Injection Molding Design Guidelines
Avoid These Common Injection Molding Design Problems
With injection molding, you must take exceptional care when designing tools and parts for manufacturability. Why? Because it requires the construction of precision tooling out of hardened steel. This process usually takes multiple weeks and requires a significant investment.
Discovering problem areas after your tooling is built can be very problematic. Tool changes tend to be very expensive and can add weeks to your project timetable. Making modifications to the surfaces of the tool can also shorten its life.
For best results, you need to get your design for injection molding right the first time. A trusted manufacturing partner with deep injection molding expertise can help. Following these tips in our injection molding design guide will help you avoid time-consuming and expensive mistakes later, during mold making and production.
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Injection Molding Design Guide
Questions? If you have any additional questions that aren’t covered in this injection molding design guide, please contact our experts. We’re here to help!
Wall Thickness //
Balance Performance and Production Efficiency
Your Challenge // Inconsistent wall thicknesses can cause a variety of defects during the molding process. It also has implications for material usage. Thick walls take longer to cool, leading to longer cycle times, more expensive parts and potential sink marks.
Our Solution // Fathom’s engineering team will help you:
- Achieve optimal wall thickness (typically 1.0-3.0mm) for your part design
- Identify areas where your design may need thickness adjustments
- Implement gradual transitions where thickness changes are needed
- Suggest design changes to prevent sink marks, warping and other potential defects
Why It Matters // Proper wall design reduces material costs, shortens cycle times and helps ensure consistent part quality.
Draft Angles //
Ensure Your Parts Release Properly
Your Challenge // During the injection molding process, ejector pins push the freshly molded parts out of the mold. Designing your parts with an inadequate draft angle can make it hard to release the part from the mold. This can cause scuffing or other damage to your parts. Textured surfaces require an even greater draft angle.
Our Solution // Our team will:
- Recommend appropriate draft angles (typically 1-3° for textured surfaces, 0.5-1° for smooth surfaces)
- Identify areas where insufficient draft may cause ejection issues
- Suggest design modifications that maintain the part’s functionality while improving its manufacturability
Why It Matters // Proper draft angles reduce tool wear, prevent part damage during ejection and help maintain consistent part quality throughout part production.
Radii & Fillets //
Strengthen Parts While Improving Flow
Your Challenge // During the injection molding process, sharp corners in the mold can cause fill problems. Fillets may need to be added to sharp corners to eliminate stress concentration points and enhance the part’s durability and longevity.
Our Solution // Fathom engineers will help you:
- Add appropriate corner radii to improve material flow
- Add fillets that strengthen your parts without adding unnecessary material
- Balance aesthetic considerations with structural requirements
Why It Matters // Well-designed radii and fillets reduce stress concentrations, improve material flow and enhance both the appearance and structural integrity of your parts.
Gate Location //
Optimize Appearance and Performance
Your Challenge // During the injection molding process, molten plastic resin is injected into the mold. Optimal gate location is critical to efficient and consistent mold fill. However, they leave behind marks that may require additional processing.
Our Solution // Our injection molding experts will:
- Recommend gate locations that optimize mold fill while minimizing visible marks on critical surfaces
- Consider how gate placement affects material flow and weld lines
- Balance cosmetic concerns with the part’s structural requirements
Why It Matters // Strategic gate placement ensures complete cavity filling, reduces the visibility of gate marks and helps prevent weak points in your parts.
Material //
A Critical Consideration for Molding Success
Your Challenge // What material should you select to build your parts? A key consideration during design for injection molding is the environment in which your parts will live. Will your parts need to withstand extreme environments? Exceptional stress? A corrosive environment? Another major consideration is cost. Is the material you’re considering the most cost-effective option for your application? Are there less expensive alternatives?
Our Solution // Our engineering team has decades of expertise:
- Understanding the applications in which your parts will be used and what physical characteristics are needed for them
- Refining material selection to improve performance (for example, a glass-filled resin may help your parts endure stress or abrasion)
- Selecting the most cost-effective material that meets all your requirements
Why It Matters // Selecting the right material is one of the biggest factors in the success of your injection molding project. It can save you money and help ensure that your finished parts have the quality, durability and other characteristics you require.
Ribs and Gussets //
Reinforce Without Over-Designing
Your Challenge // Ribs and gussets are used to add strength to your parts. But if they’re over-designed, they may use more material than necessary without improving the structural integrity of your part design.
Our Solution // Fathom will help you:
- Design ribs with appropriate thickness ratios (typically 40-60% of the adjoining wall)
- Place gussets strategically to maximize structural support
- Avoid common pitfalls that lead to sink marks or flow problems
Why It Matters // Properly designed ribs and gussets add strength without adding excess weight or material costs—critical for applications where weight reduction is required.
Bosses //
Enable Attachment Without Causing Structural or Cosmetic Issues
Your Challenge // Bosses are used for aligning parts, securing fasteners, reinforcing structures and creating assembly points within a part. They need to be strong enough to withstand the forces applied by screws or fasteners. If they’re not properly sized or placed within your part design, they can cause cosmetic and structural problems.
Our Solution // Fathom engineers will help you:
- Redesign bosses so they don’t cause sink marks on outside walls
- Add connecting ribs to strengthen them, as needed
- Separate them from the outside walls of the part to avoid overly thick wall sections, which can cause sink marks
Why It Matters // Properly designed bosses can help prevent cosmetic and structural problems with your parts.
Undercuts //
Simplify When Possible, Design Smart When Necessary
Your Challenge // Undercuts are design features in injection molded parts that prevent them from being ejected from the mold. Often, they require a side-action (a part of the mold that slides out of the way during the ejection process), which can add significant cost to your tooling and lengthen cycle times.
Our Solution // Our team will work with you to:
- Identify ways to eliminate unnecessary undercuts
- Design essential undercuts for efficient tooling and production
- Recommend side-actions or alternative solutions when undercuts are unavoidable
Why It Matters // Smart undercut design reduces tool complexity, lowers tooling and production costs and improves part quality and consistency.
Parting Lines //
Design for Minimal Visual Impact
Your Challenge // Parting lines are the boundary lines where the two halves of the mold meet, creating a line in the surface of the molded part. They are an unavoidable part of the injection molding process. With smart design, you can position them to minimize their cosmetic and functional impact.
Our Solution // Fathom’s engineering team will:
- Recommend parting line locations that hide them from visible surfaces
- Ensure parting lines don’t interfere with your part’s critical dimensions or features
- Optimize parting line design for efficient tool construction
Why It Matters // Strategic parting line placement improves part aesthetics and can reduce or eliminate the need for secondary operations, such as sanding or media blasting.
The Fathom Difference //
Engineering Support For Injection Molding Success
Unlike vendors who simply quote your design as-is, Fathom’s engineering team becomes an extension of your team. We provide these essential design for injection molding services:
Collaborative DFM Review // Our engineers work directly with you to identify improvement opportunities early in the design process—at no additional cost.
Material Selection Guidance // We’ll help you navigate material options to find the perfect balance of performance, cost and manufacturability for your application.
Scalable Production Pathways // Whether you need a few hundred parts for validation or tens of thousands for production, we offer domestic, international, and hybrid manufacturing solutions.
Side-by-Side Cost Analysis // We’ll show you multiple tooling and production options so you can make informed decisions based on your requirements.
Download the ICOMold Injection Molding Design Guide now!
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