Computer aided mould design modification and tool path regeneration for product change

In addition, the design of the plastic part may be modified at different stages of mould manufacturing process, while the mould design modification method is related to the mould machini

COMPUTER-AIDED MOULD DESIGN MODIFICATION

AND TOOL PATH REGENERATION

ZHANG LIPING (B ENG.)

A THESIS SUBMITTED FOR THE DEGREE OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2004

ACKNOWLEDGMENTS

First and foremost I would like to wholeheartedly thank my supervisors, Professor Andrew Nee Yeh Ching and Associate Professor Jerry Fuh Ying Hsi for their support (morally and academically) and for giving me invaluable guidance, suggestions, encouragement and patience throughout the duration of my graduate study in National University of Singapore I appreciate very much all they have done for me and I will gratefully remember all forever

Sincere appreciation is expressed to Associate Professor Loh Han Tong and Associate Professor Zhang Yun Feng for their kind words of advice during my research

Thanks are conveyed to National University of Singapore for providing me with the research scholarship and to Department of Mechanical Engineering and CAD/CAM/CAE Center for the use of the facilities

Finally, I wish to express my deepest thanks to some very special people in my life:

my husband, Dr Ding Xiaoming, thank to his love, patience, understanding and great support during my graduate study, for always encouraging me to do my best, for never losing faith in my abilities; my son, Ding Changzhao, of his smiles, laughter and understanding; my dear parents and in-laws, for their continuous concern, confidence and moral support This thesis is specially dedicated to them

There are many others who have indirectly contributed to my research and although I

am not mentioning any name here, I am grateful to all of them

Acknowledgements……….I Table of Contents………III Nomenclature………VIII List of Figures……… IX Summary……… XIII

Chapter 1 Introduction……… 1

1.1 Product development processes……… 3

1.2 Mould design and modification……… 6

1.2.1 Mould structure and subsystems……… 6

1.2.2 Mould design……… 8

1.2.3 Mould design modification………10

1.3 Tool path generation and regeneration………11

1.3.1 CNC machines and cutters…….………11

1.3.2 Tool path generation…….……….12

1.3.3 Tool path regeneration…….……… 13

1.4 Research objectives……….…………14

1.5 Outline of the thesis……….……… 15

Chapter 2 Literature Review………17

2.1 Mould design……….………….18

2.1.1 Parting direction and parting line design……….18

2.1.2 Insert design……….21

2.1.3 Mould system and subsystem design……… 23

2.2 Tool path generation.………24

2.2.1 CC-point method……….25

2.2.2 CL-point method……….29

Chapter 3 Mathematical Background of Curves and Surfaces…….34

3.1 Curve parameters……… 34

3.2 Surface parameters……… 36

3.3 Curves on surfaces……… ………… 40

3.4 Offset surface……….…………41

Chapter 4 Mould Design Modification………42

4.1 Basic concepts and principles of mould design modification……… 43

4.1.1 Concepts and assumptions of mould design modification……44

4.1.2 Principles of the insert and pocket design……….45

4.2 Architecture of the mould design modification system………48

4.3 Identify the solid bodies of material to be added and removed…… 50

4.4 Identify the mould insert that material needs to be added to or removed from………51

4.5 Remove material from the mould……… 53

4.6 Add material to the mould……… 53

4.6.1 Create pocket and insert……….53

4.6.2 Detect interference……….60

5.1 Terms and notations……….71

5.2 Basic concepts of tool path regeneration………73

5.3 Methodology for detecting affected CL-points……… 74

5.4 Tool path regeneration algorithms for CL-point method……….78

5.4.1 Identifying and replacing affected CL-points………80

5.4.2 Calculating scallop height values and adding new CL-points to the modified region………86

5.4.2.1 Machining scallop height and step-over size………….86

5.4.2.2 Algorithm for checking the scallop height value and adding new CL-points………90

5.5 Illustrative examples……….93

Chapter 6 Tool Path Regeneration with the CC-point Method….102 6.1 Terms and basic concepts……… 103

6.2 Methodology of identifying the affected CL-points……….104

6.3 Tool path regeneration algorithms for the CC-point method………108

6.3.1 Identifying and replacing the affected CL-points………109

6.3.2 Adding new CL-points for the modified region……… 116

6.3.2.1 Calculating the CL-points with the given tolerance…116 6.3.2.2 Calculating scallop height and adding new CL-points for the modified region……… 119

6.3.2.3 Detecting and removing gouging CL-points……… 119

6.4 Illustrative examples……… 119

Chapter 7 System Implementation and Case Studies……… 128

7.1 The computer-aided mould design modification and tool path regeneration system……… 128

7.1.1 The platform……….128

7.1.2 The architecture of computer-aided mould design modification and tool path regeneration system……… 130

7.1.3 The user interface……….133

7.1.4 The mould design modification module……… 133

7.1.5 The tool path generation and regeneration module……… 134

7.2 Case studies……… 136

7.2.1 Case 1………136

7.2.2 Case 2………142

Chapter 8 Conclusions and Recommendations………149

8.1 Conclusions……….149

8.1.1 Mould design modification……… 150

8.1.2 Tool path regeneration……… 150

8.2 Recommendations……… 151

8.2.1 Mould design modification……… 151

8.2.2 Tool path generation……….152

8.2.3 Tool path regeneration……… 152

List of Publications from this Study……….154

NOMENCLATURE

UFUN User-Function

UG Unigraphics

Figure 1.1 Plastic part development processes……… ………4

Figure 1.2 A typical injection mould structure……… ………7

Figure 1.3 A general mould design process……… ……… 9

Figure 2.1 Parting lines and parting surfaces……… …19

Figure 2.2 The iso-parametric method………26

Figure 2.3 The inverse tool offset method……… …….31

Figure 3.1 The Frenet frame……… ……… 35

Figure 4.1 Different shapes of inserts and pockets……… …47

Figure 4.2 Framework of the mould design modification system…… ………….49

Figure 4.3 Material to be added to or removed from designed mould………50

Figure 4.4 Cavity and core are affected by product design modification…………51

Figure 4.5 Designed insert and pocket……… … 57

Figure 4.6 An ejector hole interferes with the pocket……….58

Figure 4.7 Fix the insert with screw or welding process……….59

Figure 4.8 Lifter and cooling holes………….………61

Figure 4.9 Minimum distance between pocket and other holes……… 63

Figure 4.10a Old product file……… 65

Figure 4.10b New product file.……… 65

Figure 4.10c Old cavity.……… 66

Figure 4.10d Modified cavity……… 67

Figure 4.10e Old core ……… 68

Figure 4.10f Modified core……….69

Figure 5.1 Cutter contact (CC) and cutter location (CL) points……… 72

Figure 5.2 Surfaces A and its offset A o……… 75

Figure 5.3 Boundaries of A and TA……….77

Figure 5.4 Surfaces A and A′′′′……… ………78

Figure 5.5 Extreme points and affected points………81

Figure 5.6 Difference between z and z′……… 85

Figure 5.7 Overcut due to the wrong tool path direction………86

Figure 5.8 Machining scallop height……… ……….87

Figure 5.9 Calculation of tool path interval……… ………… 88

Figure 5.10 Circles interpreted from CL and CC-points……… 89

Figure 5.11 Changes of tool path direction due to an odd number of tool path lines………91

Figure 5.12a Part surface of a workpiece before modification……… 93

Figure 5.12b Tool paths of the workpiece before modification……… 94

Figure 5.12c Part surface of work-piece after modification………94

Figure 5.12d Regenerated tool paths with replaced points and added lines after modification……….………… 95

Figure 5.12e Tool paths before and after modification with replaced points and added lines……… ……….96

Figure 5.13a Part surface of bezel mould before modification……… 98

Figure 5.13b Tool paths of bezel mould before modification……….98

Figure 5.13c Part surface of bezel mould after modification……… 99

Figure 5.13d Regenerated tool paths with replaced points and added lines after modification……….100

Figure 5.13e Tool paths with replaced points and added lines before and after modification……….…………101

Figure 6.2 Curves on surface r and offset surface ro……….105

Figure 6.3 Removing undercut and interference from surface A……… 107

Figure 6.4 Identifying affected CL-points ……… ……….110

Figure 6.5 Step length L………117

Figure 6.6 Convex gouging……… 118

Figure 6.7a Part surface of a workpiece before design modification……… 120

Figure 6.7b Tool paths of the workpiece before design modification……….120

Figure 6.7c Part surface of work-piece after design modification……….121

Figure 6.7d Regenerated tool paths with replaced points and added lines after design modification……….122

Figure 6.7e Tool paths with added lines before and after design modification… 123

Figure 6.8a Part surface of T-cover mould before design modification………….124

Figure 6.8b Tool paths of T-cover mould before design modification………… 124

Figure 6.8c Part surface of T-cover mould after design modification………125

Figure 6.8d Regenerated tool paths with replaced points after design modification……….126

Figure 6.8e Tool paths of T-cover before and after design modification…………127

Figure 7.1 Framework of the mould modification and tool path regeneration system……… 131

Figure 7.2 New product selection……… 133

Figure 7.3 Input mould machining status……… 134

Figure 7.4 Item selection……… 135

Figure 7.5 Input machining parameters……….136

Figure 7.6a The old plastic part of a dairy clear door……….137

Figure 7.6b The new plastic part of a dairy clear door………137

Figure 7.6c The original cavity of a dairy clear door……… 138

Figure 7.6d The modified cavity of a dairy clear door……… 138

Figure 7.6e The original core of a dairy clear door………138

Figure 7.6f The modified core of a dairy clear door……… 139

Figure 7.7a Tool paths of the workpiece before modification………140

Figure 7.7b Regenerated tool paths with replaced points and added lines after mould modification……….141

Figure 7.7c Tool paths before and after modification with replaced points and added lines……… 142

Figure 7.8a The old plastic part of a riser……… 143

Figure 7.8b The new plastic part of a riser……… 143

Figure 7.8c The original cavity of a riser………144

Figure 7.8d The modified cavity of a riser……… 144

Figure 7.8e The original core of a riser……… 144

Figure 7.8f The modified core of a riser……….145

Figure 7.9a Tool paths of the workpiece before modification………146

Figure 7.9b Regenerated tool paths with replaced points and added lines after mould modification……….147

Figure 7.9c Tool paths before and after modification with replaced points and added lines……… 148

SUMMARY

To shorten the product development time, a new plastic part is usually sent to the mould manufacturing company before it is finalized The design of the plastic part may need to be changed many times during the mould manufacturing processes It may be necessary to modify the mould design and to regenerate the affected tool paths many times However, the existing CAD systems cannot automatically modify the mould design for non-parametric moulds In addition, the design of the plastic part may be modified at different stages of mould manufacturing process, while the mould design modification method is related to the mould machining status Moreover, with the existing CAM systems, no matter how small the portion of a mould is to be modified, the entire tool path that covers this region will need to be recalculated, which could take as much time as for generating a new tool path could increase the chance of NC programming errors Consequently, the process of mould design modification and tool path regeneration is still very time-consuming To solve these problems, this research focuses on the following aspects:

Mould Design Modification A new mould design modification algorithm that can

automatically modify the mould design for parametric and non-parametric parts according to the mould machining status has been developed in this research The developed algorithm does not rely on the product parameters that increases with the complexity of the product and the mould Therefore, it is not sensitive to the

complexity of the plastic part and the designed mould Different methods have been developed to modify the mould design according to the mould machining status When

an insert needs to be designed to add extra material, the interference between the pocket to be designed and the existing holes is detected automatically Different pockets, inserts and fasteners will be designed based on the result of interference detection

Tool path regeneration Four propositions have been made and proven in this research

for tool path regeneration The propositions indicate two important properties of a gouge-free tool path: 1) the affected CL-points are enclosed by the boundary of CL-points that is corresponding to the interference-free boundary of the modified region; 2) when projected onto the XY-plane, if one CL-point is ‘before’ another one, their corresponding CC-points follow the same topology With these propositions, the affected CL-points can be efficiently identified by only noting their x- and y-values New tool path regeneration algorithms have been developed to identify and replace the affected CL-points for 3-axis NC machining By utilizing the unaffected CL-points, the tool path can be regenerated efficiently Since the tool path regeneration method is related to the tool path generation method, and the tool path methods can be categorized into CC- and CL-point methods, two different tool path regeneration algorithms are developed for them respectively

From this research, a computer-aided Mould Design Modification and Tool Path

Regeneration System has been developed to modify the mould design and regenerate

tool paths automatically and efficiently

CHAPTER 1 INTRODUCTION

Thermoplastic parts, as they can provide properties of low density, superior corrosion resistance, electrical insulation and suitability for volume production, are widely used

in engineering and consumer products The most prevalent method for producing thermoplastic parts in large quantities is injection moulding, which is a highly cost-effective, efficient, and precise manufacturing method The process can be highly automated and produces almost no waste

Facing the challenges of an increasingly more competitive market, time-to-market plays a crucial role in new product development Technology tools such as Computer-Aided Design/Manufacturing/Engineering (CAD/CAM/CAE) have been developed to help manufacturers achieve the goals of an ever-decreasing life cycle of a product from concept to market On the other hand, as they are required to accomplish more functional and aesthetic requirements, plastic parts are becoming more complex To meet the tighter product development schedule, a plastic part may very often be passed

to the mould manufacturing company for manufacturing before it is finalized The design of a new plastic part may be changed many times during the mould design and manufacturing processes If the design of a plastic part has been changed, the mould design would need to be modified and the affected mould inserts re-machined

Chapter 1 Introduction

Although some commercial CAD/CAM systems allow for automatic modification of the design for parametric parts, they cannot automatically modify the mould design for non-parametric moulds Here, a parametric part means that geometric definitions of the design, such as dimensions, can be varied at any time in the design process by changing the parameters In addition, it may be necessary to modify the design of the plastic part at different stages of mould manufacturing process, while the method of mould design modification is dictated by the mould machining status However, this is not taken into consideration in the existing CAD/CAM systems Moreover, with the existing CAM systems, no matter how small the portion of a mould is modified, the entire tool path that covers the modified region will have to be recalculated This could take as much time as for generating a new tool path Therefore, the process of mould design modification and tool path regeneration in NC programming is still very tedious, time-consuming and error-prone

This research aims at developing a Computer-Aided Mould Design Modification and

Tool Path Regeneration System With this system, when the design of the plastic part

is changed, the modified regions are identified and located by comparing the old and the new plastic parts The mould design is then modified automatically based on the designed mould and the mould machining status The affected tool path is regenerated efficiently by identifying and replacing the affected Cutter-Location (CL) points The unaffected CL-points are reused directly to machine the modified mould, which would greatly reduce the tool path regeneration time

This chapter introduces the background and basic concepts of the proposed research The general process of plastic part development is summarized in the first section

Mould design and modification processes are analyzed in the second section This is followed by tool path generation and regeneration methods in the third section, and the objectives of the research in the fourth section

1.1 Product development processes

As shown in Figure 1.1, the processes of a product design and manufacturing include: plastic part design, mould design, mould manufacturing, surface polishing, mould assembly, mould try-out, design and manufacturing modifications if necessary, moulding and product launch

A plastic part is generally a component of a functional product After the concept design of a new product is created, the plastic part is designed based on the functional and aesthetic requirements The mould capability and manufacturability should be considered when designing the plastic part In addition, some rules for mould design should be followed to avoid the common pitfalls, e.g., design optimal wall thickness according to the function, the plastic material and the cost; keep the wall thickness of the plastic part uniform; locate the gate so that the melt plastic enters the cavity in the thickest area and flows to the thinner areas; design radius at all corners of the part; and design optimal draft angles for removing the plastic part easily for the core and cavity

Traditionally, plastic parts are designed with general CAD software based on the designer’s experience, which is time consuming and error-prone The development of CAE systems has eliminated various trial-and-error practices and greatly streamlined the product development cycle However, as there are many parameters that affect the quality of a moulded part, the design of the plastic part may not be optimal and the

Chapter 1 Introduction

design may need to be modified many times before the product can be launched For example, the design of a space mouse was changed 9 times during the mould design and manufacturing processes Over the past decade, The Engineering Change (EC) is becoming more and more frequent The reasons can be summarized as follows: (1) The new product design files are often passed to mould companies before it is finalized; (2)

Figure 1.1 Plastic part development processes

Plastic part design

related parts; (4) The feedback from the market; (5) Many new products have only six months to prove themselves in the marketplace

After a three dimension (3D) solid model of the plastic part is designed, a prototype may be created for checking and verification If the prototype meets the requirements, the mould can be designed and manufactured, which is generally done in another company Otherwise, the design of the plastic part will need to be modified and verified again

Since the plastic part and the mould are usually designed in different companies, different CAD systems may be used In this case, the designed plastic part file will be transferred from one format to another, and the design parameters may be lost during file transformation When the mould designer receives the plastic part, he/she will study and analyze the mould capability and then design the mould If the design of a plastic part is found to be not suitable for moulding, it will be returned to the product designer for modification

When the mould is designed, the material will be ordered and the mould can be machined Computer Numerical Control (CNC) milling and Electric Discharge Machining (EDM) are the two common processes that are used to machine the mould inserts with 3D profiles Mould inserts are most commonly machined with CNC milling machines The EDM process is used when the part is difficult or impossible to machine with milling cutters This research focuses on the CNC milling process

After the mould inserts are machined, the surfaces that form the profile of the plastic

Chapter 1 Introduction

part will be polished The mould is then assembled and tested If the tested part satisfies the requirements, the mould can be used for mass production and the product can be launched Otherwise, the design of the mould will be modified and the modified portion will have to be machined again If the failure is caused by the product design, the design of the plastic part will be modified, which will in turn affect the design and manufacturing of the mould

Although the time and cost have been reduced dramatically in most areas of the product development process, mould design and manufacturing is still the most time consuming and costly phase for plastic parts These aspects will be discussed in the following two sections

1.2 Mould design and modification

Mould design is an important process for a successful plastic part Proper design of an injection mould is crucial to producing a functional plastic component Mould design has great impact on productivity and part quality, directly affecting the efficiency and profitability of the moulding operation About 70% of the mould manufacturing cost is decided in the mould design process

1.2.1 Mould structure and subsystems

Injection moulding is a process that softens the plastic material with heat and forces it

to flow into a closed mould After the material cools and solidifies, a product with the specific shape is formed A mould is what determines the shape, and in most cases, the final finish of the part As shown in Figure 1.2, a typical injection mould system usually includes mould base, guiding and alignment, sprue and runner system, cooling

system, ejecting system and cavity system Among them, the mould base is used to securely retain all the inserts in the mould The guiding and alignment system ensures that the two mould halves (fixed half and moving half) remain in correct alignment and the mould is accurately positioned on the machine The sprue and runner system

Guiding & alignment

Chapter 1 Introduction

ensures that the impression can be filled properly and completely It also positions and orientates the various functional parts to ensure that the whole mould assembly works properly The cooling system makes sure that the hot material can be cooled down rapidly to a temperature, at which it solidifies sufficiently to retain the shape of the impression When the plastic material cools down, it often shrinks onto the core insert, which makes it difficult to remove The ejecting system provides a means to eject the moulded part from the core insert

The cavity system forms the shape of the plastic part It usually includes the cavity and core inserts When there is undercut in a plastic part, sliders and lifters will be designed Generally, a lifter is designed for internal undercut while a slider is designed for external undercut There are usually some cooling, ejecting, lifter and screw holes

in the inserts of the cavity system

1.2.2 Mould design

Since injection moulds are of multi-functionality and have various configurations, the design of an injection mould is a complex task In addition to satisfying the functional requirements, many other aspects, such as geometric complexities, equipment and tooling requirements, process capabilities, must also be taken into account in mould design In order to shorten the mould design lead-time, 3D CAD systems have replaced the traditional drawing boards as a design tool in most mould manufacturing companies Several mould design systems have been developed to automate some of the mould design processes and shorten the design time

Figure 1.3 shows a general mould design process: Upon receiving the design of the

plastic part, the mould designer studies the requirements and the geometries The plastic part is enlarged to compensate shrinkage The shrinkage factor is the ratio of the expected reduction of the plastic part dimension as the part solidifies in the mould and cools to room temperature The parting line and the parting surfaces are then identified and designed Based on the geometry and dimension of the part, the ordered quantity and manufacturing cost, the injection machine and the mould type are

Shrinkage

Determine parting direction and parting line Designed product model

Select the mould type and mould machine

Design the initial cavity layout

Design detailed layout, inserts and components

as the fasteners, springs, heaters, etc are added in last When the mould is designed, materials and standard components are ordered and the inserts with 3D profiles are passed to the CAM department

1.2.3 Mould design modification

When the design of a plastic part is changed, the mould design will need to be modified Since a mould has been designed and some mould inserts may have been machined, the mould design modification method is different from the mould design method Besides the geometrical and functional constraints, the structure of the designed mould also needs to be considered in modifying the mould design In addition, the product and the mould design may be modified in different mould manufacturing stages Different mould design modification methods should be applied for different mould machining status

Some parametric-based CAD systems can automatically modify the design of simple moulds as they can preserve the design history However, it may take very large storage space to preserve the design history Sometimes, this makes the file very large, and the operation becomes very slow This is a critical issue in designing complex moulds To solve this problem, some commercial CAD/CAM systems (e.g.,

Unigraphics [Unigraphics 2000]) provide a function which allows the user to remove the design parameters and history In the mould manufacturing industry, many CAD/CAM engineers do remove the design history to reduce the part file size and improve the mould design and tool path generation speeds Therefore, design modification for complex mould parts is still time-consuming and tedious

If the plastic part is designed with one CAD system while the mould is designed with another one, the parameters of the plastic part many be lost when the part is transferred, which makes it difficult to modify the mould design automatically

In addition, the design of the plastic part may be changed in different mould manufacturing stages, the existing mould design modification systems cannot modify the mould design automatically according to the mould machining process status Therefore, when a complex or non-parametric mould design needs to be changed, it is still done by the mould designer with the general CAD system based on his/her experience, which is time-consuming and error-prone

1.3 Tool path generation and regeneration

CNC milling machines are widely used in mould manufacturing industry Most mould manufacturing companies use CAM software to generate tool paths for NC machining The CNC machine, the milling cutter and the tool path are three key factors that decide the mould machining accuracy and efficiency

1.3.1 CNC machines and cutters

According to the degree of the freedom of the tool relative to the workpiece, CNC

Chapter 1 Introduction

milling machines can be classified into 3-, 4- and 5-axis machines 3-axis and 5-axis CNC machines are most commonly used in mould manufacturing A 5-axis CNC machine, with 2 additional rotating axes, can machine free-form surfaces with flat and fillet end-mills instead of ball end-mills, which can drastically reduce the machining time However, this technology is being accepted only gradually in mould manufacturing because the programming for 5-axis CNC machines is somewhat more difficult and error-prone Therefore, 3-axis milling machines are still most widely used

in the mould manufacturing industry, which is also the emphasis of this research

Flat, fillet and ball end-mills are most commonly used in mould machining Since in sculpture surface machining with 3-axis milling machines, only a ball end-mill with spherical face at its end can finish the machining task, ball end-mills are widely used in the finish machining of moulds This research will focus on ball end-mills

1.3.2 Tool path generation

The CNC programming of a complex mould part typically consists of two general sequences: rough and finish Rough machining removes most of the unwanted raw stock material while keeping the tool a safe distance from the part’s finished surface It

is during finish machining that the cutter contacts the part surface and removes the remaining unwanted stock material

Material Removal Rate (MRR) is the most important factor to be considered for rough machining Machining accuracy is not the critical issue in this process For finish machining, accuracy, surface finish and machining efficiency are the three most

important parameters Minimizing the machining time with required accuracy and surface finish is the main objective in tool path generation for finish machining

During the past decades, great improvement in tool path generation has been achieved, and many tool path generation methods have been developed Many commercial CAM systems are available Most CAM systems (e.g., CATIA, Unigraphics and Pro/Engineer) can generate gouge-free tool paths automatically with the given cutting tool and machining parameters However, despite of rapid increase in computer speed, the process of tool path generation is still time consuming It may take more than two hours to generate a finish machining tool path for a complex mould insert Since the

NC codes for rough machining are generally much fewer than those for finish machining, the time for generating rough machining tool path is also usually much less than that for finish machining

1.3.3 Tool path regeneration

When the mould design is modified, the modified mould needs to be re-machined CNC and EDM are two common processes used to machine the modified mould If the modified mould is to be machined with a CNC machine, the tool paths need to be regenerated

With the existing CAM systems, no matter how small the portion of a mould insert is

to be modified, the entire tool path that covers this region will need to be recalculated

As much time is needed to regenerate the tool path for the modified mould as that for generating a new tool path, which is highly unproductive and very time consuming Sometimes, the mould needs to be modified when it is being machined In this case,

in the existing work:

(1) Some CAD systems can modify mould design automatically for parametric parts

However, it is very difficult to automatically modify mould design for complex and non-parametric parts with existing CAD systems Moreover, the mould machining status is not considered in most CAD systems for modifying the mould design

(2) With the existing CAM systems, if the tool path needs to be regenerated, all

CL-points of the tool path will be recalculated It will take as much time to regenerate an affected tool path as that for a new one

The objective of this research is to develop a mould design modification and tool path regeneration system that can solve the above two problems This research will focus on the following two issues:

(1) Mould design modification Develop a mould design modification system that

can automatically and efficiently modify a mould design based on the designed mould and the mould machining status The system should be applicable to both parametric and non-parametric parts

(2) Tool path regeneration Develop new algorithms and methodologies that can

regenerate tool paths efficiently Since the time for generating a rough machining tool path is generally short, this research will focus on finish machining As the tool path regeneration method is related to the tool path generation method, and the tool path generation methods can be classified into CL-point method and (Cutter Contact) CC-point method, two tool path regeneration methods will be

developed for them respectively

It is assumed in this research that the product design, mould design and tool path generation are all based on 3D solid models, while an original mould has been designed and the corresponding tool paths have been generated Since 3-axis CNC machines with ball end-mills are widely used in mould machining, this research will focus on regenerating tool paths for these configurations In addition, the same size cutters will be used to machine both the original and the modified moulds

1.5 Outline of the thesis

The remaining chapters of this thesis are organized as follows Chapter 2 presents a review of the current research status in computer-aided injection mould design and tool path generation Chapter 3 introduces the mathematical background of curves and surfaces for mould design modification and tool path regeneration The mould design modification system is introduced in Chapter 4 The tool path regeneration algorithms for CL-point and CC-point tool path generation methods are introduced in Chapter 5

Chapter 1 Introduction

and Chapter 6, respectively Chapter 7 presents the computer-aided mould design modification and tool path regeneration system Some examples of mould design modification and tool path regeneration are also implemented in this chapter Conclusions and future research recommendations are discussed in the last chapter

CHAPTER 2 LITERATURE REVIEW

Nowadays, product designers integrate more sculptured surfaces into their product components to enhance product aesthetic and improve their designs The increasing complexity of manufactured parts requires more advanced CAD software and effective

NC programming capabilities to design moulds and generate tool paths efficiently Mould design and the mould manufacturing are two important factors that determine the success of a mould

During the past decades, many researchers have studied mould design and tool path generation, and numerous papers have been published in these two areas Most reported research work on mould design has concentrated on one of the following three topics: determining the optimal ejecting direction for a plastic product, automatically generating side cores, and developing interactive CAD systems for injection mould design; while research works on tool path generation have focused on how to generate gouge-free tool paths efficiently In this context, machining quality, efficiency and accuracy are the three key issues studied by most researchers

Although the reported literature can hardly be used directly for mould design modification and tool path regeneration, these researchers’ work can provide useful inputs in these two areas A review on mould design and tool path generation is given

Chapter 2 Literature Review

in this chapter

2.1 Mould design

In general, an injection mould includes cavity system, guiding and alignment system, runner system, ejection system and cooling system Among them, the cavity system is most important Determining optimal parting direction and automatically generating side cores are two of the most difficult tasks in designing a cavity system Many researchers have studied these two problems In addition, as mould design is very tedious and time-consuming, many researchers have tried to develop systems that could design moulds automatically and efficiently A brief literature review in these three research areas is given in the following subsections

2.1.1 Parting direction and parting line design

Parting direction and parting line are very important for a successful mould design as they decide the number and the shape of side cores, and this decision will affect all the subsequent steps in the design of a mould Many researchers have studied how to automatically identify the optimal parting direction and design the parting lines

As shown in Figure 2.1, a parting direction is a direction along which a mould piece is separated from the mould assembly One of the principles of selecting the parting direction is to minimize the undercut, where an undercut is the recess or protrusion region on a plastic part that prevents its removal from a mould along the parting direction When the parting direction is determined, the parting line and the parting surface can be designed A parting line is a continuous closed curve on the surface of the product part that defines the faces to be split into different mould pieces, and the

parting surface is the contact surface of two mould pieces

Chen et al [Chen 1993] developed a method to determine the parting direction based

on the minimization of local external undercuts Using the Gaussian and visibility maps, they presented an algorithm to obtain a set of feasible parting directions While their approach was able to minimize the number of external undercuts, internal undercuts were not considered Based on two levels of visibility: complete and partial, Chen et al [Chen 1995] extended their work to parts with internal undercuts by decomposing an internal undercut feature into two portions: the separable and the internal undercuts

Figure 2.1 Parting lines and parting surfaces

Core

Cavity

Parting surfaceParting lines

Parting direction Parting direction

Plastic part

Chapter 2 Literature Review

Nee et al [Nee 1997] classified the undercut features into two types, namely external and internal undercuts The external undercuts were further divided into outside external undercuts and inside external undercuts, while internal undercuts were divided into outside internal undercuts and inside internal undercuts A group of recognition criteria for undercut features were presented in their paper After all the potential undercuts were extracted, the optimal ejection direction was chosen based on the number of possible undercuts and their corresponding undercut volumes

moulded parts with planar, quadric and free-from surfaces Their hybrid method took advantage of graph-based and hint-based approaches, and various undercut features, including interacting undercut features, could be recognized

When the parting direction is determined, the parting line can be designed Ravi and Srinivasan [Ravi 1990] introduced sectioning and silhouette methods for parting line generation Chin and Wong [Chin 1996] presented a slicing strategy for generating the parting line Through a recursive uneven slicing method, several parting surfaces are generated for further evaluation Weinsten and Manoochehri [Weinsten 1997] formulated the parting line determination problem as an optimization problem Their objective function is defined as a function of the flatness of the parting line, draw depth, number of side cores required to form the undercuts, machining complexity, etc Majhi et al [Majhi 1999] presented an algorithm for computing an undercut-free parting line that is as flat as possible for a convex polyhedral object

In summary, many researchers have studied how to identify the undercut, determine

the parting direction and design parting lines Generally, if the design of a plastic part

is changed, the parting direction should not be changed so as to minimize the modification of the mould structure With the given parting direction, the possible undercut caused by the design modification can be identified with existing methods If the design change affects the existing parting line, the affected parting line can be redesigned and the modified portion of the part can be split and united with the core and cavity inserts automatically

2.1.2 Insert design

Insert design includes the design of cavity, core, slider and lifter These inserts form the shape of the plastic part The cavity and core form the profile without undercut, while the slider and lifter are designed for the undercut profiles Some researchers studied how to generate inserts automatically

Hui and Tan [Hui 1992] proposed a four-step sweeping method to create the cavity and core: 1) Generate a solid by sweeping the plastic part in the parting direction; 2) Subtract one end of the swept solid from the first mould block; 3) Subtract the other end of the swept solid from the second mould block; and 4) Subtract the result of step

2 from that of step 3 with the mould plates in the closed position

Shin and Lee [Shin1993] presented a procedure to recognise undercuts by checking the interference faces between a product and its core/cavity They also discussed the generation of side cores by using the Euler’s operations For free-form surfaces

represented by u, v parameters, their algorithm determines an interference face by

checking the normal vectors at the points corresponding to the grid points in the

Chapter 2 Literature Review

parameter domain Since it is difficult to determine the u, v parameters for trimmed

free-form surfaces, this method is not suitable for designing side cores that contain trimmed surfaces

Rosen [Rosen 1994] presented a procedure to design side cores based on the identified undercuts For external undercuts, accessibility directions were identified and used to design side cores For internal undercuts, form pins were constructed that accessed the undercuts through the core of an injection mould However, this approach is only suitable for polyhedral solid models

Zhang et al [Zhang 2002] introduced an algorithm that could create complete lifter subassemblies With their algorithm, the virtual core and cavity were generated first with the given parting direction without considering the undercuts The undercuts were then identified and grouped For each group of undercuts, the releasing direction was identified and a lifter head was designed By attaching other standard components of the lifter to the head, a complete subassembly of the lifter was designed

In summary, most researchers have studied how to automatically generate inserts of cavity, core, slider and lifter with the given parting direction and parting lines However, how to modify the design of the inserts accordingly based on the modified plastic part was not considered in their research More research is needed to automatically modify the insert design based on the modified plastic part and the mould machining process

2.1.3 Mould system and subsystem design

Even with the help of the general CAD systems, injection mould design is still a very time-consuming and tedious process, and it is highly dependent on the experience of the mould designer To automate the mould design process and shorten the design time, some researchers studied how to build up specific CAD systems for injection mould design

Yuan et al [Yuan 1993] developed an integrated CAD/CAE system for injection mould design and analysis With their system, the drawings of a plastic product were first transformed interactively into the drawings of mould impressions, the mould design was then carried out by using a group of design tools for injection moulds The system could analyze the balance of the runner and simulate the flow process of the plastic melt

Kruth et al [Kruth 1997] developed a design support system (IMES/DSS) for injection mould design The system supported the design of injection moulds through high-level functional mould objects, e.g basic assemblies, components and features The system managed the low-level CAD entities and allowed additional design information such

as process planning information to be incorporated The user could create or modify standard design objects and link them with a relational database

The above two systems could only support 2D design of injection moulds Thus, the design facilities provided by the system were actually a group of tools for editing and generating 2D drawings To solve this problem, Lee et al [Lee 1997] developed a knowledge-based injection mould design system, which supports 3D modeling The

Chapter 2 Literature Review

system contains the design libraries for mould bases and standard parts Mould design tools such as parting line selection, parting surface generation, ejecting pin design, cooling hole design, etc., are also provided by the system Similar knowledge-based systems were also introduced by Chan et al [Chan 2003] and Mok et al [Mok 2001]

Besides the entire mould design system, some mould design subsystems, such as the feeding system [Ong 1995, Ravi 1997], the ejection system [Wang 1996] and the cooling system [Lin 2001], have also been developed Most of these researches focused on how to optimally and automatically design the subsystems

In summary, some CAD/CAE systems have been developed to automate mould design processes and improve mould design quality However, none of these systems could automatically modify the mould design according to the changed plastic profile for non-parametric parts Research in this area is needed

2.2 Tool path generation

After the mould is designed, the CAM engineer generates tool paths for rough and finish machining based on the designed mould parts The purpose of rough machining

is to remove excess material from a stock, while finish machining aims to accurately machine the part shape This research focuses on tool path generation for finish machining

According to whether the topology of CC-points or CL-points is controlled, the tool path generation can be classified into CC-point method and CL-point method A brief review of tool path generation in these two categories will be given in the following

two sub-sections Since 3-axis CNC machines with ball end-mills are widely used in finish machining of sculptured surfaces, this review will emphasize on this configuration

2.2.1 The CC-point method

During the past few decades, many methods have been developed for tool path generation of compound surfaces Most of the available algorithms are based on the CC-point method With this method, a set of CC-points are planned on the compound surface CC-points are then offset along the surface normal vectors to compensate the effect of the cutter size The CL-points are thus obtained, and the final tool paths are calculated by removing the interference CL-points According to how the CC-points are planned, the CC-point tool path generation methods can be further classified into the iso-parametric method, the constant geodesic distance method, the constant scallop height method, the principal direction method, the spiral tool path method and the section curve method

The iso-parametric method [Broomhead 1986, Loney 1987, Kuragano 1992, Yu 1996], also known as the flow-line machining method, generates tool paths along the surface constant parameter lines With this method, the step-over size is first evaluated at each cutter contact point along the CC-path such that the scallop height is within the tolerance The minimum step-over size within a single path will become the step-over size By keeping one of the two parameters constant, the iso-parametric curves are formed and employed as the CC-paths The method of iso-parametric machining takes advantage of the parametric representation of the sculptured surface It is very easy to calculate tool paths for a parametric surface patch and can avoid the costly surface-to-