3D Printing

3D printing, or additive manufacturing, involves parts being created layer-by-layer using materials such as plastic filaments (FDM), resins (SLA/DLP), plastic or metal powders (SLS/DMLS/SLM).

Additive manufacturing is the formalized term for what used to be called rapid prototyping and what is popularly called 3D Printing. The term rapid prototyping (RP) is used in a variety of industries to describe a process for rapidly creating a system or part representation before final release or commercialization.

The advantages of 3D printing include its freedom of shape, applications in many sectors, accuracy, speed, and ability to cut costs and weight in parts.

Subtractive vs Additive Manufacturing

The key difference between 3D printing and CNC machining is that 3D printing is a form of additive manufacturing, whilst CNC machining is subtractive. This means CNC machining starts with a block of material (called a blank), and cuts away material to create the finished part. To do this, cutters and spinning tools are used to shape the piece. Some advantages of CNC machining include great dimensional accuracy as well as many compatible materials, including wood, metals and, plastics.

The Generic AM Process;

 AM involves several steps that move from the virtual CAD description to the physical resultant part. Different products will involve AM in different ways and to different degrees. Small, relatively simple products may only make use of AM for visualization models, while larger, more complex products with greater engineering content may involve AM during numerous stages and iterations throughout the development process. Furthermore, early stages of the product development process may only require rough parts, with AM being used because of the speed at which they can be fabricated. At later stages of the process, parts may require careful cleaning and post-processing (including sanding, surface preparation, and painting) before they are used, with AM being useful here because of the complexity of form that can be created without having to consider tooling. Later on, we will investigate thoroughly the different stages of the AM process, but to summarize, most AM processes involve, to some degree at least, the following eight steps - 

• Conceptualization and CAD 
• Conversion to STL/AMF 
• Transfer and manipulation of STL/AMF file on AM machine 
• Machine setup 
• Build 
• Part removal and cleanup 
• Post-processing of part 
• Application

Step 1: CAD

 All AM parts must start from a software model that fully describes the external geometry. This can involve the use of almost any professional CAD solid modeling software, but the output must be a 3D solid or surface representation. Reverse engineering equipment (e.g., laser and optical scanning) can also be used to create this representation.

Step 2: Conversion to STL (Standard Tesselation Language)

Nearly every AM machine accepts the STL file format, which has become a de facto standard, and nowadays nearly every CAD system can output such a file format. This file describes the external closed surfaces of the original CAD model and forms the basis for the calculation of the slices.

Step 3: Transfer to AM Machine and STL File Manipulation 

The STL file describing the part must be transferred to the AM machine. Here, there may be some general manipulation of the file so that it is the correct size, position, and orientation for building.

Step 4: Machine Setup 

The AM machine must be properly set up prior to the build process. Such settings would relate to the build parameters like the material constraints, energy source, layer thickness, timings, etc.

Step 5: Build 

Building the part is mainly an automated process and the machine can largely carry on without supervision. Only superficial monitoring of the machine needs to take place at this time to ensure no errors have taken place like running out of material, power or software glitches, etc.

Step 6: Removal 

Once the AM machine has completed the build, the parts must be removed. This may require interaction with the machine, which may have safety interlocks to ensure for example that the operating temperatures are sufficiently low or that there are no actively moving parts.

Step 7: Post-processing 

Once removed from the machine, parts may require an amount of additional cleaning up before they are ready for use. Parts may be weak at this stage or they may have supporting features that must be removed. This therefore often requires time and careful, experienced manual manipulation.

Step 8: Application 

Parts may now be ready to be used. However, they may also require additional treatment before they are acceptable for use. For example, they may require priming and painting to give an acceptable surface texture and finish. Treatments may be laborious and lengthy if the finishing requirements are very demanding. They may also be required to be assembled together with other mechanical or electronic components to form a final model or product.

AM Classification

All AM processes fall into one of seven categories; The seven process categories are presented here-

  • Vat photopolymerization: processes that utilize a liquid photopolymer that is contained in a vat and processed by selectively delivering energy to cure specific regions of a partial cross-section. 
  •  Powder bed fusion: processes that utilize a container filled with powder that is processed selectively using an energy source, most commonly a scanning laser or an electron beam. 
  • Material extrusion: processes that deposit a material by extruding it through a nozzle, typically while scanning the nozzle in a pattern that produces a part cross-section.
  • Material jetting: ink-jet printing processes.
  • Binder jetting: processes where a binder is printed into a powder bed in order to form part cross-sections.
  • Sheet lamination: processes that deposit a layer of material at a time, where the material is in sheet form.
  • Directed energy deposition: processes that simultaneously deposit a material (usually powder or wire) and provide energy to process that material through a single deposition device.

Benefits of 3D Printing-

1.) Ease of Use

3D printing is known to be simpler to use than CNC machining. This is because once the file is prepared, you just need to choose the part orientation, fill, and support if needed. Then once printing has commenced, no supervisor is required and the printer can be left until the part is finished. Post-processing requirements depend on technology.

However, CNC machining is a far more labor-intensive process. A skilled operator is required to choose between different tools, rotation speeds of tools, the cutting path, and any repositioning of the material the part is being created from. There are also post-processing techniques that can require more time.

2.) Accuracy, Size limitations & Geometric Complexity

Though there are a number of 3D printing technologies, we have chosen to compare CNC machining with SLS, industrial (not desktop) FDM, and DMLS metal 3D printing. In terms of tolerance, CNC machining is superior to all 3D printing processes, even DMLS. However, with minimum layer thicknesses, the superiority is not so profound and is not as precise as DMLS.

Powder bed fusion 3D printing processes such as SLS and DMLS are limited in their build volume, however. Even technologies with larger build volumes such as FDM cannot compete with CNC in terms of the maximum size of parts.

3D printing is well-known for its advantages in creating parts with high geometric complexity. Though supports are required for some technologies, 3D printing can create parts with geometries that no traditional manufacturing method can replicate. Technologies such as SLS and Multi Jet Fusion by HP can even do this without any support structures.

3.) Cost

Though on average 3D printing is cheaper, costs depend on how many parts are required and how quickly you need them. For larger quantities (higher double digits to 100s) CNC is likely to be more appropriate. For low volumes, 3D printing is more appropriate and lower-cost. 3D printing is also more appropriate if you need your prototypes or parts very quickly. There are additional factors however that make comparing the two technologies more difficult. These include materials varying in cost (from cheap materials like ABS to materials that can cost $500/kg like PEEK), and repairing and changing machinery (such as CNC heads).

4.) Environmentally Friendly

Since CNC involves cutting material away from an original block, there is always going to be a mess afterward. These pieces of material need to be cleaned afterward and disposed of, which isn’t required from 3D printing. Since additive manufacturing forms an object on the build platform from material fed into the machine, there is no mess except supports (if used). This makes 3D printing the more ethical of two methods as there is a less unused waste.

5.) Materials

In 3D printing, commonly used thermoplastics include ABS, PLA, Nylon, ULTEM, but also photo-polymers such as wax, calcinable or biocompatible resins. Some niche 3D printers also allow for the printing of parts in the sand, ceramics, and even living materials. The most common metals used in 3D printing include aluminum, stainless steel, titanium, and Inconel. It is also worth noting that to 3D print metal, expensive ($100,000+) industrial machines are required. Some materials such as superalloys or TPU (flexible material) cannot be created with CNC, so must be used with 3D printing or rapid tooling technology.

References: -

Additive Manufacturing Technologies 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing; by Ian Gibson, David Rosen, Brent Stucker.

Thank you.