Choose the Suitable Prototyping Process

Compare CNC machining, injection moulding, and 3D printing
in materials, strengths,  advantages and disadvantages
plastic injection molding

There are many rapid prototyping processes available today, such as CNC machining, injection moulding, and 3D printing. We need consider and compare some features of each process to determine the suitable methods.

This article explores materials, manufacturability, strength, and finishing options. By comparing the advantages and shortcomings of these major prototyping processes, help you select the best prototyping process for your product development cycle.

Selecting a Process

Use the table and definitions below to determine key factors and attributes based on the prototyping stages you are in.

Prototyping Stages: Key Factors: Attributes
Concept Model Stage: Speed Quantity
Appearance Material Choice
Surface Finish
Assembly/Fit Testing Stage: Form Material Choice
Fit Material Choice
Material Stability
Functional Testing Stage: Chemical Resistance Material Choice
Mechanical Properties
Electrical Properties
Thermal Properties
Optical Properties
Life Testing Stage: Mechanical Properties (fatigue strength) Material Choice
Material Stability
Aging Properties (UV, creep)
  • Concept Model Stage: A physical prototypes made to demonstrate an idea,  allowing people to stimulate thought and discussion.

Speed: Turnaround time to turn a design file into a real product

Appearance: Visual attribute like color, texture, size, shape, etc. 

  • Assembly/Fit Testing Stage: Making assembly parts, putting them together, and checking  if they fit properly. Tolerance tests must be performed using the actual manufacturing process.

Form: The shape and  size of prototype

Fit: How well the part fits together with other components.

  • Functional Testing Stage: seeing how a part or assembly will function when subjected to stresses representing what it will see in its actual application.

Chemical Resistance: Resistance to acids, hydrocarbons, fuels, bases, etc.

Mechanical Properties: Strength of prototype like tear resistance, impact strength, tensile strength, and flexural strength, etc.

Electrical Properties:  Dielectric, dielectric strength, dissipation factor, static decay, and other parameters

Thermal Properties: such as heat deflection temperature, thermal expansion coefficient , and vicat softening point, and other factors

Optical Properties: ability to transmit light, including refractive index, transmittance, and haze.

  • Life Testing Stage: Life testing often involves subjecting the product to extreme conditions (temperature, humidity, voltage, UV, etc.) to estimate in a shorter period of time, how the product will react over its expected life.

Mechanical Properties: include the ability to endure a vast number of loads at different stress levels.

Aging Properties: ability to tolerate UV radiation exposure with a reasonable level of deterioration;

Material and Strength

SLAStandard Resin, Durable Resin,Tough resin,  Rubber-Like,   Dental SLA Materials2,500 ~ 10,000 (psi)         17.2 ~ 68.9 (mpa)
SLSNylon, TPU5,300 ~ 11,300 (psi)          36.5 ~ 77.9 (mpa)
DMLSStainless steel, aluminum, titanium, chrome, Inconel37,700 ~ 190,000 (psi)
FDMABS, PC/ABS,  PC,   PPSU5,200 ~ 9,800 (psi)            35.9 ~ 67.6 (mpa)
MJFBlack Nylon 126,960 (psi)                        48 (mpa)
PJETAcrylic-based photopolymers, elastomeric photopolymers7,200 ~ 8,750 (psi)            49.6 ~ 60.3 (mpa)
CNC mills and lathesAluminum,  Brass, Copper, Magnesium,  Stainless,  Steel,  Titanium
Plastics(Extruded Nylon, Acetal,HDPE, PTFE and Teflon, PVC)
3,000 ~ 20,000 (psi)         20.7 ~ 137.9 (mpa)
Injection MoldingMetal,  liquid silicone rubber,  and most commodity and engineering-grade thermoplastics3,100 ~ 20,000 (psi)          21.4 ~ 137.9 (mpa)
cnc milling
CNC Milling

CNC mills & lathes

CNC mill or lathe cut a solid block (or rod stock) of plastic or metal into a finished part through a subtractive process. It generally produces superior strength and surface finish to any additive manufacturing process. Good tolerances yield parts suitable for fit and functional testing, jigs and fixtures, and functional components for end-use applications.
RpProto use 3-axis milling and 5-axis indexed milling processes along with turning to manufacture parts in a range of engineering-grade plastics and metals.

More than 40 different grades of metal and plastic materials are available, such as   Metal:  Steel,  Aluminum(6061-T6, 7075),  Stainless Steel (17-4, Inconel 625 & 718),  Titanium,  Magnesium,  Zinc, Brass,  Bronze,  Copper. Plastic:  ketone (PEEK), ABS, Acetal, nylon, polycarbonate, polyvinyl chloride (PVC), high- and low-density polyethylene.


  • CNC Machined parts have full strength, because they are made of  engineering-grade thermoplastics and metals
  • CNC Machined parts have good surface finishes and  tight tolerances
  • Custom prototypes can be manufactured in as fast as one day, because of fixed tooling and preparation


  • There are geometry limitations associated with it, Beacuse CNC process is removing material and milling undercuts is hard under some cases.
  • It has higher cost than 3D printing processes sometimes.

Injection Molding

Rapid injection molding is best option to create large quantities of production parts. It works by injecting thermoplastic resins into a mold. Molded parts are in high strength with good finishes.

Almost any engineering-grade plastic or liquid silicone rubber (LSR) can be used for prototyping process. It uses the industry standard production process of plastic parts, and prototyping in the same process has inherent advantages.  RpProto often makes rapid plastic molding from aluminium, which is much faster than from steel.


  • It significantly reduce the lead times and costs to create large quantities of parts.
  • Molded parts have excellent surface finish, because they are made of production grade materials.


  • Rapid injection molding has an initial tooling cost that does not exist with 3D printing or CNC machining.
  • So, before you begin injection molding, we recommend that you perform one or two rounds of CNC machining or 3D printing to test fit and function.
Plastic Injection Molding
Injection Molding
plastic injection molding
Plastic Injection Parts
SLA min


SLA 3D printing is an additive manufacturing technique that uses a light source— either a laser or projector—to cure liquid resin into hardened plastic.
It can create high-accuracy, isotropic, and watertight prototypes and parts in a range of advanced materials with fine features and smooth surface finish.
It is best for functional prototyping, patterns, molds, and tooling.


  • SLA can produce parts with intricate geometries and  highest resolution and accuracy, fine details.
  • excellent surface finishes and  highly versatile material selection
  • Cost is competitive and the technology is available from several sources.


Prototypes parts are not as strong as those made from engineering-grade resins, so the parts made using SLA is suitable for functional testing.

Additionally, while parts undergo a UV-cycle to solidify the outer surface of the part, parts built in SLA should be used with minimal UV and humidity exposure so they don’t degrade.


Selective laser sintering (SLS) 3D printing uses a high power laser to sinter small particles of polymer powder into a solid structure based on a 3D model.

SLS 3D printed prototypes have better mechanical properties and strength comparable to injection-molded parts, but they have a rough surface and less fine details. The finished prototypes are impact-resistant and can withstand repeated wear and tear.

Selective laser sintering is prefered by manufacturers due to its design flexibility, high productivity and throughput, low cost per part, and proven end-use materials.


  • It has low cost per part,  and high productivity
  • SLS parts are more accurate and durable than SLA parts.
  • The process can make parts with complex geometries
  • It is suitable for some functional testing.


  • The parts have rough surface and lack fine details
  • The process has a limited resin choice.
SLS parts
FDM Parts


Fused deposition modelling is a technique for creating parts by melting and extruding thermoplastic filament. It works with a variety of thermoplastics, including ABS, PLA, and their mixtures.

Compared to SLA or SLS, FDM has the lowest resolution and precision, making it unsuitable for printing complicated designs or parts with intricate features. The process is ideal for quick and low-cost prototyping of simple parts, or concept prototypes.


Lowest price of entry and materials, and can be good for some simple parts.


FDM has the lowest resolution and accuracy when compared to SLA or SLS.

The parts have a poor surface finish, with a pronounced rippled effect.


Direct Metal Laser Sintering  is a technique for precisely forming complicated structures. DMLS micro-welds powdered metals and alloys using a precise, high-wattage laser to create fully functional metal components from your CAD model.

DMLS parts commonly are printed with powdered materials including aluminum, stainless steel , titanium, and niche alloys like Nickel Alloy 718. DMLS parts are stronger and denser than investment casted metal parts.


  • DMLS parts are stronger and denser than investment casted metal parts.
  • Mechanical properties parts are equal to conventionally formed parts.
  • It eable engineers accurately forms complex geometries  that could not be cast or otherwise machined. 


  • The cost will be higher, when producing more than a few DMLS prototypes.
  • Due to the powdered metal origin of the direct metal process, the surface finish of these parts are slightly rough. 
  • The process is relatively slow and also usually requires expensive post-processing.
dmls part
MJF Part


Multi Jet Fusion is one kind of powder bed fusion industrial 3D printing technology. Parts are created by thermally fusing (or sintering) polymer powder particles layer by layer. The materials used are granular thermoplastic polymers (typically Nylon).

MJF can create functional prototyping with high dimensional accuracy, smooth surfaces, and free-form geometry. Changing the type or concentration of the fusing agent can give a part varying material characteristic. The MJF lead time is very short.


Producing functional nylon prototypes and end-use production parts in as fast as one day. 

Final parts have good surface finishes, fine feature resolution, and more consistent mechanical properties compared to SLS.


Currently MJF is limited to PA12 nylon, and SLS has better small feature accuracy (small feature tolerances).


PolyJet works by spraying layers of photopolymer resin that are then cured one by one with ultraviolet light. The layers are very thin allowing quality resolution. It has excellent resolution (up to 0.016 mm), smooth surfaces, a wide choice of materials and colours for a relatively low cost and printing time. It can print multi-material and multi-color prototypes.


This process is moderately priced, can prototype overmolded parts with flexible and rigid materials, can produce parts in multiple color options, and easily duplicates complex geometries.


PolyJet parts have limited strength (comparable to SLA) and are not suitable for functional testing. While PolyJet can make parts with complex geometries, it gives no insight into the eventual manufacturability of the design. Also, colors can yellow when exposed to light over time.

Early in the design stage, concept models are helpful. As the design progresses, a prototype that has the color, finish, strength, durability of the intended product becomes increasingly important. Therefore, selecting the appropriate prototyping process is important. Functionality, manufacturability, and viability are critical factors in validating a product design.

The attributes of the end-product are represented by the functional prototype. Material characteristics, dimensional accuracy for fit-up with mating parts, and cosmetic surface finishes for appearance are frequently included in these requirements.

It is manufacturable if your prototype design can be manufactured repeatedly and affordably in a way that supports the eventual product’s specifications. These specifications include the ability to preserve the design’s functionality as indicated above, keep piece-part costs within the required level, and adhere to the manufacturing schedule.

You can only test the viability of a design by creating a prototype. You’ll be well on your way to a successful product launch if your design passes regulatory testing and market trials.

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