What should you take into account when designing a plastic product? 2/2

In part one of this blog series, we discussed three key design rules for injection-molded products: wall thickness, corners, and draft. These basic principles form the foundation of a well-designed injection-molded product.

In part two, we highlight two additional crucial elements that product designers must consider at an early stage: ribs and undercuts. These features largely determine how strong, precise, and production-ready a design ultimately is.

Why Ribs Are Essential for Strong Plastic Parts

When a product requires additional stiffness, increasing wall thickness may seem like a logical choice. However, within the injection molding process, this often works against you: thick walls cool unevenly, create internal stresses, and lead to warpage or visible sink marks.

Ribs therefore offer a much more efficient solution. These narrow, strategically placed reinforcements strengthen the product exactly where needed, without adding unnecessary mass or thicker walls. They increase stiffness and dimensional stability, support functional details such as snap-fits, reduce torsion and bending, and save material without compromising strength.

Well-designed ribs improve both the functionality and manufacturability of a product. In the following sections, we discuss the most important design considerations for applying ribs effectively.

Rib Thickness

Rib thickness largely determines whether sink marks become visible on the exterior of the product. Because plastic always shrinks toward the thickest section during cooling, an overly thick rib will cause deformation and visible sink or read-through.

For this reason, rib thickness is always expressed as a percentage of the nominal wall thickness. Recommended values vary per material:

These percentages serve as important guidelines. When ribs are designed too thick, sink marks or read-through almost always occur. Staying within the recommended ranges keeps the surface smooth while maintaining rib effectiveness.

Rib Height and Rib Location

Ribs in plastic components provide additional strength, but their effectiveness strongly depends on proper design.

A rib must not be too tall or too thin: the plastic must be able to fill the rib properly and air must be able to escape. Due to the required draft angle (2–5°), a tall rib also becomes less effective toward the top.

Plastic always shrinks toward the thickest section. If a rib is placed on a thicker wall section, that wall will pull the rib during cooling, causing deformation. This can be prevented by positioning ribs on uniform wall thicknesses, or by making ribs narrower, splitting them, or slightly repositioning them to better distribute stresses.

In short: well-designed ribs improve stiffness without unwanted shrinkage, but only when height, thickness, and location are balanced with the moldability of the part.

Moldability

A rib is only effective if it can be produced properly. In practice, problems often arise when ribs are designed too narrow, too deep, or too close together. Very narrow ribs, especially those thinner than one millimeter, may not fill completely during injection molding. This makes the structure weaker or unpredictable.

Venting and mold wear also play a role. Over time, ribs become more difficult to fill if there is insufficient space for air to escape. A good rib design therefore takes into account both functionality and long-term manufacturability.

The Impact of Undercuts on Draft and Mold Construction

Many plastic products include features that do not align with the main draft direction. Examples include snap locks, recessed surfaces, hooks, or openings. These are called undercuts. They are often functionally necessary, but they pose a challenge within the injection molding process. A mold always opens in one direction, and anything perpendicular to that direction will lock behind the steel.

Undercuts directly affect mold complexity, cycle time, and cost. It is therefore important to consciously choose a workable solution at an early stage.

Forced Demolding: Only Suitable for Flexible Plastics

In some cases, an undercut can be applied without additional mold mechanisms. This is possible when the part can flex sufficiently during ejection to pass the undercut. This method is known as stripping or forced demolding. The part temporarily deforms during ejection and then returns to its original shape.

This technique only works when both the product and the material are suitable. There are clear limitations:

• Undercuts up to approximately 2% may be possible with moderately flexible materials, provided the walls are thin enough to flex and edges are rounded or angled.
• Undercuts must always be positioned away from rigid zones, such as corners or ribs, to allow sufficient deformation space.
• Stripping only works when the geometry is guided by rounded or angled leading edges, allowing the part to slide more easily past the mold steel during ejection.

With stiffer plastics, such as polycarbonate, PC/ABS, or glass-filled materials, forced demolding is practically never possible. These materials offer too little flexibility and are more likely to crack than deform during ejection.

When stripping is applicable, it can eliminate the need for additional mold mechanisms and keep cycle times short. However, it remains a conscious design choice, as the part is subjected to additional stress during ejection.

Sliding Mechanisms in the Mold

When an undercut is too large to strip or the material is too rigid to flex, additional mold mechanics are required. In such cases, sliding mechanisms are used. These are movable elements within the mold that hold the undercut during injection, but slide aside when the mold opens so the part can be released.

Slides make complex undercuts manufacturable, but they increase cost and complexity. They add moving components to the mold, resulting in more wear, longer cycle times, and greater maintenance requirements.

A slide is required when:

• The undercut is too large to strip,
• The material is too rigid to flex,
• The undercut is perpendicular to the draft direction, or
• There is insufficient space for forced deformation.

Because slides significantly increase production costs, it is advisable to investigate early in the design phase whether the function can be achieved without moving mold components.

Common Mistake: Snap Features Not Designed for Draft

Snap-fits, hooks, and other small features are often designed in CAD without carefully considering the draft direction. A shape may look correct on screen but turn out not to be mold-releasable once the mold is built. This leads to modifications, additional costs, or even the need to add a slide after all. Many of these issues arise because small details unintentionally lock behind the mold steel.

Typical sources of error include:

• Snap fingers or tabs pointing inward without a guiding radius or angled lead-in
• Hooks or openings positioned perpendicular to the draft direction
• Ribs or reinforcements placed too close to an opening
• Undercuts larger than the material can deform during ejection

By designing snap features to be draft-friendly from the outset and adding slight radii or angled lead-ins, the part remains manufacturable without additional mold mechanisms. This prevents surprises during mold construction and keeps production costs under control.

A Good Design Starts Early and Pays Off Twice

Ribs and undercuts may seem like small details in a product design, but they largely determine whether a plastic component is strong, dimensionally stable, and cost-effective to produce. By considering wall structure, draft directions, and the possibilities of the injection molding process at an early stage, you avoid surprises during mold construction and save both time and costs.

Like the design rules discussed in part one, these additional guidelines form a solid foundation for a reliable and efficient injection-molded product. The better the design aligns with the technical realities of the process, the smoother the path to series production.

Want to be sure your design is technically feasible and optimally aligned with the injection molding process? Please contact us. Our engineers are happy to support you from the very beginning.