Rapid Prototyping

When to use Rapid Prototyping?

A key attribute of rapid prototyping is its quick turnaround time. This means parts can be fabricated quickly and gives the design process flexibility to iterate and continuously improve the design rather than being limited to one design stage. Rapid prototyping can eliminate the need for third party involvement when manufacturing parts, which reduces production time. In addition to simply prototyping parts as part of an iterative design process, rapid prototyping allows for more creativity and freedom in the design process as it can produce parts with excellent precision, and does not have some of the limitations involved with more traditional methods [2].

Drawbacks

Rapid prototyping is an excellent way to model systems, however complex functionality can be hard to achieve. This is due to limitations in material choices, which affects factors like strength, durability, and finish. For example, in FDM 3D printing, failure along the direction of printed layers is high, and this results in increased difficulty when attaching fasteners like screws and bolts. It also typically costs more to manufacture parts as rapid prototyping technology is new and continuously evolving as opposed to more traditional established processes [2].

Additive vs. Subtractive Manufacturing

A finished part can be produced through either additive or subtractive manufacturing. An additive manufacturing processing involves the continuous addition of small amounts of material to a part to build it up and form a finished product. Subtractive manufacturing produces a finished product by doing the opposite: material is removed from a larger block to shape it into a part or component. 3D Printing and Laser Cutting are two common rapid prototyping processes, and are excellent examples of additive and subtractive manufacturing respectively.

3D Printing

3D Printing is a process where physical objects are manufactured by adding layers of material together [3]. This is performed in a variety of processes and usually begins with the design of a 3D computer-aided design (CAD) part that undergoes a "slicing" process. The design is often saved as a .STL file that contains the geometry of the part. This slicing separates the 3D object into a specific geometry file that contains layers and toolpaths that the printer reads to print. 

Material Extrusion

Fused Deposition Modelling (FDM) 

Fused Deposition Modelling is one of the most popular 3D printing processes, largely due to its low cost and easy accessibility to the public. Large databases of printable CAD models have grown over the years, thus enabling even those with minimal experience to explore FDM printing. 

FDM printing is done by extruding heated material through a nozzle onto a bed which holds the final product. Material is extruded in layers and builds upwards. In order to support the printing material while it is still hot and easily deformed, support material is printed simultaneously to provide the part some structural stability while it prints. This support material can be broken off easily once cooled, or the part can be placed in a solution that dissolves only the support material. Typically, nozzle movement can be grouped into two groups: cartesian and delta. Cartesian printers use rails in the X, Y, and Z directions to allow the nozzle to move to specified coordinates, while delta printers use three longer arms that independently move the nozzle. The print bed can either be heated on non-heated, depending on the type of material used. Heating is usually required when materials are at risk of warping. Print spaces can either be enclosed, or open air, usually depending on the material used for printing. High material melting points require enclosed spaces otherwise there is a large risk of warping. Warping can occur when the correct temperatures aren't maintained throughout the print, leading to the material cooling unevenly and causing shrinkage where it cools.

Vat Photopolymerization (VP) (Resin Printing)

Vat photopolymerization is a frequently used 3D printing method, where liquid resin is cured using various light sources. The material used is typically a thermoset plastic (rather than thermoplastic) - meaning once the resin is cured it will not be able to return into a liquid state. Two common VP processes are Stereolithography (SLA) and Digital Light Processing (DLP), with SLA being much more widely used. The main difference between the two processes is the type of light source used [3].

Stereolithography (SLA)

Stereolithography uses a UV laser to cure photopolymer resin layer by layer. A build platform sits in a vat of uncured liquid resin and can move upwards and downwards. A layer of resin (thickness of the specified printing layer) is spread over the platform and a laser scans this platform to photopolymerize and solidify it. The surrounding liquid resin maintains a cooler temperature so that it stays liquid while the laser scans it. The laser's position is controlled by a scanning mirror that specifies and directs the laser to the coordinates of the areas it needs to hit. After solidification, the platform moves in layer-thick increments for the subsequent curing of the next layer [5]. The layer thickness of SLA printed parts ranges from 25 to 100 microns [6].

Powder Bed Fusion (PBF)

Powder Bed Fusion is similar to Vat Photopolymerization as it also uses a light source to print parts, however this light source fuses powder particles together, rather than curing liquid resin.  This process produces strong, well finished parts that can be designed with low constraints, and is widely used in industrial settings due to these benefits. PBF can also be used to print with material other than polymers, such as metal. The most popular method within this family is Selective Laser Sintering (SLS) [7].

Material Jetting (MJ)

Material Jetting is often compared to 2D inkjet printing technologies due to the similarities between the two processes. This process uses photopolymer resin that is sprayed onto the built platform. The material is sprayed only in areas that are to be cured - once they are sprayed down, a curing lamp solidifies the layer and this process is repeated until the part is complete [10].

Material Jetting can print with multiple materials (simultaneously, separately, or mixed together) which allows for diverse material options, colours, and finishes. The layer height of MJ parts ranges from 16-32 microns, which is significantly smaller than other 3D printing methods. Support structures are always required, and are printed with a secondary material that is dissolvable. Material jetting is very precise and produces highly accurate parts (dimensional tolerance is typically +/- 0.1%) and chances of warping are very low. [11]

Binder Jetting

Binder Jetting works in a similar way to material jetting, however instead of the actual powder particles being fused together, a binding agent is used to join particles together. Binder jetting can print parts made out of various ceramics such as sand, metals, and metal alloys. A roller spreads a layer of material over the build platform, and a binding agent is sprayed onto designated areas that need to be fused. The binding agent acts as a glue for the particles, and usually does not require heat to do this. Unfused powder used for binder jetting is also completely reusable once removed from the printed part [12].

Binder jetting is a relatively inexpensive method to print with, and offers large build spaces that allow for big and complex parts. Metal parts printed this way have a higher porosity compared to DSLM and SLM which significantly reduces mechanical properties. Layer heights range from 35-50 microns for metal parts, and 100-400 microns for other materials. Parts do not require support material as the unfused powder acts as support, similar to SLS printing. This allows for freedom in design, especially for metal parts [13].

Laser Cutting

Laser cutting uses high intensity beams of light, lasers, to precisely cut parts and etch patterns from sheets of material. The laser heats a very small amount of material to burn or melt it away. This process is repeated over several passes to separate a part from the main material board. However, a few passes can be used to engrave or mark text and pictures onto pieces. Engraving a piece involves removing some surface material to create a cavity in the part while marking a piece only changes the colour at the surface [14].      

Laser cutters are quick and cheaper than CNCs but due to the burning of materials, can produce harmful gas byproducts. Cutting requires constant supervision to ensure fires do not start during the cutting process. Also due to the 2D nature of the technology, there is a limit to the thickness of the material that can be cut for a given machine [14].