Design Guide for Metal Injection Molding

Metal Injection Molding (MIM) stands as a remarkable manufacturing process, bridging the realms of precision engineering and materials science. Much akin to plastic injection molding in terms of tooling and machinery, MIM transforms metal powders into intricate components of astonishing complexity. However, the journey from concept to finished MIM part involves a unique set of considerations and challenges.

MIM, akin to its plastic counterpart, involves the utilization of injection molding machines and tooling to craft intricate components. This similarity in the manufacturing process, replete with characteristic artifacts like gates, ejector pins, and parting lines, underscores the importance of careful design. These artifacts, shared between plastic and metal injection molding, necessitate thoughtful integration into the design process to achieve optimal results.

Nonetheless, MIM’s distinctive character emerges in the subsequent stages of post-molding debinding and sintering. It is within these transformative phases that specific design considerations come to the fore. Factors such as cross-sectional thickness and geometric features take on added significance, demanding a nuanced approach to design.

As a guiding principle, MIM finds its forte in crafting components that weigh less than approximately 100 grams and can be comfortably cradled in the palm of your hand. Typical MIM components boast an average size of around 15 grams, though the technology is sufficiently versatile to handle parts in the minuscule realm, with weights as low as 0.030 grams. It’s essential to recognize that while MIM excels in fashioning smaller components, it simultaneously offers the advantages of thinner wall thicknesses, impeccable surface finishes, and suitability for high-volume production.

In this article, we delve into the intricacies of designing for Metal Injection Molding, exploring key considerations that unlock the full potential of this cutting-edge technology. From flats for sintering to the incorporation of decorative features, we’ll navigate the terrain of MIM design, offering insights and best practices to empower designers and manufacturers alike.

Key Design Consideration Summary

Flats for Stable Sintering

In Metal Injection Molding, achieving precise and distortion-free parts during the sintering process is a top priority. MIM materials can warp if not properly supported. To prevent this, we use “flats for sintering.”

The Purpose of Flats

A flat, built into the part design, acts as a stable anchor during sintering. It prevents distortion and allows us to use standard ceramic fixtures, which are both cost-effective and reliable. These ceramic fixtures often have holes to accommodate any bosses on the component’s flat surface.

Contoured Fixtures

When adding a flat isn’t possible, we turn to contoured fixtures. These fixtures match the component’s shape and size in its initial, green state after molding. Some are even designed to work as the part shrinks from green to sintered.

Support is most crucial during the green state because the material is softest during thermal debinding. This is when the component is most vulnerable to distortion.

Alternative Approaches

Innovations in MIM design include “molded-in” supports within the component, which can be removed after sintering. Another approach is using ceramic shims cut to the desired height of the sintered component.

“Flats for sintering” might seem like a small detail, but they play a big role in ensuring the precision and quality of MIM parts. They’re an essential part of the MIM design toolkit, helping us create reliable and distortion-free components.

Optimizing Wall Thickness in MIM Component Design

The uniformity of wall thickness plays a pivotal role in ensuring the quality and dimensional stability of MIM components throughout the manufacturing process. Deviations in wall thickness can lead to warping and variations in the final product’s dimensions. Understanding and managing these considerations are essential for successful MIM design.

The Pitfalls of Uneven Thickness

Warpage in MIM components can be attributed to variations in cross-sectional thickness, which can arise from several factors:

1. Molding Pressures: Differences in packing pressures during the molding operation can result in uneven thickness.
2. Binder Removal: Variations in the time it takes to remove binders during thermal debinding can also impact wall thickness.
3. Thermal Mass: Differences in thermal mass during sintering may further contribute to thickness variations.

Ideal Wall Thickness Range

To mitigate these issues, it is recommended to maintain uniform wall thickness wherever possible. A wall thickness exceeding 15mm (0.6 inches) should be avoided, while wall thicknesses below 10mm (0.4 inches) are considered ideal. Remarkably, some MIM technologies can achieve wall thicknesses as thin as 25–50μm (0.001–0.002 inches). However, success becomes less assured as the span of these thin sections increases, primarily due to challenges in filling and potential air entrapment. Additionally, very thin cross sections (<0.005 inches) can sinter prematurely, leading to constrained sintering, tearing, and distortion.

Achieving Consistency

Ideally, maintaining a one-to-one ratio between section thicknesses is the goal. When design constraints make this ratio unattainable, a viable alternative is to core out the thick section and introduce webs of the same thickness as the thinner section. Typically, the change in thickness should not be less than 60% of the main body of the component to ensure structural integrity.

Transitioning Thickness

In cases where a uniform wall thickness is unfeasible due to specific design requirements, a gradual transition in wall thickness should be implemented. This transition should span a distance approximately three times the desired thickness change. This approach helps maintain the structural integrity of the component while accommodating necessary design variations.

Draft Angles

Draft angles, a fundamental aspect of injection molding, take on particular significance in MIM, especially in long sections. Draft angles, which involve changes in tooling dimensions or angles parallel to tool movement, ensure the smooth release of components from molds. Understanding the nuances of draft angles in MIM design is essential to optimize the manufacturing process.

The Role of Draft Angles

Draft angles serve as a universal requirement in injection molding to facilitate the effortless removal of components from molds. In MIM, draft angles are especially crucial in long sections where intricate shapes demand careful consideration. The primary purpose of draft angles is to prevent components from sticking to the mold’s surfaces during ejection.

How to Find the Right Draft Angle

While draft angles should ideally be as substantial as the design allows, MIM introduces an interesting nuance. Some binders used in MIM can act as lubricants, reducing the need for extensive draft angles. As a general guideline, specifying draft angles in the range of 0.5–2 degrees is recommended. However, as component lengths increase or if surfaces feature textures, greater draft angles become necessary to ensure smooth ejection.

Tailoring Draft Angles

In MIM, external dimensions often require minimal to no draft angles since the material naturally shrinks away from the mold’s walls during cooling. Conversely, features that the material shrinks onto during molding typically have draft angles to ease component removal from the tooling.

An example of this concept is the use of a core pin with a draft or taper of 0.5–1 degree. The largest diameter dimension of the core pin is deepest into the tool, while the smaller diameter dimension is at the pin’s end. This design allows the component to slide effortlessly off the pin with minimal friction. As the component ejects, it becomes entirely free from all surfaces.

Smart Use of Draft Angles

MIM design often incorporates draft angles on the stationary half of the mold to ensure easy release during mold opening. In contrast, the movable half of the mold may have little to no draft angles to keep the component securely in place. This strategy enables the consistent use of ejectors to remove components from the mold, allowing for continuous operation in an auto cycle.

Moreover, the mold parting line can be strategically positioned to split the draft in two different directions. This minimizes dimensional changes from one side of the part to the other, requiring less tolerance than a configuration where the draft is applied in only one direction. In some innovative applications, reverse draft angles are used to extract components from mold features on the stationary half of the tooling, which may not be feasible on the moving half of the molding machine.

Threads

Threads, whether external or internal, are essential features in numerous applications, and MIM strives to deliver them efficiently and economically.

Crafting External Threads

External threads are a common and cost-effective feature in MIM components. These threads are typically formed using a parting line that runs the length of the thread. The parting line can either encompass each thread entirely or include a flat of 0.005–0.010 inches along its length, resulting in incomplete threads on the two sides adjacent to the parting line.

When designing for complete threads along the parting line, it’s important to consider that as the tool wears, flash from the parting line may interfere with the functionality of the threads. However, using a flat to ensure proper tool shut-off conditions prevents flash or vestige from affecting the threads. Yet, this approach may lead to reduced thread engagement strength in certain applications.

Specialized Thread Formation

For scenarios where complete threads are required without the interference of flash, specialized techniques come into play. Pneumatic or hydraulic actuated drives are used to rotate the threaded tool member during or after molding and before or during ejection to create a male thread. This approach ensures precise thread formation but may add complexity and cost to the process.

Internal Threads: A Different Approach

Internal threads in MIM are produced differently. An oversized core mimicking the thread is employed, and a pneumatic or hydraulic actuated drive is used to rotate the threaded core as part of the molding process, again either during or after molding and before or during ejection. This method, while effective, tends to be more expensive and is typically reserved for high-volume applications.

Thread Quality Concerns

Maintaining thread quality is a critical consideration in MIM. Threads formed through molding are generally of inferior quality compared to those that are machined. MIM is susceptible to anisotropic shrinkage, resulting in slight shrinkage differences across the component. This can manifest as interference in tight thread tolerances. To mitigate this, coarse threads are often preferred over fine threads in MIM.

Ensuring Functionality

To maintain thread functionality, it’s advisable to minimize thread engagement length to reduce the potential for interference between threads and the matching component due to MIM’s inherent thread variability.

When designing with exterior threads, grades e, f, and g tolerance levels should be utilized, while an internal thread design should specify a G tolerance. In general, internal thread pitch diameters should lean towards the larger side of the specification, while external thread pitch diameters should err on the smaller side. This approach accommodates minor anisotropic shrinkage and distortion while ensuring the threads retain their functionality.

Ribs and Webs

Ribs and webs serve a dual purpose: reinforcing thin sections and acting as substitutes for thick sections. These design elements play a crucial role in enhancing the MIM component’s strength, both during the manufacturing process and in its final application.

Boosting Bending Stiffness

Ribs, in particular, are instrumental in increasing the bending stiffness of the MIM component. This is achieved by elevating the moment of inertia, as expressed by the formula bending stiffness = E (Young’s modulus) x I (moment of inertia). In simple terms, ribs fortify the component, making it more resistant to bending forces. This reinforcement is valuable not only for the component’s structural integrity but also for its stability during processing, mitigating the risk of warping.

Enhancing Flow and Preventing Sinks

Ribs also serve as conduits for material flow along thin sections during the molding process, ensuring that material reaches every nook and cranny of the mold. However, it’s crucial to strike the right balance in rib thickness. If ribs are excessively thick, they can lead to undesirable sinks on the opposite flat side of the component where the rib connects with the main body. Furthermore, using the incorrect rib thickness may result in unwanted warping.

Ideal Rib Dimensions

To optimize rib design in MIM, it’s recommended to keep rib thickness within the range of 40% to 60% of the size of the section on which it is located. Additionally, the rib’s height should not exceed three times its thickness. Ensuring a proper radius at the base of these ribs is essential to prevent cracking and maintain structural integrity.

Intersection and Coring

In cases where two ribs intersect, the local thickness can become greater than that of an individual rib. To address this, coring at the intersection can be employed. This technique helps maintain uniform thickness and reinforces the component without compromising its integrity.

Applying the Rules to Gussets

Gussets, often used for strengthening purposes, should adhere to the same design principles as ribs. Their thickness and height should align with the recommended guidelines to ensure they effectively enhance the component’s structural integrity.

In summary, ribs and webs are invaluable tools in MIM design, serving to bolster both the manufacturing process and the final component’s performance. By carefully adhering to design rules regarding rib and web dimensions, engineers can harness the full potential of MIM, creating components that are not only precise but also robust and reliable in their application.

Radii

Radii, those smooth curves and fillets, play a pivotal role in metal injection molding. They offer a trifecta of benefits: stress relief, enhanced component filling during molding, and simplified tooling design. These seemingly subtle design elements have a profound impact on MIM’s success, ensuring structural integrity, efficiency, and cost-effectiveness.

Stress Relief and Brittle Materials

In MIM, the material in its as-molded state can be rather brittle and sensitive to notches or stress concentrations. This is where radii come to the rescue. By introducing these smooth curves at corners and edges, stress concentrators are eliminated. This prevents the formation of cracks during ejection, subsequent handling, and thermal processing.

The stress concentration factor (K) gradually increases as the radius-to-thickness (R/T) ratio approaches 0.4. Therefore, maintaining an R/T ratio at or below 0.5 is ideal to prevent the risk of stress-related failures. As a general rule, specifying a radius greater than 0.005 inches on inside corners is recommended, although smaller radii can be considered for special applications.

Improved Material Flow and Tooling

Radii aren’t just about stress relief; they also enhance material flow during the molding process. Sharp corners can pose challenges in achieving complete material fill, often leading to unfilled or weak spots in the final component. Radii mitigate this issue by promoting smoother material flow and reducing the potential for incomplete filling in sharp corners.

Moreover, from a tooling perspective, radii are typically easier to produce than sharp corners. Sharp corners can be achieved through intricate tooling methods, such as laminated tools with multiple pieces of steel. However, this can increase tooling complexity and cost. In contrast, radii simplify tooling design and fabrication, contributing to cost-efficiency.

Sharp Corners for Specific Scenarios

There are instances where not having a radius in the design can benefit tooling cost. This occurs when one half of the tool forms the part geometry, and the other mold half is essentially flat with no tooling features. In this scenario, a sharp corner forms naturally at the parting line, simplifying the tooling and reducing overall costs.

Bosses

Bosses may seem like unassuming features in Metal Injection Molding (MIM) design, but their strategic incorporation can greatly enhance the functionality and versatility of MIM components. Bosses serve multiple purposes, from facilitating welding and alignment to providing essential sintering support and connection points with other components.

Multi-Functional Bosses

One of the primary advantages of bosses in MIM design is their adaptability. They can serve as anchor points for welding, ensuring secure connections between components. Bosses also play a crucial role in alignment and indexing, ensuring that parts fit together precisely as intended. Additionally, bosses provide essential sintering support during the post-molding process, contributing to the dimensional stability of the component.

Striking the Right Balance

When designing bosses in MIM components, it’s important to consider their wall thickness. Similar to webs, maintaining a balanced ratio between boss thickness and localized wall thickness is essential. A preferred ratio falls in the range of 0.4 to 0.6, helping to prevent the formation of undesirable sinks and distortion in the localized wall of the component. This thoughtful design approach ensures that bosses not only fulfill their intended functions but also contribute to the overall structural integrity of the component.

Round Bosses for Load Support

One noteworthy consideration is that the MIM method often involves coring out thick sections in the component. This coring process can lead to a loss of structural support for bolts, potentially compromising the component’s stability when subjected to load-bearing tasks. To address this challenge, it’s advisable to incorporate round bosses in the design. These rounded features are specifically designed to accommodate the added load from bolt attachments, ensuring that the component remains robust and dependable in various applications.

In essence, bosses in MIM design are versatile components that go beyond their conventional roles. They serve as integral points for various functionalities, from welding to alignment and sintering support. By carefully considering boss thickness and shape, designers can harness the full potential of these unassuming features, creating MIM components that excel in strength, functionality, and reliability.

Undercuts

Undercuts, those intricate recesses and protrusions that deviate from the straightforward contours of a component, are both a challenge and an opportunity in the world of Metal Injection Molding (MIM) design. They enable the creation of complex features and geometries, yet their incorporation demands thoughtful planning and strategic execution.

External Undercuts

External undercuts can be successfully integrated into MIM components without significantly inflating tooling costs. This is particularly true when these undercuts align with the direction of pull or are positioned at a 90-degree angle from the tool’s parting line. The use of tooling slides can further facilitate the inclusion of external undercuts, provided they align with a parting line.

Tackling Internal Undercuts

Internal undercuts, on the other hand, present a different set of challenges and solutions. These features are achievable in MIM by employing collapsible cores or leachable polymeric cores.

1. Collapsible Cores: To make effective use of a collapsible core, it’s essential to design with a fairly large core that can accommodate a small collapsible core feature. This approach allows for the creation of intricate internal undercuts.
2. Leachable Polymeric Cores: Alternatively, leachable polymeric cores can be used to achieve internal undercuts. These cores are selected based on their ability to withstand the temperatures encountered during subsequent MIM overmolding. They can be removed either thermally or with a solvent that doesn’t impact the backbone polymers of the MIM material.

Sacrificial Polymers for Internal Cavities

Another method to create internal cavities with undercuts is by using sacrificial polymers. These polymers are carefully chosen for their ability to withstand the temperature of the subsequent MIM overmolding process. Once the MIM component is formed, these sacrificial polymers can be selectively removed, either chemically or thermally, during subsequent MIM operations. This process allows for the creation of complex internal geometries.

Decorative Features

The addition of decorative or functional features can elevate the appeal and utility of MIM components, opening up a world of possibilities for customization and branding.

A Canvas for Creativity

MIM offers designers the flexibility to incorporate a wide range of decorative or functional features onto components. These features can encompass knurls, texturing, lettering, logos, part numbers, cavity identification, and much more. Whether it’s adding a company logo for brand recognition or intricate texturing for improved grip, MIM can bring these visions to life with precision.

Raised Lettering: A Cost-Efficient Choice

One of the most cost-effective methods for incorporating lettering features is to have them raised on the final component. This approach involves engraving the desired lettering directly into the tool cavity or tool component. This simple yet effective technique not only reduces manufacturing costs but also ensures crisp and durable lettering on the component.

Recessed Features: Precision is Key

For situations where recessed letters or features are preferred, a slightly more intricate approach is necessary. This involves using an electrode or hard turning to remove material from the designated areas, leaving behind steel where the cavity features are meant to be located. This precision machining method ensures that the recessed features are impeccably defined and maintain the overall integrity of the component.

Adding a Splash of Color

Decorative features in MIM can also serve a dual purpose by introducing color contrast to a component. For instance, a component with a black oxide finish can be taken a step further by grinding the raised features to reveal a striking silver-on-black finish. This combination of colors not only enhances the visual appeal but also creates a tactile experience that can be both aesthetically pleasing and functional.