by Trevor Larcheveque, Branson Ultrasonics Corp.
3D printing (3DP) now is widely adopted for prototyping and developing new product designs because it allows plastic component and part prototypes to be developed, produced, assessed and modified far more quickly and economically than traditional injection-molded plastic parts, which require significant up-front investment in mold tooling. As a result, the use of 3DP is surging throughout the automotive, aerospace, consumer products and medical/medical devices industries.
But along with the surging interest in 3DP for plastic part prototyping, there also has been a surge of questions about whether components fabricated with 3DP can be joined into assemblies in the same ways as injection-molded parts. Specifically, can these parts be joined using ultrasonic plastic welding techniques?
According to current evidence, the answer is that “it depends.” To understand the factors on which the ultrasonic weldability of plastic components fabricated with 3DP depend, it is essential to look more closely at three questions:
- What demands does ultrasonic welding place on a prototype part?
- To what degree do the various 3DP processes deliver parts with the four critical physical characteristics – resolution, strength, solidity and weldability – needed for repeatable ultrasonic welding?
- Are the materials used to produce 3DP parts compatible with ultrasonic welding?
Ultrasonic welding: the basics
Ultrasonic welding is performed by applying 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 friction, which melts the material and bonds the two parts together. Ultrasonic processes also can be used to insert, stake, swage, degate and spot-weld components. Ultrasonic welding requires that parts to be joined must be made from thermoplastics. Among thermoplastic materials, weldability varies based on a variety of factors, including polymer structure, density, melt temperature, viscosity, stiffness (modulus of elasticity), thermal conductivity and chemical makeup.
Part design, specifically the design of part joints, plays a major role in weldability. Typically, ultrasonically welded parts incorporate one of the two principal types of joint designs shown in Figure 1: an energy director joint or a shear joint. Successful ultrasonic welding demands that both of these joints are produced with a high degree of resolution within the part, since feature tolerances can be quite small.
Energy director weld joints. The primary function of an energy director is to concentrate energy to rapidly initiate the softening and melting of the joining surfaces. An energy director 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. Adding an energy director to the joint helps to concentrate ultrasonic energy while significantly reducing weld time. Energy director sizes vary according to part size, but typically range from 0.010″ to 0.020″ tall, with angles that vary based on the thermoplastic being used. On the part side that is opposite to the point of the energy director, surfaces are usually textured. Molding a texture onto the mating part surface tends to improve the overall weld quality and strength by enhancing frictional characteristics and control of the melted surfaces. Usually this textured surface is quite shallow and, unfortunately, this resolution may be too fine for some 3D printing technologies to achieve.
Shear type weld joints. Occasionally, parts with energy-director type joints may not produce the desired weld results. If this occurs, use a shear joint instead. Shear joints can have a variety of appearances, but all are designed to create an interference fit between the opposing parts, retain molten material in the area of the weld, and prevent premature solidification by preventing contact with surrounding air.
A shear joint weld begins with initial contact in a small area of the opposing surfaces, where initial melting creates an interference fit. As the melt continues, it proceeds along the vertical walls of the parts, allowing the part joints to telescope and bond together under pressure. Shear joints thus result in strong structural or hermetic seals.
How 3DP fabrication technology influences part weldability
While 3DP components can provide precise part geometries that make them great for visual prototype evaluation, these parts have substantially different physical properties than those of injection-molded parts. As a result, they do not respond to ultrasonic welding as predictably or consistently. The key to understanding the differences in physical properties of 3DP part resolution, strength, solidity and weldability is to understand the 3DP technologies used to create them.
Extrusion. Extrusion is the most common and recognized 3DP technology today. Extrusion processes work by melting thermoplastic filament and passing it through a heated extruder. Extruded material is then deposited in thin layers that form two-dimensional slices of the final component. Layers are printed consecutively, one atop the other, so that the molten plastic can harden and bond to the layer below to form a 3D object.
Filament materials for extrusion include many that are already used in ultrasonic welding applications – ABS, HIPS, nylon, PC, PC-ABS, PET and PLA – with ABS and PLA being the most commonly used. Material grades are customized by different manufacturers to achieve special properties, such as greater similarity with injection-molded parts. However, the physical strength of printed parts is significantly weaker in the direction that the layers are stacked. As a result, these layers are vulnerable to separation under the stress of ultrasonic welding or during testing to evaluate the strength of the weld joint. Creating a consistent hermetic joint may also be problematic due to gaps between layers or gaps within the print paths on a single layer. While post-fabrication processes that may close surface gaps are available, these processes can run the risk of smoothing over critical features of joint geometry.
The maximum resolution (minimum layer thickness) that extrusion printers currently achieve is approximately 0.005″; however, achievable layer thickness varies based on the 3DP machine and material. For example, parts produced by the Fortus 900mc by Stratasys have an accuracy of ±0.0035″ or ±0.0015″ per inch, whichever is greater. The high tolerances required to obtain repeatable shear joint results may not be possible with FFF technology.
Figure 2 shows two pairs of parts. The top pair compares energy director butt joint specimens produced from an injection mold (left) and from extrusion technology. The extrusion part was printed with a Stratasys Dimension Elite 3D printer using Stratasys Dark Gray ABSplus-P430 material in 0.007″ 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″ tall, 0.022″ wide), rather than the preferred triangular shape.
The bottom pair in Figure 2 compares two shear joint specimens: one produced from an injection mold (left) and the other using the same extrusion process as the energy director specimen. Although shear joints do not require the sharply pointed features of an energy director joint, 3DP fabrication of shear-joint parts must maintain the dimensions needed for a precise interference fit.
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 in part design and 3DP fabrication can be overcome, the results obtained from welding these parts should more closely correlate with those of welded injection-molded parts.
Selective laser sintering. A second 3DP fabrication method, selective laser sintering (SLS), uses a focused laser directed by a mirror to melt materials – such as metal, plastic or glass – in powder form. Commonly used polymers for SLS fabrication 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 object being printed, 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 3)
The minimum layer thickness achievable by SLS processes is slightly smaller than that of the extrusion process, approximately 0.003″, so better resolution of joint detail is theoretically possible.6 However, wall thicknesses less than 0.040″ 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 can pose a major concern for weldability. Pores in final printed parts can absorb ultrasonic energy and cause part features to compress. Or, they may create stress concentrations in the component that can lead to fracturing when subjected to the high frequency vibrations characteristic of ultrasonic welding. Note that fractures can propagate from any surface of the part, not just those contacted by the ultrasonic horn or by the opposing part surface. High porosity in fabricated parts also may be problematic when it comes to achieving consistent sealing.
So, in summary: SLS processes can produce weldable parts, but achieving consistent weldability demands that part designers and fabricators carefully manage challenges associated with feature resolution, part porosity and part stress.
Stereolithography (SLA)/digital light processing (DLP)/material jetting. Multiple 3DP technologies utilize photopolymer resins, including stereolithography (SLA, Figure 4) 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, then cures it 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.
Unfortunately, a third essential element for weldability is absent. As their name suggests, photopolymer resins cure using ultraviolet (UV) energy. Unlike thermoplastics, they cannot be remelted, reshaped or joined using the friction-generated heat and pressure characteristic of ultrasonic welding.
However, photopolymer-based 3DP processes still can play a role in the production of weldable prototype parts, since they have been used to create injection molds that benefit from the high resolution and smooth surface finishes of SLA printed/material jetted processes.8 Although these plastic molds lack the durability of traditional metal injection molds, they can produce a limited number of prototype parts that replicate part features better than other 3DP processes. Further, they can use the same polymer material that later high-volume manufacturing processes will use. This approach could well enable part designers to evaluate part weldability, strength, sealing and other performance characteristics with a high degree of accuracy – a plus when it comes to reducing lead times and product development costs.8-
Designing a more weldable 3DP part
Select materials carefully. Material selection plays a primary role in weldability. Many engineered resins created specifically for 3D printing applications may mimic the behavior of more common materials but are not necessarily weldable. For example, ABS is one of the easiest polymers to ultrasonically weld. However, Digital ABS, a material created by Stratasys to mimic the properties of ABS resin, is a photopolymer that cannot be ultrasonically welded.
Evaluate different 3DP print orientations. 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 feature – such as an 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 significantly different results and also may affect the tensile properties of the parts.
Keep part walls solid. It is also important that all part walls between the joint location and the horn contact surface/supporting surface be printed with maximum infill settings (100 percent 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 voids may occur in extrusion 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 ability of the part structure to transfer ultrasonic energy to the weld joint. Print settings should be set to achieve 3D prints that are as dense as possible.
When designing parts for an extrusion-style printer, care also should be taken to avoid placing excess support material in critical weld areas. Removing this support material can damage the joint surfaces. The SLS process is self-supporting, so unwelded powder simply falls away.
Design for simple tooling and fixturing. 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 5. This will allow a generic, flat-faced horn to contact the 3D printed prototype and transmit vibrations down to the joint location. Ensure that horn contact surfaces are 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 having to produce 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.
Compared to traditional processes, such as injection molding, 3D printing offers a new and faster way to produce and evaluate prototype plastic parts. However, reliably assessing the ultrasonic weldability of 3DP prototype parts remains challenging due to the current limitations of 3DP fabrication technology and materials. Reliable and repeatable ultrasonic weldability requires not only that 3DP prototype parts be made with thermoplastic polymers, but that they offer sufficiently high resolution, strength and solidity to tolerate the ultrasonic process and retain key performance characteristics.
Of the 3DP technologies considered here – extrusion, selective laser sintering (SLS) and stereolithography (SLA)/digital light processing (DLP)/material jetting – none has yet demonstrated that it can, with currently available capabilities and 3DP materials, directly print parts with physical characteristics and weldability that match those of injection-molded parts.
However, by managing the limitations of 3DP technologies, it may be possible for part designers and fabricators to produce prototypes that reduce current resolution, performance and weldability differences. At present, though, this remains the exception, not the rule. Statements made herein regarding the design of prototypes, weld joints and other factors are intended as a guide 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.
Trevor J. Larcheveque leads Bransons Ultrasonic Application Development team, which specializes in plastics joining methods that utilize ultrasonic welding. Larcheveque works with customers to deliver robust engineered solutions for their application challenges. Before joining Branson in 2014, Larcheveque worked four years as an application engineer for Dresser-Rand, a Siemens business in Wellsville, New York, where he supported steam turbine customers with rebuilding and upgrading their equipment. For more information, email [email protected] or visit www.bransonultrasonics.com.
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