Can 3D printed parts be ultrasonically welded? In some cases, they can. But differences in resolution, strength, and solidity from the materials and 3D printing technology are key to successful ultrasonic welds. Here’s what you need to know.
Trevor Larcheveque, Applications Development Engineer, Branson Ultrasonics Corp.
As 3D printing (3DP) has become more accessible, affordable, and practical, it has been adopted in many industries including automotive, aerospace, consumer products, and medical. Many manufacturers actively prototype and develop new product designs using 3DP because the technology allows plastic component and part prototypes to be assessed and modified more quickly and economically than traditional fabrication methods such as plastic injection molding.
Along with the swelling interest in 3DP, there has been a surge in questions and requests to the makers of ultrasonic welding (UW) equipment about whether and how this commonly used assembly and joining technology can be used with components fabricated using 3DP. To answer those questions, it is necessary to consider the current state of 3DP technology and materials and assess several issues:
–Nature of UW and the dimensional and physical demands it places on a part.
–Ability of 3DP processes to provide the part resolution, strength, and physical characteristics needed for repeatable UW using 3D-generated parts.
–The weldability of materials used to fabricate 3DP parts.
Can 3D printed parts be ultrasonically welded? In general, the answer is “sometimes,” but not with the reliability and repeatability characteristic of ultrasonic welds involving injection-molded parts. All 3DP processes and materials yield parts with some of the key characteristics needed for reliable and repeatable ultrasonic welding—high resolution, strength, solidity, and weldability–but none yet can consistently produce them in a full range of parts. However, given the continued rapid evolution of 3DP materials and technologies and the substantial installed base of ultrasonic welding equipment, it seems likely that these current limitations will be addressed and overcome.
The basics of ultrasonic welding
Ultrasonic welding is the application of high-frequency vibrations to two parts or layers of material using a tool commonly called a “horn” or “sonotrode.” These vibrations travel to the interface of the two parts and produce heat through hysteresis and frictional heating, which melts the material and bonds the two parts together. The technique is fast and efficient, and eliminates the requirements of consumables. Ultrasonic processes can also be used to insert, stake, swage, degate, and spot weld components.
Ultrasonic welding is best done on thermoplastic materials. Because thermoset materials undergo an irreversible chemical change and cannot be reformed, they cannot be ultrasonically welded.
Additional factors may affect the ultrasonic energy requirements and weldability of a material. Major factors include polymer structure, density, melt temperature, viscosity, stiffness (modulus of elasticity), thermal conductivity, and chemical makeup. Both amorphous and semi-crystalline polymers may be welded; however, amorphous materials are generally easier to weld, as they have broad softening temperatures and can transmit ultrasonic vibrations to the weld joint.
There are two major types of ultrasonic weld joint designs: an energy director type joint, and a shear type joint (Figure 1). Both require a high degree of resolution in the 3D parts, as feature tolerances can be quite small.
Figure 1. Example of an energy director type joint (left) and a shear type joint (right).
Energy director type-weld joints. An energy director concentrates energy to rapidly initiate the softening and melting of the joining surfaces. It is typically a triangular bead of raised material located on one of the mating joint surfaces. During the weld process, the energy director melts and flows throughout the joint area and mixes with the opposing melted surface. It significantly reduces weld time. Energy director joints are the most commonly used joints for amorphous materials, though they may also be used for semi-crystalline thermoplastics too.
Energy director size will vary depending on part size, but typically range from 0.010 in. to 0.020 in. tall, with an included angle of 90° for amorphous materials and 60° for semi-crystalline materials. It is important that the energy director maintains a sharp point; a radius of 0.002 in. or less is preferred on injection molded parts.
A textured surface is often implemented on the surface opposing the energy director. Molding a texture on the mating part can improve the overall weld quality and strength by enhancing frictional characteristics and melt control. Usually the texture is only 0.003 in. to 0.006 in. deep (MoldTech 11040 to MoldTech 11050-6), which may be impossible for some 3D printing technologies to achieve.
Shear type weld joints. Semi-crystalline resins often have better results than shear type weld joints. Semi-crystalline resins change rapidly from a solid to a molten state, and back again, over a relatively narrow temperature range. As an energy director melts, molten material flowing away from the heated zone could re-solidify before fusing to the adjoining surface; a shear joint prevents premature solidification as the molten material is retained in the weld area and prevented from contacting the surrounding air.
Welding a shear joint is initiated by a small, initial contact area that creates an interference fit between the two parts. Once melting starts, it continues down along the vertical walls of the parts, allowing the parts to telescope together for a strong structural or hermetic seal.
The interference of the weld joint may be as small as 0.008 in. with a recommended tolerance of ±0.001 in. for parts with a maximum part dimension of less than 0.75 in. in size. Larger parts (1.5 in. to 3.0 in.) should have a weld interference of approximately 0.014 in. with a tolerance of ±0.003 in. Shear joints require rigid side-wall support to prevent deflection during welding, and the interfering surfaces should come into contact with flat, 90° surfaces.
How 3DP technologies affect UW part fabrication
While 3DP components can provide precise part geometries, these parts have substantially different physical properties than those of injected, extruded, and machined parts. As a result of differences in resolution, strength, solidity, and weldability, parts produced using 3DP do not respond to ultrasonic welding with the consistency and predictability of injection molded, extruded, or machined parts. The key to understanding the differences in 3DP parts is to understand and identify the 3DP technology used to create them.
Extrusion. Extrusion is the most common and most recognized 3DP technology today. It processes work by melting thermoplastic filament and passing it through a heated extruder. The extruded material is deposited in thin layers that form two-dimensional slices of the final component. These layers are printed consecutively on top of one another, allowing the molten plastic to harden and bond to the layer below, forming a 3D object.
Figure 2. Depiction of the fused filament fabrication process.
Filament materials for extrusion include those frequently used for ultrasonic welding applications, such as ABS, HIPS, Nylon, PC, PC-ABS, PET, and PLA, with ABS and PLA being the most commonly used 3DP filament types. Material grades are customized by different manufacturers to achieve special properties. Even if properties are made to mimic injection-molded parts, the physical strength of the printed parts is significantly weaker in the direction that the layers are stacked. As a result, these layers may separate during the ultrasonic welding process, or during testing to evaluate the strength of the weld joint. Creating a consistent hermetic joint may be impossible due to gaps between layers or even gaps between print paths of a single layer. Post processes are available that may close surface gaps, but use of these might also smooth over critical joint geometry.
The highest resolution (minimum layer thickness) that extrusion printers can achieve is approximately 0.005 in.; however, achievable layer thickness varies based on the 3DP machine and material. Achievable tolerances also vary depending on the size, shape, and orientation of the printed part. For example, parts produced by the Stratasys Fortus 900mc have an accuracy of ±0.0035 in., or ±0.0015 in. per inch, whichever is greater. The high tolerances required to obtain repeatable shear joint results may not be possible with extrusion technology.
Figure 1 shows two large energy director butt joint specimens: one produced from an injection mold and the other printed with extrusion technology. The part was printed with a Stratasys Dimension Elite 3D printer using Stratasys Dark Gray ABSplus-P430 material in 0.007 in. thick layers. Note that due to the limitation of extrusion width, the energy director of the 3DP part is created in two single passes, resulting in a rectangular shape (0.014 in. tall, 0.022 in. wide).
Shear joints do not require a sharp-pointed feature as there is no energy director; however, maintaining precise interference is important for obtaining repeatable results. Figure 3 also shows two shear joint specimens: one produced from an injection mold and the other using the same extrusion process as the energy director specimen.
Figure 3. Injection molded specimens (left) and extrusion printed specimens (right).
While it is possible to weld extrusion printed parts, energy requirements, weld strength, flash results, and sealing capability may be quite different in comparison to injection molded parts of similar material. In summary, the weldability of extrusion parts may be limited due to variance in the strength of the layers, the inability to build a repeatable shear joint feature due to variance in the interference fit, and the variability in the shape of the energy director. If these limitations can be overcome in part design and 3DP fabrication, the parts themselves should be weldable.
Selective laser sintering. Selective laser sintering (SLS) uses a focused laser directed by a mirror to melt materials in powder form, such as metal, plastic, or glass. Commonly used polymers include variations of nylon and polystyrene. Within a heated enclosure, powder is pushed from a powder supply by a roller and spread in a thin layer across a build surface. A mirror directs a laser through a 2D trace of the printed object, lifting the temperature of the focus point just enough to melt the powder. The build surface is then lowered and another thin layer of powder is deposited on top. The process repeats until the object is completed.
Figure 4. Depiction of the selective laser sintering (SLS) process.
SLS processes can produce parts that are more accurate than extrusion processes, although the surface finish may not be as fine as those produced by 3DP processes that use photopolymer resins. The chosen SLS material will dictate the size of the powder granules. The minimum layer thickness achievable by SLS processes is slightly smaller than that of extrusion, approximately 0.003 in., so better resolution of joint detail is theoretically possible. However, wall thicknesses less than 0.040 in. in size are generally not recommended for SLS processes and fine details, such as the sharp point of an energy director, may be “smoothed over” or lost as a result of the SLS layering process.
The sometimes high levels of porosity of SLS-fabricated parts may pose a concern for weldability. Pores in the final printed part can absorb ultrasonic energy and cause part features to compress. Or, they may create stress concentrations throughout the component that can lead to fracturing under the high-frequency vibrations required in the ultrasonic welding process. Fractures can propagate from any surface of the part, not just those that are contacted by the ultrasonic horn or the surfaces of the weld joint. As would be expected, significant porosity in fabricated parts might also make it difficult or impossible to evaluate the consistency of design features such as a hermetic seal.
So, although SLS processes can produce parts that are weldable, achieving consistent weldability demands that part designers and fabricators carefully manage and overcome challenges associated with feature resolution, part porosity, and part stress.
Stereolithography (SLA) / Digital Light Processing (DLP) / Material Jetting. There are multiple technologies that take advantage of photopolymer resins, such as stereolithography (SLA) and digital light processing (DLP). These processes use focused light to cure photopolymer resins layer by layer into a solid object. A third process, material jetting, applies a thin layer of photopolymer with an inkjet-style printing head, and cures the photopolymer immediately with a UV light source. Parts produced with these methods have high accuracy and smooth finishes, two of the essential elements required for consistent weldability.
Figure 5. Depiction of the stereolithography (SLA) process.
Unfortunately, a third essential element for weldability is absent. As their name suggests, photopolymer resins cure using ultraviolet light (UV) energy. They cannot be melted, reshaped, or joined using the friction-generated heat and pressure characteristic of ultrasonic welding.
Though photopolymer-based 3DP processes cannot directly produce ultrasonically weldable parts, they do offer part designers another option. These processes have been used to create injection molds that benefit from the high resolution and smooth surface finishes of an SLA printed/material jetted process. Because these molds are made from plastic instead of metal, they typically produce only a limited number of parts. But, parts fabricated in these molds more accurately replicate part features and can use the same material that later high-volume manufacturing processes will use. So, part weldability, strength, sealing, and other performance characteristics can be evaluated with a high degree of accuracy – a plus when it comes to reducing lead times and product development costs.
Materials with favorable flow characteristics and low melt temperatures (< 300°C), such as ABS, PS, PE, and PP, can use a mold created by a material jetting process as many as 100 times, as compared to only a few uses (roughly 5 to 15) when using more demanding plastics, such as glass-filled nylon or PC.
The resolution available with SLA/DLP 3D printed parts is extremely small: the Form 2 created by FormLabs can print with 25 µm photopolymer layers. Material jetting technology, such as PolyJet technology by Stratasys, can achieve layer thicknesses as low as 16 µm.
Part design considerations for ultrasonic welding
As noted, different 3DP printing technologies produce different resolutions. But claims of higher resolution do not always equate to better or sharper product or joint designs.
Figure 6 shows detailed views of energy director joint specimens using injection molding and three different 3DP technologies. The extrusion-processed part was printed with a Stratasys Dimension Elite 3D printer using Stratasys Dark Gray ABSplus-P430 material in 0.007 in. thick layers. The part created with an SLS process was printed with a 3D Systems sPro 60 production printer using Duraform PA, a nylon powder developed by 3D Systems. The part produced with material jetting was printed with a Stratasys Objet 260 Connex 2 printer using Vero White, a rigid opaque photopolymer developed by Stratasys. Note that, although the material jet component has smoother surfaces and is more consistent along its length, it uses a photopolymer material and therefore will not ultrasonically weld.
Figure 6. Energy director specimens created with an injection mold and three 3DP technologies.
When designing parts for an extrusion printer, avoid support material in critical weld areas. Removing support material can damage the joint surfaces. The SLS process is self-supporting, unwelded powder simply falls away.
Material selection. Material can play a major factor in the weldability of an application. Many engineered resins are created specifically for 3D printing to mimic the behaviors of other materials, and should not be confused with weldable materials. For example, ABS is one of the easiest polymers to ultrasonically weld. Digital ABS, created by Stratasys, mimics properties of ABS resin; however, it is a photopolymer and will not ultrasonically weld.
3DP print orientation. Depending on the 3DP technology used, joint design geometry can vary significantly when parts are printed in different orientations. Joints do not always follow straight paths, and the orientation of a single energy director may lie in more than one direction. This large variance is created by the layer height typically being shorter than the minimum layer width and the tolerances achievable by the printer. Printing a weld joint in three different orientations will produce different results and may also affect the tensile properties of the parts.
Generic tooling contact and support. Typically, 3D printed parts are created to reduce time and cost when evaluating part designs. Creating custom ultrasonic tooling for each prototype design would defeat the advantages of 3D printing. To evaluate a joint design, the surfaces directly above the joint should be raised so that all horn contact surfaces are flat and above any other part geometry, as demonstrated in Figure 2. This will allow a generic, flat-faced horn to contact the 3D printed prototype and transmit vibrations down to the joint location. Furthermore, horn contact surfaces should be as close to the weld joint as possible to reduce the amount of energy absorbed by the material before reaching the weld joint.
Ultrasonic welding also requires rigid support from the fixture. To avoid a custom-designed fixture, the bottom half of the assembly should have a flat surface below the weld joints so that it can support itself on a hard, flat surface.
Part infill. It is also important that all part walls between the joint location and the horn contact surface/ supporting surface should be printed with maximum infill settings (100% solid). Some 3D printed parts are designed with internal voids and thin-walled geometries to reduce the amount of material required by the print; however, such voids inside a part can make ultrasonic welding more difficult or impossible by preventing transmission of ultrasonic energy to the weld joint.
Even when printed solid, small holes and voids may occur in extruded printed parts along the edges of the layers and between layers. These irregularities may reduce the effectiveness of a shear joint, cause welded parts to leak, or reduce the print’s ability to transfer ultrasonic energy to the weld joint. Print settings should be set to achieve 3D prints that are as dense as possible.
Figure 7. Flat contact area above (red) and below (blue) weld joints.
3D printed parts offer a new and exciting way to evaluate new product designs. However, the ability to use 3DP parts to directly evaluate their ultrasonic weldability is at present limited, primarily due to the current limitations of 3DP fabrication technology. Reliable and repeatable ultrasonic weldability requires plastic parts that offer high resolution, strength, and solidity, and that are formed using weldable polymers. To date, parts produced using injection molding have offered this degree of predictability.
Of the 3DP technologies noted here – Extrusion, Selective Laser Sintering (SLS) and Stereolithography (SLA) / Digital Light Processing (DLP) / Material Jetting – none have yet demonstrated that they can, with currently available capabilities and 3DP materials, directly print parts with physical characteristics and weldability that match those of injection molded parts.
By managing the limitations of 3DP technologies, it may be possible for part designers and fabricators to produce some prototype parts that reduce the resolution, performance, and weldability differences relative to injection molded parts, but this is currently the exception, not the rule. Conclusions made regarding these weld joints may be erroneous and may not reflect final production results.
Given the latest advances in new 3D printing technologies and materials, 3D printed injection molds may offer a cost-effective solution to producing prototype parts whose ultrasonic weldability and performance can more accurately predict final production results using injection molded parts.
Branson Ultrasonics Corp.
www.emersonindustrial.com
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