2. Hybrid manufacturing processes (HMP)

#### 2.1 Casting - subtractive

While the modern definition of HMP focuses on the collection of production processes integrated together using computer-assisted systems engineering tools, the first instances of 'hybrid manufacturing' were originally much more sequential in nature. From literature, some of the first reported instances of using a sequential 'hybrid' approach were found in the finish machining of cast components (a.k.a. castings) [2, 3]. When combined together, casting and subsequent machining provides numerous advantages including: reduced material waste, tighter achievable tolerancing, and increased overall geometric complexity. This is because this unique

#### Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing DOI: http://dx.doi.org/10.5772/intechopen.88560

combination takes advantage of the capabilities of both processes. However, the material properties can be sacrificed compared to just machining.

Since the material flows into a mold cavity, casting enables the production of complex internal and external geometries that are net- or near-net shape. Parts fabricated using casting are often limited in other ways. For example, the surface roughness of castings is directly correlated to the roughness of the mold cavity walls, which in the case of sand casting is the roughness of the sand. Additionally, consideration must be given to process inherent defects that affect the mechanical performance and geometrical and dimensional accuracy of the casting such as shrinkage cavities, inclusions of air or foreign matter due to turbulence from pouring, etc. Machining can allow users to manufacture parts with increased accuracies as compared to casting alone. For example, parts often exhibit better flatness and smaller radiused corners when machined. Machining using computer numerical control (CNC), means that the process is highly repeatable and easily scalable due to the incorporation of computer-guided automation. While the accuracy is better for machined components, there is a sharp reduction of the geometric complexity possible, particularly with internal features, when compared to cast parts. This is because machining is limited to a straight line of sight from the cutting tool, which limits the features that are accessible for finishing. Additionally, unless combined with another process, machining is associated with larger amounts of material waste from transforming rectangular or cylindrical billets into final geometries.

Combined, these two processes can produce parts that are better able to meet the final part specifications in an economical way as outlined by the advantages mentioned previously. In this category of HMP, there are special considerations that must be given to the incorporation of machining after casting. For example, engineers should decide if small holes in the casting should be filled (i.e. not produced in the casting) to ensure drills would be able to accurately finish holes without tool walking. Another possible consideration is the method for fixturing cast parts to a milling machine, since each individual castings' defects (flash, shrinkage, pores, etc.) could impact this. Additional factors and where they should be addressed in the casting-subtractive category of HMP are outlined in Figure 1.

Figure 1.

in process engineering time. For many of these polymer components, the mechanical properties come directly from the combination of the polymer material and the processing parameters. The geometric shape and dimensions comes from a combination of the computer-aided design (CAD) model developed during Product

Unfortunately, polymers have a limited use for only certain products. As better mechanical properties and finer geometric tolerances are required, the use of metals becomes necessary. Although metal AM has been around for two decades, the geometric accuracy of metal AM frequently falls short of the engineering specifications and the mechanical properties of AM produced metal parts are often highly dependent on the surface conditions. The result of these specifics is that metal AM production typically requires multiple post-production processes and machines. Metal AM machines have typically been used to create "near net-shape" components that require additional processes to enhance both the tolerances and surfaces as well as the mechanical properties of the AM printed component. This has slowed the adoption of metal AM for many high-performance components, especially those

To increase the performance of engineered parts with complex geometries which use processes such as metal AM, Hybrid Manufacturing Processes (HMP) are used which incorporate a secondary post process. HMP can significantly reduce time to customer, waste, and tooling costs per part while increasing possible part geometries and material availability for small batch parts. Examples of hybrid manufacturing for this chapter include Casting-Subtractive, Injection-Molding-Subtractive, and Additive-Subtractive processes. HMP usually have accurate results

but require extra layers of complexity including process plan development.

plan that demonstrated automation of an optimized process plan.

2. Hybrid manufacturing processes (HMP)

2.1 Casting - subtractive

118

required to design parts and develop process plans for the complex processes. Although HMP is largely comprised of an additive process followed by a subtractive process, two other manufacturing methods are discussed since they have similar complexities in the process planning phase. Finally, a feature-based advanced hybrid manufacturing process planning system (FAH-PS) is discussed. This framework uses feature-specific geometric, tolerance, and material data input to generate automated process plans based on user-specified feature precedence for additivesubtractive hybrid manufacturing, a hybrid manufacturing process. Plans generated by FAH-PS can optimize process plans to minimize tool changes, orientation changes, etc., to improve process times. A case study of a patient-specific bone plate is described at the end of the chapter for proof of concept of the framework. Imploring a strategy of minimizing tool and orientation changes generated a process

This chapter outlines several hybrid manufacturing processes and the intricacies

While the modern definition of HMP focuses on the collection of production processes integrated together using computer-assisted systems engineering tools, the first instances of 'hybrid manufacturing' were originally much more sequential in nature. From literature, some of the first reported instances of using a sequential 'hybrid' approach were found in the finish machining of cast components (a.k.a. castings) [2, 3]. When combined together, casting and subsequent machining provides numerous advantages including: reduced material waste, tighter achievable tolerancing, and increased overall geometric complexity. This is because this unique

Engineering and the dimensional capabilities of the AM machine used.

requiring certification.

Mass Production Processes

## 2.2 Injection molding - subtractive

Injection molding is most commonly used to create small to large sized polymer, and in some cases metal, parts in large batches. The parts themselves are typically ready to use, once injection parameters have been optimized to reduce; voids, shrinkage, warping, short shots, burn marks, and flash. However, the most complex, expensive, and time-consuming part of the injection molding process lies in manufacturing the mold itself. There are many methods used to fabricate injection molds, including traditional machining, casting, and additive manufacturing methods. It is imperative for injection molds to maintain extremely tight tolerances and be manufactured of materials which can withstand the repeated pressures and temperature cycles from the injection molding process of large batch size parts. Traditionally machined molds satisfy these requirements but because machining is a line-of-sight finishing method there is often an inability for intricate or complex cooling geometries within the mold. Therefore, a more modern approach is to use additively manufactured molds with complex cooling features for large batches of parts. This approach is best suited for production of smaller batch sizes where lengthy mold manufacturing times are not cost effective on a per part basis. Both of these methods require post processing, usually machining, to achieve tolerance and surface finish requirements of an injection mold.

or the addition of features that cannot be manufactured using any other method. However, when compared to subtractive CNC finishing the achievable tolerances of an as-built AM component are much lower [5]. These tolerances may not be acceptable and require further finishing; however, the complex designs possible with additive manufacturing can pose challenges for subtractive CNC finishing, which requires the tool to have line of sight to the region that it is finishing [6]. Design considerations for Additive - Subtractive HMP include location of

Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing

DOI: http://dx.doi.org/10.5772/intechopen.88560

Figure 2.

Figure 3.

121

Additive-subtractive HMP.

Process flow for creating injection molded parts for large batch scenarios.

Although injection molds are typically made from metal, molds can be created from other materials such as UV cured polymer manufactured via vat photopolymerization processes or material jetting processes. These parts will need the appropriate post curing time and conditions. This recipe of post curing will directly affect the life of the mold and the accuracy of the parts [4].

Injection molding typically requires several large investments in machinery. Specifically, the process of creating the mold, although this is typically outsourced, have their own mold fabrication shop to cut down on costs. These fabrication shops require several milling and turning machines, tools to assist in fixturing and precise measuring, as well as experienced and competent operators to design and maintain the molds. Also required for injection molding is the injection molding machine itself. Injection molding machines are typically very large, even for small parts.

Although injection molding is a complex process, this chapter will focus on the methods for process planning of hybrid manufactured molds. Figure 2 depicts the flow in which injection molded parts are developed. Note the important considerations for process planning are related to the mold design and fabrication steps.

#### 2.3 Additive - subtractive

With growing popularity and improving resultant parts, AM is driving renewed development in process planning and optimization for hybrid manufacturing processes. Additive manufacturing is classified by the layerwise addition of material to create a near-net-shape or final part. A variety of additive manufacturing processes exist that can manufacture polymers, ceramics, or metals with varying precision. Initially, AM was considered a prototyping technology that enabled accelerating design changes due to the relatively quick turnaround from CAD model to final part. Advances in additive manufacturing and design methods have facilitated growth in the area and additive manufacturing is now being adopted as a production manufacturing technology in aerospace, medical device, and automotive manufacturing among others.

Additive manufacturing allows for components that have highly complex designs or are made from materials that are difficult to process using other methods. This often allows for the reduction in the number of components, weight reduction, Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing DOI: http://dx.doi.org/10.5772/intechopen.88560

or the addition of features that cannot be manufactured using any other method. However, when compared to subtractive CNC finishing the achievable tolerances of an as-built AM component are much lower [5]. These tolerances may not be acceptable and require further finishing; however, the complex designs possible with additive manufacturing can pose challenges for subtractive CNC finishing, which requires the tool to have line of sight to the region that it is finishing [6]. Design considerations for Additive - Subtractive HMP include location of

Figure 2.

2.2 Injection molding - subtractive

Mass Production Processes

surface finish requirements of an injection mold.

the life of the mold and the accuracy of the parts [4].

2.3 Additive - subtractive

manufacturing among others.

120

Injection molding is most commonly used to create small to large sized polymer, and in some cases metal, parts in large batches. The parts themselves are typically ready to use, once injection parameters have been optimized to reduce; voids, shrinkage, warping, short shots, burn marks, and flash. However, the most complex, expensive, and time-consuming part of the injection molding process lies in manufacturing the mold itself. There are many methods used to fabricate injection molds, including traditional machining, casting, and additive manufacturing methods. It is imperative for injection molds to maintain extremely tight tolerances and be manufactured of materials which can withstand the repeated pressures and temperature cycles from the injection molding process of large batch size parts. Traditionally machined molds satisfy these requirements but because machining is a line-of-sight finishing method there is often an inability for intricate or complex cooling geometries within the mold. Therefore, a more modern approach is to use additively manufactured molds with complex cooling features for large batches of parts. This approach is best suited for production of smaller batch sizes where lengthy mold manufacturing times are not cost effective on a per part basis. Both of these methods require post processing, usually machining, to achieve tolerance and

Although injection molds are typically made from metal, molds can be created from other materials such as UV cured polymer manufactured via vat photopolymerization processes or material jetting processes. These parts will need the appropriate post curing time and conditions. This recipe of post curing will directly affect

Injection molding typically requires several large investments in machinery. Specifically, the process of creating the mold, although this is typically outsourced, have their own mold fabrication shop to cut down on costs. These fabrication shops require several milling and turning machines, tools to assist in fixturing and precise measuring, as well as experienced and competent operators to design and maintain the molds. Also required for injection molding is the injection molding machine itself. Injection molding machines are typically very large, even for small parts. Although injection molding is a complex process, this chapter will focus on the methods for process planning of hybrid manufactured molds. Figure 2 depicts the flow in which injection molded parts are developed. Note the important considerations for process planning are related to the mold design and fabrication steps.

With growing popularity and improving resultant parts, AM is driving renewed development in process planning and optimization for hybrid manufacturing processes. Additive manufacturing is classified by the layerwise addition of material to create a near-net-shape or final part. A variety of additive manufacturing processes exist that can manufacture polymers, ceramics, or metals with varying precision. Initially, AM was considered a prototyping technology that enabled accelerating design changes due to the relatively quick turnaround from CAD model to final part. Advances in additive manufacturing and design methods have facilitated growth in the area and additive manufacturing is now being adopted as a production manufacturing technology in aerospace, medical device, and automotive

Additive manufacturing allows for components that have highly complex designs or are made from materials that are difficult to process using other methods. This often allows for the reduction in the number of components, weight reduction,

Process flow for creating injection molded parts for large batch scenarios.

Figure 3. Additive-subtractive HMP.

Figure 4.

Sample part which could replace multiple components and become part of an assembly after finishing (reproduced from [8]).

machining fixturing, part location in the machine due to variability in the AM processes, support structure removal, required tolerances, and required surface finish. Although there are additional considerations that must be made to accommodate the use of additive manufacturing in hybrid processes, the buy-to-fly ratio and costs can be lower than machining alone due to the material waste associated with subtractive only manufacturing [7]. Figure 3 shows the flow chart and key considerations for additive-subtractive HMP processes.

using traditional casting, machining, and then finishing, the process engineer would first determine how much additional material would be necessary to use in the near net-shape casting. Once this is done, the process engineer would create a "new pattern" that would be used in the green sand casting process. This pattern would allow for enough material of the critical features (faces and cylinders) for subsequent steps; this is the machining allowance. Next, the machining processes would be planned, where drilling, boring and milling operations would typically be used to create the next step in the production. Finally, finishing operations of the highly toleranced surfaces would be conducted. Planning each of these activities requires experience and a detailed understanding of the precision of each process. Tolerance stacks must be identified and used to properly sequence the operations

Engine block with some assembled components. (reproduced from [9]. Photo by Garett Mizunaka on

Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing

DOI: http://dx.doi.org/10.5772/intechopen.88560

The planning of each of these processes can be both time consuming and expensive. For each of the three production activities illustrated in this example (casting, machining and finishing), these activities represent "fixed costs" associated with each of these activities. Planning time for each of these activities would typically be on the order of 3–10 days depending on the complexity, tolerances and experience with similar products. For very small quantities of parts, process engineering can be

The final cost of any manufactured component will be the sum of the costs at each step of the production plus the materials, holding and overhead costs. At each step, the production cost must be determined. In general, we can define the cost of a

Product Cost ¼ ðOne � time CostsÞ þ ðBatch Setup CostÞ þ ð Þ Processing Cost

In order to put cost as a function of volume, we can express this as cost per

þ

Cmotset�up nb

þ ð Þ Batch Set � up Cost =ð Þþ Batch Size ð Þ Processing Cost

þ Cmo tp

Product Cost=Part ¼ ð Þ One � time Costs =ð Þ Total Parts Produced

Cp <sup>¼</sup> <sup>C</sup><sup>1</sup>�time nt

(1)

(2)

(3)

that will be used.

Figure 5.

Unsplashed).

product as:

part or:

123

the dominant cost component.

Or in terms of variables:

The component shown in Figure 4 is an excellent example of an additive manufacturing component that could be used in a functional assembly. However, the tolerances of the functional surfaces would not meet the requirements as is and would need to be finished before assembly.
