The Most Common Plastic Part Design Flaws That Cause Injection Mold Tooling Revisions

Many part quality issues in plastic injection molding are blamed on the manufacturing process. In practice, the root cause is usually the design itself. If a plastic part is not designed for injection molding, the tool can’t really be blamed when defects start to pop up. 

Design decisions directly affect how a part fills, cools, and ejects from the mold. When those decisions are flawed, the result is visible on the production line: warped parts, sink marks, poor surface finish, difficult ejection, and high rejection rates. These issues often lead to tooling changes, additional trials, and delays that could have been avoided earlier in development.

This article outlines the most common plastic injection molding design flaws that lead to tooling iterations and production problems. Each example focuses on how specific design choices impact part quality and why fixing these issues at the design stage is critical to achieving stable, repeatable production.

Insufficient Draft Angles

One of the most common plastic injection molding design flaws is insufficient draft angles. Draft angles allow the part to release from the mold as it opens. When draft angles are missing or too small, parts tend to stick to the core or cavity, making consistent ejection difficult or impossible.

In production, this shows up as parts getting hung up in the tool, ejector pin marks on cosmetic surfaces, surface scuffing, or operators having to manually remove parts. Over time, this leads to higher rejection rates and inconsistent cycle times. From a tooling perspective, the fix often requires steel rework to add draft, which can be costly and may impact critical dimensions or surface finish.

Draft issues are rarely visible in CAD unless they are reviewed specifically for manufacturability. What looks acceptable in a 3D model can quickly become a production problem once the tool is built. Proper draft angles, adjusted for material type, surface texture, and part depth, are essential for stable, repeatable molding and avoiding unnecessary tooling iterations.

Inconsistent or Excessive Wall Thickness

One of the most common plastic injection molding design flaws is poor wall thickness control. When a part has walls that are too thick or vary significantly across the design, material does not cool uniformly. This leads to cosmetic defects such as sink marks and surface distortion, as well as functional issues like warpage and dimensional instability.

From a tooling and production standpoint, uneven wall thickness increases cycle time and makes the process harder to control. Thicker sections hold heat longer, which can cause internal stresses that only show up after ejection or during downstream assembly. In many cases, molders are forced to adjust processing parameters or modify the tool to try to balance cooling, neither of which fully solves the root problem.

These issues are especially problematic in high-volume production, where small inconsistencies quickly turn into high rejection rates. Designing for uniform wall thickness wherever possible, and using proper transitions instead of abrupt thickness changes, is critical to producing stable, repeatable parts without unnecessary tooling iterations.

Poor Rib Design

Ribs are often added to plastic parts to increase stiffness without increasing overall wall thickness, but poorly designed ribs are a frequent source of tooling and quality problems. When ribs are too thick, too tall, or placed incorrectly, they create sink marks on cosmetic surfaces and uneven cooling that leads to warpage.

From a tooling perspective, thick ribs concentrate material and heat, making it difficult to achieve consistent cooling across the part. This often forces molders to slow down cycle times or request tooling changes to thin ribs, adjust steel, or add additional cooling, changes that could have been avoided with better upfront design.

Rib-related issues are especially problematic because they tend to show up late, often during T1 or T2 trials, when cosmetic expectations become clearer. Proper rib-to-wall ratios, controlled rib height, and smooth transitions into the nominal wall are essential to avoid visible defects and repeated tooling iterations during production ramp-up.

Sharp Corners and Lack of Radii

Sharp corners are a common plastic injection molding design flaw that create multiple problems during molding and production. Internally, sharp corners restrict material flow and concentrate stress, making it harder for plastic to fill the cavity evenly. Externally, they increase the likelihood of cracking, deformation, and cosmetic defects once the part is ejected.

From a tooling standpoint, sharp internal corners are difficult to machine accurately and can lead to premature tool wear or breakage in those areas. They also increase the risk of short shots and incomplete filling, especially with higher-viscosity materials or glass-filled resins. These issues often result in tool steel modifications to add radii after initial trials.

Adding appropriate radii improves flow, reduces internal stress, and strengthens the part without increasing material usage. Even small fillets can significantly improve moldability and part consistency, helping avoid tooling rework and quality issues that would otherwise appear during early production runs.

Undercuts That Are Not Designed for Tooling

Undercuts are not inherently bad in plastic injection molding, but they become a major problem when they are included in a design without considering how the part will be released from the tool. Unintentional undercuts often prevent straight pull ejection, making it impossible to remove the part without additional tooling complexity.

When undercuts are discovered late, the solution usually involves adding side actions, lifters, or hand-loaded inserts. These changes increase tooling cost, extend lead times, and introduce more points of failure in production. In some cases, the tool must be partially rebuilt, or the part design must be revised after T1 trials, resulting in additional iterations.

From a production standpoint, complex undercut solutions can also reduce cycle time stability and increase maintenance requirements. Designing undercuts intentionally, or eliminating them entirely when they are not functionally necessary, is critical to keeping tooling simple, reliable, and cost-effective for mass production.

Poor Gate Location or Gate Type Selection

Gate location and gate type have a direct impact on how material flows into the cavity, and poor decisions here are a frequent cause of tooling changes. When gates are placed incorrectly or the wrong gate type is selected, the part may fill unevenly, creating weld lines, flow marks, air traps, or short shots that affect both appearance and strength.

These issues often become apparent during early sampling, when parts show cosmetic defects on visible surfaces or fail functional testing due to weak knit lines. Fixing the problem typically requires modifying the tool to relocate the gate, change its size, or switch gate types altogether. These adjustments add cost and delay while the tool is reworked and requalified.

Proper gate design should be driven by part geometry, cosmetic requirements, material behavior, and ejection strategy. Addressing gate location early in the design phase helps ensure consistent filling and reduces the likelihood of tooling iterations once the mold enters trial and production stages.

Inadequate Ejection Surface Area

Even when a part fills and cools correctly, poor ejection design can derail production. Parts that lack flat, reinforced ejection surfaces often deform during ejection or show visible ejector pin marks, especially on cosmetic faces. In more severe cases, the part sticks in the tool, forcing manual removal or repeated tool stops.

From a tooling perspective, insufficient ejection surface area limits where ejector pins can be placed. This concentrates force in small areas, increasing the risk of stress whitening, pin push-through, or cracked features. The typical fix involves adding pads, thickening local areas, or relocating pins, changes that require steel rework and additional trials.

Designing intentional ejection landings early allows the tool to eject parts evenly and consistently. Providing enough flat area for ejector pins, keeping pins off cosmetic surfaces, and aligning ejection with the part’s natural release direction are critical to avoiding tooling iterations and maintaining stable production output.

Overly Tight Tolerances in Non-Critical Areas

One of the most overlooked plastic injection molding design flaws is applying tight tolerances across features that are not functionally critical. While tight tolerances may look good on a drawing, they often add unnecessary risk once the part moves into tooling and production.

In molding, dimensional variation is influenced by material shrinkage, flow behavior, cooling, and tool wear. When non-critical features are held to overly strict tolerances, even minor variation can push parts out of spec, leading to increased scrap rates and constant process adjustments. Tooling changes are often requested in an attempt to chase dimensions that do not affect part performance.

A better approach is to clearly define which features are truly critical to function, fit, or assembly, and relax tolerances elsewhere. This gives the molder room to control the process effectively and reduces the likelihood of tooling revisions driven by cosmetic or non-functional dimensional variation.

Designing Without Considering Material Behavior

A plastic part that looks fine in CAD can still fail in production if the design ignores how the selected material behaves during molding. Different resins shrink at different rates, flow differently through the tool, and respond uniquely to wall thickness changes, ribs, and fiber reinforcement. When these behaviors are not accounted for, dimensional issues and cosmetic defects are almost guaranteed.

This is especially common with glass-filled or mineral-filled materials, where fiber orientation can cause warpage, twisting, or uneven shrinkage across the part. During tooling trials, these problems often lead to repeated steel adjustments, process tuning, or even part redesigns in an attempt to control deformation that originates from material behavior, not tooling quality.

Successful injection molding requires the design, material selection, and tooling strategy to work together. Accounting for shrink rates, flow direction, and reinforcement effects early in the design phase reduces tooling iterations and results in parts that are easier to control, inspect, and scale into mass production.

Conclusion: Plastic Injection Molding Design Flaws 

Most tooling iterations in plastic injection molding are not caused by poor tooling or inconsistent processing. They are driven by design decisions that were made without fully considering how the part would be molded, ejected, and produced at scale. When those design issues surface during T1 or T2 trials, the only options left are tooling rework, process compromises, or acceptance of higher scrap rates.

The design flaws covered in this article all point to the same conclusion: part quality is largely determined before steel is ever cut. Draft, wall thickness, ribs, gates, tolerances, ejection, and material behavior all influence whether a tool runs consistently or becomes a constant source of rework and delays. Addressing these factors early through proper DFM review and supplier collaboration reduces risk, shortens development timelines, and leads to more stable production.

Injection molding works best when design and manufacturing are treated as a single system. Optimizing the design for how the part will actually be molded is the most effective way to minimize tooling iterations, control quality, and achieve repeatable, production-ready parts.