Guide To 3D Printing Rapid Prototyping

Stereolithography (SLA)

Stereolithography, abbreviated as SLA, is an additive process that takes a 3D CAD model and converts that digital model into slices as thin as 25 microns. The formation of the physical 3D model is a process of photopolymerization of liquid resin which reacts to a laser-focused beam that bonds the molecules together which cures to form a very specific solid shape.

Here’s an SLA printer, the first of our featured 3D printing technologies, in action:

Image source: www.3dnatives.com

The laser beam moves in the X-Y coordinates of the slices generated from the digital model, each time a slice has been cured a table drops down by the slice thickness and the laser completes the next slice. By repeating this cure and drop process within a vat of the liquid resin, the final three-dimensional part is produced with very accurate details.

Most SLA parts need to be cleaned and then have an element of post-finished applied. This could include surface finish smoothing, fine detailing, additional curing, and painting.

 

The SLA Printing Process

SLA printing process utilizes a vat of liquid resin and a laser to cure the resin in a very controlled and precise manner. The laser follows a defined path at the bottom of the vat of liquid resin that has been generated from slicing the 3D model into fine layers. Once the laser has completed a layer, the model is raised by the slice thickness, this is repeated until the part has been completed.

 

Advantages and Disadvantages of SLA

Advantages

• Early visibility of prototype designs throughout the product development.
• Minimal material waste.
• Very low production quantities.
• High definition and accurate dimensions achievable.
• Both rigid and flexible parts can be produced.
• Cost-effective part production.
• Customized colors are possible.
• Multiple part production possible in one manufacturing process (one process time can produce multiple parts depending upon what can fit inside the vat and extremities of the laser head movement).

Disadvantages or Limitations

• Depending upon the resin, parts may become unstable and brittle over time.
• Resin is affected by moisture, heat, and chemicals.
• Post-processing can be time-consuming and messy (dust is generated when sanding parts to remove support features for example).

 

Typical Applications for SLA

SLA’s are used for the following:

• Fit and Form testing
• Proof of concept prototypes
• Design change verifications
• Product development iteration prototypes
• Investment casting master models
• Vacuum casting master models
• Scale models and exhibition demonstration units

Examples of SLA Prototype parts

Some of the typical industries that utilize SLA models for their prototype testing are:

• Aerospace
• Agriculture
• Automotive
• Construction
• Dental
• Electronics
• Firearms
• Military
• Transportation

---

 

Selective Laser Sintering (SLS)

Selective Laser Sintering is the process of bonding together particles of plastic, glass, or ceramic with heat from a high-powered laser which forms a three-dimensional part. The laser heats the particles to just below boiling point, which is known as sintering, or to the point just above the material boiling point which is known as melting. It is this sintering or melting of the material that fuses the particles to form a solid in this 3D printing technology.

Here’s an SLS 3D printer:

The most common material used for SLS printing is a polyamide (PA) or commonly known as nylon. Two grades are used more than any other, they are PA11 and PA12, both of which are excellent general engineering plastics. The PA11 has better impact resistance and flexibility properties, whereas PA12 is better when it comes to strength and abrasion-resistant properties.

The mechanism by which the 3D models are produced is very similar to SLA, instead of the vat of liquid resin, there is a vat of the appropriate powder. The laser traces the slice from the digital model and once the initial layer of powder has been bonded together, the table drops by the slice amount, 0.1mm for example. The process continues until the three-dimensional solid part is completed.

Due to the powder particles being self-supporting, SLS parts do not need any additional supporting structures as with the SLA process for example.

 

The SLS Printing Process

Selective laser sintering utilizes vats of powdered polymer, typically nylon, and a laser to heat the powder particles to the point they fuse together in a precise formation. Once a layer has been created, the platform in the vat with the part created moves down the thickness of a slice or layer. A roller or blade then moves from one side to the other depositing more powder into the build vat where the laser can create the next layer of the part. This process is repeated until the part is completed. A simple graphical representation of this is shown below:

 

Advantages and Disadvantages of SLS

Advantages

• No support structures – as mentioned above, the powder particle supports the solid object as it is being produced, therefore complex structures with overhangs and thin sections are reproducible.
• Fast model building – due to the polymer powder used and the quick bonding time between particles that have been subjected to the laser, SLS printing is a very quick and efficient process.
• Excellent mechanical properties – because SLS parts have virtually isotropic mechanical properties, they are excellent in tensile strength, hardness, and elongation and are almost equal in all directions.
• Easily colored – due to the physical structure of the SLS solid model, the particles create a porous surface, which is prone to moisture intake. This moisture intake aspect is the very feature that allows SLS parts to be dipped in liquid dyes allowing for part color modification. If the part is to be used or exposed to moisture conditions, a waterproof coating may be required.

Disadvantages or Limitations

• Porous and brittle – SLS printed parts are, by nature of the process, porous as the powder particles are not fully melted to form a completely solid object. The form has natural voids that reduce the structural integrity of the printed part and reduce the mechanical properties of the model. It best to avoid testing SLS models where there is a chance the printed part could fracture. The diagram below shows how the SLS model is full of voids.
• Shrinkage and warping – the powder used in SLS need to be subjected to high temperatures for it to go through the sintering process. This also means the solid finished layer that has just been printed needs to cool down which could result in shrinkage. If the shrinkage is not uniform it could result in the part warping out of shape and dimensional tolerance. Shrinkage and warping issues can be overcome with design considerations including applying a material-specific shrinkage percentage rate to the digital model and ensuring no large flat areas are printed (these are most prone to warping).
• Cleaning – When the printed part is finished, it needs to be removed from the vat of powder. The part will generally be surrounded by excess material that that needs to be removed to expose the finished printed part. The best way to remove all the additional powder is with compressed air, this process can get messy and needs to be done in an airtight cleaning chamber to avoid powder particles from getting everywhere.
• Powder not reusable – For the laser beam to sinter the powder particles quickly and efficiently, the initial batch of powder needs to be pre-heated. This pre-heating process changes the molecular structure of the particles thus using the powder more than once will result in brittle and unstable parts. At best it is recommended that any batch of SLS powder should have a maximum of 50% recycled powder.

Typical Applications for SLS

SLS models are commonly used for the following:

• Fit and Form testing
• Design verification and iteration proofing
• Architectural models from part build to complete village layouts and skyscrapers
• Wind tunnel testing models
• Short-run production models
• One-off complex component
• Investment casting master models
• Vacuum casting master models

Examples of SLS Prototype parts

Some of the typical industries that utilize SLS models for their prototype testing are:

• Aerospace
• Agriculture
• Architecture
• Art
• Automotive
• Construction
• Dental
• Electronics

Here’s an example of a prototype part made by SLS printing:

Image source: www.accutech

---

 

Fused Deposition Modeling (FDM)

Fused Deposition Modeling (FDM) is a process where a part is built up by adding layers of melted polymer in pre-determined areas like the slices or layers we talked about in the SLA and SLS process. The polymer, in this case, is a filament that is extruded out of a pre-heated nozzle which is at a temperature that melts the polymer.

You can see an FDM printer below:

Each layer of the part to be produced is generated from the 3D CAD model and the layer is a digital footprint that the print head follows. As the melted filament is deposited onto the previous layer, it is pressed down to form an oval shape which has the effect of melting the surface of the previous layer, thus bonding the two layers together.

As each layer is formed the machine table will drop down the pre-determined amount, generally 75% of the filament thickness. This is repeated until the part is completed.

As the part is produced in free space, support structures are required during the build. These supporting structures can either be from the same material or a different material, in which case this alternative material can be a soluble material than dissolves on a liquid after the part has been completed. If the support material is the same as the main part, post finishing will be required to trim and remove these supporting features.

 

Materials used in FDM Printing

There are a multitude of materials available today for printing 3D models utilizing the FDM technology. The material selection will be highly dependent upon the part specifications and properties the part is designed for. The table below shows some of the popular polymers available and their key characteristics:

MATERIAL	KEY CHARACTERISTICS
ABS	Acrylonitrile Butadiene Styrene has high impact resistant properties, it’s tough and exhibits good overall engineering polymer properties.
PLA	Polylactic Acid is an organic material used to generate FDM filaments, the finished parts in PLA offer a high gloss surface perfect for non-functional test models.
PET	Polyethylene Terephthalate commonly used in the production of drinking bottles is used to produce items such as utensils, cups, and food storage container with the FDM process
PA	Polyamide or Nylon offers excellent flexibility, durability, wear-resistant, and strength properties and is a very popular material for FDM printing.
PVA	Polyvinyl Alcohol dissolves in water and is perfect for building support features on parts that have overhangs. It is also biodegradable
HIPS	High Impact Polystyrene dissolves in a solution of liquid hydrocarbon and is perfect for building support features on parts that have overhangs. It is also biodegradable
CARBON FIBER	Carbon fiber filament of a blend of PLA and carbon fiber strands. This material has very good layer adhesive properties which result in high quality dimensionally accurate hard-wearing parts.
TPE	Thermoplastic elastomer filaments produce flexible parts, just like rubber molded parts. This is perfect for printing gaskets, or any part that needs elastomer characteristics.

There are more materials available in filament form and the list will grow over time as new materials get developed and introduced into the market.

 

The FDM Printing Process

Fused deposition modeling utilizes a polymer filament which is extruded through a heated nozzle which melts the polymer as it is laid in a precise pattern derived from software which has sliced the original 3D CAD model into fine layers. The nozzle lays the molten polymer filament down to create the desired part. A simplified graphical representation of this is shown below:

 

Advantages and Disadvantages of FDM

Advantages

• Most used – FDM is one of the most known and used 3D printing methods available for both everyday model makers as well as consumer prototype users.
• Low cost – The low cost of entry as printing machines are nowhere near as complex as other prototyping technologies.
• Ease of use – Simple to use and user-friendly machines and software allows virtually everyone to have access and create 3D printed models.

Disadvantages

• Low print quality – The print quality of FDM 3D prints are not as good as those by SLA or SLS.
• Slow print speed – 3D printing with fused deposition modeling is slow. This makes it unusable in some industries when large numbers of parts are needed quickly.
• Warping and shrinkage – The layer-by-layer printing in FDM can sometimes lead to problems with warping and minor shrinking.
• Large layer height – The layer height is determined by the filament thickness and is generally larger than a layer in SLA or SLS printing.

 

Typical Applications for FDM Prototypes

FDM models are commonly used for the following:

• Fit and Form testing
• Low volume production runs
• Engineering and concept models
• Functional testing of products
• Design change verifications
• Product development iteration prototypes

Examples of FDM Prototype parts

Some of the typical industries that utilize FDM models for their prototype testing are:

• Medical – pre-surgical models
• Architecture
• Art
• Automotive
• Construction
• Hobbyist
• Film – costumes and props

You can see another example of a small-scale FDM printer creating a part below:

---

 

Direct Metal Laser Sintering (DMLS)

The last of the 3D printing technologies featured here is direct metal laser sintering (DMLS). This is a process that partially melts metal powder particles for them to bond together which forms a completed metal part.

You can see a DMLS printer interior with its metal part being created below:

These metal sintered parts can be used in all stages of the prototype phase as well as in final product assemblies. This provides fully functional testing to be carried out in the same material as the production components.

The process starts by generating slices from the 3D CAD model, these slices are the path the laser takes that melts the metal powered particles to create a sold surface. The metal powder is held in a build chamber where the high powered laser traces the slice path to fuse the metal particles. Once a layer is complete the table drops by a predetermined amount which is generally in the range of  15 to 50 micrometers. This process is repeated until the final part has been completed.

 

Materials used in Direct Metal Laser Sintering

There are several metals currently available for the DMLS process, they are; aluminum, bronze, alloy steel, titanium, stainless steel, tool steel, and cobalt-chrome. There will be more materials available as powder technology advances.

 

The DMLS Printing Process

Direct metal laser sintering is a similar process to that of SLS where the latter’s vats of powder are now replaced with metal powder particles. The part being printed is in the build chamber where the platform lowers by a very small amount which is the slice dimension. The metal powder is moved from one side of the build chamber to the other via a roller or blade which deposits the powder into the build chamber ready for the laser to create the next layer of the part. A simplified graphical representation of this is shown below:

 

Advantages and Disadvantages of DMLS

Advantages

• Reduced lead-time to production – With the utilization of DMLS, you can test, validate and verify the fit, form, and function with production materials during the prototype stage, this reduces the development time and allows you to get to market quicker.
• Part accuracy – With the metal powder particles being just 0.00059 inches (15 microns), the slices of the part are very fine. This level of granularity provides very good accuracy and finished part tolerances.
• Strength – DMLS parts are just as durable as traditionally manufactured metal parts, however, it is advisable to carry out specific tests to determine part strength to compare against your final part requirements and product specifications.
• Complex geometry – This process offers virtually unlimited design and builds capability from a complexity point of view.
• Multiple part creation – As long as you can fit multiple parts into the manufacturing chamber, they can all be produced at the same time. This reduces the production time to get final products.

Disadvantages

• Size limitation – The manufacturing chamber of DMLS machines has a limited size constraint, therefore the part size is determined by this factor.
• Expensive – The metal powders are expensive compared to a solid billet of metal, for example, stainless steel 316L powders costing $350-450/kg.

 

Typical Applications for DMLS Prototypes

DMLS models are commonly used for the following:

• Fit and Form testing
• Low volume production runs
• Engineering and concept models
• Functional testing of products
• Design change verifications
• Product development iteration prototypes
• Production parts
• Complex geometry where tooling or machining can not achieve the designed part

Examples of DMLS Prototype parts

Some of the typical industries that utilize DMLS models for their prototype testing are:

• Medical
• Automotive
• Aerospace
• Prosthetics
• Dental work
• Tooling
• Jigs and fixtures

Take a look at a dental development utilizing Direct Metal Laser Sintering below:

---

 

3D Printing Technologies Compared

If you are looking to invest in your own 3D printing technology, there are different elements to consider before making that decision, here we have put some tables together to make it easy to compare the 3D printing rapid prototyping technologies that are available.

 

The Cost of 3D Printing Machines

As you can see, the entry-level 3D printing system is fused deposition modeling (FDM) with the machines and the raw material being very affordable, however, there are limitations with this process as you can see later in the article. The top-end method is clearly direct metal laser sintering (DMLS) with machine costs being up to half a million dollars and the metal powder around four times the cost of polymer powders. However, DMLS does provide production metal parts that cannot necessarily be produced by any other method.

3D Printing Material Cost Comparison

3D Printing Process Comparison

Here we’re showing a comparison of the different 3D printing technologies we are examining with respect to cost, ease of use, what the precision of the finished parts are like, and how easy it is to obtain the desired surface finish (smooth or specific texture finish).

Rating Key:

• Cost (1⭐️ = high cost)
• Ease of use (1⭐️ = difficult)
• Precision (1⭐️ = low precision)
• Surface texture (1⭐️ = less choice)

 

Example of the same part being produced by the different 3D printing technologies

So, if the same part were to be produced using the different 3D printing technologies we are looking into, we come up with the following comparison: 

Rating Key:

• CAD/CAM (1⭐️ = difficult)
• Machine preparation (1⭐️ = difficult)
• Print time (1⭐️ = longer)
• Post finishing (1⭐️ = more finishing required)
• Part cost (1⭐️ = more expensive)

Each of the elements we compared allows for an overall rating of the printing methods, this rating is shown in the graph below:

The elements we compared were:

1. How easy it is to take the 3D CAD model and convert that into software that generates the slices to create the 3D printing layers.
2. How easy it was to prepare the machine for printing.
3. How long it takes to print a part, we campared times for printing the same part with the different technologies.
4. If there were any post printing processes and how eay it was to carry out the post finishing.
5. Overall part cost

The ratings went from 1 Star for poor or difficult to 5 Star to very good or easy.