7 Common Mistakes in Plastic Part Design (and How to Fix Them)

Bringing a new product from a “napkin sketch” to a retail-ready reality is a journey fraught with technical hurdles. In the world of plastic injection molding, the difference between a successful product launch and a costly production delay often comes down to the initial design phase. At Delaney Manufacturing Services, we have spent over 50 years refining the art of Design for Manufacturability (DFM). We see inventors and established manufacturers alike stumble over the same recurring design pitfalls. These mistakes don’t just affect the look of your part; they lead to structural weaknesses, cosmetic defects like sink marks, and unnecessarily high tooling costs. To help you navigate your next project, we have compiled the seven most common mistakes in plastic part design and, more importantly, how to solve them before they hit the production floor.

1. Non-Uniform Wall Thickness

The golden rule of injection molding is simple: keep your wall thickness uniform. When a part has thick and thin sections, the plastic cools at different rates. This differential cooling creates internal stresses that lead to warping, sink marks, and voids. The Fix: Design your part with a consistent nominal wall thickness. If your design requires varying thicknesses, ensure the transition between thin and thick areas is gradual. Use a taper or a long chamfer to allow the plastic to flow and cool more predictably. If a section needs to be thicker for strength, consider using ribs instead of a solid block of plastic.

2. Inadequate Draft Angles

Imagine trying to pull a straight-sided ice cube out of a perfectly square tray. It sticks. In injection molding, draft is the slight taper applied to the vertical faces of a part. Without it, the vacuum created during cooling causes the part to grip the mold, leading to “drag marks,” scuffing, or even part breakage during ejection. The Fix: As a standard rule of thumb, apply a minimum of 1 to 2 degrees of draft to all surfaces parallel to the direction of the mold opening. For parts with textured finishes, you may need as much as 3 to 5 degrees to ensure the part releases cleanly without damaging the surface aesthetics.

Technical cross-section illustration of a plastic part comparing uniform and non-uniform wall thickness with callouts for thin wall design, thick wall sink mark risk, and plastic flow path.

3. Sharp Internal Corners

Sharp corners are the primary cause of mechanical failure in plastic parts. They create “stress concentrators”: points where the physical load is magnified, making the part prone to cracking under pressure. Additionally, sharp corners impede the flow of molten resin, leading to air traps and structural weak spots. The Fix: Replace sharp corners with radii and fillets. A rounded corner allows the plastic to flow smoothly and distributes stress across a larger area. For maximum strength, aim for an inside radius that is at least 50% of the adjacent wall thickness, and an outside radius of 150% to maintain a uniform wall.

4. Over-Designing Ribs and Bosses

Ribs are excellent for adding structural rigidity, and bosses are essential for assembly points (like screw holes). However, making them too thick is a common error. If a rib or boss is as thick as the wall it’s attached to, it will pull the material inward as it cools, creating a visible sink mark on the cosmetic “A-side” of your part. The Fix: Follow the 60% rule: keep the thickness of ribs and bosses to approximately 40% to 60% of the thickness of the main wall. This ensures the features are strong enough for their function but thin enough to cool quickly without causing cosmetic deformities.

5. Neglecting Gate Placement

The “gate” is the entry point where molten plastic enters the mold cavity. Poor gate placement can result in unsightly “weld lines” (where two flow fronts meet) or “gate vestige” (a small nub of plastic) in highly visible or structurally critical areas. The Fix: Plan your gate placement during the CAD design phase. Ideally, gates should be located in non-cosmetic areas, such as the inside of a housing or under a label. Strategic placement ensures that any weld lines are pushed to the edges of the part or areas where they won’t compromise the part’s integrity.

Technical split-view illustration of a plastic part showing a sharp internal corner with stress concentration on one side and a proper filleted radius corner with stress distribution on the other.

6. Creating Impossible Undercuts

An undercut is any feature (like a side hole or a snap-fit) that prevents the part from being pulled straight out of the mold. While undercuts are often necessary, they require complex mold mechanisms like “slides” or “lifters,” which significantly increase the cost and lead time of your tooling. The Fix: Challenge your design to see if the undercut can be eliminated. Often, a small change in the parting line or the addition of a “shut-off” (a hole through the part) can allow the feature to be molded with a simple straight-pull tool. If an undercut is unavoidable, work with your manufacturing partner to minimize its complexity.

7. Choosing the Wrong Material for the Application

A part designed perfectly for Polypropylene (PP) may fail miserably if molded in Polycarbonate (PC). Every resin has unique shrinkage rates, flow characteristics, and environmental tolerances. Designing a part without a specific material in mind often leads to dimensional inaccuracies and failure in the field. The Fix: Select your plastic material early. Consider the operating environment: Will it be exposed to UV light? High heat? Harsh chemicals? Once the resin is selected, the design must be adjusted to account for that specific material’s shrink rate. At Delaney, we often help clients with metal-to-plastic conversion, identifying heavy-duty polymers that can outperform traditional metal components at a fraction of the weight and cost.

Technical cutaway illustration of a plastic part showing rib and boss design rules, including a correct 60 percent wall-thickness rib, an overly thick rib with a labeled sink mark, and gate placement callouts.

The Delaney DFM Advantage

Designing for success requires a holistic view of the entire product lifecycle. Many designers focus solely on the “look” of the product, but at Delaney Manufacturing Services, we focus on how it will be built, assembled, and shipped. Our product development services bridge the gap between concept and creation. We offer:

CAD Design & Prototyping: We use 3D printing to validate designs before cutting steel.
Tooling Expertise: We handle mold production and management with a focus on longevity and efficiency.
End-to-End Fulfillment: From plastic injection molding to assembly and drop-shipping, we handle the details so you can focus on growing your business.

Whether you are an entrepreneur with a new invention or an established manufacturer looking for a more reliable production partner, our team is ready to provide the expert guidance needed to avoid these common mistakes.


Images for illustrative purposes.