3d rapid realization of initial design for...

Journal of The Institution of Engineers, Singapore Vol. 44 Issue 4 2004

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3D RAPID REALIZATION OF INITIAL DESIGN FOR PLASTIC INJECTION MOULDS

Maria L.H. Low1 and K.S. Lee2

ABSTRACT To provide an initial design of the mould assembly for customers prior to receiving the final product CAD data is a preliminary work of any final plastic injection mould design. Traditionally and even up till now, this initial design is always created using 2D CAD packages. The information used for the initial design is based on the technical discussion checklist, in which most mould makers have their own standards. This technical discussion checklist is also being used as a quotation. This paper presents a methodology of rapid realization of the initial design in 3D solid based on the technical discussion checklist, which takes the role of the overall standard template. Information are extracted from databases and coupled with the basic information from customer, these information are input into the technical discussion checklist. Rules and heuristics are also being used in the initial mould design. A case study is provided to illustrate the use of the standard template and to exhibit its real application of rapid realization of the initial design for plastic injection moulds.

INTRODUCTION The most established method for producing plastic parts in large quantities is plastic injection moulding. This is a highly cost-effective, precise and competent manufacturing method, which can be automated. However, costly tooling and machinery are needed in this manufacturing process. The design of a plastic injection mould is an integral part of plastic injection moulding as the quality of the final plastic part is greatly reliant on the injection mould. A plastic injection mould is a high precision tooling that is being used to mass produce plastic parts and is by itself an assembly of cavities, mould base and standard components etc. An example of an injection mould assembly is shown in Figure 1. Over the years, much research work using computer-aided techniques had been done from studying the very specific areas of mould design to studying mould design as a whole integrated system. Knowledge-based systems such as IMOLD (Lee et al. 1997), ESMOLD (Chin et al. 1997), IKMOULD (Mok et al. 2001), etc were developed for injection mould design. Many commercial mould design software packages such as IMOLD, UG MoldWizard, R&B MoldWorks, etc are also available today in the market for mould makers. However, the systems and software packages mentioned above did not consider the initial design prior to actual mould design. These software packages

1 Research Student, Department of Mechanical Engineering, National University of Singapore, Singapore 119260 2 Associate Professor, Department of Mechanical Engineering, National University of Singapore, Singapore 119260


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assist in the preparation of the detailed mould design that includes the core/cavity creation, cooling and ejection design. As a result, mould designers hardly used the mould design software packages when they are doing their initial design because the software does not catered for such a design process.

Figure 1: An injection mould assembly

There is not much research being done on the initial design of plastic injection moulds except for Ye et al. (2000) who presented an algorithm for the initial design. The researchers first determine the parting line for the plastic part followed by the calculation of the number of cavities required. The cavity layout is created based on the input information of the layout pattern and the orientation of each cavity. The mould base is loaded automatically to accommodate the layout. The researchers also proposed to use their initial design as a guide tool for the quotation of the mould. However, the research that is being done may not be applicable for most plastic injection moulding industries. The calculation of the number of cavities required is mostly determined by the customers who provides the product CAD file and they seldom seek opinion from the mould makers, thus this step could be omitted to save time. Although external undercuts are identified in the product, the research did not consider the standard components that are required in producing such undercuts, which in this case, the use of sliders. The research also did not consider internal undercuts where lifters are required. Thus, the quotation derived would not reflect the correct costing of the mould, and thus could be very misleading, since the


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use of these types of standard components can increase the cost of producing such a mould substantially. Alternatively, the authors (Low et al. 2002) proposed a methodology of standardizing the cavity layout design system for plastic injection mould such that only standard cavity layouts are used. When only standard layouts are used, their layout configurations can easily be stored in a database for fast retrieval later in the mould design stage. This research is being incorporated into the rapid realization of the initial design for plastic injection mould in this paper. There is a need to introduce a faster method of mould design since there are fewer very experienced mould designers and coupled with the fact of today’s market demands of having shorter lead-time and higher quality products. This is fulfilled by the introduction of standardization into mould design, since the design processes are repeatable for every mould design project. This paper presents a methodology of rapid realization of the initial design in 3D solid instead of 2D drawings using standardization method. The initial design in 3D solid will be based on the technical discussion checklist that acts as the overall standard template. Every sub-design such as cavity layout design, gating system, mould base design etc will have its own respective standard template. This is to enable timesavings in the design stage as the final mould design can be obtained directly by making minute changes to the initial design.

INITIAL DESIGN OF PLASTIC INJECTION MOULD The customers and the mould designers have to work closely together to obtain a mould that could produce what is desired suitably. It would be costly to rectify the errors after the mould is manufactured completely. Thus the initial planning of how the layout design of the mould is likely to be is important. A typical mould design project workflow chart is shown in Figure 2. When the customers have decided to engage a particular mould-maker, the CAD file of the product have to be provided to them. However, the mould-maker is always prepared to receive newer versions of the product CAD file as changes are constantly made to it. The downside of this is that the lead-time given to complete the mould still remains as it is. Thus, the time that was left to complete the final mould design and manufacture the mould becomes shorter. When the product CAD file is first received, the assigned project engineer or mould designer fills up a technical checklist during their first technical discussion with the customers. The checklist records information such as the resin material to be used and its shrinkage value, the number of cavities required by customers, the gating system, and the moulding machine to be used, the required type of mould base and other information needed to provide the basis of the initial design of the mould. Since this checklist contains most of the basic information, it doubles up as a quotation. This allows customers to decide whether to modify their product CAD file to produce a simpler mould that is cheaper. After that, the mould designer prepares an initial design based on the product CAD file and information in the checklist.


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Figure 2: Typical mould design project workflow chart

Traditionally and even up till now, mould designers are using 2D CAD packages to create the initial design, although 3D CAD packages are readily available. Ironically, mould designers would then use the 3D CAD package only in their final mould design. When this initial design is completed, it will be presented on the next technical discussion. Modifications made to the initial design are normally done by marking and sketching the changes on the printed drawing paper. Though there is no final product CAD file at this stage, the mould-maker could go ahead to purchase the raw materials and standard components subject to approval of the customers. After the final product CAD file has been received, the mould designer would start the actual mould design afresh using the 3D CAD package that they have. This is a time consuming method since the initial design is not related to the final mould design.


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Figure 3: Proposed design structure

THE DESIGN STRUCTURE Figure 3 shows the overall structure of the proposed system. In the proposed approach, the standardization method utilizes standard mould designs, which are derived from the information listed in the technical discussion checklist. This checklist takes on the role of the overall standard template and must be used for every new mould project. The sub-designs will have their own templates. Databases are used to record information such as types of standard components, types of design, geometrical parameters and project data etc. Mould-making industries can easily adopt the proposed approach since they are able


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to customize the databases to include their own standards. A standard mould design uses only standard components such as the mould bases; ejector pins and other accessories. Standard configuration of cavity layouts, that produces only the same products in a balanced layout are used in a standard mould design. Calculations used in a standard mould design are based on rules and heuristics, which can be applied universally to any organizations. Rules and heuristics make up an important sector of mould design since they determine if the mould that is designed, is able to fit into the specified moulding machine or be able to mass produce the product without any problems due to bad design. The hierarchical organization of the assembly files of a standard mould design should also follow a single mould assembly structure. Provision had been made in this system to present representations of the core, cavity, slider head and lifter head as blocks in the initial design. These blocks can be edited to trim to the profile only when the final product CAD file has been received and confirmed. During the initial design, ejector pins/blades and cooling lines are still not included because these depend greatly on the final product CAD file. Since this paper focuses on the rapid realization of initial design, core/cavity parting, profile creation, addition of ejector pins/blades and cooling lines will not be discussed here. 3D solids would be used as they have their advantages. The advantages of solid modeling are better visualization, simplified simulation, improved producability, faster drawing production and facilitates an integrated design process.

Standardization method Standardization method involves using standard mould designs, standard components and a standard working method of mould design. This means that every mould designer will design moulds in exactly the same method, use the same design assembly hierarchy tree, and use standard components from a specified supplier. This allows the different teams involved in the mould project to speak the same language. The advantages are as follows: a) Easy following-up of mould project, b) Lower cost and faster delivery of components and c) Proper mould project management.

Databases Four different types of databases are used in this system (Figure 3): a) Library database is a collection of all standard components commonly used by the mould-making industry. b) Configuration database is used for all standard components and cavity layout design. All the different configurations are already pre-defined in the 3D solid files and only the required configuration will be activated. c) Project database is a collection of all data that is input into the technical discussion checklist and sub-designs interfaces, thus enables tracking and retrieval of information that are unique to a particular project. The quantity of the various components and their types that are to be used in the project are also recorded here, thus enabling an easy generation of an initial bill of materials (BOMs) when desired and d) Geometrical parameters database is utilized where there is a need to change the geometrical parameters such as distances between different cavities and locations of standard components etc.


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SYSTEM IMPLEMENTATION A prototype of the rapid realization of initial design system for plastic injection mould has been implemented using a PIII PC-compatible as the hardware. This prototype system utilizes SolidWorks 2001 as the CAD software, Microsoft Visual C++ V6.0 as the programming language and the SolidWorks API in a Windows environment. The rules, heuristics and formulations used in this prototype system are based on the local mould making industries in Singapore.

Figure 4: Completed “Technical Discussion Checklist”

Technical discussion checklist & mould assembly structure Before the sub-designs are utilized, the overall standard template, “Technical Discussion Checklist”, must be used first (Figure 4). This enables the mould design to follow a


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standard mould assembly structure using the initial product CAD file, the basic information and requirements from customers. It is important to adhere to the same mould assembly structure for every mould design project within an organization to ensure every mould designer follows the same working methods of designing. It also enables other mould designers to be able to locate a certain design of a component easily. Table 1: Short extraction of injection moulding machine database TYPE COMPANY MC MODEL MC TONN TIE BAR (H*V) PLATEN DIM OPEN ST MOULD HT MAX DAYLT EJECT ST

Toggle Engel ES 350 K-SL 350 735 x 735 1080 x 1080 700 250-660 1360 240



Toggle Engel ES 500 K 500 840 x 840 1200 x 1200 700 250-750 1450 240







Toggle Toshiba EC 20 20 280 x 280 390 x 390 230 130-250 480 50








Hydraulic Toshiba IS 90 B 90 375 x 375 545 x 545 475 160 635 63



Hydraulic Toshiba IS 550 E 550 860 x 860 1230 x 1230 1200 400 1600 160

Hydraulic Toshiba IS 630 CNII 630 930 x 930 1350 x 1350 1200 600 1800 180

The section “Project Tracking” records down the basic details of the project. In the section known as the “Moulding Material”, the resin to be used is chosen from the list. The shrinkage value of the resin is entered into the space provided so that the initial product CAD file can be scaled accordingly prior to mould design. The section on “Moulding Machine Details” allows users to select the moulding machine to be used. The customers provide this information. Upon selecting the required machine from the list, it extracts data from the moulding machine database and reflects the relevant dimensional information onto the appropriate spaces in the interface. A short extraction of the injection moulding machine database is depicted in Table 1. The section on “Mould Information” records down information such as the type of cavity layout and the number of cavities that is required. All these information in this section will eventually be used in their own individual sub-designs templates where users are able to input more information or to edit the current selection or the geometrical parameters. The last


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section records down the mould materials and the type of surface finishing required. This information is needed for the initial quotation and the procurement of materials for the mould project. Finally, entering the person’s name that had recorded the information and the date when the information is recorded ensures that a proper record is kept. All the information that is entered is listed into a mould project database.

Shrinkage factor & core/cavity creation A representation of the core and a representation of the cavity in the form of blocks are simultaneously created to encapsulate the scaled product CAD file. A default-offset value is applied to the range box of the scaled part to give the approximate size of the blocks (Figure 5). These values are commonly used by mould designers locally and can be edited.

Figure 5: Default-offset value between range box of the scaled part and core/cavity


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Other accessories/secondary components The secondary components that may need to be selected during the initial mould design are sliders, lifters and special inserts. Sliders and lifters will be the source of concern in the layout assembly level as they played a part in determining the overall size of the layout assembly. They are initially categorized into the general types and are available in three basic sizes: Small (S), Medium (M) and Large (L). The respective sub-design templates for the sliders and lifters are used to place the components to the appropriate undercuts. They also functions as an interface to allow users to edit the geometrical parameters or configurations of the secondary components. As this is the initial design, very accurate positioning is not required but the secondary components have to be at its correct location and orientation.

Cavity layout Only standard types of cavity layout are used in this prototype system (Low et al. 2002). As the types of available layouts are fixed, the different types of layouts can be listed into a configuration database that enables the required layout to be activated in the mould assembly during the mould design. This provides a faster method of designing. The desired new configuration can be reloaded via the sub-design template for cavity layout. In addition, the orientation, the distances between cavities, which are known as geometrical parameters can also be edited through the same interface.

Selection of mould base size The secondary components and cavity layout are assembled in the layout assembly. The overall size of the layout assembly needs to be known in order to select the appropriate mould base size. The system automatically loads the smallest possible mould base from the available configurations of the specified model of mould base that has been selected. Simultaneously, the system has to check the compatibility of the chosen mould base with the targeted injection moulding machine that is to be used for moulding the products. Parameters that are checked are the tie-bar dimensions, platen dimensions, mould height and maximum daylight of moulding machine. A representation of the clamping unit of the chosen injection moulding machine can be activated to allow the user to verify the design visually.

Gating & runner design The standard design of gates and runner are pre-created and store in a library database. Depending on the number of cavities that are indicated earlier, the appropriate configuration will be activated in the initial mould design. In this prototype system, some rules and heuristics are also set for the gating and runner design. The available options can be selected only after the type of mould base has been chosen since they are dependent on the type of mould base.


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The rules for selection of the gating are:

1. For 2-plate mould, types of gating that can be used are all but pin-point gating and hot-runner gating.

2. For 3-plate mould, only pin-point gating and side gating can be used and not

the rest of the options.

3. For hot runner moulds, only the different choices of hot runner gating can be used.

The general rules in the design of runners are:

1. For 2-plate moulds, runners with round cross-sections are being used.

2. For 3-plate mould, runners with trapezoidal cross-sections are being used.

3. For hot runner moulds, no runners are needed.

Figure 6a: External undercuts of Figure 6b: Internal undercut of test part and sliders test part and lifter

A CASE STUDY The test part that is used in this case study is a phone casing. The approximate size is 180mm × 60mm × 35mm. ABS that has a shrinkage value of 0.5% is used. The


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moulding machine used is a Toshiba 280-ton toggle type injection moulding machine (Table 1). The dimensional information of the clamping unit is given in Table 1. A closer observation of this test part reveals that there are three undercuts, thus requiring additional accessories such as lifters and sliders. Figure 6a illustrates the external undercuts that require sliders and Figure 6b shows an internal undercut that requires a lifter. A 2-plate mould with two cavities is also required for this test part. The information as supplied by the customer, are entered into or selected from the “Technical Discussion Checklist” interface that acts as a standard template for all mould projects (Figure 4). Then, the original product CAD file of the test part is scaled accordingly to the shrinkage factor provided by the interface. The core and cavity are also created at the same time to encapsulate the test part. The selected type of the mould base, gating system and the required accessories are copied into the respective levels of the mould assembly structure.

Figure 7: Application of sub-design template for lifter to test part

Secondary components are attached to the core/cavity assembly. Figure 7 shows the application of the sub-design template for the lifter. A similar sub-design template is used separately for sliders. Since the requirement for this test part is to have a two-cavity layout, the linear two-cavity configuration is activated. Since there are sliders between the two cavities, an allowance is given between the cavities (Figure 8). This distance can be edited using the cavity layout sub-design interface. In this case study, the overall size of the layout is approximately 520mm × 250mm × 320mm (Figure 9). Thus, a mould base must have an ejector plate and ejector retainer plate that is larger than 520mm × 250mm. Since the mould base type that was chosen is the DME SF series, a smallest possible mould base is of the 4060 configuration. As the distance between the tie bars of


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the moulding machine is 730mm × 730mm, the selected mould base can be secured onto the moulding machine without any difficulty. The completed initial design of the injection mould for the test part is shown in Figure 10. A representation of the clamping unit of the moulding machine can be activated to allow verification that the mould is designed correctly since it is easy to visualize them in 3D (Figure 11).

Figure 8: Providing allowance between cavities

Figure 9: Overall size of layout assembly of test part


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Figure 10: Completed initial design of the injection mould for the test part

Figure 11: Designed mould of test part with clamping unit


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CONCLUSIONS An approach to using the standardization method is applied to the rapid realization of the initial design of plastic injection moulds in this paper. Design processes that are the same for every mould design project are consolidated into a standard template. The technical discussion checklist takes on the role of the overall standard template while the sub-designs have their own sub-templates. The use of databases allows the flexibility in allowing customization. Approximate costing of the mould can also be derived from the information based on the checklist. Other advantages include having a faster approach to design, designing a mould that functions and easy visualization. However, this rapid realization of initial design system has its limitations. As technology advances, more databases, rules and heuristics needs to be built into the system to accommodate for mould designs meant for the newer forms of plastic injection moulding such as multi-colour moulding and thin-wall moulding. Much effort and money needs to be invested by organizations to customize their systems to consider the new technologies. The databases for the materials and moulding machine also had to be constantly updated and checked to account for the newer materials and machines that are introduced into the industry. If there is a wrong entry in the databases, the results that are obtained can be disastrous. An experienced designer would know at once when the design is not right but to a novice designer, he/she may just accept the design without much thought, believing that the system would always provide the correct solution. The authors are currently researching into improving the system so as to enable an easier approach of customization.

REFERENCES CHIN, KWAI-SANG and T. N. WONG, 1996, Knowledge-based evaluation for the conceptual design development of injection molding parts. Engineering Application of Artificial Intelligence, 9(4), 359-376 LEE, K. S., Z. LI, J. Y. H, FUH, Y. F. ZHANG and A. Y. C. NEE, 1997, Knowledge-based injection mold design system. CIRP International Conference and Exhibition on Design and Production of Dies and Moulds, Turkey, June, 45-50 LOW, MARIA L. H. and K. S. LEE, 2002, A parametric-controlled cavity layout design system for plastic injection mould. The International Journal of Advanced Manufacturing Technology, Accepted for publication Li Pang, Kamath G M, Wereley N M, Dynamic Characterisation And Analysis Of Magnetorheological Damper Behaviour, SPIE Conference on Passive Damping and Isolation SPIE Vol. 3327, pp 284-302, 1998 MOK, C. K, K. S. CHIN and JOHN K. L. HO, 2001, An interactive knowledge-based CAD system for mould design in injection moulding processes. The International Journal of Advanced Manufacturing Technology, 17, 27-38


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YE, X. G., K. S. LEE, J. Y. H, FUH, Y. F. ZHANG and A. Y. C. NEE, 2000, Automatic initial design of injection mould. International Journal of Material & Product Technology, 15(6), 503-517

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