white paper - overcoming additive manufacturing surface ... · the range of materials and...
TRANSCRIPT
WHITE PAPER
Overcoming Additive Manufacturing Surface Finishing Challenges with Automated SRF Technology
www.postprocess.com
CUSTOMER SPOTLIGHTCONTENTS
I. INTRODUCTION TO THE IMPORTANCE OF SURFACE FINISHINGThe Legacy Need for Surface Finishing: Traditional manufacturing methods have re-lied on surface finishing as a secondary operation. This is particularly true for metals. Processes such as casting, manual milling and lathes, and CNC machining produce a ‘raw’ part that typically needs additional processing to meet specifications. These processes can be either subtractive: blasting, sanding, polishing, grinding, tumbling - or additive: painting, dyeing, plating, and beyond. Typically, the subtractive processes must meet specifications (like dimensional and surface tolerances or smoother sur-faces) before the subsequent step (painting, plating, etc.) can be done. This surface finishing step is usually required to be truly ‘customer-ready.’
Plastic processes such as injection molding streamline some of the traditional metal process issues and are valued for tight tolerances and uniform, high-quality surface finish. High-volume producers have relied heavily on injection molding for its inherent cosmetic advantages, including a wide array of color options. However, even with in-jection molding, geometries can be limited, and designs and applications may require additional machining. This is usually done with techniques carried over from metal fabrication and results in burrs, surface flaws, or even breakage, leading to additional work or scrap.
The Promise, Possibilities & Reality of AM: Even with advancements in traditional manufacturing and plastics processing, manufacturers continued to research ways to solve labor and tooling costs, geometry restrictions, limitations of workflow flexibility and consistency. Additive Manufacturing (AM) was invented specifically to solve these problems. Significant advancements have been made through additive manufactur-ing. However, as inventors, manufacturers, and service providers incorporate additive solutions into their workflow, there is growing recognition of the additional processing required after they come off the printer. This is true regardless of print technology.
© PostProcess Technologies, Inc., 2018. All rights reserved. The contents of this white paper is owned by PostProcess. You may not use or reproduce it in any type of media, unless you have been granted prior written consent thereto by a person authorized to represent
PostProcess for such purpose.
2
Introduction to the Importance of Surface FinishingIn-Depth Review of Traditional Surface Finishing Surface Finishing Designed for AdditiveConclusion
I.II. III.IV.
www.postprocess.com
3
Table 1 - Ra Walk
The Ra of a part is impacted by the type of technology and printer, the build material, and the part geometry. For example, PolyJet technology can have layers as thin as 630µin (0.016mm) and a typical Ra near 400µin. Fused Deposition Modeling, or FDM, has a minimum layer thickness of 0.004in (0.1mm) resulting in a rougher surface fin-ish, closer to 500 µin. Stereolithography, or SLA, leaves visible ‘bumps’ after removing breakaway supports. And metal printing such as Electron Beam Melting (EBM) can see initial Ra values greater than 500µin. In addition, thermodynamic challenges with even heat distribution can lead to varying roughnesses throughout the same part.
The user can make some adjustments to mitigate surface roughness through the software operating the slicing program. Adjusting selected parameters such as layer thickness and Z-seam location to may improve print results but will not be enough to achieve the ‘traditional’ finishes that manufacturers are accustomed to. AM users need to address these post-print challenges and meet finishing specifications while avoiding some of the common drawbacks like labor cost, process variation, and in-creased scrap rates.
All 3D print technologies (fusing, depositing, jetting, curing, sintering, etc.) have two shared process realities:
1. Additional material or structures are required to support certain geometries during print.2. Geometries are generated one layer at a time.
There are different ways to tackle the first challenge of removing support materials, ‘cleaning’ the parts, and exposing the build surface. Once secondary material and sup-ports are removed, the second challenge, surface finishing, is exposed. Surface fin-ishing requirements vary by print technology and material, whether metal or polymer, but the single biggest driver is the layer-by-layer build process. This layering process creates small variances in surface finish, while many parts require absolute uniformity across the surface.
A Common Understanding of the Common Standard for Smoothness: The current industry standard for quantifying the smoothness of a surface is the Roughness Aver-age, or or Ra, measured with a profilometer in either µin or µm. For reference, the table below is a generalization of finishes according to Ra values:
CUSTOMER SPOTLIGHT
www.postprocess.com
4
This is increasingly important as Additive Manufacturing moves beyond prototyping to produce ‘customer-ready’ parts (with higher surface requirements) at scale. This requires protecting fragile geometries and preserving critical feature detail in a repeat-able manner, without expensive, variable, manual labor. Solving this allows all indus-tries to revolutionize their manufacturing processes today while advancing towards Industry 4.0.
II. IN-DEPTH REVIEW OF TRADITIONAL SURFACE FINISHINGAs manufacturing transitions from the machine age into the digital age, capabilities are advancing at different paces. In 3D printing, the raw part build has advanced more quickly than surface finishing treatments. This means that manufacturers are current-ly relying on traditional equipment, tooling, and tribal knowledge to address these new problems. An overview of these traditional processes follows:
MANUAL:· Hand Sanding - Manual force applied with varying levels or gritted paper · Manual Bead Blasting - Loose, dry grit cycled through a hose at high-pressures (50-90 PSI) into an enclosed cabinet. Part handling and hose control are manual operations.· Hand Polishing/Buffing - Using manual force with a series of polishing compounds and motorized buffing wheels to brighten a metal surface and/or achieve Ra values less than 35µin
MACHINE:· Centrifugal Barrels - High-energy systems that leverage rotary speed with variously shaped abrasives to create high friction environments. Centrifugal barrels have single-part chambers and are not considered mass finishing options. · Vibratory/Rotary/Tumbling - Chamber of variously shaped abrasives oscillated to create continuous friction between abrasive and submerged part. This is considered a mass finishing option.
While highly successful with traditional manufacturing processes, these traditional finishing approaches have limitations in an automated additive model. This is not surprising since they were never designed with Additive Manufacturing in mind. It is like taking a commuter car to the race track. The car has value, but it was never meant to race. The parts will be subjected to extreme conditions and would require a design overhaul to adapt. Even if there are short-term successes, it is not sustainable.
5
1. Throughput. Parts are typically printed in batches. To maintain a balanced workflow, it is ideal to do surface finishing in batches as well.
2. Consistency. The process needs to be repeatable and reproducible. Consistent sur-face finish is essential, not just part to part, but from operator to operator. If only one person can consistently meet specifications, the process is not in control.
3. Efficiency. Relying heavily on manual labor and tools introduces fatigue, reliability factors, and general human error. To maximize efficiency, it is valuable to digitally control forces to generate efficient friction patterns and protect parts.
4. Flexibility. Printers are designed to provide flexibility. Every part in a batch can be unique. Surface finishing needs to be adaptable too. If each part requires new training, lengthy setup, or effort to try and remember what worked last time, the process is not scalable.
In the table below, we evaluate the five criteria listed above against the common tradi-tional surface methods:
Table 2 - Traditional surface finish comparison
Relying heavily on the first three methods of manual labor, hand sanding, manual bead blasting, and hand buffing are not sustainable options for optimal surface finishing. Like-wise, the fourth option, centrifugal barrels – even with the potential for efficiency with their high frequency -- are limited to a single part per chamber, not a scalable, mass fin-ishing option.
Bottleneck: The weak link in a process that limits the capacity of the entire workflow.
Therefore, it is important to look at four addi-tional key areas from a process standpoint. If the surface finishing solution is unable to check each of the boxes below, it can easily become the bottleneck.
6 7
It is worth taking a closer look at rotary, vibratory, and tumbling systems. These tech-nologies can range from an open-top donut shaped bowl that vibrates parts horizon-tally, to a log shaped chamber that tumbles vertically, or a high-speed rotary disc that rushes an abrasive over the part. Because these systems can process parts in batches at a constant frequency, some consider them viable for ‘throughput’ and ‘consistency’.
This perspective is missing their shortfall in efficiency, flexibility, and lack of Addi-tive Manufacturing prowess. Although these systems have some overhead advantag-es over manual systems (because they do not require an operator at the machine at all times), they do have some deficiencies. First, they operate at a single frequency throughout the cycle. This limits their ability to simulate varying force and friction which is the level of control needed to increase efficiencies and adapt to fragile geometries. There is also an efficiency gap in maintenance. These machines lack the intelligent software to proactively identify maintenance issues which leads to excess downtime.
These systems also lack flexibility. They are not able to easily adapt to changes in ma-terials and geometries upstream, or special one-off or sporadic projects which require adjusting parameters such as motor speed, abrasive type, cycle time, and lubrication frequency. These are typically cumbersome adaptations.
Ultimately these machines were not designed to handle the complexities of Additive Manufacturing. What if all bridges were designed before anyone knew what a car was? They would not be suitable for the advancements in technology. This is where tradi-tional finishing equipment falls short.
The range of materials and geometries offered in 3D printing is widespread. From flex-ible polymers with fragile thin walls to metals with fine feature details, and each tech-nology offering varying levels of layer lines and roughness. Traditional systems were designed primarily for robust metals with geometries limited by molds and milling machines within very predictable workflows and loud manufacturing environments. When introduced into a scaling Additive Manufacturing environment, it often results in high scrap rates due to extreme forces and under-trained operators, downtime for changeover and maintenance, and decibel levels too loud for a lab setting.
III. SURFACE FINISHING DESIGNED FOR ADDITIVETo summarize, a surface finishing solution designed for Additive Manufacturing should provide the following benefits:
- Ability to process in batches to align with printer throughput- Agility to quickly adapt to varying geometries and materials - Repeatability for consistent results with minimal operator intervention- Versatility for both lab and manufacturing settings
7
This is required to deliver the consistent quality needed for customer-ready parts, which in turn makes additive manufacturing scalable to a degree never before possi-ble. There is a surface finishing solution specifically designed for additive that meets all these requirements. PostProcess Technologies has engineered a technology called Suspended Rotational Force (SRF). SRF is a comprehensive solution that combines hardware, software, and chemistry designed to work together in unison. At a high lev-el, it can be described as digitally tuned oscillations creating a horizontal or vertical circular motion in a chamber consisting of a composite, or single material, abrasive and fluid mixture. The result is an even, controlled mechanical force applied to each part at the surface level. A primitive schematic below provides a visualization for the description.
Image 1: Right-side cross-section view of SRF technology schematic
The science of the effectiveness of this technology is the digitally controlled oscilla-tion, or frequency, which is mapped to the amplitude. As the frequency is increased, the amplitude is being converted from potential energy to kinetic energy. This works together with the proprietary properties of PostProcess abrasives, or media, to deter-mine the amount of force applied to a part. As seen in Image 1, when the media crests it produces an additional shear force inducing additional rotation. This process is akin to a solar system: The part orbits throughout the abrasive (RPM cycle), and with this additional force it gently rotates about its own axis as well. This optimizes the dwell time and ultimately the Rate of Removal (RoR) of each cycle. RoR is the metric Post-Process uses to measure the efficacy of their comprehensive solutions. In surface fin-ishing, this is defined as Roughness Average (Ra)/Time. Understanding this relation-ship and managing these forces throughout the cycle is fundamental to minimizing breakage or diminishing fine feature details.
This level of control is achieved through PostProcess’ AUTOMAT3D™ software. An Agitation Algorithm (AGA), which operates like a human brain, with the ability to learn based on the range of results achieved from each cycle and continuously
8
adjusting system performance. AUTOMAT3D operates the SRF tech-nology and other PostProcess tech-nologies. In essence, this allows PostProcess to digitize the ‘tribal knowledge’ of manual processing through real-time adjustments. In this case, AUTOMAT3D allows SRF to sim-ulate the natural variation of force a craftsman sanding by hand achieves by adapting and optimizing friction to different geometries and materials.
Image 2: AUTOMAT3D HMI Screen
The difference is that with the software platform, fatigue is eliminated, and the process is repeatable part to part, and easily reproduced from operator to operator with the touch of a button. Leveraging software to this capacity is critical to workflow evolution and creating a digital factory.
Chemistry plays a critical role in high quality, repeatable surface finishing, and efficient processing as well. Lubrication and media selection directly impact the RoR. If the mix-ture is too dry, parts can be damaged easily and diminish the life of the abrasive. If the mixture is oversaturated it can cause foaming and inconsistent, extended cycle times. PostProcess technology is designed with this in mind. Upon starting a cycle, each sys-tem in the SRF technology family auto-doses PostProcess’ lubricant, PG3, and can be digitally adjusted based on the abrasive and cycle time. As for the media, it is important to understand the impact of material, density, and shape. A basic scenario of ceramic abrasive weighing 110lbs/ft shaped like a star would be far too aggressive for, and most likely break, a material-jetted part, but it may be suitable for a less complex, more robust ABS part.
The SRF technology is packaged in three main hardware configurations:
· The NITOR incorporates the AUTOMAT3D software and can run two different abra-sives simultaneously, or process parts near 5 feet in length. · The RADOR is a gentle, lab-quiet, and easily maneuverable.· The LEVO is a compact high-frequency centrifugal system for small robust parts.
Each of these comprehensive systems provides flexibility in processing multiple mate-rials and geometries simultaneously. Chart 1 below shows an average Rate of Removal of four examples of common print technologies processed using the SRF technology.
9
Chart 1 - SRF RoR examples
Average Rate of Removal for Various Print & Material Technologies
Roug
hnes
s Av
erag
e (R
a, µ
)
The data depicted in Chart 1 demonstrates the capabilities of the SRF technology:
· Actual technician labor time was 5 minutes or less for each example, which includes loading and unloading parts. Time shown represents unattended operator time. · Measuring the Ra values against the build orientation with a Mitutoyo SJ-410, results show an average Ra reduction of 80%-95% for polymers and 60% for metal, processed to meet customer specifications. · These examples were run in batches of 3 to 50 parts with each batch size optimized based on throughput requirements. With minimal technician time and the ability to process in batches, parts were completed with as little as 5 seconds of labor time per part.
Four different abrasives were used. Each consisted of geometries with either a ceramic or plastic composite. The abrasive was determined based on the part’s material prop-erties, geometry and surface finish requirements to optimize RoR. In this scenario, both the FDM and SLA examples including a polishing step to meet the desired Ra. With a split chamber, the NITOR can run two different abrasives, eliminating the need to change media when the process requires two steps.
Time (hours)operator unattended
- FDM & SLA include a polishing step- PolyJet wall thickness - 0.072” (1.8mm)
10
POLYJET Example
DMLS Example DMLS Example
In Process Surface Finish Ra = 75
In Process Surface Finish Thin Wall - Ra = 100
As Printed Ra = 250
FDM Example FDM Example
In Process Surface Finish Ra = 30As Printed - Ra = 550
Also worth noting is that the lubricant, PG3, was used across all processes allowing manufacturing to process multiple materials at the same time without changing chemistry. This is another way SRF reduces downtime and increases efficiency.
This single technology has the flexibility to address a wide range of finishing challenges including the ability to:
· Polish FDM components to a near injection-molded finish (Ra < 35)· Improve clarity and roughness on clear SLA parts; prepped for clear coat· Protecting delicate walls on fragile PolyJet geometries· Smooth robust alloys with unique geometries
With an automated process, these results are repeatable with a push of a button and designed keep up with the versatility of Additive Manufacturing. Therefore, PostProcess SRF technology achieves a level of throughput, efficiency, consistency, and flexibility unique to the promise of additive manufacturing.
11
Section IV: ConclusionAlthough surface finishing has been around for many decades, the methods have not kept up with the advancements in technology. Manual surface finishing has become a bottleneck for users attempting to fully leverage the flexibility of their 3D print tech-nology. Hand sanding is an art and not scalable, and semi-automated manufacturing equipment was never designed with Additive Manufacturing in mind. This results in inconsistent surface finishes, breakage, and an unpredictable workflow.
Surface finishing requires and deserves the same degree of a step-change that ad-ditive brought to traditional manufacturing. In short, it requires a digitized approach. Software and data are required to achieve the intense variability in surface finishing that is the natural companion to the variable geometries and materials native to Ad-ditive. Likewise, software and data are critical to aligning the post-printing step with the upstream design and print phases. This is a natural progression toward a digital factory.
The Suspended Rotational Force technology has the intelligence to learn and adapt to a wide range of materials with minimal operator intervention and downtime. Com-bined with the AUTOMAT3D software, SRF mimics the ‘art’ of surface finishing in a manner that frees up skilled labor to further scale the operation.
Consistency off the 3D printer does not mean much if it does not make it to the cus-tomer with the same consistency. The PostProcess SRF technology allows users to meet these needs, for today and for the future. In doing so, it unlocks the transforma-tive power of additive manufacturing, making customer-ready parts at scale possible.
SLA Example SLA Example
In Process Surface Finish Ra = 19
As Printed Ra = 93
