The Definitive Guide to Fabrication

While manufacturing encompasses the broad overall picture of turning raw inputs into a finished product ready to sell to a consumer, fabrication is slightly more focused. Fabrication deals with the making of components and tooling that are ultimately used to build the finished product and is a critical piece of the overall manufacturing process. It is important to understand the advantages and disadvantages of each fabrication method to ensure that the part is being built in the most efficient way possible. Fabrication starts at design; designers must weigh a variety of factors including: cost, speed, and quality. Some fabrication methods can hit tighter dimensional tolerances, produce higher volumes, create more complex features and utilize different materials. So why should one process or processes be chosen over the other?

5 Fabrication Methods

Fabrication can broadly be broken down to 5 categories, with dozens upon dozens of sub-processes for each method:

  1. Machining: Drilling, Turning, Milling
  2. Molding: Casting, Injection Molding, Rotational Molding
  3. Forming: Brake Form, Hydroform, Drop-Hammer Form, Roll Form
  4. Joining: Welding, Bonding, Fastening
  5. Additive: FDM, SLM, SLS, SLA, Binder Jetting

What are the advantages and disadvantages of each? When should each be used and when can methods be combined?

Machining

Machining is a subtractive process that commonly utilizes a computer numerical controlled (CNC) cutting tool to remove material from a solid block to create an object. CNC Machining is utilized for parts that require tight tolerances at medium volumes (dozens to hundreds). Machining also has excellent dimensional accuracy with the ability to hold tolerances of +/- 0.005 or better. There are several advantages and disadvantages to consider during the design and sourcing process.

  • Advantages
    • Ability to hold tight tolerances
    • Wide range of materials; almost any solid material can be machined.
    • Excellent mechanical properties
    • Automated, computer controlled.
    • Low labor cost to operate.
  • Disadvantages
    • Higher complexity directly translates to higher cost.
    • Internal geometric features are impossible for the tool to reach.
    • Low economies of scale for high volume orders
    • Large amount of wasted material

Molding

Molding is a process that forces a molten material of either plastic or metal into a mold to create a part. Injection molding is common for plastics and casting is common for metals. Molding has high fixed costs due to the requirement of having a mold tool, and as a result is typically only used for high volume orders.

  • Advantages
    • Good for high volume orders (thousands)
    • Can normally hold dimensional tolerances of up to +/- 0.010 IN
    • Good mechanical properties
  • Disadvantages
    • High fixed costs
    • Bad for low volume production
    • Internal geometric features are impossible for the tool to reach.
    • Tolerances and mechanical properties are not as good as machined parts

Forming

Forming is a process by which a sheet of metal is cut to size, either with a laser or water jet, and bent, often using a specially designed die, to form the sheet to the required shape. Because sheet metal forming typically requires the use of a die, high volume batches are usually required to amortize the fixed cost across the individual units. Sheet metal tolerances are typically +/- 0.030 IN.

  • Advantages
    • Good economies of scale for high volume orders
    • Excellent mechanical properties
  • Disadvantages
    • Not good for applications requiring tight tolerances
    • Large amount of wasted material
    • Cannot be used for thicker materials

Additive

Additive manufacturing, otherwise known as 3D printing, has recently emerged as a new manufacturing method with several capabilities that are not available with the other conventional methods. Additive manufacturing can produce parts with high complexity, high customization, in low volumes with an increasingly wide range of materials. Additively manufactured parts have good dimensional accuracy and can usually hit tolerances of up to +/- 0.010 IN, depending on the machine.

  • Advantages
    • Ability to produce highly complex parts with internal features.
    • Does not require tooling, so low volume production orders are economical with AM
    • Very little wasted material
    • Automated build process
  • Disadvantages
    • Mechanical properties are not as good as injection molded or machined parts
    • Dimensional accuracy is less repeatable than injection molded or machined parts
    • Can require a large amount of post-processing

Joining

Joining is series of processes that involves linking together separate components, typically through welding, bonding or fastening. Components are machined, formed, molded or printed and then are joined together to make an assembly component. Although there are some automated welding and joining operations, most joining operations are still manual.

  • Advantages
    • Can use a variety of methods to join machined, molded and formed parts
    • Good mechanical properties
    • Good dimensional accuracy
  • Disadvantages
    • Joints are not as strong as other areas on an assembly and are prone to leaks in the case of gas or fluid transfer systems
    • Typically, joining is a labor-intensive process

So Which Process Should You Choose?

Volume is a key factor to consider when deciding on a fabrication method. Will the product be needed in high volume, medium volume or low volume? Additive manufacturing is great for low volume production orders from 1 to a few dozen, due to its low fixed costs. Whereas machining is good for medium volume orders in the hundreds and molding and forming are best suited for high volume orders so that the high fixed costs can be amortized over a higher volume of parts.

Dimensional accuracy is also a key consideration when deciding on a fabrication method. CNC machining is the most dimensionally accurate and repeatable. Casting, injection molding, and additive are similar in their abilities in terms of dimensional accuracy, with a variety of factors at play that could lead to variations; the design of the part and machine used being the primary determining factors. With 3D printing, for instance, laser sintering methods are more accurate than extrusion and industrial printers are more accurate than desktop 3D printers. Sheet metal forming is generally not as accurate as other methods, but it of course depends on the machinery, the tooling, the material, and the skill of the operator.

Complexity is also an important factor to consider. Additive manufacturing can easily produce highly complex parts in low volume but cannot compete with other methods for parts with low complexity that require high volumes.

Hybrid Methods

There are also some possibilities to use hybrid methods to obtain more design freedom than if one method was used alone. For instance, 3D printed patterns for investment casting can create highly complex casted parts that would not be possible with traditionally made patterns.

3D printed molds are also a great hybrid method as they allow conformal cooling channels and other complex features that would be difficult or impossible to machine.

Hybrid tooling is also becoming an increasingly utilized application, whereby a combination of machined and 3D printed components are combined. For instance, a metal baseplate could be fastened to 3D printed component features, allowing the ability to benefit from both methods.

Thinking about using additive manufacturing to build your next product? Get in touch with ZABFAB to get started!

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