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Live Sinter Case Study - Fiber™ heater body

It might seem like just a simple cylinder at first glance, but the story behind the design and manufacture of the heater body in Desktop Metal’s Fiber™ printer is a complex one.

The heater body in the printer assembly
[The heater body in the printer assembly]

It might seem like just a simple cylinder at first glance, but the story behind the design and manufacture of the heater body in Desktop Metal’s Fiber™ printer is a complex one.

A key component in ensuring the Fiber printer can make strong and stiff composite parts, the heater body houses a heater that channels hot air (up to 600oC) toward the continuous fiber composite material to fuse layers of material together.

The combination of high-temperature requirements and geometric complexity - the heater body must bend around other components inside the compact μAFP tool head and mate with a nozzle tip - drove the requirement to 3D print the part out of 17-4 stainless steel.

While that material and process combination minimized challenges and costs associated with traditional machining or casting, printing the part came with its own problems - namely how to keep it from distorting as it sintered.

The final step in powder metallurgy processes - like metal injection molding, press and sinter, and MIM-based metal 3D printing - sintering involves heating parts to near-melting temperatures, causing metal powder to fuse together and form solid parts.

In the process, parts shrink by as much as 20 percent and experience an array of forces, from gravity to reactions to friction, compounded by elastic and plastic creep strain and more, which can leave parts cracked and deformed.

For the heater body, the challenges were two-fold. First, the part had to meet relatively tight tolerances to ensure a tight fit around the heater cartridge, and secondly, its long, tubular shape and thin walls increased the chances it would sag or collapse during sintering, rendering it useless.

Distortion that's possible during sintering
[Distortion that's possible during sintering]

For decades, the low-tech solution to avoiding distortion has been simple trial-and-error - testing various combinations of part geometry, ceramic setters and support rafts to find one that works.

To manufacture the heater body, however, engineers took an innovative new approach.

Using Desktop Metal’s breakthrough Live Sinter™ software, they were able to proactively deform the parts by precise amounts, allowing them to return to their intended shape as they sintered - all while minimizing the use of supports.

Example of a bracket that was printed with a negative offset but sintered to straight
[Live Sinter Example]

Example of a bracket that was printed with a negative offset but sintered to straight

Part Design and Challenge

Another challenge engineers faced when designing the heater body had to do with size.

From the start, one of the goals for the Fiber printer was to provide users with the largest-possible build volume. Producing the heater body via traditional methods, however, resulted in a significantly larger part, one that would require redesigning the print head, a costly and time-consuming process that would also shrink the build volume.

The heater body needed to fit into a tight space
[The heater body needed to fit into a tight space]

By designing the part for 3D printing, engineers were able to create a more sophisticated part which included an internal channel, while still maintaining a minimal footprint.

The printed part was also designed as a consolidated assembly which required fewer parts than a traditionally manufactured part, resulting in a better - and better performing - part.

Designing the part for printing, however, also came with challenges.

To ensure they sinter successfully, parts are often printed with supports intended to prevent sagging or slumping.

While it was possible to print independent support structures that would be manually inserted into the heater body chambers, many times printed or assembled support inserts get stuck inside parts as they sag and deform during sintering. After sintering, those support structures would need to be either removed by hand or machined away, adding significant cost and lead time to production.

Using ceramic setters to support the cylinder in the furnace resulted in a similar issue. While they kept the part from collapsing, they were difficult to remove from the cylinder after sintering.

The solution came when engineers applied Live Sinter™ to the problem.

heating body hole
[Heater body printed and sintered]

Left: Printed negative distortion of the heater body

Right: Final sintered heater body

Rather than simply scaling up the cylinder in the X, Y and Z directions, resulting in a shape whose sections were circles and ovals, the software deformed the part to have non-uniform profiles having complex curvature that resembled those of a banana or horse's back.

As it sintered, gravity and the unsupported sides of the part caused the shape to drop more in the center than the ends, resulting in a cylindrical part with straight sides.

Heater body printed and sintered
[Heater body printed and sintered]

Left: Printed negative distortion of the heater body

Right: Final sintered heater body

The software, though, was only part of the answer to questions of how to manufacture the heater body.

The second half of the solution came in the form of the Shop System and its ability to quickly print parts at scale.

Built around binder jetting technology and utilizing a high-speed, single-pass print engine, the system is capable of producing parts up to 10x faster than laser-based systems.

By tightly nesting the parts in the build box, engineers are able to print as many as 80 heater bodies in a single print run, all without the need for complex tooling.

The tooling-free nature of the system makes for a far more agile manufacturing environment - parts only need to be printed when needed, and designs can be stored in “digital warehouses,” and retrieved as needed.

Enabling Smart Manufacturing with Live Sinter™

The benefits Live Sinter™ delivers, however, go well beyond a single part, and might even rewrite some of the fundamental rules of manufacturing.

For decades, one of those rules held that, once designed, many parts had to undergo a redesign to make them manufacturable. Features like undercuts, internal channels or complex curves might be eliminated as too expensive or time-consuming to produce.

While additive manufacturing offers new opportunities for design freedom, many parts still undergo a similar process. Parts are scaled up to account for shrinkage during sintering and additional material may be added in some areas so they can later be machined.

Incorporating Live Sinter™ into that manufacturing preparation process could play a crucial role in driving the future of smart manufacturing.

While other efforts have been made to simulate how parts behave during sintering, the challenge lies in replicating the laundry list of factors - from material properties to varying density, the effects of gravity and friction, to stress induced plastic creep strain, and more - which are at play.

Heater body at different stages of sintering simulation
[Heater body at different stages of sintering simulation]

To do it, Live Sinter™ uses the novel approach of borrowing an NVidia GPU-based multi-physics engine from the gaming world. The software uses that engine to create a very fast approximation - the software simulates a 20-plus hour sintering cycle in just minutes - of the physics at work in the furnace. That approximation is accompanied by a meshless FEA engine to analyze the model at regular intervals.

Before releasing the software for customer use, Desktop Metal tuned the application using a series of generic parts. After printing and sintering, the parts are scanned to measure their distortion. The results of those scans are used to set the physics parameters that define Live Sinter’s™ behavior.

Once tuned, Live Sinter™ may then be used on any part shape to produce the signature “negative offset” geometry that will print and sinter to a straight shape that meets design tolerances.

Enabling Additive’s Advantages

Additive manufacturing offers a host of benefits to manufacturers - from increased design complexity to tooling-free manufacturing to assembly consolidation and more.

It’s clear - the way we make things today is changing, and additive manufacturing is helping to drive that change. If those changes are to take root, though, sintering must be easier and more predictable, and Live Sinter™ will play a critical role in that change.