle time, so ribs are typically the better option.
The key constraint is that rib thickness must be controlled relative to the main wall. A rib that is too thick at its base creates a concentrated mass that cools more slowly than the surrounding material, which can produce a sink mark on the opposite surface. Rib spacing also matters, as over-ribbing a region can create uneven cooling and lead to warpage.
Bosses are cylindrical features used for screws, inserts, and alignment. They are one of the most common sources of sink marks when not designed carefully. Connecting a boss directly to an exterior wall creates a thick section that is prone to sinking. The better approach is to support the boss with ribs while keeping it clear of the main wall surface.
Undercuts and Side Actions
An undercut is any feature that prevents the part from being ejected straight out of the mold in the direction of pull. Common examples include holes or slots perpendicular to the parting direction, internal threads, hooks, and certain snap-fit geometries.
When an undercut cannot be designed out, the mold requires an additional mechanism to release it, typically a side action (also called a slide), a lifter, or, in more complex cases, a collapsible core. These mechanisms add tooling cost and complexity.
The design decision should happen early. Whenever possible, redesigning a feature to eliminate an undercut is more cost-effective than adding a side action. When a side action is unavoidable, its location and direction of travel need to be accounted for as part of the mold design, which is another reason why identifying undercuts at the DFM stage matters.
Parting Line Placement
The parting line is where the two halves of the mold meet. It is often treated as a mold-maker’s decision, but its placement is directly influenced by part geometry and has real consequences for cosmetics, flash risk, and mold complexity.
On cosmetic surfaces, the parting line will typically leave a faint witness mark. Designing the parting line to fall on a non-visible edge, a natural break in the geometry, or an intentional step in the part can minimize the cosmetic impact.
Parting line placement also affects where flash can occur. Flash forms when molten plastic bleeds into the seam between mold halves. A parting line that runs across a complex surface is harder to seal tightly than one that follows a clean, flat edge. Designing parts with this in mind, and keeping parting lines on flat or simply curved surfaces where possible, reduces flash risk and simplifies tooling.
Weld Lines
Weld lines form where two separate flow fronts of molten plastic meet inside the mold. This happens when material flows around a hole or obstruction, or when plastic enters from multiple gates and converges. At the meeting point, the material bonds, but that bond is inherently weaker than the surrounding material and is often visible on the part surface.
Where weld lines form depends on gate placement and part geometry. Repositioning a gate can shift where flow fronts converge, potentially moving a weld line away from a structurally or cosmetically sensitive area. Wall thickness variations can also redirect flow and change where weld lines appear.
Weld lines cannot always be eliminated, but knowing where they are likely to form and designing with that in mind is an important part of good DFM practice.
Snap Fits and Living Hinges
Snap fits and living hinges are common plastic part features, but both have specific design requirements that are closely tied to material selection.
A living hinge is a thin, flexible section connecting two parts of a molded component, allowing the part to flex repeatedly without failure. Polypropylene is the standard material for living hinges due to its fatigue resistance and natural elasticity. The geometry of the hinge, including its thickness, radius, and orientation relative to the mold fill direction, directly affects flex life. A living hinge produced in the wrong material, or with geometry that does not account for how plastic fills and orients during molding, will fail prematurely.
Snap fits rely on controlled deflection during assembly. The geometry of the snap, including the deflection arm length, catch angle, and undercut depth, determines whether the feature assembles without cracking and whether it locks or releases as intended. Material selection is equally important here. A rigid resin that lacks the elongation needed to flex without fracturing will fail in a snap-fit application regardless of how well the geometry is designed.
Both features are worth identifying early in the design process. They can introduce undercuts that affect mold design, and their performance is difficult to predict without knowing the mechanical properties of the chosen material upfront.
How DFM Review Fits In
Many of the issues described above, including insufficient draft, problematic undercuts, weld lines in critical areas, and bosses merged into exterior walls, are typically caught during a Design for Manufacturability (DFM) review. The purpose of that review is to surface these problems before tooling begins, when changes are still relatively inexpensive to make.
That said, applying these guidelines early in the design process reduces the number of revisions required for the DFM review. A design that already accounts for draft, uniform wall thickness, parting line placement, and undercut management will move through review faster and into tooling sooner.
For detailed specifications on all of these design elements, including recommended draft angles, rib-to-wall ratios, and boss geometry guidelines, read the full design guide.
