Designing Effective Corners for Injection-Molded Parts

Table of Contents

Corners are common areas when stress concentration occurs and result in sink marks , warpage and aesthetic part defect if not well designed. The design of corners for injection molded parts is very crucial in determining part quality, structural integrity and enhanced performance for part requirement application.

Effective corner designing offers several advantages which include, optimizing part performance, preventing molding defects such as sink marks and warpage, and enhancing structural integrity of parts.

To guarantee your success in manufacture of injection molded parts, we will go through everything you need to know about designing corners, from factors affecting corner design to best practices to adopt for corner design.

Corners 1

Injection Molding Machine (Image Source: Dreamstime)

The Importance of Corner Radii and Filets

A corner radii refers to the curvature that is used in corners of a part. The choice of radius of a part depends on factors such as material properties being used, thickness of the part, aesthetic requirements, application requirement and cost production efficiency. Effective corner radii offer several key advantages:

Improved aesthetic appearance of parts though smooth and rounded edges, prevention of part distortion such as warpage through promoting uniform cooling and filling of the mold cavity. 

Optimum part performance where effective corner radii minimizes stress concentration preventing part defect and increases its life cycle. Part strength and durability is enhanced through uniform distribution of stress throughout the part.

Filets are rounded corners formed between two features of a part. They are found at the base of compartments and rounded tops of walls. They have several purposes in part design which include, stress reduction, effective molten material flow and enhances aesthetics requirements.

Smooth and rounded corners are widely used and preferred in the manufacture of injection molded parts. Here are some reasons why.

  • Proper mold filling: Smooth, rounded corners ensure efficient molten plastic flow into the mold cavity promoting uniform filling and packing.
  • Promote even distribution of stress: These corners reduce stress concentration in one area preventing potential part defect and failure.
  • Enhancing aesthetics: Smooth,  rounded corners contribute greatly to the overall appearance of the final part giving it a polished surface.

Challenges associated with sharp corners 

Sharp corners in injection molded parts have several key challenges that impact part accuracy strength and aesthetics.They include:

Mold filling and packing

Sharp corners interfere with the smooth flow of molten plastic into the mold cavity leading to incomplete filling. This can bring about part defects such as sink marks and warpage. 

Stress concentration

Sharp corners lead to localized stress concentration within part. This leads to cracking or premature failure. 

Aesthetic defect

Sharp corners don’t bring out the desired appearance of the final part due to visible knit lines and mold seams which can lead to customer disatisfaction.

Understanding Wall Thickness in Relation to Corner Design

This aspect in injection molded parts is very crucial in achieving optimal part performance and quality. Wall thickness has a great impact on the behavior of corners. Here is how wall thickness relates to corner design.

Stress concentration 

In sharp internal corners stress concentration is very common. Thicker areas directly opposite sharp corners tend to resist deformation better while thinner sections experience higher stress concentration. Premature failure at the sharp corners may be observed as a result of uneven distribution of stress.

Stress absorption

Adjacent wall thickness plays an important role in absorbing and distributing stress within sharp internal corners. Thicker adjacent walls possess more material to absorb stress reducing the risk of stress concentration. Thinner adjacent walls offer less resistance to stress which lead to stress concentration and eventual failure of the corner.

Molten plastic flow and packing

Properly designed corners with balanced wall thickness enhance smooth material flow ensuring proper filling. In parts with varying wall thickness  sharp corners can affect the material flow resulting in incomplete filling leading to part defect.

Cooling and solidification

Adjacent wall thickness affects the cooling and solidification rate around sharp corners. Thin adjacent walls take shorter time to cool than thick adjacent walls leading to uneven cooling rate. This can result in warpage and sink marks in parts.

Disadvantages of increased wall thickness.

Here are some of the design challenges that may result due to increased wall thickness.

Increased material usage

 More material is needed for creation of thicker walls which brings about increased production cost.

Increase in part weight

Parts with thicker walls tend to be heavier which may not be desirable in industrial application where light parts are required for efficiency. This also leads to increased shipping cost due to the added weight.

Geometry accuracy

It is very difficult to maintain dimensional accuracy and tight tolerances in production of complex parts. This can lead to inconsistencies affecting part quality and structural integrity. 

Longer cooling time

Thicker sections take longer to cool leading to increased cycle time and reduce production efficiency. It can also lead to uneven cooling rates which lead to warpage and other part defects.

Achieving the Right Radius

Achieving the right radius in injection molded parts involves careful consideration of various factors. These factors are:

Design requirement

Start by understanding the functional and application requirements of the part. Consider the mechanical load, regulatory standards, assembly and aesthetic preferences that are to be achieved.

Material properties

Different materials possess varying hardness, viscosity and thermal stability which affect the choice of radius. Select a material that best suits your parts requirement.

Wall thickness

The choice of radius is directly influenced by part wall thickness.Thicker walls require larger radii to avoid stress concentration and prevent molding defects. To maintain part integrity make sure the radius is proportional to the wall thickness.

Stress analysis

This can be achieved through use of simulation softwares to evaluate stress distribution and identity stress concentration areas. It helps manufacturers make the necessary adjustments to meet part application requirements.

Smooth transitions

Proper radius design should offer smooth transition between the intersecting surfaces. Sudden changes in the dimensions lead to stress concentration and molding defects. This reduces the structural integrity of parts.


The radius size and shape should be consistent throughout the part to ensure there is consistent appearance and part performance. This ensures efficiency in production.

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Injection Molded Parts Design (Image Source: Pinterest)

Addressing Manufacturing Challenges with Proper Corner Design 

Proper corner design helps reduce manufacturing challenges in injection molding in various ways. 

Preventing mold defect : Properly designed radius promotes a smooth flow of material into the mold cavity ensuring complete filling. This reduced the risk of part defects such as warpage.

Efficient part ejection : Sharp corners pose challenges during part ejection from the mold. They lead to distortion and damage of the finished part. Well designed radius facilitates smooth and efficient part ejection increasing chances of producing an overall high quality part.

Enhancing aesthetics : Proper corner design promotes the appearance and texture of the final part. This enhances customer satisfaction which promotes the organization image.

Optimizing material usage : When manufacturers incorporate well designed corners, material required to fill in the sharp corners  is reduced. This reduces the overall cost of material.

Facilitate streamlined assembly : Rounded corners are easy to handle and assemble. They facilitate effective assembly specifically in application where parts need to be fitted together. This improves the overall part performance

Machining Limitation

These are the challenges associated with the machining understanding these limitations manufacturers can make informed decisions about corner design, material selection and manufacturing process.

Here is how they affect.

  • Cost and time : For parts with complex geometries creating the desired internal corner radii can be time consuming and costly. They need specialized machining and longer machining time to achieve desired radii which increases the manufacturing cost and lead time.
  • Geometry accuracy : Achieving high levels of dimensional accuracies  may be difficult due to machine dimension variation. Maintaining accurate and consistent radii across parts require well designed setup and monitoring of the machining process.
  • Material properties :  Different materials have varying hardness. Specialized techniques and tools are required to achieve the desired internal corner radii depending on different materials used.

In production of injection molded parts manufacturers can use the following alternative processes to create complex geometry parts.

Electrical discharge machine (EDM) : This is a manufacturing process that is used in the creation of complex geometry parts . It is used mostly in hard materials. Bolt on inserts are used to achieve these complex details and features.

Bolt-on inserts are essentially pre-developed components that are incorporated into the mold cavity to create the details in the final plastic part. 

These Inserts are made from hardened and durable material with the properties of withstanding high pressure and temperature  involved in the injection molding process. These features offer precision, flexibility and cost effectiveness in producing high quality parts.

Best Practices for Corner Design in Injection Molding

To achieve high quality parts with optimal performance and durability, best practices are essential for effective corner radii. Here are some tips.

Corner radii design

Use filets to avoid sharp corners  which help distribute stress  evenly minimizing stress concentration. You need to maintain a uniform wall thickness to ensure consistent material flow and to prevent molding defects.

Another factor to Consider is material properties to determine which material will offer the required characteristics. Make use of simulation and analyzing tools for corner design to validate performance and stress distribution in parts. incorporate  use of filet transitions for smooth material flow and uniform stress distribution. 

Material selection tips

First you need to consider the stiffness, flexibility and shrinkage properties of material when selecting them for injection molding. Another factor is to evaluate material compatibility with the injection molding process to produce high quality parts. 

Select a material that can create the desired corner radii and provide required strength and durability for part application.

Manufacturing methods tips

Evaluate alternative manufacturing methods such as EDM or the use of bolt inserts for creating complex geometries that are difficult to create. Prototype and test manufacturing methods to assess part quality and to optimize the production process for efficiency and cost effectiveness.

Strategies to avoid common pitfalls

Ensure uniform wall thickness  to prevent varying cooling rates and reduce the risk of warping. Use proper filet radii to help distribute stress evenly and prevent stress concentration. 

You can incorporate the use of computer softwares to simulate the injection molding process to evaluate potential issues that may arise such as warpage.

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Corner Designs in Injection Molded Parts (Image Source: Pinterest)


Well designed corners  is one of the key factors that ensure part performance manufacturability and aesthetics. Properly designed corners facilitate the even distribution of stress throughout the part reducing the risk of stress concentration and potential part failure.

Smooth rounded corners promote the smooth material flow ensuring complete mold filling and packing reducing defects such as sink marks.another role of well designed corners is geometry stability and accuracy. They have uniform thickness and smooth transition enhancing the structural integrity of parts.

Well designed corners contribute to the overall visual appearance of injection molded parts. This gives the parts a professional look in applications where aesthetics are significant in customer satisfaction. Through following these guidelines in corner design you can  optimize injection molding process, achieve superior part performance and meet the highest standard of quality and reliability.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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