外文翻譯--在多材料注射模設計中要綜合考慮可制造的工藝性 中文版

1、<p>　　Incorporating Manufacturability Considerations during Design of Injection Molded Multi-Material Objects </p><p>　　Ashis Gopal Banerjee, Xuejun Li, Greg Fowler, Satyandra K. Gupta1 </p><

2、p>　　Mechanical Engineering Department and </p><p>　　The Institute for Systems Research </p><p>　　University of Maryland, College Park, MD 20742, U.S.A. </p><p><b>　　ABSTRAC

3、T </b></p><p>　　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.

4、Therefore, designing multi-material molded objects requires addressing many additional manufacturability considerations. In this paper, we first present an approach to systematically identifying potential manufacturabili

5、ty problems that are unique to the multi-material molding processes and design rules to avoid t</p><p>　　Keywords: Automated manufacturability analysis, generation of redesign suggestions, and multi-material

6、 injection molding. </p><p>　　1 INTRODUCTION </p><p>　　Over the last few years, a wide variety of multi-material injection molding (MMM) processes have emerged for making multi-material objects,

7、 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 re

8、duction in assembly operations and production cycle times. Furthermore, the product quality can be improved, and the possibility of manufactu</p><p>　　There are fundamentally three different types of multi-m

9、aterial 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

10、 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 sequenc

11、e, w</p><p>　　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

12、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 mo

13、ldable using multi-material molding. By moldable we mean that the assembly (or part) ca</p><p>　　On the other hand, there are molded multi-material assemblies where at least one of the parts would have not b

14、een 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 mo

15、lding 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 </p><p>　　The reason why MMM appears

16、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 add

17、ition to the metallic mold pieces 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 depicti

18、ng the two molding stages in rota</p><p>　　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

19、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 w

20、hy, simply ensuring that the first stage part and the gross shape are moldable do not solve this problem either. Fig</p><p>　　Based on the above discussion, we conclude that a new approach needs to be develo

21、ped 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

22、 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 </p><p>

23、　　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 mo

24、lding 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

25、of such rules and generate feasible redesign suggestions in Se</p><p>　　2 RELATED RESEARCH </p><p>　　A wide variety of computational methods have emerged to provide software aids for performing

26、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 manuf

27、acturing cost. At the other end are sophisticated tools that perform detailed manufacturability analysis and offer redesign suggestions. For analyzing the manufacturability of a design, t</p><p>　　Several le

28、ading 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

29、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 genera

30、ted their own guidelines for the design of plastic</p><p>　　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

31、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

32、 shows an example of how part design needs to be altered to get rid of sharp corners. </p><p>　　b) The parting line must be chosen carefully so that “parting” and metal “shut-off” flashes can be minimized. T

33、ypically, 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 rede

34、signed 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, de</p><p>　　c) Thin and uniform section

35、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 s

36、ections 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, interna

37、l stresses can lead to warpages </p><p>　　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 f

38、eatures. 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. </p><p>　　e) Draft angles need to b

39、e 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 (toward

40、s the core side) resolves this issue 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 la

41、rge extent. Typically, the required</p><p>　　Computational work in the field of manufacturability analysis of injection molded parts mainly focuses on two different areas. The first area deals with demoldabi

42、lity 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 t

43、o deciding if there exists a combination of main parting direction, side cores and split cores such that the criterion</p><p>　　Ahn et al. [Ahn02] describe mathematically sound algorithms to test if a part i

44、s, 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 asp

45、ect 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</p><p&g

46、t;　　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, f

47、inite 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

48、during multi-material injection molding are the same as those ex</p><p>　　3 IDENTIFYING SOURCES OF MOLDING PROBLEMS </p><p>　　Many different reasons can contribute to manufacturability problems

49、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 m

50、anufacturability 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 c</p&g

51、t;<p>　　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 diff

52、erent 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

53、used to mold the last part. Thus, it is clear that specific process-depende</p><p>　　Let us now try to systematically identify the manufacturability problems so that corresponding design rules can be framed

54、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 ma

55、nufacturability problems using state transition diagrams and studying failure mode matrices is presented below. The effectiveness of this technique is first v</p><p>　　Fig. 17 shows the state transition diag

56、ram for SMM. It is clear that five states have to be completed successfully so that an acceptable quality part is obtained. Four common failure modes are possible (shown in Figure 17 and Table 1). Even if any one of them

57、 occurs, a defective part will be formed. These modes have been mapped into corresponding causes in the matrix shown in Table 1. From that, design rules have been derived to alleviate each of the problems. It may be note

58、d here that a particu</p><p>　　1) Incomplete filling of the mold cavity: Solution is that sharp corners should have fillets. A simple method can be used to detect such corners and round them. If the two tang

59、ent vectors at the common intersection point between two edges create a sharp angle, then this corresponds to a sharp corner. In that case, the two mating edges have to be truncated and a small radius circular arc (fille

60、t) has to be created to join the truncated segments such that the tangent vectors change gradually along</p><p>　　2) Occurrence of flashes: The parting line should not cross faces on which flash is not permi

61、tted to avoid this problem. An uneven slicing method suggested by [Wong98] can be used to form the parting line for parts having planar as well as curved surfaces. Since flashes result in ergonomic and aesthetic problems

62、, parts should be designed so that the flashes generated can be easily scraped out or minimized. </p><p>　　3) Presence of residual stresses: The wall thickness should be uniform and as thin as possible to mi

63、nimize this problem. A medial surface transform of the 3D part geometry can be computed to obtain the associated radius function, i.e. the distance from the medial surface to the closest point on the surface boundary. Th

64、is function directly gives the values (and hence the variation) of wall section thickness all along the periphery of the part. </p><p>　　4) Collision among the mold pieces and the molded plastic component: T

65、he first design rule is that parts should be redesigned in order to remove such undercuts wherever possible. If this is not possible, then feasible side actions must exist to mold such regions. </p><p>　　The

66、 second important solution is that faces with direction normals almost perpendicular to the main parting direction should have adequate draft angles. Usually parts are so aligned that the main parting direction coincides

67、 with the vertical axis. The problem of identifying faces that need to be tapered and then actually drafting them has been studied extensively in the mold design literature. Mathematically speaking, we want to give draft

68、 angles to all the part faces whose normals are inclined </p><p>　　5) Tool wear and part damage – Exactly identical design rules as proposed for the previous problem are needed to tackle this problem as well

69、. </p><p>　　Thus, we can summarize that this methodology enables us to identify all the five published traditional molding design guidelines listed in Section 2. Hence it makes sense to extrapolate this idea

70、 for multi-material molding processes also. Fig. 18 shows the state transition diagram for MMM processes. Here instead of four, as many as eight failure modes can result in a defective part and formation of an acceptable

71、 quality MM object entails successful completion of nine states. Mapping of the eight</p><p>　　However, as can be expected the scenario is much more complex in this case. Two kinds of problems are involved h

72、ere- some that are common to single material molding and some that are unique to multi-material molding. Moreover, all the design rules for traditional molding cannot be applied everywhere during the two stages of moldin

73、g. Some need to be applied during the first stage, some during the second stage and some during both the stages. Similarly, some design considerations need to be taken </p><p>　　All these observations correl

74、ate directly to the inferences drawn in the introductory section, where we concluded based on practical examples that an altogether new manufacturability analysis framework has to be developed for MMM processes. In fact,

75、 our matrix clearly reveals that all the new MMM design rules as well as modifications in the existing SMM guidelines are necessitated by the fact that the second molding stage is fundamentally different from what is enc

76、ountered in conventional inject</p><p>　　1. Incomplete filling of the mold cavity – Sharp corners should have fillets in both the components. </p><p>　　2. Occurrence of “parting” and metal “shut

77、-off” flashes – Parting lines should not cross faces on which “parting” and metal “shut-off” flashes are not permitted in either of the two components. </p><p>　　3. Presence of residual stresses – Each of th

78、e two components must have thin, uniform wall sections. </p><p>　　4. Collision among mold pieces and gross object, tool wear and part damage during mold opening after completion of the final stage – Non-mati

79、ng vertical walls should be tapered, i.e. they should have adequate draft angles. </p><p>　　5. Additional possibility of collision among mold pieces (and finished/partially completed object), tool wear and p

80、art damage during mold opening and closing in both the molding stages – A feasible molding sequence should exist in order to eliminate these problems. Henceforth this set of manufacturability problems is referred to as i

81、nfeasible molding sequence problem for the sake of brevity and easy understanding. </p><p>　　6. Undesired friction during mold opening and closing prior to beginning of the second stage – All the mating face

82、s whose normals are perpendicular to the mold opening direction should be tapered so that it is reduced. </p><p>　　7. Occurrence of additional plastic “shut-off” flashes only in the second molding stage – Ae

83、sthetically important mating faces should not be present where metal meets plastic. Crush grooves (i.e., grooves designed to provide a tight seal between the already molded part and the mold) should be created between su

84、ch coplanar faces so that the second material is blocked by the mold and cannot flash on the finished faces of the first component. </p><p>　　8. Excessive interface deformation – Unsupported, thin sections s

85、hould not be present. Instead, supporting pads need to be provided in order to lend extra rigidity to the first component to withstand the pressure of injected plastic during the second stage. </p><p>　　9. E

86、xtra residual stresses in the gross object – Thorough thermal stress and fluid flow analysis must be carried out using some simulation software to determine the best possible thermal management solution (external mold he

87、ating and cooling) so that the insulation effect of the already molded plastic is appropriately taken care of during the second molding stage. </p><p>　　A detailed description of each of the five new manufac

88、turability problems and design rules is given as follows.</p><p>　　?Infeasible molding sequences: Infeasibility of molding sequences implies that the multi-material object cannot be manufactured because the

89、proposed molding sequence is infeasible. Usually this problem is caused by the presence of undercut features and it manifests itself in five different ways. They are listed as follows. All such reasons for the different

90、types of multi-material molding processes have been summarized in Table 3. </p><p>　　The two components are made of different materials and one of them has a higher melting point than the other. Then this co

91、mponent has to be molded first and if it is non-moldable then multi-material molding cannot be carried out. The moldability of the other component and the gross object is immaterial in this case. </p><p>　　L

92、et us now consider that the two components are made of same material (different colors) or different materials having comparable melting points. If none of the two components are separately moldable, then no feasible seq

93、uence exists as the molding process cannot be started at all. </p><p>　　If the faces that need to be demolded in the second stage create impossible to mold undercuts multi-material molding is not possible. &

94、lt;/p><p>　　In addition to the above three conditions, if the core-side faces of the first stage component are in contact with any face of second stage component, or if those core-side faces do not create enoug

95、h contact with the core, it creates a problem in the rotary platen multi-shot molding. </p><p>　　Analogously if in addition to the first three conditions if there does not exist any portion on the first stag

96、e component that is not shared by the second stage component or if the non-shared portion cannot be grasped by the index plate properly, index plate multi-shot molding cannot be done. </p><p>　　Fig. 19 clear

97、ly highlights this infeasible molding sequence problem for different MMM processes. Assuming that we are allowed to follow the sequence of molding A first and then B in all the cases from material considerations, Fig. 19

98、(a) clearly shows how removal of the internal undercuts (caused by faces that need to be demolded) in component A enables us to perform overmolding operation. In Fig. 19(b), sequence A, B is infeasible for rotary platen