When it comes to manufacturing high-performance components, plastic part design is the foundation for success. Whether you need parts for aerospace, medical devices, semiconductors, or fluidics systems, the design phase for your plastic parts determines not only the functionality and durability of the finished product, but also its manufacturability. A thoughtfully designed part can even streamline the production, minimize waste, and optimize performance in demanding applications.

However, even experienced injection molders can encounter design challenges that lead to costly mistakes, extended lead times, or parts that fail to meet specifications. Let’s take a look at some of these common design pitfalls and how you and your manufacturers can avoid them in order to ensure consistent, high-quality results.

Plastic Part Design Mistake #1: Poor Wall Thickness Design

Wall thickness is one of the most critical factors in successful plastic part design. If your part has inconsistent or overly thick walls, you could run into a number of production and performance issues, including:

Warping — Uneven cooling rates across the part lead to distortion, causing the final part to deviate from the intended shape.
Sink Marks — Excessive wall thickness can result in surface imperfections as the material cools and contracts unevenly.
Extended Cooling Times — Thick walls take longer to cool, increasing cycle times and production costs.

Beyond this, even if your part isn’t subjected to these problems, varying wall thicknesses make components more prone to stress concentrations, which can compromise functionality over the long term.

The Solution

To combat these issues, injection molders often approach plastic part design with a focus on uniform wall thickness wherever possible. Incorporating features like ribs and gussets also add strength and rigidity without unnecessarily increasing the material volume. For parts that do require varied wall thicknesses, manufacturers should apply gradual transitions to avoid abrupt changes and minimize stress buildup.

Plastic Part Design Mistake #2: Improper Rib and Boss Design

In plastic part design, ribs and bosses are extremely important for adding structural integrity and supporting attachment points. However, if you don’t get the design of these features right, it can undermine their purpose and lead to defects. Incorrect rib placement or oversized bosses, for instance, can create stress concentration points and make the part more susceptible to cracks under load. Similarly, ribs that are too thick or positioned too closely to adjacent surfaces can cause visible surface defects as the material cools unevenly.

The Solution

To optimize rib and boss design, there are a few best practices that engineers should be mindful of. First, it’s a good rule of thumb to keep rib thickness between 50-70% of the adjacent wall thickness to ensure support and avoid sink marks. Maintaining enough spacing between ribs and from the part edges is also critical for even material flow and cooling. Rib height should be limited to 2.5-3 times the wall thickness and incorporate a slight draft angle (usually only 1-2°) to aid in mold release. And finally, use fillets and radii at the base of the bosses to reduce stress concentrations and improve durability.

Plastic Part Design Mistake #3: Overlooking Draft Angles

Draft angles might seem like a minor detail in plastic part design, but they play an important role in ensuring smooth manufacturing and part integrity. Neglecting draft angles can lead to several problems, such as parts getting stuck in the mold, increased mold wear, and part breakage during ejection.

The Solution

Incorporate appropriate draft angles into your part design to help ensure smooth part ejection and a long-lasting mold. Aim for 1-2° in general; for deeper parts, a steeper angle might be needed. In either case, consistent draft angles across similar features are critical for maintaining part symmetry and avoiding uneven stress.

Plastic Part Design Mistake #4: Incorrect Gate and Runner Design

In plastic part design, gate and runner design is essential for ensuring an efficient and high-quality injection molding process. Improperly designed runner systems and poor gate location, however, can lead to a number of issues including short shots, weld lines, and uneven material flow. All of these can significantly impact the quality and performance of the final part.

The Solution

Optimizing gate and runner design ensures better material flow, reduces defects, and enhances part quality. There are several ways to achieve this including choosing the right type of gate, designing a runner system that minimizes material waste, and optimizing gate placement for material viscosity and part geometry.

Plastic Part Design Mistake #5: Neglecting Tolerances and Fits

While tolerances are an important and necessary element in plastic part design, overly tight tolerances can negatively affect production costs and efficiency. When tolerances are too tight, several issues can arise, such as increased production costs, higher rejection rates, and extended lead times.

The Solution

To avoid these pitfalls, it’s essential to specify tolerances that are appropriate for the application and material — but be careful not to increase complexity unnecessarily. To get your tolerances right, it’s always important to evaluate your application needs, ensure your tolerances align with material properties, ensure you’ve run a cost vs benefit analysis, and use draft angles and clearances wisely.

Plastic Part Design Mistake #6: Ignoring Environmental Factors

And finally, when working on plastic part design, failing to account for environmental conditions can lead to some serious issues. Chief amongst them are premature material degradation, increased wear and tear, and part performance failure.

The Solution

Addressing this mistake mostly comes down to making the right material selection, but there are several things that go into that. To ensure your parts can withstand the conditions they’ll be exposed to, you’ll want to be sure your parts are designed with operational conditions in mind, select a material with the right properties, test for environmental stress, and consider environmental regulations.

Avoid Design Mistakes in Your Plastic Parts with Ensinger

At Ensinger, we specialize in helping our customers avoid common plastic part design mistakes by leveraging our expertise in materials selection, part design, and manufacturing optimization. Our team ensures your parts meet the highest standards for performance, durability, and manufacturability.

With our advanced injection molding and CNC machining capabilities, we deliver precision-engineered solutions across industries like aerospace, medical, and oil & gas. We also provide high-performance material expertise to help you choose the best resins for your application, ensuring your parts perform in even the most demanding conditions.

6 Common Mistakes in Plastic Part Design and How to Avoid Them