Innovative grid molding and cooling using an additive and subtractive hybrid CNC machine tool

doi:10.1016/j.cirp.2017.04.093

CIRP Annals

Volume 66, Issue 1, 2017, Pages 401-404

https://doi.org/10.1016/j.cirp.2017.04.093 Get rights and content

Abstract

An innovative injection mold making and cooling approach using a hybrid additive and subtractive machine tool is proposed in this paper. Approximated mold geometry is quickly formed using prefabricated blocks. Directed Energy Deposition (DED) followed by finish milling is then used to form the continuous free-form surface of the mold cavity. The innovative technique enables quick fabrication of a mold with a conformal cooling channel resulting in a dramatic improvement in cooling performance of the mold. A mold prototype was fabricated and the performance was evaluated compared to a mold fabricated by the traditional subtractive machining method.

Introduction

Additive Manufacturing (AM) especially for metal applications such as Selective Laser Melting (SLM) and Directed Energy Deposition (DED) has gained a lot of attention recently in academia as well as industry. According to a CIRP keynote paper in 2016 [1], the AM market is worth more than $4 billion and is expected to grow up to $20 billion in 2020. Due to high expectations for AM technologies, substantial efforts have been made by many researchers. Kakinuma et al. studied the influence of powder characteristics on clad quality of Inconel 625 [2]. Paris et al. reported the environmental impacts of AM as compared to traditional machining processes [3]. Newman et al. introduced a new concept of process planning for additive and subtractive processes [4]. Choi and Chang studied characteristics of laser deposited H13 tool steel [5]. Choi et al. studied the effect of specific energy input on microstructure [6]. Moreover, Thompson et al. summarized recent trends, opportunities, considerations, and constraints with regards to AM design [1].

One potential application for AM technology is to manufacture plastic injection molds. An example is fabrication of so called conformal cooling channels in the mold using SLM, which can improve cooling efficiency [7], [8]. Cooling is one of the most important design considerations for injection molding as it constitutes up to 75% of the shot cycle time for plastic part manufacturing [9]. Conformal cooling technology creates a complex cooling channel which cannot be made by traditional machining processes. However, there are some drawbacks in the SLM based manufacturing of a mold. Production time is one issue as it is necessary to build the mold from scratch with a large number of layers each having a thickness of only a few microns. Adjustment after manufacturing is also impossible for SLM, so the mold must be fabricated from scratch to make changes, which is a time-consuming process.

While the AM process achieves near-net shape parts with minimal material waste, the parts typically require post-machining processes such as cutting and grinding to ensure accurate geometry and to produce a good surface finish. Multi-axis hybrid CNC machine tools that combine both additive and subtractive processes have emerged that can optimize the advantages and reduce or eliminate the disadvantages of each process. One of the most prominent advantages of such hybrid machines is the ability to switch between additive and subtractive operations at any time. However, very few studies have been reported to fully utilize the unique capabilities of the machine and develop novel applications due to the complexities of both the AM process and the machine itself.

This paper proposes an innovative grid based injection mold fabrication and cooling technique in order to study the great potential of a hybrid additive and subtractive CNC machine and to introduce a unique mold design methodology. The overall concept of the grid mold and cooling are discussed, and feasibility testing of the mold is performed by fabricating a prototype using a hybrid CNC machine tool and testing it compared to a traditional mold.

Section snippets

Overview of grid molding and cooling

Fig. 1 shows the overview of the grid mold concept. Instead of removing material from a block stock with milling or EDMing or building the mold up from powder stock using SLM, the mold cavity geometry is discretized into a grid and approximated by a collection of blocks. Each block has a cutaway as shown on the left of Fig. 1, which may vary between blocks. By aligning these cutaway sections of each block, a cooling channel can be generated through the grid as shown on the right of Fig. 1.

Additive and subtractive hybrid CNC machine tool

Since the grid molding technique requires deposition of metal layers on a pre-defined three-dimensional array of blocks, SLM or similar powder bed methods are not well suited to the application. In this study, a DMG Mori LASERTEC 65 3D was used for fabricating prototypes. The machine setup is shown in Fig. 5. This machine is a 5-axis hybrid CNC machine tool which enables both DED and traditional subtractive cutting to create complex metal geometry with a single machine setup. A laser head can

Plastic injection and setup

Injection testing was conducted using the grid mold in order to assess the cooling performance as compared to the traditional mold. The injection test setup and plastic part produced by the mold are shown in Fig. 10. The plastic for this test was J-3021GA polypropylene, with an injection temperature of 225 °C at 52.5 MPa pressure and ejection temperature of 60 °C, with a total cycle time of 42 s. The core side of the mold used in this test had a traditional cooling channel created by drilling.

Cooling performance

Conclusion and future work

The innovative grid molding and cooling method was proposed utilizing a hybrid additive and subtractive CNC machine tool. The grid mold prototype has been successfully fabricated and tested by comparing cooling performance with a mold made by traditional fabrication methods. Conclusions are as follows:

•
DED and subtractive machining hybrid process can successfully produce functional molds using a framed grid of individual blocks as substrate.
•
Simulation results showed very uniform cooling

References (9)

M.K. Thompson et al.
Design for Additive Manufacturing: Trends, Opportunities, Considerations, and Constraints
CIRP Annals—Manufacturing Technology
(2016)
Y. Kakinuma et al.
Influence of Metal Powder Characteristics on Product Quality with Directed Energy Deposition of Inconel 625
CIRP Annals—Manufacturing Technology
(2016)
H. Paris et al.
Comparative Environmental Impacts of Additive and Subtractive Manufacturing Technologies
CIRP Annals—Manufacturing Technology
(2016)
S.T. Newman et al.
Process Planning for Additive and Subtractive Manufacturing Technologies
CIRP Annals—Manufacturing Technology
(2015)

There are more references available in the full text version of this article.

Cited by (43)

A review on additive/subtractive hybrid manufacturing of directed energy deposition (DED) process
2022, Advanced Powder Materials
Additive manufacturing (AM) processes are reliable techniques to build highly complex metallic parts. Direct energy deposition (DED) is one of the most common technologies to 3D print metal alloys. Despite a wide range of literature that has discussed the ability of DED in metal printing, weak binding, poor accuracy, and rough surface still exist in final products. Thus, limitations in 3D printing of metal powder and wire indicate post-processing techniques required to achieve high quality in both mechanical properties and surface quality. Therefore, hybrid manufacturing (HM), specifically additive/subtractive hybrid manufacturing (ASHM) of DED has been proposed to enhance product quality. ASHM is a capable process that combines two technologies with 3-axis or multi-axis machines. Different methods have been suggested to increase the accuracy of machines to find better quality and microstructure. In contrast, drawbacks in ASHM still exist such as limitations in existing reliable materials and poor accuracy in machine coordination to avoid collision in the multi-axes machine. It should be noted that there is no review work with focuses on both DED and hybridization of DED processes. Thus, in this review work, a unique study of DED in comparison to ASHM as well as novel techniques are discussed with the objective of showing the capabilities of each process and the benefits of using them for different applications. Finally, new gaps are discussed in ASHM to enhance the layer bonding and surface quality with the processes' effects on microstructures and performance.
Review of Computer-Aided Manufacturing (CAM) strategies for hybrid directed energy deposition
2022, Additive Manufacturing
Citation Excerpt :
Hybrid manufacturing enables additive manufacturing processing to occur on a multi-axis machine tool. Hybrid manufacturing is transforming the way engineers design and manufacture components from new, functionally graded materials to finished, internal features that were once impossible to machine any other way [189,190]. Modern CAD softwares are capable of representing geometries that were once impossible to manufacture from complex internal passages to gradient material concentrations within a single piece of material.
Hybrid additive manufacturing intertwines both additive and subtractive manufacturing layer by layer to digitally fabricate parts with complex geometries, improved surface finish and tight dimensional accuracies, the sum of which is difficult to obtain with any single process. Computer-Aided Manufacturing (CAM) software is required to orchestrate the machine toolpathing for both the deposition as well as the machining processes and is crucial for the successful fabrication of high quality structures. CAM requires substantial operator input to account for challenging aspects of each fabricated structure. For example, deciding at which layer of deposition will the machining process continue to maintain access to complex cavities for finishing - internal features that would otherwise be unfinishable due to reach limitations or obstructions. Moreover, of the many commercially-available hybrid systems, each has a unique kinematic environment which can benefit from specific optimization of toolpath planning and substantial research has directly correlated toolpathing with the microstructure evolution, mechanical properties, porosity and residual stress state of the final fabricated part. This review explores the available strategies for CAM in the context of hybrid direct energy deposition, discusses the advantages and disadvantages of each and considers future CAM trends for this transformational digital manufacturing technology.
A technical overview of metallic parts in hybrid additive manufacturing industry
2022, Journal of Materials Research and Technology
Additive manufacturing technologies have emerged as the promising alternatives of conventional manufacturing techniques. Conventional manufacturing techniques involves cutting and removal of material by mechanical procedures to achieve final product. Whereas, discrete chunks of material in any form are combined point by point and layer by layer for the fabrication of final product in additive manufacturing processes. Numerous advantages and inefficiencies of these manufacturing techniques are reflected in factors such as the design, fabrication, material properties and working condition etc. Therefore, development of a production technology by combining the benefits of both conventional and additive techniques is significantly important. “Hybrid Manufacturing” jointly apply additive and conventional production methods to attain final products. Hence, this short overview covers the operation aspects of both additive and subtractive manufacturing of metallic materials.
Fundamentals of stereolithography: techniques, properties, and applications
2022, Tribology of Additively Manufactured Materials: Fundamentals, Modeling, and Applications
Stereolithography (SLA) has become an important additive manufacturing technique nowadays. High-level accuracy, availability of a wide range of resin materials, and diversified applications from the medical industry to the prototyping sectors made SLA an efficient 3D printing process. Like other 3D printing processes, SLA printing also has the same three primary steps: design of the specimen, printing, and finally, postprocessing treatment. These steps are vital for creating prototypes and workpieces with desired mechanical, tribological, and corrosion properties. Typically, layer thickness, the orientation of the constructed piece, and the hatch spacing influence mechanical and tribological properties. In terms of corrosion and tribo-corrosion behavior, the addition of secondary substances, such as graphene, alumina, and zirconia, have been proven helpful to increase the conductivity, improve the strength of the material, and improve the texture for dental applications. Over the past decade, the SLA process has benefited rapid prototyping, preoperative planning, research, biocompatible organ printing, and many other engineering and biomedical applications.
Design and fabrication of conformal cooling channels in molds: Review and progress updates
2021, International Journal of Heat and Mass Transfer
Citation Excerpt :
Recently, a large step in the development of the equipment of hybrid additive/subtractive manufacturing was made by the company DMG Mori wherein it combined LPD and conventional cutting onto a CNC platform. The laser head and cutting tools can be exchanged discretionarily and automatically in the spindle, leading to the development of the first commercially-used hybrid LPD/cutting machine tool (DMG Mori LASERTEC series) worldwide [190]. It further increases the manufacturing flexibility and improves the dimensional accuracy and surface quality of both outer and inner surfaces significantly.
Conformal cooling (CC) channels are a series of cooling channels that are equidistant from the mold cavity surfaces. CC systems show great promise to substitute conventional straight-drilled cooling systems as the former can provide more uniform and efficient cooling effects and thus improve the production quality and efficiency significantly. Although the design and manufacturing of CC systems are getting increasing attention, a comprehensive and systematic classification, comparison, and evaluation are still missing. The design, manufacturing, and applications of CC channels are reviewed and evaluated systematically and comprehensively in this review paper. To achieve a uniform and rapid cooling, some key design parameters of CC channels related to shape, size, and location of the channel have to be calculated and chosen carefully taking into account the cooling performance, mechanical strength, and coolant pressure drop. CC layouts are classified into eight types. The basic type, more complex types, and hybrid straight-drilled-CC molds are suitable for simply-shaped parts, complex-shaped parts, and locally complex parts, respectively. By using CC channels, the cycle time can be reduced up to 70%, and the shape deviations can be improved significantly. Epoxy casting and laser powder bed fusion (L-PBF) show the best applicability to aluminum (Al)-epoxy molds and metal molds, respectively, because of the high forming flexibility and fidelity. Meanwhile, laser powder deposition (LPD) has an exclusive advantage to fabricate multi-materials molds although it cannot print overhang regions directly. Hybrid L-PBF/computer-numerical-control (CNC) milling pointed out the future direction for the fabrication of high dimensional-accuracy CC molds, although there is still a long way to reduce the cost and raise efficiency. CC molds are expected to substitute straight-drilled cooling molds in the future, as it can significantly improve part quality, raise production rate and reduce production cost. In addition to this, the use of CC channels can be expanded to some advanced products that require high-performance self-cooling, such as gas turbine engines, photoinjectors and gears, improving working conditions and extending lifetime.
Mechanical performance of 316 L stainless steel by hybrid directed energy deposition and thermal milling process
2021, Journal of Materials Processing Technology
Citation Excerpt :
The effects of processing parameters on the machining features and the residual stress were analysed, but the mechanical properties of the parts fabricated by the ASHM process were not analysed. Soshi et al. (2017) studied an injection mould manufacturing and cooling method using DED and CNC machines. The hybrid technology rapidly fabricated a mould with conformal cooling channels, thus substantially improving the cooling performance of the mould.
Due to its excellent temperature resistance, 316 L stainless steel (SS) is widely used in integral impellers; however, as the engine performance requirements have increased, the shape of integral impellers has become increasingly complex. Directed energy deposition (DED), with a high production rate and low cost, is used to directly fabricate complex structural geometries. Nevertheless, the surface quality is lower than that achieved by conventional methods, i.e., milling. To overcome this problem, a novel hybrid approach with DED followed by a subtractive milling process within a single workstation is developed. This method can directly produce internal and highly complex structural parts with ideal dimensional accuracy. However, the process parameters and mechanical properties of DED and subtractive thermal milling (starting milling temperature of 200−300℃) after each deposition of a set of layers have rarely been evaluated. The purpose of this study is to address these research limitations. The densification, phase composition, microstructure and mechanical behaviour are studied, and a correlation between process parameters and performance is newly established. The results indicate that the nearly fully dense 316 L SS specimens exhibit high microhardness and tensile strength under the optimum process parameters, which is attributed to the high density and fine microstructure. Moreover, the highest tensile strength (683.3 MPa) among all tensile samples is obtained with v = 8 mm/s. The tensile strength values for wrought (hot work-annealed), wrought (cold-worked), cast samples, and for the industry requirement for 316 L SS are 480, 574, 552 and 450 MPa, which are 42.97 %, 19.56 %, 24.33 %, and 52.51 % lower, respectively, than that for the hybrid DED and thermal milling process. The test results and a comparison analysis show that the components from the hybrid DED and thermal milling process can satisfy the industry requirements for 316 L SS.

View all citing articles on Scopus

View full text

Innovative grid molding and cooling using an additive and subtractive hybrid CNC machine tool

Abstract

Introduction

Section snippets

Overview of grid molding and cooling

Additive and subtractive hybrid CNC machine tool

Plastic injection and setup

Cooling performance

Conclusion and future work

CIRP Annals—Manufacturing Technology

CIRP Annals—Manufacturing Technology

CIRP Annals—Manufacturing Technology

CIRP Annals—Manufacturing Technology