Material jetting uses Inkjet technology to deposit materials selectively ‘drop by drop’ on to a build platform where they solidify. Once the base layer is printed the build platform is lowered and the process is repeated to form a second solid layer and so on until the build is complete. The materials used are generally thermoplastics or photopolymers that are solidified by cooling or exposure to UV radiation respectively, although other materials that are of low enough viscosity to be printed and can be triggered to undergo a liquid-solid transition following deposition can be used. Another related method is binder jetting, which is a powder bed technique like powder bed fusion but instead of using a laser or electron beam the particles are fused together using a binding agent which is deposited from a print head.
As with 2D inkjet technology a fundamental requirement is that the material is jet-able and should form discreet drops prior to hitting the substrate. These requirements depend on material viscosity and in the case of drop formation, surface tension also. For more complex materials which contain high molecular weight polymers or dispersed components the materials may become non-Newtonian and viscoelastic which may affect printing performance.
Measuring the rheology and viscosity of the pre-printed material is important for determining the optimum print temperature and printer settings. Such measurements can be made on a rotational rheometer which gives both shear and temperature dependent viscosity profiles. A rotational rheometer can also provide information about the viscoelastic characteristics of the material in both its liquid and solid states, allowing any temperature or radiation induced transitions to be followed in real time and the modulus of the cured material to be determined.
Gel Permeation Chromatography (GPC) is another important technique that can be used to determine the molecular weight and molecular structure of oligomers or polymers present in the formulation and can aid the development of new photopolymer resins. GPC can also be used to monitor polymer degradation or scission resulting from mechanical stresses in the print head or the circulatory system (pump), which may affect jetting behavior and the structural integrity of the final material.