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A computer-aided approach for gating system design for multi-cavity dies

by Ranjit Singh (Author) Rohit Kumar (Author) Sukhwinder Singh Jolly (Author)

Master's Thesis 2017 91 Pages

Engineering - Mechanical Engineering

Excerpt

Contents

1. Introduction
1.1 Main stages in the die-casting part manufacturing
1.1.1 Part design
1.1.2 Die-design
1.1.3 NC code generation
1.1.4 Die-manufacturing
1.1.5 Die-casting part manufacturing
1.2 Die-casting machines
1.2.1 Hot Chamber Die-Casting:
1.2.2 Cold Chamber Die-Casting:
1.3 Die-Casting Die
1.4 Design-Manufacturing Integration
1.4.1 Introduction
1.4.2 Design-Manufacturing Integration of Die-Casting Process
1.5 Computer Aided Die-Casting Die Design
1.5.1 Parting Line Generation:
1.5.2 Gating system
1.5.3 Side Core Design
1.6 Need and Motivation of the Proposed Work
1.7 organization of the thesis

2. literature review
2.1 Identification of undercuts features
2.2 Determination of the Parting line
2.3 Determination of Gating System Design
2.4 Side Core Design
2.5 Research Gaps
2.6 Objective of Proposed Work
2.7 Methodology for the Design-Manufacturing Integration
2.8 Benefits of present Work

3. Data initialization
3.1 Part information
3.2 material data
3.2.1 Material data of die-casting part
3.2.2 Material of Die
3.3 die-casting process data
3.4 production data
3.5 die-casting machine data
3.6 summary

4. Cavity design
4.1 determination of number of cavities
4.1.1 Number of cavities based on production time [Npt]
4.1.2 Number of cavities based on cost [Ncost]
4.1.3 Number of cavities based on Machine parameters [ ]
4.1.4 Number of cavities based on Part geometric features [ Ngeo ]
4.1.5 Selection of number of cavities
4.2 Selection of Feeding system
4.3 Selection of layout pattern
4.3.1 Factors affecting the layout pattern
4.3.2 Methodology for selection of layout pattern
4.4 orientation and placement of gate
4.5 Die base design
4.6 summary

5. Gating-System Design
5.1 Design Guidelines for Gating System Design
5.1.1 Gate design
5.1.2 Runner design
5.1.3 Overflow design
5.1.4 Biscuit design
5.2 gating-system design
5.2.1 Determine die-casting process parameters
5.2.2 Determination of Gating Parameters
5.3 System architecture for gating-system design
5.4 implementation and results
5.5 summary

6. Conclusions and Future Research Directions
6.1 Conclusions
6.2 Future Scope directions

7. References

LIST OF FIGURES

Figure 1.1: A block diagram depicting manufacturing process

Figure 1.2: Stages of die-casting part manufacturing

Figure 1.3: Hot Chamber Die-Casting Machine [6]

Figure 1.4: Cold Chamber Die-Casting Machine [6]

Figure 1.5: Various parts of Die [24]

Figure 1.6: Computer Aided Die-Casting Process Flow Diagram

Figure 1.7: Die Design Process [1]

Figure 1.8: Parting line for a cored component [8]

Figure 1.9: Gating System

Figure 3.1: Data Initialization Process

Figure 3.2: A Sample Part I

Figure 3.3: A Sample Part II

Figure 4.1: a.) Single cavity die b.) Multi-cavity die [41]

Figure 4.2: A flow diagram depicting various activities of cavity design

Figure 4.3: Flow Diagram depicting selection of number of cavities

Figure 4.4: Type of Feeding System

Figure 4.5: Types of Cavity Layout Pattern a) Symmetric b) Circular c) Series d) In-line

Figure 4.6: Layout pattern for both feeding system

Figure 4.7: Information flow diagram for selection of cavity layout pattern

Figure 5.1: A schematic of the distributed flow pattern [50]

Figure 5.2: A schematic of the Directed Flow Pattern [50]

Figure 5.3: A schematic of the shortest path of metal flow [50]

Figure 5.4: A schematic of the effect of gate shape on metal flow pattern [50]

Figure 5.5: A flow diagram to Determine probable position of gates

Figure 5.6: Part having different feature with parting line

Figure 5.7: Dimensional parameters of a chisel gate

Figure 5.8: Information Flow diagram of the computer-aided system for design of gating system for multi-cavity dies

Figure 5.9: Case study I

Figure 5.10: Multi-Cavity Multi-Gating system design for case study 1

Figure 5.11: Gating system design for case study 1

Figure 5.12: Part Case Study 2

Figure 5.13: Gating System Design for case study 2

LIST OF TABLES

Table 3.1: Part Information

Table 3.2: Material for part can be used with required properties

Table 3.3: Material of die and their properties

Table 3.4: Production Data for Die-casting process

Table 3.5: Machine Size and Information

Table 4.1: Clearance database for a die-casting die [44]

Table 5.1: Recommended gate velocities for common die-casting alloys

Table 5.2: List of part parameters

Table 5.3: List of process parameters and cavity design

Table 5.4: List of gating-system parameters

Table 5.5: List of part parameters

Table 5.6: List of process parameters and cavity design

Table 5.7: List of gating-system parameters

PREFACE

Introduction: Gating-system design is one of the crucial activities of die-design for die-casting dies. It involves determination of gating-system parameters, and modelling of gating-system features. Traditionally, a die-casting engineer uses his knowledge and experience along with information like material properties, machine and process parameters to determine gating-system parameters for a die-casting part. The gating-system parameters are then used to model the gating-system for die-casting part. Thereafter, the gating-system for the die-casting part is tested using filling simulations. The necessary modifications in the gating-system design are made till the desired results are obtained. Finally, the gating-system is tested on the die-casting machine and the final modifications in gating-system design are made.

Need and Motivation: The procedure of the gating system design, discussed above, makes the process iterative and time consuming. Moreover, the involvement of huge data processing makes gating-system design a tedious process. Therefore, there is a need of an automated system which would handle the huge data and reduce the iterations and the time required for gating-system design. In the recent past some of the researchers have focused their attention to automate some of the activities of gating-system design. A number of computer aided system are available for gating-system design for die-casting dies. Though, these system are helpful in automating some of the gating-system design activities yet the level of automation needs enhancement. Some of the limitations of the present systems are: (i) gating system design for multi-cavity dies that require multi-gates for each cavity has not been reported, (ii) automated placement of gate, which is an important factor for sound filling of the cavity, has not given due attention, and (iii) automatic determination of shape of the gate has not been reported. The issues related to traditional gating system design and available literature are discussed in Chapter 1 and Chapter 2 respectively.

Proposed system: This work presents a system that would help in automatic design of multi-gates for multi-cavities dies. The system is able to automate: (i) the determination of the placement of gate, and (ii) determination of the shape of gate for die-casting parts having simple shapes. The system has to be implemented in GUI of MATLAB 7.10 using the best industry practices and recommendations from NADCA for gating-system design. The proposed system is able to generate parameters of multi-cavity gating-system for parts requiring multi-gates from the part model data. The other information required for working of the system includes material properties, die-casting process parameters and die-casting casting machine parameters which comes under the data initialization process. The parameters of the gating-system which are determined by the system are used for design of gating-system for the multi-cavity dies. This is done by updating the parameters of pre-modeled gating-system features stored as feature library. Chapter 3 , 4 and 5 present the various modules of the system along with its implementation on case-study parts. The results from the case-studies are quite encouraging and are in-line with the best industry practices.

Originality / Value: The present work describes a novel methodology for computer-aided automatic placement of the gating system for a die-casting part. it uses the parting line information to identify the probable positions for the placement of the gating system. This aspect of die-design has not received due attention of the researchers in the past. Moreover, the automated gating-system design for die-casting parts that require multiple agates in a multi-cavity die has not been attempted. Both these features provide the originality to the present work.

Keywords: die-casting, die-design, gating-system design, multi-cavity, multi-gate.

Chapter 1

1. Introduction

Manufacturing is an activity of transforming raw materials, parts or components into finished and fine products that fulfil a certain purpose besides meeting customer requirements and specifications. The customer requirements relate to the functions which the component must do, and the specification are the qualities of the components that define its form and geometry. A manufacturing process can be understood by a simple block diagram as illustrated in Figure 1.1.

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Figure 1 . 1 : A block diagram depicting manufacturing process

Metallic components are abundantly used in various products available today. A wide variety of manufacturing processes exist that can be used to produce a variety of parts with different sizes and shapes [1]. These manufacturing processes are broadly classified into the following categories:

- Casting
- Forming and shaping
- Machining
- Joining
- Finishing

Usually there is more than one manufacturing process available to make a component of a given material. Each of these manufacturing processes has its own advantages, disadvantages, limitations, cost, and production rates. The selection of manufacturing process for producing a component is made by taking a number of factors into considerations such as form, geometry, material, production quantity, accuracy and tolerances. However, a single manufacturing process may not be capable to produce finished parts and additional processes maybe necessary. For example, rolled part may not have the desired dimensional accuracy and surface finish; therefore additional manufacturing processes such as machining or grinding maybe necessary. [1]

The additional processes or operations contribute significantly to the total cost of manufacturing. Consequently, there is trend of near net-shape or manufacturing processes [1]. Near net shape manufacturing processes utilize single manufacturing operations to manufacture a part that required almost no further processing and is ready for use. Some near net shape manufacturing processes are powder metallurgy, injection molding and die-casting. Few advantages of near net shape processes are as follows [2].

i. Parts requiring tight tolerances can be manufactured easily
ii. Cost of production is usually lower than metal removal processes
iii. Steps in the production are reduced
iv. Process variation is less
v. Combining of two and more components is possible, thus reducing the inventory

Die-casting is popular near net-shape manufacturing process which is capable of producing, sharply defined, accurately dimensioned and smooth or textured surface parts [3]. The die-casting process utilizes permanent metal molds called dies to produce thousands of parts in succession. In the die-casting process, the molten metal, which is typically a non-ferrous alloy is injected at high pressure ranging from 0.7 to 700 MPa into the die [4]. After sometime, the metal cools down and solidifies to produce a die-casting part. The die-casting part then ejected out of the die. The metal used for die-casting are: aluminum, zinc, magnesium, lead, copper and tin. The die-casting process is especially suited for applications where large quantity of small-to-medium-size parts are needed with good details, a fine surface quality and dimensional consistency. General weight range of die-casting parts is from 30 gm. to 20 Kg. Some common application of die-casting parts are: (i) housings for variety of equipment’s and appliances, (ii) automobile parts like cylinders, pistons, and engine block, and (iii) other parts like propeller, gear, valves, bushing and pumps [5]. This level of veracity has placed die-casting parts among the highest volume products made in the metalworking industry [3]. Following sections present main stages of the die-casting process, machines and tooling used in die-casting process equipment’s.

1.1 Main stages in the die-casting part manufacturing

This section explains main stages in the die-casting part manufacturing starting from part design to manufacturing of the die-casting part. The main stages of die-casting process are shown in the Figure 1.2 and are explained in the following paragraphs.

1.1.1 Part design

It refers to the design of the part or component which is to be manufactured using the die-casting process. Now days, the design of die-casting part is usually carried out with help of a CAD modelling software. The designer defines geometry, topology, material and other specification of part.

1.1.2 Die-design

It consists of a group of activities which are normally done by a die-designer. Example of various die-design activities are, cavity design, parting design, side-core design, gating system design and ejector system design. The die-designer utilizes his experience and applies industry best practices to prepare design for die-casting die. The die-casting die is also molded using CAD modelling software. A schematic showing various activities in the die-design is presented in section 1.4.

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Figure 1 . 2 : Stages of die-casting part manufacturing

1.1.3 NC code generation

Gemrally, NC and CNC machines are used to manufacture die-casting dies by using processes, such as machining. CAD model of die is used to generate part program for manufacturing of the die. The part program or NC code is prepared by using either manual or automated part programming.

1.1.4 Die-manufacturing

The die-casting die is generally composed of two halves, named core half and cavity half. As mentioned earlier, normally, NC and CNC machines are used to machine various elements of the die such as core, cavity, gating system, cooling channels, and ejector pin holes.

1.1.5 Die-casting part manufacturing

The manufacturing of die-casting part is done on manufacturing machine. The first step is to mount the die-casting die for the part on die-casting machine. Thereafter, the following procedure is adopted to complete the die-casting process cycle [5].

i. Die Preparation and Clamping: - The first step of die-casting process is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part. The lubrication time increases with part size, as well as the number of cavities and side-cores. Also, lubrication may not be required after each casting, but after 2 or 3 casting, depending upon the material. After lubrication, the two die halves, which are attached to the die casting machine, are closed and securely clamped together. Sufficient force must be applied to the die to keep it securely closed while the metal is injected. The time required to close and clamp the die is dependent upon the machine (those with greater clamping forces) and die size - larger machines and dies will require more time. This time can be projected from the dry cycle time of the machine.

ii. Injection: - The molten metal, which is kept at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used. The difference in this equipment will be detailed in the next section. Once transferred, the molten metal is injected at high pressures into the die. Typical injection pressure ranges from 1,000 to 20,000 psi. This pressure holds the molten metal in the dies during solidification. The amount of metal that is injected into the die is referred to as the shot. The injection time is the time required for the molten metal to fill all of the channels and cavities in the die. This time is very short, typically less than 0.1 seconds, in order to prevent early solidification of any one part of the metal. The proper injection time can be determined by the thermodynamic properties of the material, as well as the wall thickness of the casting. A greater wall thickness will require a longer injection time. In the case where a cold chamber die casting machine is being used, the injection time must also include the time to manually ladle the molten metal into the shot chamber.

iii. Cooling : - The molten metal which is injected into the die will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is molded. The die cannot be opened until the cooling time has over and done, and the casting is solidified. The cooling time can be estimated from several thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the die. A greater wall thickness will require a longer cooling time. The geometric complexity of the die also requires a longer cooling time because the additional resistance to the flow of heat.

iv. Ejection : - After the pre-determined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity. The time to open the die can be projected from the dry cycle time of the machine and the ejection time is determined by the size of the casting's envelope and should include time for the casting to fall free of the die. The ejection mechanism must apply some force to eject the part because during cooling the part shrinks and adheres to the die. Once the casting is ejected, the die can be clamped shut for the next injection.

v. Trimming : - During cooling, the material in the channels of the die will solidify which are attached to the casting. This excess material, along with any flash that has occurred, must be trimmed or cut from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the additional material can be projected from the size of the casting's envelope. The scrap material that results from this trimming is either rejected or can be reused in the die casting process. Recycled material may need to be overhauled to the proper chemical composition before it can be combined with non-recycled metal and reused in the die casting process.

The following section explains the types of die-casting machine. The term die-casting machine in this thesis refereed to the high pressure die-casting machines only.

1.2 Die-casting machines

The die-casting machines are classified mainly into two types, hot chamber and cold chamber machines. Selecting a type of machine depends upon die-casting part material to be manufactured. Types of die-casting machines are explained briefly in following paragraphs.

1.2.1 Hot Chamber Die-Casting:

Hot-chamber die-casting is the more popular of the two die casting processes. A schematic of hot chamber die-casting machine is shown in Figure 1.3. In this machine process, the cylinder chamber of the injection mechanism is completely immersed in the molten metal bath. A gooseneck metal feed system draws the molten metal into the die cavity. While direct immersion in the molten bath allows for quick and convenient mold injection, it also results in increased corrosion susceptibility. Due to this fact, the hot-chamber die casting process is best suited for applications that utilize metals with low melting points and high fluidity. Good metals for the hot-chamber die casting process include lead, magnesium, zinc and copper.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 . 3 : Hot Chamber Die-Casting Machine [6]

1.2.2 Cold Chamber Die-Casting:

The cold-chamber die casting machine is very similar to hot-chamber die casting machine. A schematic of cold chamber die-casting machine is shown in Figure 1.4. With a design that focuses on minimizing machine corrosion rather than production efficiency, the melted metal is automatically- or hand-ladled into the injection system. This eliminates the necessity for the injection mechanism to be immersed in the molten metal bath. For applications that are too corrosive for the immersion design of hot-chamber die casting machine, the cold-chamber machine can be an excellent alternative. These applications include the casting of metals with high melting temperatures, such as aluminum and magnesium alloys.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 . 4 : Cold Chamber Die-Casting Machine [6]

1.3 Die-Casting Die

A die-casting die consists of two halves termed as cavity and core. The cavity half is fixed and the core half is moveable. The core half is moved towards the cavity half to assemble the die. Molten metal is poured into the space left between the two halves termed as cavity. Figure 1.5 show various parts of a die with shot sleeve and plunger.

There are four types of dies which are generally used in the die-casting process:-

- Single cavity: A single cavity die has only one impression of the part in the die, and therefore can produce one part in a die-casting cycle.

- Multi-cavity: A multi-cavity die has a number of identical impressions of the part. A multi-cavity die is useful to produce a number of parts in a die-casting cycle.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 . 5 : Various parts of Die [24]

- Unit die: A unit die has a number of impressions on a die-base, which are not identical. A unit die is useful to produce a number of different parts in a die-casting cycle.

- Combination die: A combination die also has a number of non-identical impressions on a die-base to produce a family of parts for an assembly in a die-casting cycle.

1.4 Design-Manufacturing Integration

1.4.1 Introduction

Everyday new products are being launched in all the segments of the market. This is possible due to the adoption of the new design and manufacturing processes by the manufacturing companies. Modern design and manufacturing processes are different from traditional one. Traditional design and manufacturing processes are the manual processes in which a lot of paperwork involved and it was very time consuming. Now the days all these things are changed due to a lot of introduction of the computer systems, NC and CNC machines, and the information technology in design and manufacturing processes. The design section has been changed with hi-fi computer systems with 2D and 3D software like AUTOCAD, SOLIDWORKS, Pro E, CATIA etc. In manufacturing all the floor or system is changed with hi-tech machines controlled by computer and embedded systems. All this result for the development of the CAD/CAM which stands for the Computer Aided Design/ Computer Aided Manufacturing. CAD deals with managing and generating the design information and CAM deals with the planning, controlling and managing the manufacturing processes. Though, in last some decades a lot of development is made in CAD/CAM system. Normally some of the designers uses CAD with a little knowledge/understanding of the CAM system. Sometimes the design of the has to be change various time or modified as new properties/improvement to be made which leads to too much time consumption and the increase in the cost of the product. Therefore, if the designer will solve the all the machining and manufacturing problems at the design stage it will lead to the great saving in the machining and manufacturing timing and the manufacturing cost. It can only be achieved by integration of the manufacturing with design i.e. by using fully integrated CAD/CAM system. The following subsection will describe a little about the design-manufacturing integration, die-casting process and design manufacturing integration in the die-casting process.

As explained above about importance of the design-manufacturing integration, this section clarify in detail about the idea and give insight into the integration process. Design-integration involves the generation of the manufacturing information and data from the design of the product.

Many researchers made effort for the integration of the CAD/CAM system. Now the days, systems are available in the market which are capable of generating the manufacturing information data for the CAD model but they require a lot of human interference and the level of automation is limited. The main focus on the integration of CAD/CAM system to generate the manufacturing data information for the NC and CNC machines by utilizing the CAD information and information like part tolerance, material type, tool parameters, type of machine, etc. to generate the CAM database. On the other hand with the generation of CAM database we can also integrate the some other functions like process planning, management and robotic control.

Design-manufacturing integration has many benefits. Some of the benefits are such as increased productivity and quality of the product, eliminate costly mistakes and waste, less time consumption, better design analysis and greater accuracy in design calculations, improved process/product planning management and control, scheduling of tools and components etc.

1.4.2 Design-Manufacturing Integration of Die-Casting Process

In die-casting process involves large use of the CAD tools parts and die design. In this process with the help of the CAD tool first model of the part is prepared and by using the CAD model of the part, designing and making the CAD model of die takes place. For designing of the die-casting die needs lot of knowledge and expertise as the die designer. Design-Manufacturing integration leads to if we able to develop a system that is capable of generating a die design by using or from the 3D model of part itself. In this system will take the 3D model as input and from this 3D model it will recognize the various features of the part model and determine different aspects of die such as, parting line, parting direction, parting surface, side-core, undercut, gating system design, cavity layout etc. Figure 1.6 shows the important stages in the die-casting process.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 . 6 : Computer Aided Die-Casting Process Flow Diagram

1.5 Computer Aided Die-Casting Die Design

The die-casting die-design involves several non-trivial tasks. Fuh et al. [1] identify seven major steps of computer-aided die-casting die-design which are briefly summarized here and are illustrated using Figure 1.7:

i. Setting shrinkage and draft: The molten metal, which is injected in the die, contracts during solidification. Therefore, the negative impression of the part in the die (or the cavity) must be scaled by a certain factor to compensate the material shrinkage. Furthermore, to facilitate easy ejection of the die-casting part from the die, those surfaces of the cavity, which are parallel to the direction of die opening are slightly tapered; this taper is also known as draft.
ii. Determining the cavity number and layout: The number of cavities are determined accounting for different factors, such as part shape and dimensions, machine type, machine size limitation, and machine clamping force.
iii. Designing the gating system: Shape, size and location of different parts of the gating system, such as gate, runner and overflow wells are determined keeping in view the part geometry and cavity layout to achieve proper filling in the die cavity. Flow paths and filling conditions are also analysed at this stage.
iv. Designing the die-base: After the number of cavities and their layout is decided, a suitable die-base is selected. Size of the die-base is decided based on the die design requirements, such as accommodation of all the cavities and provision of clearances.
v. Parting design: The parting design here means to create parting surfaces along the selected parting line, which eventually helps to split the containing box or the die-base in which the negative impression of the die-casting part is formed into two halves, namely core-half and cavity-half.
vi. Designing the side-core mechanism: If the die-casting part has an undercut, the die may require a side-core. The design of side-cores is also required to complete the design of a die-casting die.
vii. Designing the cooling system: The purpose of the cooling system of a die is to keep it at a pre-determined uniform temperature. The cooling system comprises of a set of waterlines drilled within the die that takes the heat continuously being exhausted from the molten metal away from the die. The cooling system should be positioned and sized properly so as to achieve rapid and uniform cooling without interfering with the ejection system and side-core mechanism.

The role of CAD/CAM tools for die-design has become important to keep pace with the latest technology, demand for low cost, high quality, and fast delivery. Although, CAD/CAM tools are quite useful in preparing CAD model of a die-cast part, they lack many aspects. The mold tool applications of available CAD systems allow the user to use their functionalities for preparing CAD models of different components of the die. However, much needed design knowledge and automation of design steps especially for the die-design of multi-cavity die-casting is lacking.

First six steps of the die-casting die-design namely, (i) setting shrinkage and draft (ii) determining the cavity number and layout (iii) designing the gating system (iv)designing the die-base (v) parting design (vi) designing the side-core mechanism, are very important for design of a die-casting die. These steps not only are responsible for designing major component of a die-casting die, but also effect design of its other systems, such as cooling and ejection.

The designing of the gating system for the die-casting design is most crucial activity such that is directly affects the whole of the die-casting process which includes its manufacturing as well as the quality and the cost of the part produced. The gating system design is prepared by the casing engineer by considering the factors such as material properties, process parameters and the other die-design principle which are specified by NADCA (North American Die Casting Association). The designing of the gating system includes the design of the gate, runner, overflow and biscuit etc. Gating system design also further affected by parting line determination and side core design. For better gating system design to achieve one should have to work first on the parting line determination and side core design and they are discussed in following paragraph.

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Figure 1 . 7 : Die Design Process [1]

1.5.1 Parting Line Generation:

Parting line is the line where parting surface meets with the casting surface of the mold or in other words parting line is the plane in which two halves of the mold meet. It is the line which can be seen on the cast part where two halves of the die (core and cavity) meets. To the extent possible, all features should be oriented perpendicular to the parting line to facilitate removal from the mold. Normally, the parting line is transferred to the surface of the part as a witness line, an unavoidable result of two mating mold members. In some cases, as in the example shown in Figure 1.8, the full part geometry can be maintained in the upper half of the mold, in which case the parting line would be along the bottom edge of the component and no witness line would be created. At other times, the mold may be designed to separate along an inconspicuous edge, thus “hiding” the parting line. A parting line that can be contained in a single plane is preferred. However, it is sometimes necessary to modify the simple shape in order to mold desirable features. The added complexity, although it increases the cost of fabricating and maintaining the tools, may be cost effective when the features it molds would otherwise require machining or assembly operations. [7]

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 . 8 : Parting line for a cored component [8]

Parting line determination is based on the selection of the parting direction of the mold. There are numerous factors affecting the selection of the parting direction. Selecting of parting direction takes into account the draw distance, volume and number of undercut features, and type and shape of undercut features etc. Once the parting direction is final we can move to determination of the parting surface or parting line. The parting line is generally the maximum perimeter of the part as seen from the parting direction and projected on the plane perpendicular to the parting direction.

So a method must be devised to determine the maximum perimeter and thus the parting line. It should also be considered that the resultant parting line should meet the rules of thumb for the same.

1.5.2 Gating system

The gating system for the die-casting die is most crucial element such that is directly affects the whole of the die-casting process which includes its manufacturing as well as the quality and the cost of the part produced. The gating system design is prepared by the casting engineer by considering the factors such as material properties, process parameters and the other die-design principle which are specified by NADCA (North American Die Casting Association).

The main elements of the gating system design are explained below and shown in Figure 1.9:-

i. Gate: Gate is the thin passage for molten metal which connects runner to the mold cavity or controlled entrances from the runners into the mold cavities. Generally, it provides small restriction to the molten metal.

ii. Runner: A runner is the path or passage which connects sprue to gate. Sprue is the vertical part of the gating system through which molten metal pours into the die. A runner can be subdivided into three parts:

- Gate-Runner: the portion of the runner that connected to the gate is known as gate-runner. A gate runner leads the molten metal into the mold cavity.

- Branch-Runner: a branch runner connects the gate runner to the main-runner.

- Main-Runner: a main runner is the passage which connects the branch-runner to the biscuits.

iii. Overflow well: an overflow well is a small reservoir cavity added to exterior of the die cavity to receive impure or cold metal during cavity filing. It is also used to generate added die heat in the local area.

iv. Biscuit: a biscuit is the excess of the molten or solidified metal that left in the shot sleeve of the cold chamber die-casting machine. It is the part of cast shot and is removed from the die along with the casting.

v. Flow angle: flow angle is the angle at which the metal flows into the cavity.

illustration not visible in this excerpt

Figure 1 . 9 : Gating System

1.5.3 Side Core Design

A side core is a local core which is normally mounted at right angle to the mold axis for forming a hole or recess in the side face of a molding. Any portion of a molding, which blocks the opening of its mold or prevents its removal from a core, is called an ‘‘undercut.’’ To form a molding with undercuts, side cores are required, which are withdrawn along releasing directions by a slider mechanism.

Traditionally, side core design is carried out manually, which is tedious, time consuming, and error-prone, not to mention enormous experiences required. To design a side core, a designer first has to identify an undercut region, and then carefully extract the geometric entities from the undercut region to form the side core. However, for complex shapes in moldings, undercut regions may not easily be detected by the designer. On the other hand, the design-to-market lead time becomes vital to the success of products. Automated side core design, therefore, becomes necessary for the mold industry. [9] Based on the discussion on parting line generation, gate runner system and side core design next section deals with the need and motivation for proposed work.

1.6 Need and Motivation of the Proposed Work

As discussed earlier, die-casting die design is very tedious, time consuming and costly as it involves lot of human being interference and efforts. Therefore to achieve the objective of design-manufacturing integration this gap must be filled. Also the parting line determination, gating system design, and side core design need lot of knowledge and human expertise. Now the days CAD systems are extensively used in industry for the designing of a die-casting die but these system are still not efficient and rarely provide requisite knowledge for the automated die design. The die designer have to take decisions for the selecting required various parameters for the design. So there is need of system that can bring the knowledge and expertise of the die designer together into one system. Such a system which would perform activities like determination of parting line, gating system design and side core design automatically. If such a system is developed it would greatly help to bridge the gap between the design and manufacturing of die-casting process. The present work is a step towards developing a computer-aided system for multi-cavity dies wherein cavity design and gating system design are determined in an automated manner.

1.7 organization of the thesis

Organization of the rest of the thesis is discussed in the following paragraphs.

Chapter 2 reviews previous research attempts on computer-aided die-casting die-design, primarily covering following four important aspects of it: (i) determination of parting line, (ii) identification of undercut features, (iii) gating system design, and (iv) integrated framework for the die-casting die-design. Furthermore, a few research papers related to injection molding process, relevant to the present research are also discussed. On the basis of the literature review, the shortcomings found in the available system are identified, and the information is summarized in the form of research gaps. Lastly, research objectives of the thesis are also discussed.

Chapter 3 presents a brief detail about the data initialization for die-casting parts. The chapter discusses: (i) part information, (ii) material data, (iii) die-casting process data, (iv) production data, and (v) machine data. The information of the proposed data is presented in the form of Tables and its working is discussed. Capabilities of the developed data are demonstrated with the help of industrial case study parts.

Chapter 4 presents a system for cavity design. The chapter discusses four modules of the proposed system: (i) determination of the number of cavities, (ii) selection of feeding system, (iii) selection of layout pattern, and (iv) orientation and placement of gate. Information flow diagram of the developed system is presented and its working is discussed. Capabilities of the developed system are demonstrated by presenting results for industrial case study parts.

Chapter 5 presents a computer-aided system for gating-system design for die-casting parts with multi-cavities that require multi-gates. The chapter discusses the following modules, namely: (i) design guidelines for gating system design, (ii) gating-system design, (iii) system architecture for gating-system design and (iv) generate CAD models of gating-system features using the feature library. Information flow diagram of the proposed system is also discussed. Capabilities of the proposed system are demonstrated with the help of industrial case study parts.

Chapter 6 discusses a brief summary of the systems and framework presented in this thesis. Major contributions of the research work are also discussed. Lastly, the chapter discusses future research directions.

Chapter 2

2. literature review

From past few decades researchers are working on design-manufacturing integration. So, literature available on the design-manufacturing die-casting die design was also referred here. Some of the literature related to the plastic injection molding also studied due to the similarities between both of the processes. In this section literature review on the general topics of the die-casting i.e. identification of undercuts features, parting line determination, gating system design and side core design is presented. Research gaps also discussed at the end of this section. Some of the literature surveyed is discussed in following paragraphs.

2.1 Identification of undercuts features

Fu et al. [10] presented the definition, classification and a concept to identify the undercut features for injection mold design system. The Visibility maps (V-maps) were used to find out the draw range and direction of the undercut. A criterion of virtual edges was introduced to deal with blending surfaces. The methodology was limited to the determination of the undercut draw range and direction. Optimum parting direction and parting surface determination were not considered.

Chen and Chou [11] defined the levels of visibility being complete and partial. An algorithm was developed for computing augmented visibility map of a non de moldable surface and then to find out the direction with the minimum number of under cuts. An algorithm was also developed for computing the volume of the undercuts in a particular direction. Only polyhedron surface was considered for application of the algorithm.

Nee et al. [12] presented a method to detect the undercut features and extract the same using geometric and topological relations. The undercuts were also classified and defined. The system was capable of finding the direction of the undercut feature. The optimum parting direction was determined based on the number as well as volume of the undercuts. The methodology could not handle free form surfaces.

Woon and Lee [13] developed a system for die-casting die design. The system has seven modules that work on the different areas of die design. A parametric designing system is used to create a 3D model using B-rep and to extract the geometric and topological information. The system was implemented in API of solid works 2000 and was written in C++. The system is very helpful in designing a die from scratch or modifying the existing design. The feature library is not sufficient. The system could not perform design of gate and venting system.

Choi et al. [14] developed a system that dealt with the cast design, die layout design and die generation. The system has sound database for material and shrinkage, gate-runner and overflow, air vent, and cavity block. The application of the system was studied on a cap shaped product and a motor pulley the system was no implemented on parts with undercuts and automatic selection of parting line was not taken into account.

Kumar et al. [15] have applied polyhedron face adjacency graph technique to recognize completely and partially visible undercuts on the basis of the angle between the adjacent polyhedron. They classified undercut features into completely and partially visible undercuts.

2.2 Determination of the Parting line

Madan et al. [16] use die-casting feature recognition to determine the parting direction parting line. Geometric reasoning and rules are applied to the part model to identify the die-casting features. A B-rep file of the die-casting part is used for this purpose. These features then used to identify and classify the undercut features. The withdrawal directions of these features are also determined. The withdrawal directions are used as candidate parting directions. The parting directions are evaluated on the basis of criteria that considers the factors of projected area, draw distance, undercut volume, and draft surface, etc. surfaces of parts are classified into three categories based on their moldability. Generally the common outer edge-loop of the core and cavity surfaces is the parting line. The system also determines parting line for the parts having vertical surfaces with protrusion features. However, the proposed technique for parting line section has limited application on die-cast parts with free form surfaces.

Kumar et al. [17] present methodology to recognize parting surface of the molded part. The methodology first identifies the undercut surfaces using polyhedral face adjacency graph technique. The undercut feature is then classified as visible and partially visible undercuts. Thereafter, the parting surfaces are determined in each candidate parting direction. These parting surfaces are normal to the selected parting directions. Each parting surface for given parting direction is then evaluated on the basis of number of undercuts presents. To select an optimal parting surface the criteria of minimum number of undercuts, flatness of parting line and draw system is used.

Singh et al. [18] developed a system for automated determination of parting line for die cast parts. In this system, discussed classification of the die-cast part surfaces, identification of undercuts and protrusions, identification of parting line regions, and determination of the parting line. The system generates a number of feasible parting lines in a given parting direction after applying the die-casting process requirements. Finally, the most suitable parting line is determined from the feasible parting lines considering the industry best practices. The results obtained from the system are similar to those of the industry. The proposed system would prove to be a major step towards automation of the die-casting die design, leading to design–manufacturing integration of the die-casting process.

Wong et al. [19] proposed an algorithm for parting line formation based on uneven slicing on the product model. The main function of the algorithm is to locate the parting line in predetermined parting direction. The parting direction is usually selected in the direction of principle axis. The optimum parting line is selected based on parting line selection criteria. The system was implemented on a virtual helmet. The user can also modify the profile of selected parting line. The system cannot take parting direction other than the principle axis.

Nee et al. [20] proposed a methodology to generate 3D parting lines and parting surfaces for the injection molded parts. An optimal parting direction was considered first. Based on the topological relationship of the surfaces to the optimal parting direction these were classified into three groups. A plane perpendicular to the optimum parting direction was taken to calculate the projected area of an edge loop on it. Then the parting surfaces were determined by extruding the 3D parting lines. The method did not consider application on the free form surfaces.

Fu et al. [21] apply surface visibility and moldablity to generate the parting line for die-casting and injection molded parts. The proposed methodology first identifies the edge-loops on the part into single edge-loop (SEL), and multi-edge loop (MEL). These edge-loops are further classified as internal edge-loop (IEL) and external edge-loop (EEL). The parting line is determined by identifying EELs of the part. However, external edge-loop of the part, may not result in a unique parting line in those cases when surfaces parallel to the parting direction are present in the part.

Priyadarshi and Gupta [22] develop an algorithm for an automated design of a multi-piece permanent mold. The algorithm first determines the candidate parting directions. The candidate parting directions are: (i) direction of the coordinate axis, (ii) normal to the planner faces of the part, and (iii) axis of cylindrical and conical features. Thereafter, the accessibility of each facet in candidate parting directions is checked. They also developed an algorithm, ISOBSTRUCTING, which is used to check the obstruction of the front facing facets by other facet in the candidate parting direction. Thereafter, the mold-piece regions are identified from the part boundaries. Each mold-piece region is a set of the facets accessible in a candidate parting direction. However, a facet may be present in more than one mold-piece region. Therefore, the next step is to find the optimal set of mold-piece regions with minimal number of mold-pieces regions. Lastly, the mold pieces are constructed from the gross mold by using the method of slicing. The boundary of the mold piece regions is selected as the parting line. However, there is no separate treatment for undercut detection for two-piece permanent molds.

Zhao et al. [23] present an iterative surface growth algorithm for geometric moldability analysis of the injection molding parts. The algorithm first classifies the surface on the basis of geometric moldability and visibility. The surface are classified as: (i) core surfaces, (ii) cavity surfaces, (iii) zero draft surfaces, and (iv) invisible surfaces. Thereafter, the core, and cavity surfaces groups are identified. The next step is to identify the outer parting line (OPL) loops and inner parting line (IPL) loops. The OPL with the maximum projected area is preferred. The flatness factor is considered to determine the preferred OPL. The undercut surfaces are regrouped to form undercut features. The algorithm is applicable to free form surfaces. However, the effect of the presence of an undercut features on the part’s surface which are parallel to the parting direction is not accounted for.

Li et al. [24] purpose a system for generation of a smooth parting line by using meshed models. The triangular facets of models are first classified as: (i) up facets, (ii) down facets, and (iii) neutral facets. The up and down facets are further categorized as visible and occluded facets. The facets which are occluded from the undercut features. The parting line runs through the facets which have their normal perpendicular to the parting direction. Weighted shortest path algorithm is used to select the parting line. The triangulation renders some of the mesh points outside the parting line thus creating undercuts. These undercuts are removed by first identifying them, then adjusting their direction normal and finally updating the position of their vertices. The generated parting line is based on geometric computation only. However, the system does not consider the die-casting process requirements.

2.3 Determination of Gating System Design

Singh et al. [25] proposed a computer aided system for gating-system for die-casting die-design with multi-gates which helps to generate parameters of the multi-cavity die taking into account a multitude of factors, such as part design, material properties, process data and die-casting machine information. The parameters so generated are applied on selected gating-system feature existing in the library to complete the design of gating-system. The system generated results for industrial case-study parts are in tandem with the industry practices. The proposed system would greatly help bridge the existing gaps between design and manufacturing for die-casting die-design.

[...]

Details

Pages
91
Year
2017
ISBN (eBook)
9783668542723
ISBN (Book)
9783668542730
File size
2.5 MB
Language
English
Catalog Number
v375250
Grade
8
Tags
die-casting die-design multi-cavity multi-gate

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Title: A computer-aided approach for gating system design for multi-cavity dies