Additive vs. subtractive manufacturing – what’s the difference?
Additive Manufacturing adds material to create an object. Traditional forms of manufacturing start with a ‘block’ of material and remove pieces by milling, machining, carving or shaping, this is known as Subtractive Manufacturing.
Additive manufacturing has become an integral method of making prototypes of new products and is best recognised in the emerging field of 3D Printing. Which in itself is only one form of additive manufacturing. As most product developers will gravitate towards 3D printing to produce their first prototypes lets briefly discuss the other process Subtractive manufacturing.
Product developers start with a sketch of their idea and translate this into a Computer Aided Design (CAD) which becomes the instruction code for a Computer Numerical Control (CNC) machine (milling machine, lathe, etc.) which produces the item. These machines start with a solid block of material such as steel, alloy or plastic and remove material until the finished product is made.
Until 3D printing emerged the majority of prototypes were made using Subtractive Manufacturing.
The primary interest for product developers is 3D Printing.
3D printing builds thin layers using a variety of different materials such as carbon, nylon, metal and different plastics. The automotive, aerospace, mechanical engineering and civil engineering sectors are all developing new techniques with other materials.
The most common applications for 3D printing are:
- Rapid Prototyping
Used to test concepts, form and fit for products, verify the product and test its viability.
- Tooling and moulds
Used to create custom designed final products often of a complex nature. Moulds are used to cast tools and create more prototypes.
- Direct Manufacturing
Used to create a final product.
Steps in 3D printing
- Create a design for the product using one of the following:
- CAD software
- Free-form modelling software
- Sculpting software
- Converting the Design to an STL file format and transferring it to the 3D printer
STL special file format that is used to allow the communication between a computer and the 3D printer. The STL format is the standard file format for 3D printing and all major CAD software packages support it.
- Set up the 3D printer by specifying
- Material to be used
- Power sources
- Construction of the object
This process is assisted by the automation of the 3D printer.
- Removing and cleaning
Depends on the 3D printing method used.
Removal of support structures that may have been designed to hold the piece in position.
3D Printing Technologies
3D printing can be divided into three different technologies:
- Fused Filament Fabrication (FFF) – uses melted plastics, known as a filament
- Stereolithography (SLA) – uses a liquid resin as a starting base
- Selective Laser Sintering (SLS) – uses a powder composite that is melted to form the model.
Fused Filament Fabrication (FFF)
FFF is the most commonly used additive manufacturing method. It utilises a filament (coiled plastic) that is melted through a heated nozzle and creates the product layer by layer. FFF is mainly used for rapid prototyping as materials are affordable.
Materials than can be used with the FFF process include:
- ABS – Acrylonitrile Butadiene Styrene
A popular durable, lightweight and flexible plastic widely used mainstream products. ABS is used for architectural models, concept product models, manufacturing, fixtures and general DIY projects. ABS has a high-temperature requirement and this can create fumes.
- PLA – Polylactic acid
PLA is a material derived from biodegradable compounds and is widely used in additive manufacturing. It is safer to use than ABS and is used in the food and medical industries for prototyping low-cost models and functional models. PLA is not as heat resistant as ABS and has a rough texture that can degrade over long periods of time.
- PV PVA – Polyvinyl alcohol
A water-soluble plastic used in 3D printing primary as a support structure material in the construction of complex structures. PVA is an expensive material compared to the other plastics and can also release toxic fumes if the temperature settings are too high.
- HIPS – HIPS High Impact Polystyrene
HIPS is a filament that is very similar to ABS but unlike ABS, it is soluble in d-limonene. It also has the advantage of being lighter than ABS making it a useful filament for various applications. HIPS is used both to build prototypes but has the additional benefit of being able to be a support material for complex prints. HIPS requires that your printer has both a heated bed and is capable of reaching high temperatures for effective printing.
- PETG – PETG polyethylene terephthalate (glycol)
PETG is a very durable material that is used in 3D printing. It has high strength and high heat resistance. It is safer than ABS when using it alongside food products or medical instruments. PETG is used for prototyping models, functional prototypes and end-use products. It is also used for mechanical parts since it is not affected by shrinkage, warping and it is fairly flexible. Although highly durable, the disadvantage of using PETG is that it requires careful calibration to achieve high- quality prints.
- Nylon – Nylon Polyamide
Nylon is utilized heavily due to its industrial strength, flexibility and durability. It is stronger than both ABS and PLA and additionally has the added advantage of being affordable. Nylon is used for many applications due to its versatility. It is used for both prototyping, manufacturing, tooling and machine parts. Nylon requires proper storage because it absorbs moisture easily and can also produce fumes if exposed to high temperatures.
Stereo-Lithography and Digital Light Processing (SLA)
SLA uses a photosensitive liquid resin that is hardened by using a light source. In contrast to FFF, an object is created through a build platform being lowered into the resin and a light source above or underneath hardening the material. The objects created are usually more accurate and smoother than FFF, however this method is mainly used for intricate small objects and has issues with larger objects.
Materials that can be used with the SLA process include:
- General purpose resins
Unlike FFF, SLA uses liquid resins that can be categorised by use and not specific material types. This is due to the variety of companies that produce their own variants and resin combinations. General purpose resins have similar finishes to standard plastic and colour choices are limited. General purpose resins have a number of standard uses. The main advantage they bring is they allow for high fidelity prototypes to be built. However, SLA has issues with building larger models. General purpose resins are used to produce, jewellery, functional prototypes and art models. SLA uses a variety of chemical solvents, and UV light which require safety precautions to be strictly followed.
- Engineering resins
Engineering resins can come with a number of attributes that are dependent on the development requirements. Some are similar to ABS (Tough), others have high-temperature resistance or overall durability. These resins are used for both rapid prototyping and also direct manufacturing. Although these resins have various properties that aim to reproduce the durability of injection moulded parts, they can be expensive. Tough Resins: Prototypes and visual prototypes. These resins are designed for heavy use cases and case where the structure will be subjugated to high stress. High-temperature resistant resins: Tooling or moulds that require heat-resistant properties. Overall durable resins: Rapid prototyping and visual prototypes.
Selective Laser Sintering (SLS)
SLS uses a laser to solidify a powder material. It works by having a build area that is filled with a powder composite and storage compartment. The small amount of powder in the build area is heated to just below its melting point by a laser. As the laser melts the first layer, the build platform moves down as the powder store area moves up. This action adds a new layer that can be melted by the laser. The other powder that remains in the build area also acts as a support for the object. This method is suitable for functional components and end products. Generally, SLS is used with different polymer materials while metallic components are produced using DMLS (Direct metal laser sintering). This process is similar but has different power requirements and slightly different processes due to the metal. Traditionally SLS is a method that has been strictly used by large institutions due to the high price of an SLS printer.
Materials than can be used with the SLS process include:
- Plastic Powder
The most commonly used polymer is nylon, however, there are equivalent composites that have the properties of ABS, PLA and other standard printing plastics. Often glass or other materials are added in SLS powders to induce specific material requirements. The applications for plastic powders for SLS are to produce a wide variety of standard prototypes, visual concepts and functional prototypes. These prototypes can have heat resistant properties or be flexible. One example is printing shoe components. SLS standard plastics have different properties and deficiencies. Depending on the composite, the deficiencies are similar in nature to standard polymers. The variety of materials are limited however for SLS printing.
- Metallic Composites
There are metal composites like Alumide which is a composite of nylon and aluminium particles. Like plastics, everything in terms of characteristics is dependent on end-user requirements. So long as the metal can be melted, then it is viable for DMLS. Functional prototypes and tooling for aerospace, medicine, electronics and rapid manufacturing. The usage of these parts is for highly customizable situations that require unique engineering solutions that are not easily found in the standard industry. Better materials are expensive depending on the composite. SLS can be risky due to possible combustion due to the metals and powder inhalation.
TECHNIQUE COMPARISON TABLE
Source: (3D PRINTING TECHNIQUES AND RAPID PROTOTYPING – Kyocera White Paper 2018)
|Raw Material Used||Solid||Liquid||Powder|
|Raw Material Handling||Easy||Medium||Hard|
|Print Complexity||High, with soluble support||High, with support||High|
|Support Structures||Can be removed by hand
or by dissolving in water
|Have to be remove
|Post Processing||Low to none||Medium, curing||Medium, removing powder|
|Instant||Curing, 15 minutes||Removing powder,
|Machine Preparation||Low, 5 minutes avg.||Very low, 1-minute avg.||Low, long heat up?|
|Machine After-care||Medium, 10 minutes avg.||Medium, 10 minutes avg.||Low, 15 minutes avg.|
|Recommended Protection||None||Gloves||Gloves, mask|
Additive and Subtractive Manufacturing working together – Hybrid Manufacturing
Hybrid systems combine the versatility of additive manufacturing with some of the advantages of subtractive methods. Specialist machines can perform both operations, meaning that complex parts can be made more easily. Hybrid manufacturing is particularly good for repairing worn or broken parts, as the material can be added in layers, then finished with milling tools.
This device can leverage any number of tools, including drills and saws, to remove large amounts of raw material and create prototypes or finished products at scale.
3D Printing enters the Building and Construction sector
Conventional structures are now being created using 3D printed concrete with the result that walls, openings and complex structures can be created with minimal human input.
Additive manufacturing (3D printing) is a fast-evolving methodology that is increasing in its flexibility, materials used, size and speed. Each method has its own attributes and it is inevitable that we will see its spread across many sectors. As speed increase and costs are reduced it will replace conventional methods in some production processes.