How to Avoid Undercuts in Injection Molding?


Table of Contents

1. Introduction

2. Understanding Undercuts in Injection Molding

A. Definition and causes of the undercut problem

B. Influence of Undercut Problem on Mold Design and Demolding

C. Limitations and Challenges of the Undercut Problem


3. Design Considerations to Avoid Undercuts

A. Reasonable parts design skills

B. Material Selection and Shrinkage Factors

C. Mold Design Considerations

4. Mold Design Solutions to Avoid Undercuts

A. Use core pull mechanism and side action

B. Overmolding and Insert Molding Techniques

C. Mold Surface Treatment and Coating

5. Conclusion

1. Introduction

Injection molding is a common manufacturing process used in a wide variety of industries. However, undercutting is a common problem in the injection molding process, which may cause parts to not be released from the mold smoothly, affecting production efficiency and part quality. This article will detail how to avoid the undercut problem in injection molding, providing practical advice on all aspects from design to mold manufacturing.

2. Understanding Undercuts in Injection Molding

A. Definition and causes of the undercut problem

What is an undercut?

Undercutting is a situation during injection molding where one or more specific areas of a part are attracted to the mold and cannot be completely demolded. Typically, undercuts can result in damaged, deformed or unusable parts.

Common causes of backcharge issues include:

Friction and Adhesion: During injection molding, friction and adhesion between the part and the mold surface can cause undercuts. When the contact area between the part and the mold is large or the surface roughness is high, the friction and adhesion force increase, and the undercut problem is more likely to occur.

Improper undercut feature design: Some specific shapes or configurations in part design may increase the risk of undercut. For example, features such as grooves, thin-walled areas, and tapered structures that are easily absorbed by the mold.

Improper material selection: The surface tension and thermal expansion and contraction properties of the material will affect the occurrence of the undercut problem. Some materials have high surface tension and tend to stick to the mold surface. At the same time, the thermal expansion and contraction properties of the material will also lead to close contact between the part and the mold, increasing the risk of undercutting.

Improper mold design: The structure and surface treatment of the mold can also have an impact on the undercut problem. For example, mold surface roughness is too high, there is no proper mold release angle, uneven mold cooling, etc. may lead to undercut problems.

Unreasonable injection molding process parameters: Unreasonable selection of injection molding process parameters may also lead to undercutting. For example, high injection molding temperature and insufficient cooling time may cause undercuts.

B. Influence of Undercut Problem on Mold Design and Demolding

In mold design, the following factors need to be considered to solve the undercut problem:

Stripping angle: Reasonable selection of the stripping angle of parts and molds can reduce the occurrence of undercut problems. Larger draft angles help the part separate from the mold and reduce friction and sticking forces.

Mold surface treatment: Through proper surface treatment, such as applying a release agent, using a special coating, or applying a texture, etc., the friction and adhesion between the part and the mold can be reduced and the release performance can be improved.

Mold structure optimization: For features that are prone to undercuts, the risk of undercuts can be reduced by optimizing the mold structure. For example, increase the ventilation hole, optimize the groove structure, increase the undercut release angle, etc.

Cooling system design: Reasonable design of the cooling system of the mold can increase the cooling speed of the parts and reduce the occurrence of undercut problems. Uniform cooling can avoid buckling caused by localized overheating or insufficient cooling.

Challenges to part release from undercuts include:

Damaged parts: In the case of undercuts, parts may be damaged or deformed, affecting their quality and functionality.

Reduced production efficiency: Undercuts cause parts to be unable to be demolded smoothly, which may lead to a decrease in production efficiency. Additional time and manpower are required to deal with back-off issues, thus extending the production cycle.

Die wear and damage: Undercuts increase friction and stress on the die surface, causing wear and damage to the die. Frequent undercut issues may require more frequent mold maintenance and replacement, increasing production costs.

Part quality problems: Undercuts may cause blemishes, scratches or adhesions on the surface of the part, affecting the appearance quality of the part.

C. Limitations and Challenges of the Undercut Problem

Undercut problem of complex parts:

Complex parts often have multiple features, curves, and details, which add to the difficulty of undercutting problems. Parts with complex shapes may have more contact area, resulting in greater friction and adhesion. In addition, the demoulding angle selection and mold structure design of complex parts are also more complicated, requiring higher technical level and experience to solve the undercut problem.

Undercutting of special materials:

Some special materials have high adhesion, high coefficient of thermal expansion and contraction or special surface tension characteristics, which make the undercut problem more serious. These materials may be more likely to adhere to the mold surface and difficult to completely release from the mold. Solving the undercut problem of special materials requires an in-depth understanding of the characteristics of the material, and taking appropriate preventive measures, such as changing the injection molding process parameters, selecting the appropriate mold material, etc.

Restrictions on Mold Manufacturing and Processing:

Tool making and tooling constraints can also pose challenges to undercut issues. The processing of complex part shapes may require higher technical requirements and more complex processing equipment.

3. Design Considerations to Avoid Undercuts

A. Reasonable part design skills

Add molding angle: The molding angle refers to the angle between the side wall of the part and the direction perpendicular to the demolding direction. By adding an appropriate molding angle to the part design, it can help the part release smoothly from the mold. The role of the molding slope is to reduce friction and adhesion with the mold surface, making it easier for the part to release from the mold. Usually, the general molding slope is 2-5 degrees, but the specific slope depends on the size, material and processing requirements of the part.

Simplify part geometry: In the part design process, minimize complex features and sharp corners, simplify the design, and reduce the risk of undercutting. Complex geometries and sharp features can easily lead to sticking and seizure between the part and the mold.

B. Material Selection and Shrinkage Factors

Choose a material with low shrinkage: Shrinkage refers to the degree to which a part shrinks in size after injection molding. Different materials have different shrinkage characteristics, so this characteristic needs to be considered when designing the part. Choosing a material with low shrinkage can reduce the dimensional change of the part during cooling and solidification, and reduce the risk of adhesion and undercut with the mold. Common low shrinkage materials include polypropylene (PP) and polyethylene (PE).

Understanding Material Flow Behavior During Injection Molding: The flow behavior of different materials during injection molding can affect part formation and release from the mold. The fluidity of a material is determined by its melt index (Melt Flow Index, MFI) or melt flow rate. Materials with higher fluidity have better filling ability and can fill the mold cavity smoothly, but they are also prone to undercut problems. When selecting materials, it is necessary to balance the filling performance and undercut risk according to the structural complexity and requirements of the part.

C. Mold Design Considerations

Simplified mold geometry: In mold design, simplifying the geometry of the mold cavity can reduce the difficulty of manufacturing and demoulding, and reduce the risk of undercuts. The complex cavity structure can easily lead to material accumulation and jamming, which increases the difficulty of part demoulding.

Adjust gate location and runner design: Optimizing the location of gates and runners is important for uniform filling and smooth release. The gate is the entrance of the material into the cavity during the injection molding process, and the runner is the channel that guides the material from the gate to the cavity. Reasonable selection of the gate location and design of the runner can ensure that the material fills the cavity evenly, avoiding bubbles and defects, and reducing the risk of undercuts.

Modify the parting line and mold split direction: The parting line is the boundary in the mold that separates the cavity and core. Optimizing the location of the parting line and the direction of mold splitting can reduce undercutting of parts.

4. Mold Design Solutions to Avoid Undercuts

A. Core pull mechanisms and side actions

Incorporating slides, lifters, and hydraulic mechanisms: By utilizing slides, lifters, or hydraulic mechanisms, undercuts can be eliminated. These mechanisms allow for the movement of additional cores or slides within the mold, enabling the creation of complex features without the risk of undercuts.

Implementing collapsible cores or expandable cavities: Collapsible cores or expandable cavities are specialized designs that can be employed to eliminate undercuts. These features can collapse or expand during ejection, allowing the part to be released without interference from undercuts.

B. Overmolding and insert molding techniques

Using inserts or secondary components to eliminate undercuts: Inserts or secondary components can be incorporated into the mold design to cover or fill the undercut areas. During the molding process, the primary material is injected around the inserts, effectively hiding or eliminating the undercuts.

Proper placement and design of overmolding features: Overmolding involves the application of a second material onto a pre-molded part. By strategically placing the overmolding features, undercuts can be concealed or avoided. Careful consideration of the part design and mold configuration is crucial for successful overmolding.

C. Mold surface treatments and coatings

Applying release agents or coatings to facilitate part ejection: The application of release agents or coatings on the mold surface can help in the smooth ejection of the part, reducing the risk of undercuts sticking to the mold. These agents or coatings reduce friction and improve part release.

Utilizing textured or patterned surfaces to hide undercuts: Creating textured or patterned surfaces on the mold can effectively hide or camouflage undercuts on the final part. By incorporating intentional surface patterns, the presence of undercuts can be visually obscured.

5. Conclusion

While undercuts can be unavoidable in some injection molding applications, understanding their challenges and potential solutions can help in minimizing their impact on production. By leveraging clever design and advanced molding techniques, it's possible to mitigate the complications posed by undercuts, leading to smoother production processes and higher-quality parts.