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Incorporating Manufacturability Considerations during Design of Injection Molded Multi-Material Objects Ashis Gopal Banerjee, Xuejun Li, Greg Fowler, Satyandra K. Gupta1 Mechanical Engineering Department and The Institute for Systems Research University of Maryland, College Park, MD 20742, U.S.A. ABSTRACT The presence of an already molded component during the second and subsequent molding stages makes multi-material injection molding different from traditional injection molding process. Therefore, designing multi-material molded objects requires addressing many additional manufacturability considerations. In this paper, we first present an approach to systematically identifying potential manufacturability problems that are unique to the multi-material molding processes and design rules to avoid these problems. Then we present a comprehensive manufacturability analysis approach that incorporates both the traditional single material molding rules as well as the specific rules that have been identified for multi-material molding. Our analysis shows that sometimes the traditional rules need to be suppressed or modified. Lastly, for each of the new manufacturability problem, this paper describes algorithms for automatically detecting potential occurrences and generating redesign suggestions. These algorithms have been implemented in a computer-aided manufacturability analysis system. The approach presented in this paper is applicable to multi-shot and over molding processes. We expect that the manufacturability analysis techniques presented in this paper will help in decreasing the product development time for the injection molded multi-material objects. Keywords: Automated manufacturability analysis, generation of redesign suggestions, and multi-material injection molding. 1 INTRODUCTION Over the last few years, a wide variety of multi-material injection molding (MMM) processes have emerged for making multi-material objects, which refer to the class of objects in which different portions are made of different materials. Due to fabrication and assembly steps being performed inside the molds, molded multi-material objects allow significant reduction in assembly operations and production cycle times. Furthermore, the product quality can be improved, and the possibility of manufacturing defects, and total manufacturing costs can be reduced. In MMM, multiple different materials are injected into a multi-stage mold. The sections of the mold that are not to be filled during a molding stage are temporally blocked. After the first injected material sets, then one or more blocked portions of the mold are opened and the next material is injected. This process continues until the required multi-material part is created. Nowadays, virtually every industry (e.g., automotive, consumer goods, toys, electronics, power 第 1 页 共 67 页tools, appliances) that makes use of traditional single-material injection molding (SMM) process is beginning to use multi-material molding processes. Some common applications include multi-color objects, skin-core arrangements, in-mold assembled objects, soft-touch components (with rigid substrate parts) and selective compliance objects. Typical examples of each class of application are shown in Fig. 1. There are fundamentally three different types of multi-material molding processes. Multi-component injection molding is perhaps the simplest and most common form of MMM. It involves either simultaneous or sequential injection of two different materials through either the same or different gate locations in a single mold. Multi-shot injection molding (MSM) is the most complex and versatile of the MMM processes. It involves injecting the different materials into the mold in a specified sequence, where the mold cavity geometry may partially or completely change between sequences. Over-molding simply involves molding a resin around a previously-made injection-molded plastic part. Each of the three classes of MMM is considerably different. Each specific MMM process requires its own set of specialized equipment; however, there are certain equipment requirements that are generally the same for all types of MMM. Techniques described in this paper are applicable to over-molding and multi-shot molding.Currently only limited literature exists that describes how to design molded multi-material objects. Consequently very few designers have the required know-how to do so. Consider an example of a two piece assembly consisting of part A and part B to be produced by multi-material molding. In fact, many new users believe that if part A and part B meet the traditional molding rules then assembly AB will also be moldable using multi-material molding. By moldable we mean that the assembly (or part) can be molded using one or more MMM (or SMM) processes such that basic functional and aesthetic requirements for the part or assembly are satisfied and the mold cavity shape can be changed (i.e. mold can be opened, pieces may be removed or inserted and then mold can be closed) without damaging the mold pieces. However, this notion is not always correct. Fig. 2 shows an assembly to be molded by MMM. In this case, both parts can be individually molded without any problem. However, molding them as an assembly using over-molding process leads to manufacturability problems. After molding the inner part in the first stage, it is not possible to carry out second stage molding as the injected plastic will flow over the inner part and damage the surfaces of the already molded component. This emphasizes the need for developing new design rules that are specific to addressing manufacturability problems encountered in multi-material molding. Detection of this problem and corresponding redesign suggestion will be described in sub-section 5.3.On the other hand, there are molded multi-material assemblies where at least one of the parts would have not been moldable as an individual piece using traditional molding. However, this part can be molded when done as a part of the assembly. Fig. 3 highlights such a case. Although application of traditional plastic injection molding rules would have concluded that component B cannot be manufactured, it is possible to mold assembly AB by choosing an appropriate molding sequence. For example, in this case we first need to mold part A and then mold part B using overmolding operation.The reason why MMM appears to be significantly different from SMM can be explained as follows. The part that has been molded first (component A) acts as the “mold piece” during the second molding stage. Thus, a plastic mold piece is present in addition to the metallic mold pieces 第 2 页 共 67 页during this molding stage. Hence, this second stage is fundamentally different in nature from conventional single-material injection molding. Fig. 4 illustrates this condition by depicting the two molding stages in rotary platen multi-shot molding. Although the shape of the core remains identical in both the stages, the cavity shape changes and already molded component A acts as an additional “mold piece” in the second shot. Moreover, the first stage part that acts as plastic “mold piece” is not separated from the final assembly. This forces us to avoid applying some of the traditional molding design rules on certain portions of the gross shape of the overall object also referred as gross object. By gross object, we mean the solid object created by the regularized union of the two components. That is why, simply ensuring that the first stage part and the gross shape are moldable do not solve this problem either. Fig. 5 illustrates this fact; blindly checking all the faces of the gross object for presence of undercuts leads us to wrongly conclude that it cannot be molded. In reality, this is not the case and we should only test the faces that need to be demolded (i.e., separated from the mold pieces) during that molding stage while determining a feasible molding sequence.Based on the above discussion, we conclude that a new approach needs to be developed to analyze manufacturability of molded multi-material objects. In the current paper we only consider manufacturability problems arising due to the shape of the components and the gross object. Fig. 6 shows an example where undercuts create problems; they need to be eliminated in order to form a feasible molding sequence. The gross object shown in that figure cannot be made by any MMM process, because neither of the two components is moldable due to the presence of deep, internal undercuts. Slight redesign of component A enables us to carry out MMM operation component A can be injected first and then component B, provided they have similar melting points or A melts at a higher temperature than B. Section 3 systematically derives five such new manufacturability problems that arise in multi-material molding from the state transition diagram representing the process flow.The next task in developing a systematic manufacturability analysis methodology is to develop a detailed approach for applying these new rules. A comprehensive approach to outline how and when the new multi-material molding design rules need to be applied and traditional single material molding rules have to be applied, modified or suppressed has been proposed in Section 4. Finally, algorithms have been presented to detect violations of such rules and generate feasible redesign suggestions in Section 5. All the algorithms have been implemented in a computer-aided manufacturability analysis system. We conclude this paper by stating its contributions and limitations in Section 6.2 RELATED RESEARCH A wide variety of computational methods have emerged to provide software aids for performing manufacturability analysis Gupt97a, Vlie99. Such systems vary significantly by approach, scope, and level of sophistication. At one end of the spectrum are software tools that provide estimates of the approximate manufacturing cost. At the other end are sophisticated tools that perform detailed manufacturability analysis and offer redesign suggestions. For analyzing the manufacturability of a design, the existing approaches can be roughly classified into two categories. In direct approaches Ishi92, Rose92, Shan93, shape-based rules are used to identify 第 3 页 共 67 页infeasible design attributes from direct inspection of the design description. In indirect or plan-based approaches Gupt95, Gupt97b, Gupt98, Haye89, Haye94, Haye96, the first step is to generate a manufacturing plan, and then to evaluate the plan in order to assess the manufacturability of design. This approach is useful in domains where there are complex interactions between manufacturing operations.Several leading professional societies have published manufacturability guidelines for molded plastic parts to help designers take manufacturability into account during the product design phase Bake92, Truc87. Poli Poli01 has also described qualitative DFM rules for all the major polymer processing processes including injection molding, compression molding and transfer molding. Moreover, companies such as General Electric Gene60 have generated their own guidelines for the design of plastic parts. Such guidelines show examples of good and bad designs. It is left to the designers discretion to apply them as and when necessary. Basically, there are two types of guidelines. The first type deals with manufacturability issues, whereas the second type deals with part functionality. We will only cover the first type of guidelines here. They are listed as follows.a) Fillets should be created and corners should be rounded so that the molten plastic flows smoothly to all the portions of the part. Use of radii and gradual transitions minimize the degree of orientation associated with mold filling, thereby resulting in uniform mold flow Mall94. Moreover, this also avoids the problem of having high stress concentration. Fig. 7 shows an example of how part design needs to be altered to get rid of sharp corners. b) The parting line must be chosen carefully so that “parting” and metal “shut-off” flashes can be minimized. Typically, flashes (solidified leakages of plastic material) occur along the parting line, where the mold pieces come in direct contact with each other. Fig. 8 illustrates how the stiffening ribs on a part have to be redesigned in order to change the location of the parting line. This consequently changes the flash formation position. In the first case, flashes run all along the part, destroying the part quality. However, they occur on the top surface of the part in the second design, and hence can be easily removed later on. c) Thin and uniform section thickness should be used so that the entire part can cool down rapidly at the same rate. Thick sections take a longer time to cool than thin sections. For example, in the first part shown in Fig. 9, the thicker, hotter sections of the molding will continue to cool and shrink more than the thinner sections. This will result in a level of internal stress in the portions of the part where the wall thickness changes. These residual, internal stresses can lead to warpages and reduced service performances. If possible, the part must be redesigned to eliminate such thickness variations altogether. Otherwise, tapered transitions can be used to avoid residual stresses, high stress concentrations and abrupt flow transitions during mold filling. Whenever feasible, wall section thickness must be reduced by coring out sections of the molding, and by using ribs to compensate for the loss in stiffness of a thinner part Mall94. d) Side actions (side cores, split cores, lifters etc.) must be used to create undercut features on the part or the part should be redesigned to eliminate undercut features. Fig. 10 shows an example of a plastic part, whose undercut region cannot be molded by any side action. A simple redesign shown in this figure solves this problem. e) Draft angles need to be imparted to vertical or near-vertical walls for ease of removal of the part from the mold assembly. Fig. 11 shows that incorrect draft angles make it impossible to eject the part. Tapering the side walls inward (towards the core side) resolves this issue 第 4 页 共 67 页satisfactorily. Drafting also reduces tool and part wear considerably sliding friction as well as scuffing or abrasion of the outer (cavity) faces of the part are eliminated to a large extent. Typically, the required draft angle ranges from a fraction of a degree to several degrees and depends on a lot of parameters such as depth of draw, material rigidity, surface lubricity and material shrinkage Mall94. Computational work in the field of manufacturability analysis of injection molded parts mainly focuses on two different areas. The first area deals with demoldability of a single material part. The demoldability of a part is its ability to be ejected easily from the mold assembly (core, cavity and side actions) when the mold opens. Deciding if a part is demoldable is equivalent to deciding if there exists a combination of main parting direction, side cores and split cores such that the criterion of demoldability is satisfied. Chen et al. Chen93 describe a visibility map based approach to find a feasible parting direction that minimizes the number of side cores. Hui Hui97 describes a heuristic search technique for selecting a combination of main parting, core and insert directions. Approaches based on undercut-feature recognition have also been developed Gu99, Fu99, Lu00, Yin01. The basic idea behind these approaches is to find potential undercuts on the part using feature recognition techniques. Each type of feature has its own set of candidate parting directions. The optimal main, parting direction is then chosen on the basis of some evaluation functions.Ahn et al. Ahn02 describe mathematically sound algorithms to test if a part is, indeed, moldable using a two-piece mold (without any side actions) and if so, to obtain the set of all such possible parting directions. Building on this, Elber et al. Elbe05 have developed an algorithm based on aspect graphs to solve the two-piece mold separability problem for general free-form shapes, represented by NURBS surfaces. McMains and Chen McMa04 have determined moldability and parting directions for polygons with curved (2D spline) edges. Recently, Kharderkar et al. Khar05 have presented new programmable graphics hardware accelerated algorithms to test the moldability of parts and help in redesigning them by identifying and graphically displaying undercuts. Dhaliwal et al. Dhal03 described exact algorithms for computing global accessibility cones for each face of a polyhedral object. Using these, Priyadarshi and Gupta Priy04 developed algorithms to design multi-piece molds. Other notable work in the area of automated multi-piece mold design includes that by Chen and Rosen Chen02, Chen03.The second area of active work deals with the simulation of molten, plastic flow in injection molding process. Many commercial systems are available to help designers in performing manufacturability analysis. Also, finite element analysis software like ANSYS, ABAQUS, FEMLAB etc. can be used to predict and solve some problems, such as whether the strength of some portion of the part is adequate. Since these types of problems arising during multi-material injection molding are the same as those experienced in case of single material molding, appropriate commercial packages can be used to overcome them.3 IDENTIFYING SOURCES OF MOLDING PROBLEMS Many different reasons can contribute to manufacturability problems during MMM. These reasons include material incompatibility, interactions among cooling systems for different stages, placement of gates, demoldability, and ejection system problems. In this paper we mainly focus on the manufacturability problems that result from the shape of the multi-material objects. Specifically, we focus on those manufacturing complications that arise due to the presence of plastic material inside the mold cavity during the second shot. The work presented in this paper is 第 5 页 共 67 页applicable to multi-shot rotary platen (shown in Fig. 4 and Fig. 12), multi-shot index plate (shown in Fig. 13 and Fig. 14), and over-molding processes (shown in Fig. 15). Appendix A describes each of these processes in details.It is important to note here that part designs need to be modified significantly depending upon the nature of the MMM process that will be used to mold it. Fig. 16 illustrates this idea by using three different part designs. The first object can be molded by overmolding process only, whereas the second object can be molded using either overmolding or index plate multi-shot molding process. Rotary platen process should be used to mold the last part. Thus, it is clear that specific process-dependent design rules are essential in multi-material injection molding.Let us now try to systematically identify the manufacturability problems so that corresponding design rules can be framed to handle them. These design rules will be later utilized by the algorithms in Sections 4 and 5 to offer meaningful solutions once the problems have been detected. A new way of identifying all the potential sources of manufacturability problems using state transition diagrams and studying failure mode matrices is presented below. The effectiveness of this technique is first validated by comparing the identified failure modes and corresponding design rules with the established guidelines for traditional injection molding. Afterwards, the same approach is adopted in order to detect and diagnose all the potential manufacturability problems in MMM processes. This scheme is comprehensive with respect to the state transition model; it does not overlook any of the potential probl
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