What is material jetting?

There is abundance of 3D printing technologies on the market, with many more in the pipeline pending patents approval. One such 3D printing technology is called Material Jetting (MJ) which includes MultiJet Printing (MJP) and PolyJet or Drop-On-Demand (depending on the manufacturer). Although material jetting is relatively new compared to other 3D printing methods, it’s considered to be one of the most futuristic and impressive methods available today.

How does material jetting work?

People familiar with material jetting love to compare it with 2D printers, which is useful if you know how an inkjet printer works. If you don’t, that’s fine, we’ll explain it here. MJ 3D printers operate with viscous materials: they spread tiny drops of the material across the build platform and cure it with UV-light. After the first layer is completed, the platform drops, and the machine deposits the second layer. This process continues until the object is formed, layer-by-layer. MJ 3D printers vary in their construction but typically there are several key components:

  • Print heads. All print heads are extremely small – to deliver resolution up to 16 microns. They are attached to both sides of a carrier system to cover the entire area under it.
  • Build platform. The build platform is equipped with an elevating system that moves it down after each layer. The system is capable of changing the Z-axis movements according to the height of a layer.
  • Material containers. All materials are stored as liquids in separate containers – modern systems can handle up to 4 at the same time. Material containers are attached to the carrier system with tubes, which enables mixing or changing materials during the production process.
  • UV-light source. UV-light is located near the print heads as it cures the material drops after each layer, solidifying the structure.

The working principle of material jetting printers plays a role in the types of materials they can work with – it’s possible to operate with just liquids of certain viscosity to create an object from tiny drops. Furthermore, these materials have a photo-resistant nature, which means that they change their properties (solidify) under a UV-light.

Material jetting 3D printers form objects based on a code created in a slicing program – the code essentially is a combination of commands for how to maneuver the machine’s components and drop the “inks”. One crucial point about material jetting technology is the need of supporting structures. Supports are created from a special material which can either be dissolved or melted away after the printing process. For some printing modes, supports are only required for overhanging parts to counteract the effects of gravity. For other printing processes, supports are required to fill all cavities and fully surround the object.

What materials are used with material jetting?

All material jetting “inks” are photo resistors – that’s why they are stored as liquids and then solidified during printing. At the beginning, MJ materials were waxes and some photopolymers, but nowadays, the variety is huge thanks to mixing (blending) of the aforementioned materials in differing combinations. The three main groups of materials are as follows:

Base resins

As the name implies, these resins form the base of all others. They can be used without mixing or combined to reveal new properties. Base resins carry the main functional features and colors, each one – its own, so a machine can then use them as a palette to receive some hybrid materials. Base resins usually include:

Rigid general-purpose material– depending on the manufacturer, properties and colors may vary but all MJ printers have this - “the most general” opaque resin type. Objects created with this material are rigid and can be compared with plastic parts that exhibit relative stiffness. The older MJ printers used to operate with only 1-2 colors of general-purpose resin, while their more modern counterparts can handle Cyan, Magenta and Yellow colors of rigid material and fragment them into hundreds of different shades and colors.

Transparent rigid resin– we have decided to separate transparent resin from other rigid resins due to its unique features that influence the printing process. Clear resins are more complicated to create and work with as they usually tend to have glimpses while exposed to too much UV-light. Resin can become yellow, orange or cloudy when over cured, so when a clear part is printed, it’s better to switch to higher speeds. A special design approach also helps: the more flat and leveled objects are, the less likely the resin will fail. Transparent resins can be post-processed to near glass-clarity but would require gentle and continuous sanding, polishing and moisturizing.

Strong and Tough Resins– another base photopolymer which improves stiffness and toughness. These resins are typically rigid and are used for precision fitted parts. Some materials of this category simulate Polypropylene, delivering more wear and temperature resistant results.

Flexible Resins– for tangible objects, flexible resins with greater elasticity are used. The final product performs like rubber, so prints can be deformed and stretched without damage.

Biocompatible resins – these are manufactured specifically for use in medicine and dentistry. If other resins can be safe for use, these go one step further, so they can be subjected to water, wear and other human body’s conditions. Biocompatible resins are typically not used for internal implants but rather dental and ear prosthetics, bio testing purposes, or other functions where a printed object has contact with medical devices and materials.

Strong engineering resins– this family of photopolymers was created for mechanical and manufacturing applications. With better strength and performance, the category is supposed to deliver a precise and high-detailed part suitable for higher temperatures and usage in injection and blow molding.

Castable Resins– these resins exhibit similar yet vastly improved properties as that of casting wax and are suitable for creating master models which will undergo lost wax casting with metals.

Composite resins

Created by mixing different base resins. The blending process usually happens before printing so the composite material can be loaded into a machine, ready to go. Composite resins can be achieved by combining 1,2,3 or more base resins for a certain result. For example:

Flexible + Rigid resins can be mixed in different proportions to achieve certain shore hardness. Playing with the amount of elasticity creates more engineering potential, since flexibility influences the final performance of an object.

Clear + Opaque mixture in the process is popular for printing anatomical and design prototypes, since it delivers transparent parts with a rigid image captured inside.

Colored opaque + various resins in certain proportions can deliver deep realistic patterns and surfaces like a 2D full-color printer would but from all sides and inner parts of a print.

Other possibilities are also available depending on the machine’s capabilities of handling several material containers at the same time and mixing them.

Support material

Since supporting structures are a must for material jetting, some resins are optimized for easier removal. Commonly, dissolvable materials are used. There are several types of them with different parameters of solubility that influence the range of chemicals required to fully dissolve the structures.

Some supporting resins can also become soft during the printing process, which makes it possible to remove them manually, and water jet is used for the final touches. In other cases, supporting material appears like wax, so to remove it simply requires heating.

What are the suitable applications of material jetting?

Before some improved materials were introduced to the market, material jetting was believed to be more suitable for display purposes as the mechanical properties weren’t that great.

Today, the technology maintains the aesthetic function thanks to color realism but has also expanded to manufacturing, functional prototyping, medical, production and other markets. The key features of material jetting technology include:

  • High resolution. Even compared with other 3D printing methods that use photopolymers, material jetting wins hands-down as print heads are capable of forming objects more accurately.
  • Smooth surface. One of the MJ printing modes is called “glossy” thanks to the final appearance of prints. The liquid allows the 3D printer to form smoother parts, which appear shiny if no supporting structures are used. If an object is printed with matte mode, it would have a specific texture, but layer lines still won’t be so visible.
  • Ability to combine and mix materials. Material jetting is nearly the only technology which can print different materials in one go, and is the only method capable of mixing resins during a print. Currently, this is the only technology that can offer such versatility.
  • Color and texture realism. The key feature MJ methods have is that objects appear perfectly realistic. It’s widely used for design, prototyping, demonstration models and training or educational applications. Resin combinations are capable of mimicking other materials in color, texture or both, so MJ printers deliver precise pieces for many industries.

With all these benefits, material jetting became an effective way to create dental and anatomical models, jigs and fixtures, fashion prototypes (for glasses, shoes and more), functional mock-ups for presentations and demos, and end-use molds for plastic.

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What are the limits of material jetting?

As with all manufacturing and prototyping tools, material jetting has several disadvantages and things to keep in mind while working with it. So the main limits of MJ are as follows:

  • Photosensitive properties of material. Photopolymers open great possibilities when printing them, but outside of the build plate, this can sometimes prove to be problematic. Like with clear resins specifically, over curing prints with UV-light can lower the quality of the final properties. This results in parts losing their mechanical properties and degradation over time. The effect can be reduced by coating options.
  • Weak mechanical properties. Older MJ 3D printers are capable of working with just general-purpose base resins, which leads to prototypes having more of a display role than a functional one. Newer materials and systems overcome the problem but still some limits exist in this field – compared to some other manufacturing and 3D printing technologies, MJ printed parts have less elongation strength.
  • High costs of materials and printers. Material jetting technology is a complicated one to use, and as long as the components and materials are expensive, the process requires greater investments compared to other technologies, which already started spreading as open-source systems.

What post-processing is required?

Post-processing resin parts created with MJ printers is an important aspect of fabricating objects with these machines. Almost all parts require at least some basic steps, and for releasing the great potential of some materials, further work is necessary.

The required post-processing steps are:

Supports removal. Depending on the supporting resins used, the step will vary. For wax-like supports, removal is performed by burning it out in an oven or heating device. The easier the object’s geometry, the faster the process will be. In addition, for structures with wax-like supports trapped inside complex mazes or so, heating step can last hours.

If gel-like dissolvable supports are used, removal may start by simply scratching an object with fingers to separate the outer shells. Next, a pressurized water stream can be used. Water jet helps to remove massive gel areas and release a part – for simpler objects completely.

Next, for parts, made with wax-like supports comes the ultrasonic heated oil bath and a bath with detergent (to remove oils). Dissolvable supporting structures are processed in a bath with Sodium Hydroxide (with ultrasound for better results) and through a quick water washing. Other processes depend on the usage of the printed parts. They can be coated, painted, polished.

A special treatment, however, is required for clear parts. On top of the supports removal, the post-process includes: dry sanding and wet sanding (using soap water or oil) with several types of sanding paper with different grit, going from the highest to lowest grit. Afterwards, a plastic polishing compound should be applied and cleaned with a soft fabric. For frosted parts, a clean resin print can be sand blasted.

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