According to international standards (ASME AM223, DIN 8580), manufacturing processes can be divided into three categories: subtractive, additive and formative (Fig. 1) [3, 5, 31, 32]. More specifically, SM processes, such as turning, drilling and milling, start with a stock part or bulk material, removing material until the desired geometry is created. In AM processes, material is selectively deposited in successive layers until the final geometry is reached. Finally, in formative processes, such as casting and forging, the bulk material is shaped into the desired geometry through molds and tools. Table 2 presents a generic comparison of manufacturing processes categories.
Table 2
Comparison of manufacturing processes categories [31, 32].
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Formative
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Subtractive
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Additive
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Process
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The material is formed into the desired shape by temperature and pressure, e.g. injection molding.
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The desired part geometry is shaped by removing chips from bulk material through cutting tools, e.g. milling.
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The material is built through phase change in a layer bylayer manner until the entire part is constructed, e.g. 3D printing.
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Cost
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Incomparable low cost to produce large volumes of identical parts, however the investment in tools (molds) is high.
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Often the best choice for production low to medium volume components (10 to 100 parts). Initial investments in machinery are not cheap, but then individual spare parts can be produced at a relatively low price per unit.
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Offers a cost-effective way to create intricately designed small or medium sized components that cannot be made with other manufacturing processes.
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Lead Time
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Steel tools for mass production are complex and their creation is time consuming, increasing part or batch delivery time to weeks.
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Quick lead times, usually within 5 days.
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Depends on the part, sizes, complexity, material, machine tool, needed post-processing etc.
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Material properties
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Ability to produce relatively complex components with high tolerances and a wide range of materials ideal for functional parts.
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Almost anything with no internal features can be processed with great precision, with very tight tolerances and retains excellent material properties.
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A number of materials available depending on the AM technology, however, it is generally not possible to produce spare parts with material properties equivalent to forming or material removal techniques.
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Design Constrains
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Design is limited by the need for mold features, such as spurs, runners, corner design and uniform wall thickness.
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Many machines are limited to relatively simple geometries, although complex geometries become cheaper as technology evolves.
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The parts can be produced with almost any geometry, complexity and internal features.
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Manufacturing processes can be combined, thus creating hybrid fabrication processes. The most common hybrid production method is the serial combination of additive with subtractive processes. The component is built entirely through AM, almost to a roughly shaped geometry of the object, then subjected to a finishing SM process to perfect the object’s surfaces and geometric tolerances. The second hybrid manufacturing method alternates iteratively between AM and SM during the construction process on an existing or a new object. AM and SM alternate either at every material layer or at every couple of layers to achieve accuracy in any external and internal geometrical features of an object, e.g. engine cooling pipes or mold cooling/heating channels.
Also, a hybrid system can combine materials, such as polymers, metals, composites, etc, during the additive stages and apply different ones for the creation of a single part thus creating what is called “multi-material parts” [3, 33, 34, 74]. However, this can be a serious challenge when melting temperature is significantly different between two materials, where special actions must be taken to successfully perform that [35]. A great deal of materials that can be used in additive processes, either as filament, usually polymers, such as ABS, nylon, PLA, PETG, etc, or as powder, metals, such as aluminum, copper, stainless steel and titanium, and polymers. As a result, by using different metals in a single multi-stage process, the workpiece acquires better characteristics without compromising its integrity. There are currently two types of HMT. The first is an off-the-shelf HMT offering both AM and SM, while the second is to add AM capabilities to a SM machine tool. By adding one or more AM heads to an existing CNC machine allows them to work in parallel with the standard set of subtractive tools. Although the total number of HMT available is still relatively small, as presented in the previous section, it is a technology that will only be expanded and developed, due to its inherent flexibility of combining both processes into one working space, which leads to increased productivity and ability to create parts that otherwise it is either impossible or very difficult to produce [7, 36, 37].
Hybrid technology offers several advantages over the material removal or additive process alone. Due to the ability to add material to existing parts, it allows the repair of damaged parts, since selective addition of material to the parts is performed. At the same time, AM allows the combination of different materials, changing its mechanical properties layer by layer which can improve its durability, e.g. car engine connecting rods. Besides, it reduces the need for extensive post-processing machining processes, as material is added only where required, for part finishing (Fig. 2). Finally, the ability to switch between AM and SM without having to move the piece to another machine tool saves significant time and cost (set-up and down phases) and minimizes all related errors.
As with any new technology, HM technology faces a set of pre-challenges, learning curve and questions. The first challenge is the investment and implementation cost associated with the equipment, and whether these costs can be kept low, so that any business can invest in a HMT. Also, another question that arises is whether HM technology is able to meet production requirements in a timely manner. HM has been proven in the production of models and unique parts in small batches, as well as in repairs of high quality and complexity parts. Custom on-demand production performed with HM eliminates the need of inventory and minimizes production time, thus resulting in overall cost reduction of the total production. Moreover, HM can effectively produce complex parts or perform complex repairs, e.g. in molds by adding material to enhance it for added durability. Over time, more and more industrial and other sectors are becoming aware of the benefits of using HM systems.
HM technology is at the beginning of a new technology leap, which fits smoothly with the Industry 4.0 and Industry 5.0 transition that will dominate the manufacturing world in the following years. The combination of both AM and SM in one machine enables innovative ideas and leads to production disruption. Instrumental in the HM success is process optimization, which can be applied seamlessly, so as to provide design and manufacturing flexibility while maintaining strict specifications for the produced parts.