Designing 3D Printed Hinges and Interlocking Components

The ability to produce fully functional assemblies in a single step is one of the unique benefits of 3D printing. By leaving small gaps in the 3D designs of Multi Jet Fusion parts, interlocking and moving components are produced right out of the machine. These can range from single-axis rotation, ball joints, living hinges, interlocking parts, and even complex fabrics of small, interconnected design elements. As engineers have become more familiar with these possibilities, the design rules have become refined and clear. Well-designed 3D-printed hinges deliver motion without the need for assembly or additional accessories.

This article describes the most common methods and design steps for adding motion using 3D-printed hinges or interlocking elements in MJF manufacturing.

Key Advantages of MJF for 3D Printed Hinges and Joints

There are several key advantages to directly 3D printing moving elements in your parts.

Design complex movements at no additional cost: Because 3D printing doesn’t have the geometry constraints of conventional manufacturing, movements can be controlled and complex. A part can limit the angle of movement, have teeth or burrs lock into place, or increase the resistance of the design during the motion. All features are directly integrated into the 3D-printed plastic.
Streamline assemblies to take advantage of distributed production: When parts from the MJF machine are fully functional, production can occur anywhere. This is a huge advantage for cross-border production to counter less reliable supply chains and scale-up/scale-down capacity.
Control motion without additional accessories: Once the powder is removed from the parts, you can have fully interlocked hinges and rotating elements with no additional sourcing or assembly. For more complex components from third-party suppliers, being able to handle production in-house brings big gains in sourcing time.
Gain part repeatability by eliminating expertise: Parts made with Multi Jet Fusion are repeatable and reliable, no matter where or when they are produced. HP delivers machines built for volume production. These machines deliver production parts. Combine this with the repeatability of eliminating assembly and procurement, and you have a winning combination.

Design Rules for 3D Printing Hinges and Interlocked Elements

To manufacture 3D-printed hinges and moving elements, the design must follow the basic principles of design for additive manufacturing (DfAM). The approach also shares much of the same methodology for preparing assembly consolidation. For the interlocking elements in hinges, the two primary DfAM considerations that need focus are maintaining gaps and removing powder. Keeping sufficient wall strength retains the long-term performance and overall dimensional accuracy of the part.

Maintaining proper gaps is the key to function

When MJF 3D printing fuses powder, there is always a risk of fusing elements. Leaving at least 1/64″ (0.5mm) between the different elements in the part ensures that when the special binder is applied and cured, it does not fuse the two sides together. These minimum clearances are especially important when working with an external manufacturing partner, as they may not be aware of all the areas of the part that need to be inspected after production.

In some cases it may be necessary to work to tighter tolerances. It is possible to produce smaller gaps in certain orientations and use a light force after production to mechanically “snap” the hinge free from thin bonds. However, these tight-tolerance parts are not a guaranteed success, so extra time should be scheduled to 3D print one or more tests. The gap size and print orientation can then be adjusted to achieve a good balance between tightness in the design and repeatability of properly moving parts.

Account for Powder to be Removed

MJF is a member of the powder bed family of 3D printing processes. This means that parts are fused layer by layer in a packed “cake” of fine plastic powder. Any material not fused during production must be removed from the part afterward to allow for smooth movement.

In practice, this means that a 3D-printed hinge or interlocking component cannot be completely enclosed. It must have a slot above or below the joint, or holes at the top and bottom to allow powder to drain out of the part. As a general rule, drain holes should be 3/16″ or larger, which is why gaps are more commonly used to ensure that powder can be removed by air pressure around the hinge point.

Wall Strength

The minimum wall thickness for hinges is the same as the standard design recommendations of 1/32″ in the X-Y direction and 1/16″ in the Z direction. However, since these applications will experience motion and therefore more pressure than a standard wall, it is quite likely that the minimum wall thicknesses will not be sufficient. The more use a hinge will receive, especially if it is moving significant weight, the thicker the surrounding walls should be. Simulation software is an excellent starting point and increasingly covers 3D-printed parts.

Another option is to look at the size and thickness of similar conventionally manufactured components. If you are replacing a metal hinge with a 3D-printed alternative using Multi Jet Fusion, the thicknesses will need to be adjusted to account for the material change. If the design allows, the more the better, as the additional material cost is minor in comparison to the long-term performance benefit.

Overall Dimensional Accuracy

The basic rule of thumb is to work with an IT grade of 13. Within this range, 1/64” (0.5mm) should be considered the absolute minimum for all details such as holes and gaps within the part. In some cases, it is possible to achieve even tighter accuracies if the part is oriented so that these surfaces are flat to the X-Y plane. However, this should be the exception, not the rule, as these design elements are more likely to fail and may require additional calibration to ensure success.

Best Practices for 3D Printed Rotational Hinges

By adjusting the axis of rotation and the angle of movement with design, 3D printed parts can provide precisely controlled motion without any supporting hardware or assembly.

Rod and Pin (2-Dimensional) Rotation

Using a rotary axis is an easy way to define the plane of motion in a part. The standard method is to use a circular component (sleeve) around a rod with two end caps (as shown in the figure). There is a free range of motion on the X-Y plane in 360°. In this example, there can be either three rotating elements (top image) or the top and bottom elements can be fixed with only the middle element free to rotate (bottom image).

Within a single assembly, different 3D-printed hinges can be produced, allowing movement to occur on different axes within the part. To provide more strength, a part will often have two or three attachment points to the shaft to increase strength during use. It is a similar concept when compared with traditionally manufactured parts, as an individual part will typically have two or three attachment points, just like every door in your home. Because the complexity of 3D printing comes at no extra cost, working with multiple hinges adds strength without adding expense.

3D printing can easily define a limited angle of motion for a hinge by using pins or hard stops to end the rotation. Two basic methods can be used to limit the amount of movement. First, as shown in the image, an extended pin can be placed in a channel so that when it hits the wall, it immediately stops the motion. This is a simple but effective method of defining the range of motion in a component. The advantage is that the connecting pin/clip is usually fairly thick so it can be stopped repeatedly against the wall without breaking. This is similar to how the knee works, where the kneecap provides a hard stop for the leg bone as it moves forward.

The other method is for a pin to extend from the inner rod and control the motion by being blocked within the rotating element. This is a more elegant solution in many ways because the pin is hidden and protected. However, there are a few drawbacks for 3D printing. First, it can be difficult to remove the excess powder in this scenario, so there must be a larger planned gap between the rotating element and the rod. If the gap is too loose, then the movement of the element will tend to “wobble”. This is a balance that must be determined by testing. Second, the pin is by definition smaller because it is inside the rotating element. This means that it has less overall strength and can be more easily snapped if there are higher mechanical pressures on the part.

Ball Joint (3-Dimensional) Rotation

The next stage of movement is a ball joint, which allows rotation similar to that of a shoulder socket. In 3D printing, the concept is to create a circular shape as a cap that wraps around the ball so that it cannot be removed. It can then rotate freely 360° around the main attachment point and over the top until it hits the rod at the bottom. This is a very effective way to allow quite free rotational movement.

As with the two-dimensional axis examples above, the angle of motion of a ball joint can also be limited by design elements. The same two principles can be applied. First, limiting walls can be added to the master element so that if the extension is too far in a particular direction, then no further movement is possible. This is similar to the shoulder of the body, where the arm has a “hard stop” when it extends to the limit of the tendons connected to it.

For a ball joint, the idea of an internal pin to limit movement can be very effective. Usually pins in ball joints are easier to execute because the powder can be drained from the same gap that allows the general rotation. There is just an additional hollowed-out area in the “cap” where the pin attaches to the rest of the part.

Products with Living Hinges, Flexible Structures and “Fabrics”

Living hinges, flexible structures, and 3D-printed fabrics are the next level of complexity beyond the more mechanical rod and ball-joint hinge types. These methods require more attention to detail and typically benefit from a testing phase to ensure the long-term performance of these types of 3D-printed hinges.

The benefits are compelling. Living hinges are an integral part of product design, and the MJF PA11 material in particular, with its high ductility, allows the reproduction of classic part designs without breakage. The flexible ULTRASINT® TPU01 can also be used to produce very resilient living hinges as long as basic design rules are followed.

When creating flexible structures in parts or designing 3D-printed fabrics, movement can happen over an entire surface without complicated manufacturing or assembly processes. The cost savings can be significant. Use cases for parts with flexible structures or fabrics are not as prevalent. However, since advanced DfAM parts tend to compete at the high end of the market, adding these premium features can often be the key to a valid business case. In addition, the tooling for creating flexible structures in standard manufacturing is often expensive. Since this complexity is included for free in 3D printing, this is an area where additive manufacturing can add strong value.

5-Parrallel-Motion-Through-3D-Printed-Hinges

An array of parallel motion living hinges. Source: HP

3D Printing Living Hinges with Hard Materials

A traditional living hinge is manufactured as a thin layer of material that connects two sections of a part. Typically, living hinges are used to connect a cap, lid, or two halves of an outer housing or shell. A key advantage of living hinges is that the movement can be controlled so that the parts always meet at the same point. This is especially important for complex shapes where the final part is opened and closed repeatedly. It also eliminates the assembly and coordination costs of sourcing hardware for a hinge or producing and sorting multiple types of parts. Of the MJF 3D printing materials portfolio, polypropylene (PP) is the most common choice, as it is also used in conventional manufacturing. Polyamide 11 (PA11) for Multi Jet Fusion, with its good ductility, can also be a good choice for 3D-printed living hinges. The flexible TPU material is also often used for sealing elements within larger systems.

The key to success is to adhere closely to the recommended design rules for living hinges, which have been developed over many years for injection molding.

The hinge needs a certain minimum thickness to ensure repeated performance. For 3D printing, generally, 0.5 – 0.7 mm is recommended to get a clear print and enough powder mass to have the expected performance. The minimum thickness includes multiple layers of material bonded by the MJF process.
Parts should be printed with the living hinge perpendicular to the X-Y plane to maximize the hinge strength. The flat plane is exposed in a single pass to fuse the material. This gives strong bonds in this direction and is similar to the added strength in injection molding when the living hinge is in the same flow direction as the material.
A radius should be added to the outside of the bend and an option to add a dent in the middle where the hinge folds. These two tricks help to reduce the stress on the hinge when it bends. Adding the radius between the thin and thick material reduces the possibility of tearing along sharp corners. Removing a line of material at the bend point as a “V” or “U” shape reduces the pressure at that main fold.

Unfortunately, 3D printing still doesn’t provide the same consistent material strength as injection molding, especially where bending stresses are placed repeatedly on the part. Even when following the guidelines above, the fact that 3D printed parts are bound together by individual particles rather than a molten mass of material like injection molding makes the performance less rather than a molten mass of material like injection molding makes the performance less robust and not suitable for continuous use over long periods.

PA 11 foldable clip. Source: HP

Because we strive for a balance of thin material (for good motion) and thick functional areas (for robustness in use), final physical testing is the key to success. Living hinges that are 3D printed can be made in multiple designs and thicknesses in one batch, so testing and validation of those various designs can occur immediately. The best design in terms of usability and durability can then be released for production. Improved resolution and better material properties to support bonding between layers have made 3D-printed hinges a more common part of end-use products.

Flexible, Functional Living Hinges using 3D Printing

The production of flexible living hinges using TPU thermoplastic material increases the chances of long-term use in functional products. The key difference is that the flexible TPU material allows for thicker living hinges. This thickness translates directly into strength and durability. These types of flexible hinge elements can be used in caps and covers or to connect hard parts and provide specific movement.

Chemically smoothing TPU parts improves visual surface quality and seals the rough surface into a single, stronger layer. Like rounding the corners between the thin and thick parts, chemical smoothing removes the sharp “powder edges”, making them less tear-resistant and therefore much more durable over time. Chemical smoothing also increases the resistance to water, dirt and other contaminants to improve the reliability and robustness of the final parts.

3D Printed Fabrics and Moldable Forms

A broader definition of hinges includes flexible fabrics. These can be made by 3D printing interlocking elements. One part completely encloses another bar or loop on another part, allowing the two to move freely but remain connected. 3D printed fabrics have hundreds or thousands of these interlocking connections, so the hard plastic elements behave like a piece of fabric (which is also made up of thousands of interlocking loops). The simplest visual reference is a chain link fence. The difference with 3D printing is the ability to add design elements to create a unique visual style or adjust the interlocking elements to influence the mechanical performance.

While 3D-printed fabrics have mostly found a home in fashion or architecture, the idea of working with multiple interlocking “hinges” can have interesting industrial applications. The key advantage is that no additional hardware or assembly is required to create motion. For example, you can add a door with two or three hinges printed directly into the part. Another example would be to create three long metal ovals that connect two larger parts with pins at the edges. Then the two parts can swing back and forth along the X-Y plane (perpendicular to each other) but in a larger forward/backward motion than the simple rod and pin rotation. Especially when thinking about decentralized sourcing of parts for large organizations, these types of all-in-one solutions bring tremendous value to the organization, even though they require more design work upfront.

Adding Value to Parts with 3D Printed Hinges

Integrating 3D-printed hinges into parts brings four major benefits to your production parts.

Complex motion can be incorporated into parts at no additional cost.
Distributed production of parts across multiple facilities around the world or with trusted manufacturing partners is possible and easy because all functionality is included in a single design.
Accessories like hinges or brackets are eliminated, so you are not dependent on third-party delivery schedules, and you can adjust design and performance with a simple CAD design change that you control.
Single-source procurement for spare parts is especially important for items that may be needed 10 to 15 years down the road, or for low-volume production of specialty components. It eliminates the need to manage assembly documentation or maintain a backlog of subcomponents.

For many decades, 3D-printed components from a single source have been a clear goal. Advances in manufacturing quality, material performance, and the development of a clear design language make these parts attainable. The design advantages of 3D-printed hinges, long used in injection molding, are now being added to parts in industrial applications around the world.

Designing 3D Printed Hinges and Interlocking Components