Introduction to 3D Printing: What Are Composites?

How do you decide what material is best for a manufactured part? This simple question has a complicated answer. An engineer must weigh criteria across several domains, including intended function, loading scenarios, work environment, production quantity, available manufacturing processes, and more. 3D printing is appropriate for some production applications, and less so for others. Here, we explore a new process for fabricating end-use parts: 3D printing composite materials.

Comparing manufacturing processes 

We’ll use Dixon Valve’s gripper as our example part, which enables their production line to make couplings efficiently at a massive scale. It must be strong enough to transmit the clamping force, durable through repeated loading cycles, and non-marring to the valves. Since the gripper is not a production part but rather a component of the manufacturing line, the production quantity is only in the 10s or 100s. Manufacturing processes for this part are compared as follows:

• Machining would be appropriate for the geometry, except that the smaller production quantity reduces the cost effectiveness for fine-tuning fixtures and paths. The company also lacked machine bandwidth, and contracting a third-party manufacturer for workholding fabrication increased lead time for ramping up production. 
• Cast parts would require internal cores and post-machining for the fasteners, and there is much longer lead-time to pour a new batch. A metal part might also damage the sandcasted finish of the valves. 
• Fiberglass and carbon fiber layups are often used to provide a lightweight, stiff, and strong skin around an internal frame, which is not in line with the desire for a compact, robust gripper. The layup process is also inherently far less repeatable.
• Thermoplastics such as ABS and PLA are easily printable in small production runs, but possess none of the desired strength or toughness. They will quickly degrade — if not snap — under regular use.

What if there were a way to take advantage of the accessibility of 3D printing, and produce end-use parts that meet functional requirements for strength and toughness? 

This is now made possible by incorporating continuous reinforcement fibers inside a printed part, as you can see in the rendering below. The plastic matrix offers all the advantages of small-batch fabrication and rapid iterability, while the strands of Kevlar® make the composite part mechanically stable and sufficiently durable for long-term use on the production floor. 

Systems reinforced with material composites are not new. Composites are characterized by the structural integration of multiple unique materials such that the resultant mechanical properties of the combined part are optimized for the loading scenario. They exist everywhere from construction — rebar transmits tensile loads throughout the strong-in-compression concrete base — to biomechanics — the muscles and connective tissues in the human body which distribute loads around the skeleton. 

The great advantage of composites is that the engineer is not limited to a single set of material properties. They leverage the characteristics of both constituents.

Composites in 3D printing take advantage of the compressive strength of the plastic matrix — the support structure which comprises most of the part volume — and the tensile strength of embedded fibers. These two materials are mutually dependent: without fiber, the plastic part is only as strong as the adhesion within and between extruded plastic strands. Without the matrix, the fiber has no structure and therefore won’t maintain its shape. The matrix creates space so that the fiber has a lever arm to stabilize against the load. When combined, they synergize to form a composite with greater strength in both compression and tension than either can offer individually. 

What properties characterize matrix and fiber?

The matrix material must extrude easily, offer a smooth surface finish, and adhere well to itself as infill so that it is stable under compression. It should not be brittle once it sets, otherwise it will risk fracture. Most importantly for embedded fibers, the matrix must be ductile enough to allow the fibers to be loaded. (Metals are therefore too stiff to be effective as a matrix.) Consider the tensile moduli of steels (200 GPa), carbon fiber (60 GPa), and nylon (1 GPa). Since nylon’s modulus is less than that of carbon fiber, it will elongate before the fiber, thereby transferring the load to the stiffer material, in this case the fiber. (This assumes similar lengths and cross-sectional areas, which means that this is always true locally.)  

How about the material properties of fiber? Since its function is reinforcement, the primary factor is tensile strength. This is especially true when parts are designed to maximize axial loading of the fiber. Unlike the plastic matrix, fiber does not need to create a nice surface finish or hold stable under compression. Other qualities such as heat deflection and gradual yield modes distinguish one fiber from another.

Remember that there is a mutual dependence: fiber requires the space created and held by the plastic in order to do its job, so both must work together to form a strong part.