Rapid prototyping has changed almost every industry for the better, and companies that don’t use it soon fall behind the competition.
But rapid prototyping is a rather vague term that could refer to multiple processes.
Older forms of rapid prototyping included simple but time-consuming processes like papier-mache or cardboard modelling that nullified the word “rapid.”
Modern approaches are much more time-efficient and affordable, with far better turnaround times.
The three most common rapid prototyping techniques are Stereolithography (SLA), Fused Deposition Modelling (FDM), and Computer Numerically Controlled Machining (CNC Machining).
But these are just three of eight popular processes, each with distinct advantages, disadvantages, and ideal usage scenarios.
Let’s compare the eight standard rapid prototyping manufacturing processes.
Stereolithography
Stereolithography, or SLA, is an additive manufacturing process that involves a liquid photopolymer resin that is cured and solidified, layer by layer, using an ultraviolet laser. After the entire model has been “scanned” by the laser, it is fixed in an ultraviolet oven to maintain its final form.
SLA has many advantages, including being accurate, relatively affordable, and freely available. The models also have smooth and detailed textures, making it easy to paint or add different finishes. You can add a lot of fine detail to SLA-printed models, which you can’t do as well with many other processes. The result is also heat resistant to an extent.
On the downside, SLA doesn’t create the strongest of models. It’s not the ideal solution if the rapid prototype needs to handle a lot of strain, and UV light and moisture can also degrade its quality.
Stereolithography printers can be hefty since they have tanks to hold the photopolymer resin. The size has an added advantage since the larger your printer, the bigger the models you can print.
Fused Deposition Modelling
FDM is the most common and popular 3D printing technology because of its simplicity and affordability. It’s the rapid prototyping technology most commonly taught at schools and most often used at home for business or pleasure.
The process involves thermoplastic filament, usually bought on a spool, that gets fed into a nozzle where it’s melted and extruded onto a printing bed in layers. These layers build the model from the bottom up.
FDM has several benefits, not least of which is its cost. The machines and filaments are exceptionally affordable compared to other 3D printing technologies. They are also relatively fast, and the building quality is decent.
Unfortunately, FDM is not a good solution for prototypes that require high durability. The textures can also be a bit rough, but you can tweak many settings to improve this. Another potential drawback is that you must prepare support structures, where needed, for FDM to work correctly since molten plastic will sag if there’s nothing to support it.
CNC Machining
CNC machining, or Computer Numerically Controlled machining, is another typical rapid prototyping technique that’s often used, but it works oppositely from most others. Instead of building a model using various materials, it starts with a base material and uses a subtractive method to remove parts of it.
CNC starts with a rod stock (a fancy way of saying “solid block”) of either plastic or metal. The block is clamped into a CNC lathe or mill, which then moves around three axes to remove layer after layer of the stock material to manufacture the model.
Manufacturers often use CNC to add finishing touches to models manufactured using other additive methods, but it’s also possible to create a model entirely using only CNC processes.
CNC models are remarkably strong because they aren’t formed using layers. You start with a solid block of whichever material you’re using, so the material will retain its structural integrity for the most part. The results can also look highly polished, or you can add fine details by hand after the process.
Selective Laser Sintering
SLS, or Selective Laser Sintering, is similar to SLA because it uses a laser to 3D print your rapidly prototyped models. Unlike SLA, though, it uses a CO2 laser to do this instead of a UV laser. SLS also uses resin in a powdered form rather than liquid.
The CO2 laser “draws” your model in the powder layer by layer, fusing the powder into a solid unit (a process called “sintering”). The model doesn’t require baking in a UV oven after it’s finished.
SLS creates very sturdy models. Some engineers use SLS to manufacture replacement parts for larger machines rather than just models. The plastic parts can be fully functional and have isotropic mechanical characteristics. There’s a lot of fine detail in the prototypes.
However, the finish isn’t smooth, and your prototype will have a rough surface with no transparent options. Material is also more challenging to find than other 3D printer materials. But the process of SLS is so effective that it gave rise to other rapid prototyping processes that use different materials but the same method as SLS.
Direct Metal Laser Sintering
DMLS stands for Direct Metal Laser Sintering. It works on the same principle as SLS, but instead of using resin powder, it uses powdered metal. You can use DMLS to make prototypes using aluminum, stainless steel, titanium, Inconel, maraging steel, or cobalt chrome. A high-powered laser beam fuses the powder to “draw” your model one layer at a time.
Because it uses actual metal, DMLS is perfect for heavy-duty and final-prototype projects. This is ideal if you want a model that can withstand lots of strain and would be suitable as a production unit. People have used DMLS to 3D print many models, including a titanium electric guitar.
As can be expected, DMLS isn’t cheap. The machine is most practical for industrial use since that’s the only arena where the price can be justified. The materials can also be quite expensive, and it is relatively slow. To get the most out of your model, you should post-process it using a method like hot isostatic pressing.
However, DMLS can make models that would be impossible to machine or cast, and you can print with almost any metal alloy.
Multi Jet Fusion
MJF, or Multi Jet Fusion, is a process that produces relatively strong functional prototypes similar to those made through SLS but much faster. MJF printers can use a range of materials, notably TPUM95A, a material suitable for most rapid prototyping needs.
More than just rapid prototyping, MJF is also great for limited production purposes. The machine can produce up to 250 small to medium-sized models or products daily.
There are a few steps in the MJF process. First, the machine uses powder to draw the first layer of the design. Then a fusing agent is sprayed over the powder, after which a fuser passes over it to fuse the powders. When the model is complete, it is bead blasted to remove excess powder, then dyed if needed as the parts come off the machine grey.
PolyJet (PJET) Prototyping
PolyJet is an additive manufacturing technology similar to FDM and SLA in some ways but also unique in many aspects. The method uses a print head that sprays layers of photopolymer resin onto a print bed, followed by a UV light to cure it. The model is then built up from the bottom one layer at a time.
Layers can be as thin as 0.032mm, which is mind-blowingly thin, allowing for insane resolutions and fine details in your prints. Unlike SLA, a PolyJet model is already cured when it’s done, and there’s no need to cure it further in an ultraviolet oven.
These aspects make PolyJet the perfect rapid prototyping process for highly detailed models, but the material isn’t very strong. You shouldn’t use the models in any functional testing scenarios. This means the most common use of PolyJet is to make a model mainly for display or visual purposes, like an architect’s model.
Rapid Injection Moulding
Injection moulding has been a go-to manufacturing method for decades, but the development of rapid injection moulding has led to it becoming a viable rapid prototyping process.
Rapid injection moulding (IM) works by injecting thermoplastic resins into a pre-made mould, giving it its shape. IM has never been seen as a rapid prototyping solution since it wasn’t practical to make and change moulds too frequently since the moulds are made out of stainless steel, making the mould-making process slow and expensive.
Rapid IM is slightly different since it uses aluminium moulds instead of stainless steel. This makes the moulds cheaper and easier to manufacture or change, if possible, as the need arises.
However, rapid IM is still not something that most businesses would do on-site. It’s a highly specialised field, and the fact that it’s faster and cheaper than standard injection moulding doesn’t make it fast or cheap compared to 3D printing, for example.
Injection moulding is usually one of the final steps in the prototyping process after sizes and shapes have been confirmed through 3D printed or CNC machined prototypes.
Conclusion
Rapid prototyping is a vital aspect of modern product development, with different processes to suit different needs.
Each process has its own advantages, disadvantages, and ideal usage scenarios.
It is essential to understand and evaluate these processes, based on the specific requirements of your project and the resources available, to make an informed decision when choosing a prototyping process.