Undercut Injection Molding: Techniques and Considerations

Table of Contents

Undercuts injection molding mold
Undercuts injection molding mold

I. Introduction

Undercut injection molding is a term that can become a nightmare for a manufacturer in the injection molding world. They seem harmless, but we can bet that many manufacturers will want to avoid them if they can help it. However, they seem impossible to evade but can be adequately managed.

According to the Society of Plastic Engineers, almost seventy percent of injection molding flaws can be traced back to these unfortunate circumstances. But how do manufacturers regain control of these near misfortunes? That is what we are about to unveil.

So, fasten your seatbelts and enjoy the ride into the world of undercut injection molding mastery. An interesting part of the journey includes techniques, considerations, and practical examples of undercut injection molding.

II. What Is an Undercut in Injection Molding?

If you are familiar with injection molding, you know that once cooling is done, the ejection of the molded part from the mold is the next part to conclude the process. There are times when ejection becomes difficult or impossible usually due to the presence of undercuts. Undercuts are extensions and indentations or protrusions from molded parts within the mold, preventing easy removal of the part from the mold.

The funny thing about undercuts is they typically reveal their presence in complexly designed molds. Whether it’s in the crevice of a medical device molded part or an automobile part, undercuts usually have special plans by appearing in intricate parts.

Now the presence of undercuts changes the dynamics of things in the ejection process during injection molding. It may look impossible, but it’s not. The complexity it presents further puts designers in a more creative position to think of how the part will be gently and meticulously removed from the mold. Just like a poker game, it requires strategy and finesse.

plastic injection mold making design 1
plastic injection mold-making design

III. The Injection Molding Process

For those already with the knowledge of how the injection molding process works, you know the drill. Nonetheless, we keep in mind those who have no prior knowledge of this manufacturing brilliance. The injection molding process entails the flow of material into deftly crafted molds. These molds carry the template for the project, and it is through this template the part is formed in the mold. The molten material is then allowed to cool. Cooling precedes the existence of our flag-bearer, the undercut injection molding.

The injection molding process is typically broken down into these magnificent stages. It starts with the clamping of the mold shut, with a firm tightness that ensures no spilling of the next stage. Then comes the free-flowing material into the mold with intense pressure by injection. The next phase sees the molten material cool down, and ejection completes the cycle.

While this process is meticulously executed, the injection stage represents the part where these undercuts reveal their stubbornness. When the material is been injected into the mold, undercuts show up with a different mission. This mission is to hinder your smooth-flowing process by creating extra indentations that stop you from ejecting your parts seamlessly. Luckily, there are creative maneuvers to solve these problems.

IV. Challenges and Considerations of Undercuts

Every mold designer’s dream is to see their masterpiece work flawlessly, but you see these undercuts can be the devil’s spawn. Now you don’t want to imagine how manufacturers feel when they encounter these challenges. Why this is so is because part release can be complicated and the risk of damage to the part can be heightened. In addition, maneuvering this hurdle must be meticulously performed. On the flip side, with these challenges come opportunities to work out innovative solutions ensuring efficiency and quality are maintained.

As far as mold design goes, especially in high-quality mold manufacturing, undercuts often appear in the most unwanted positions. This eventually hinders a smooth part release, and the quality of the part is in danger. The chances of losing part quality are higher when the mold is unable to release the molded part as it should all thanks to undercuts.

The interesting part about undercut injection molding is that they don’t just appear magically, they are a function of cause and effect. Undercuts result from the irregular flow of molten materials or materials that get stuck in the mold. Its effect is rejection due to defects and reduction in part quality. We’ll be considering ways to avoid undercuts, and how it can be resolved if unavoidable.

V. Strategies for Mold Design

In the high-quality mold manufacturing industry, the mold is often considered the most important part of any injection molding project. Hence, designing the mold is of greater importance in achieving quality and excellence in part production. Especially when undercuts are the main villain to be defeated, the mold design takes all the attention.

The mold design lays the foundation for material flow and the intended project’s design. This means it becomes the determining factor between a hitch-free process and a production nightmare for designers, engineers, and manufacturers alike.

Tackling the problems caused by undercuts as earlier mentioned requires some careful maneuvering. One such maneuver is the addition of draft angles to the mold’s design. Similarly, a few more techniques can be employed. They include deploying slides, lifters, and collapsible cores. The meticulous adoption of these techniques assists engineers and designers to be cleverer than the problem in undercut injection molding.

To further elaborate on the functions of the slides, lifters, and collapsible cores, these techniques work systematically to enable the release of the part from the mold. The mold which is typically a rigid tool becomes a flexible yet versatile component when these techniques are applied. The slides glide along the part where it is stuck, while the lifters elevate to unhook the undercuts, and collapsible cores can either shrink or expand as the case may be.

Plastic Injection Molding material
Plastic Injection Molding material

VI. Material Selection and Mold Release Agents

The next most important thing after the mold is the type of material to be used. Given the challenges faced with undercuts, it is enough reason for manufacturers to carefully select the materials to be used based on material properties. There are special materials that are self-lubricating that can aid the triumph over challenges posed by undercuts. With the help of these self-lubricating materials, ejection of part becomes easier and the friction between mold and part is limited.

Specialty materials are materials that offer unique properties that can combat the dangers of losing part quality. The effects of specialty materials with self-lubricating qualities are positive when looking at possible solutions for undercuts. This eliminates friction while limiting the adoption of external agents. External agents such as mold release agents should be a secondary option to specialty materials, and they can be deployed when specialty materials don’t work.

When two separate parts are in an interlocked position and all efforts to separate them fail, we typically turn to grease or lubricants to ensure they come apart without force. Such is the use of mold release agents in separating the mold from the molded part. This is to ensure no damage comes on the part. The applications of mold release agents range from customized coatings to spray-on applications. One goal is shared, and that’s the seamless release of the part from mold without blemish.

VII. Ejector Pins and Unscrewing Mechanisms

Before we forget, a major part of the mold that can also tackle the stubbornness of undercuts is ejector pins. These strategically positioned tools are included when crafting the mold for the main purpose of situations like this. Undercuts are a menace to part ejection in injection molding. Even so, with the right amount of pressure and precision, they can safely eject the parts from the mold. Ejector pins are the heroes without a cape during the injection molding process.

Furthermore, there are specialized undercuts such as threaded undercuts. These types of undercuts go through the trouble of entangling within the mold making ejection a lot more difficult. But with every problem, there’s a solution. That is where unscrewing mechanisms enter the fray. Unscrewing mechanisms likewise follow the pattern of untangling critically threaded parts. Rotating and releasing these threads permits material ejection from the mold eventually.

Undercuts are in variations of complexity, and the complexity they bring has also warranted the use of complex mechanical solutions. It is no feat that any traditional method could resolve, and that is why mechanical solutions are employed. Hydraulic cores are one of the mechanical solutions available. Another is the deployment of intricate slides. Drastic challenges they say deserve drastic measures, and that is why such measures are called for in cases like this.

VIII. Advanced Technologies and 3D Printing

Looking into advanced technologies that can minimize the occurrence of undercuts in injection molding, we can confidently say the future is bright. Continuous research and development are ongoing in the manufacturing world.  With designers and engineers looking at how to optimize processes, innovative means such as simulations and Computer-Aided Designs (CAD) are helping to understand it better. The best way to counter undercuts is to anticipate possible areas where they can pop up and make necessary adjustments before they happen.

By the same token, other technologies available include 3D printing technology as well. This is an additive manufacturing technique that changes the dynamics of mold making. 3D printing does not only manufacture prototypes but also assists in crafting complex parts to render undercuts powerless. This way, mold designers and engineers can level the playing field with mold designs that beat undercuts at their own game.

To crown it all, multi-material and multi-shot molding takes the game to a whole new level. This entails merging multiple materials in a single mold. This expands the possibilities available in colors, material properties, and part quality. Implementing these technologies allows manufacturers to likewise control the behavioral properties of materials in tackling their undercut problem.

Mold Manufacturing Quality
Mold Manufacturing Quality

IX. Quality Control and Inspection

Quality control has been established as the watchdog that ascertains the safety of manufactured components. From the get-go when you commission a tooling method like high-quality CNC machining, quality control already takes its course. With undercuts, quality control takes the security watch more seriously. It has been charged with the responsibility of maintaining quality and overseeing that these undercuts do not impair the quality of parts. As far as quality control is concerned, even if undercuts happen, how well they are handled in alignment with the precision required is its top priority.

Inspection processes and quality assurance are like detectives on a crime scene, looking for the slightest hint of clues. In this case, the inspections being exercised is to guarantee quality is maintained. The best practices in ensuring this is by various methods ranging from CT scans to optical measurement of dimensions and intricate geometries. This is a necessary step to be sure undercuts do not hide any flaws.

Part quality is ascertained once necessary quality control/quality assurance exercises have been perfectly executed. Once the quality is guaranteed, certifications are issued by the highest standard regulatory body. That is the International Organization for Standardization (ISO). ISO certifications indicate that the molded parts are approved for public consumption, and that marks the parts as tested, trusted, and fit for use.

X. Applications and Real-World Examples

At this point, it is obvious that undercuts continuously prove itself to be a pain in the neck for manufacturers. For that reason, manufacturers have tried to find a way to hone its flaws to their advantage by utilizing these faults in creating complex parts.

A typical example is the automotive industry where undercuts have been used to boost the aesthetics and functionality of molded parts. Also, the medical industry has likewise found a way to use undercuts’ powers against it by crafting devices that are precise and perform excellently. Here, challenges have been transformed into opportunities for groundbreaking innovations.

Considering that theoretical inferences don’t do complete justice to the idea, practical examples do. Here is a perfect success story. A manufacturer of medical parts creates molds with slides incorporated in the mold. A move like that already defeats the threats undercuts might attempt to present.

Unscrewing mechanisms have also been deployed in another scene where threaded undercuts attempt to endanger the manufacturing of automotive parts. The application of the unscrewing mechanisms steps up to save the day. Challenges may turn to ideas for improvement, and that has been the case in these examples.

XI. Conclusion

In a nutshell, undercuts are protrusions that prevent the detachment of parts from molds, and such extensions risk damage to the part. The major challenges posed by undercuts are linked to mold design, part release, and part quality. However, strategies for mold design have been discovered, and they include the inclusion of slides, lifters, or collapsible cores. In addition, ejector pins and unscrewing mechanisms also play crucial roles in tackling advanced undercut issues.

Mastering undercuts has led to the innovation of 3D-printing techniques, including multi-material and multi-shot technologies. With a robust quality control mechanism in place, undercuts could further serve as an advantage rather than a problem as duly highlighted in the case studies. Conclusively, challenges should not be magnified but seen as a chance for innovative solutions. 


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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