5 Design Tips to Make Your Injection Molded Parts More Manufacturable

Design flaws are one of the most common reasons why injection molding projects run into trouble. You might start with a great-looking CAD model, only to face unexpected mold revisions, production delays, or cost overruns once manufacturing begins.

These issues often stem from overlooking manufacturability during the design phase. Details like wall thickness, draft angles, or undercuts may seem minor on screen, but they can create serious challenges during molding.

This article offers five practical design tips to help you prevent common production problems. By addressing these factors early, you’ll improve production efficiency, reduce waste, and keep your project on schedule and within budget.

Tip 1: Optimize Wall Thickness for Uniform Flow and Cooling

Wall thickness plays a critical role in how your part will mold, cool, and perform after production. Inconsistent thickness can lead to several common defects during injection molding:

• Sink marks from uneven cooling and material shrinkage
• Warping due to internal stress buildup
• Void formation where air gets trapped inside the thicker sections

To avoid these issues, here are some key design guidelines:

• Maintain uniform wall thickness wherever possible. Consistency helps plastic flow evenly and cool at the same rate across the entire part.
• Use gradual transitions when changes in wall thickness are unavoidable. Sudden thickness shifts can cause flow hesitation and trapped air pockets.
• Apply tapering between thick and thin sections to reduce stress concentrations and improve filling.

For example, imagine designing a part with a 2mm wall next to a 6mm rib. Without a gradual transition, the thicker rib will cool much more slowly, increasing the risk of sink marks and warping.

By optimizing wall thickness early in your design, you’ll help ensure better mold filling, fewer production defects, and shorter cycle times—all of which contribute to lower manufacturing costs. These considerations are a routine part of professional mold design services and are often reviewed before tooling begins.

Tip 2: Add Proper Draft Angles to Aid Part Ejection

Once your part cools inside the mold, it needs to be ejected without damage. This is where draft angles come into play. A draft angle is a slight taper applied to the vertical walls of your part. Without it, parts can stick to the mold surfaces, making ejection difficult and increasing the risk of damage.

Here’s why ignoring draft angles creates problems:

• Difficult ejection that may require excessive force
• Surface scratches or drag marks caused by friction between the part and mold walls
• Increased wear on mold components, leading to higher maintenance costs and production downtime

The amount of draft needed depends on your part’s material and surface finish. As a general guideline:

• For smooth surfaces: Aim for at least 1° to 2° per side
• For textured surfaces: Increase to 3° or more, depending on the texture depth
• For soft plastics: You may get away with smaller angles, but it’s safer to follow standard recommendations

If you’re unsure whether your design includes enough draft, most CAD software offers draft analysis tools. Running a quick check can help you identify problem areas before the mold build begins.

Taking time to apply proper draft angles will reduce the risk of part defects, minimize mold wear, and help keep your production running smoothly.

Tip 3: Use Radii Instead of Sharp Corners to Minimize Stress Concentration

Sharp corners may look clean in your CAD model, but they create real problems during injection molding. When you design with sharp internal or external corners, you’re introducing points of stress concentration. This makes your part more prone to cracking, especially under load or during ejection.

Beyond structural concerns, sharp corners also cause processing issues:

• Material flow hesitation: Plastic struggles to fill sharp corners evenly, increasing the risk of voids or incomplete filling.
• Increased mold wear: Sharp corners accelerate tool wear, leading to higher maintenance and shorter mold life.
• Cooling inconsistencies: Corners trap heat, causing uneven cooling and warping.

To minimize these risks, follow these guidelines when applying radii:

• For internal corners: Aim for a minimum radius of 0.5 to 1 times the wall thickness, depending on part geometry and material flow needs.
• For external edges: A small radius of 0.25 to 0.5 mm usually helps reduce stress without affecting part fit or aesthetics.
• For rib bases and boss intersections: Always include generous radii to avoid stress risers.

One example: A customer once submitted a part design with multiple sharp internal corners around load-bearing features. After production, several parts cracked during assembly. A simple redesign—adding 1mm internal radii solved the issue and reduced their defect rate by over 80%.

So, by softening your corners with proper radii, you’ll improve both moldability and long-term part performance.

Tip 4: Design with Ejection and Gating in Mind

Ejection and gating are two aspects of mold design that directly affect the quality and appearance of your injection molded parts. If you ignore them during the design phase, you may end up with visible surface marks, deformation, or costly secondary operations.

Ejector Pin Location: Minimize Surface Defects

Every molded part needs to be pushed out of the mold after cooling. This is done using ejector pins that contact the part surface. Poor ejector pin placement can lead to:

• Visible pin marks on cosmetic surfaces
• Deformation or warping if the ejection force isn’t evenly distributed
• Part sticking if the pin coverage is insufficient

When designing your part, think about where the pins will land. Try to position critical visual surfaces away from likely ejection points.

Gating Considerations: Control Flow and Minimize Cleanup

Gating determines how molten plastic enters the mold cavity. Poor gate placement can cause flow imbalances, air traps, or weld lines in visible areas.

Here are some gating design tips:

• Balance flow paths to ensure even filling and avoid short shots or weld lines.
• Place gates on non-cosmetic areas whenever possible to reduce visible gate marks.
• Consider gate type (e.g., edge gate, pin gate, hot tip) based on part geometry and material.
• Plan for easy gate trimming to minimize manual finishing work after molding.

For example, placing a gate at the center of a cosmetic surface may leave a visible blemish even after trimming. Moving it to a hidden edge could eliminate the need for secondary surface treatment.

By considering both ejection and gating early in your design, you’ll reduce scrap rates, improve part aesthetics, and lower your post-processing costs.

Tip 5: Minimize Undercuts or Design for Easier Mold Actions

Undercuts are any part features that prevent a simple, straight ejection from the mold. While sometimes necessary, undercuts complicate mold design, increase tooling costs, and add to cycle time.

Here’s why undercuts matter:

• They require additional mold actions, such as slides or lifters, to release the part without damage.
• They increase mold complexity, leading to longer build times and higher tooling costs.
• They slow down production cycles, since more time is needed for mold movements during each shot.
• They raise maintenance needs, as moving mold components are more prone to wear and failure.

If your design absolutely needs an undercut, here are common engineering solutions:

• Slides: Side-action components that move in and out during molding to release undercut features.
• Lifters: Angled pins or inserts that lift the part out of the undercut area during ejection.
• Collapsible cores or unscrewing mechanisms: For threaded or complex internal undercuts.
• Design modification: Often, a small change in geometry—like shifting a feature position or using snap-fit alternatives—can eliminate the need for undercuts altogether.

For example, one project involved a deep side groove that required a costly side-action slide. By redesigning the part with a snap-fit hook instead, the customer saved both on tooling and per-part cost.

By minimizing undercuts or designing them with simple mold actions in mind, you’ll reduce upfront tooling expenses, shorten lead times, and lower long-term production risks.

Common Mistakes Designers Make When Ignoring Manufacturability

Skipping manufacturability considerations during the design phase can lead to a series of costly problems once production starts. Here are some of the most common issues you may face:

• Frequent mold modifications: Design flaws often force multiple rounds of tool rework, each adding time and expense.
• Unexpected cost overruns: Additional mold actions, longer cycle times, and increased scrap rates drive up manufacturing costs.
• Project delays: Design revisions and tooling changes can push your production schedule weeks behind.
• High defect rates and customer complaints: Parts that fail in the field or don’t meet cosmetic standards can result in returns, rework, or lost business.

Taking time to address manufacturability early in your design process can save you significant time and money later. Spending an extra hour on thoughtful design now could prevent days or even weeks of production delays down the line.

Final Thoughts

Good design doesn’t end with part geometry—it extends to how easily and efficiently your part can be manufactured. By focusing on manufacturability from the start, you’ll help reduce production costs, improve part quality, and keep your project timeline on track.

Each of the five design tips covered can make a measurable difference in how your parts perform in production.

If you’re unsure whether your current design meets manufacturability standards, now is the time to address it. Many potential issues can be identified and resolved during the early stages of project evaluation, long before you request an injection molding quote. A thorough DFM review can help you avoid costly surprises later in production.

Read more about CAD, product design and related technology at SolidSmack.com

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