Innovative grid molding and cooling using an additive and subtractive hybrid CNC machine tool
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:
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DED and subtractive machining hybrid process can successfully produce functional molds using a framed grid of individual blocks as substrate.
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Simulation results showed very uniform cooling
References (9)
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Design for Additive Manufacturing: Trends, Opportunities, Considerations, and Constraints
CIRP Annals—Manufacturing Technology
(2016) - et al.
Influence of Metal Powder Characteristics on Product Quality with Directed Energy Deposition of Inconel 625
CIRP Annals—Manufacturing Technology
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Comparative Environmental Impacts of Additive and Subtractive Manufacturing Technologies
CIRP Annals—Manufacturing Technology
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CIRP Annals—Manufacturing Technology
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