研究聚合物纤维材料和木质材料钣金成型模具的强度和可行性.pdf

journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 4753 journal homepage: Study on the usability and robustness of polymer and wood materials for tooling in sheet metal forming M ario Pintoa, Abel D. Santosa,b, Pedro Teixeiraa, P.J. Boltc a INEGI, Institute of Mechanical Engineering and Industrial Management, R. Barroco 174, 4465-591 Lec a Balio, Portugal b FEUP, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias s/n, 4200-110 Porto, Portugal c TNO Industrial Technology, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands a r t i c l ei n f o Article history: Received 20 April 2007 Received in revised form 27 July 2007 Accepted 23 August 2007 Keywords: Sheet metal tooling Tooling materials a b s t r a c t The traditional market of sheet metal parts is based in high production volumes normally greater than 10.000 parts by using high-speed presses and highly automated operations, being the tooling usually made from steel or cast iron, which ensures the needed robustness for such production volumes. Current trends in stamping industries can be characterized by the increasing demand for individualization of products which leads to a growing devel- opment towards an increasing number of product variants, lower production volumes and shorter innovation cycles. These trends have triggered research in the development of faster andcost-effectivetoolmanufacturingtechniquessuitableforlowproductionvolumes,since high investments in steel tools can only be compensated by huge volume series of one and the same part during long life cycles. A promising approach for production of small series of complex sheet metal parts is the use of non-conventional tooling materials such as poly- mers,woodandlowmeltingpointmetals.Theusageofthesealternativematerialsintooling is still looked with some suspicion, due to the lack of know-how on its processing and also on its wear durability, infl uenced by the required product geometry, blank material and expected production volume. In this paper a study on the use of non conventional materials for tooling, to be used for producing a complex sheet metal part, is performed. The evolution of tool wear, roughness and geometrical changes in punch and die radius were measured and its infl uence in the stamped part geometry evaluated, for an imposed production volume target of 500 parts. Also a comparative study of the tooling costs is presented in order to assess the aptitude and economical feasibility of the use of these tooling materials for low production series. 2007 Elsevier B.V. All rights reserved. 1.Introduction Sheet metal forming processes are among the technologies characterized by high production volumes and typically con- nected to important economical fi elds of application such as automotive industry, white goods, kitchenware, electrical Corresponding author at: FEUP, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias s/n, 4200-110 Porto, Portugal. Tel.: +351 22 508 17 42; fax: +351 22 508 14 45. E-mail address: abelfe.up.pt (A.D. Santos). appliances, etc. For such huge production volumes of the same part during long product life cycles, well-established forming methods enable a secure, cost-effective manufac- turing of complex parts, compensating high investments in required complex tools and equipment as well as long prelim- inary development times until production (Hirt et al., 2006). 0924-0136/$ see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.08.082 48journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 4753 But the current market demand for individualization of prod- ucts has activated research in the development of faster and cost-effective tool manufacturing techniques suitable for low production volumes. This demand along with strong competi- tionamongseveralproducersandthecontinuousreductionof products life cycle requires a faster and cost-effective devel- opmentofhighqualityproductswithhighfl exibilityfordesign changes supporting the innovation imposed to those products (Yangetal.,2002).Asaresult,sampleparts,prototypesandlow volume series parts have to be available at very short term. In this prototype or low production series, the conventional way of tool manufacturing can not be applied, due to associated high tooling costs and consequent high lead-time. Taking as an example a prototype series of a kitchen appliance housing or a series of classic car parts, the tooling cost can be respon- sible for 8090% of the fi nal product costs and the lead-time can be up to 6 months. Generally, the manufacturing of sheet metal parts prototypes is associated to methods of “rapid tool- ing” (RT), where the traditional materials of production of the tools, generally steel or cast iron, are substituted by less resis- tant materials, cheaper and more easily machinable, as the light alloys and polymeric materials. These methods can be classifi ed as direct or indirect manufacturing methods accord- ing to the process chains. Direct methods are those where the sheet metal forming tool is the direct product of the specifi c manufacturingtechnology.Althoughthetoolismanufactured directly from the process, further processing is often required in order to obtain a strengthened tool (e.g. heat treatment, curing, sintering, infi ltration of a “green” part with a low melt- ing point alloy metal, etc). Indirect methods are characterized by the need of one or several intermediate products in order to obtain the stamping tool. This intermediate product (pat- tern) is then converted by conversion techniques to obtain the fi nal tool. An overview of such methods can be seen in Table 1. Anotherpossibleclassifi cationofthosetoolmanufacturing methods may be the division between removal and additive methods, a common classifi cation from the Rapid Prototyping (RP) viewpoint. Removal methods imply the removal of mate- rial for tool manufacturing, for example, by using high speed milling.Theadditivemethodsareusuallybasedonlayerman- ufacturing techniques. Many additive processes are available such as Laminated Object Modelling (LOM), Stereolitography (SLA), Fused Deposition Manufacturing (FDM), Selective Laser Sintering (SLS), Solid Ground Curing (SGC), etc. A review of these techniques is presented by Rosochowski and Matuszak (2000). Nevertheless, both of these methods, additive and removal, may be seen under RP range of technologies if the method implies digital manufacturing, obtained from direct transfer from CAD data to pattern (Donnachadha and Tansey, 2003). Since the offer of tool manufacturing techniques is diversifi ed and numerous, the selection of the best method shouldbetakenconsideringseveralvariableswhichwillrange from economical issues to dimensional accuracy and tool durability. An example of RT additive method applied to sheet metal forming was performed by Harris et al (Harris et al., 2003). In their study, a tooling for a complex geometry was manufac- tured using metal powder binding techniques. The punch was obtained from a mixture of steel powder and binder, then was sintered and infi ltrated by a low melting point alloy metal, while the die was produced by direct metal powder laser sin- tering EOSint. During testings in the press, the tool began to show signs of severe wear after a small number of stampings and production has ceased after 17 stamped parts. As indi- cated by Wohlers (2003), “(additive) methods of Rapid Tooling (RT) were once thought to be a key to RPs potential, but in many of these have yet to meet expectations. In many cases the applications and goals not the technology have been the limiting factors”. So,theremoval/machiningmethodusingalternativesofter tooling materials, such as densifi ed wood, polymers and low melting points metals, seems to show a better agreement for tool manufacturing to be used in low volume productions of sheet metal forming parts (Neubauer et al., 1997; German, 1999), offering a better dimensional accuracy and wear resis- tance (van der Hide et al., 2003) when compared to layer manufacturing techniques. Also, it provides higher removal rates, fewer machining time as well as less need of tool fi nish- ing when compared to conventional tooling materials. Table 1 Classifi cation of different methods to obtaining active parts for tooling (Pinto, 2002) MethodGeneric technologyFabrication methodProcess chainTooling material Direct Machining technologyMillingSteel, aluminium, Zinc alloys, polyurethane machinable boards, woodHigh speed milling Rapid prototyping tooling (RPT) Direct RPT LOMPaper SLSPolymeric materials DMLSBronze or steel based materials SLS rapid tool Steel with infi ltration of copper Indirect Metal shell forming on model Metal spraying on modelMost RPT or machined model using several spraying techniques Zinc alloys Metal deposition on modelMost RPT or machined model Niquel Non metal castingCasting on RP moulds or machined models Epoxy or poliurethane casting Epoxy or poliurethane resins Low melting point metal casting Casting on RP moulds or machined models Low melting point metal casting Low melting point metals journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 475349 Table 2 Properties of tool materials used in current study (OBO-Werke) MaterialHardnessFlexural strength (MPa) Density (Kg/m3) Working temperature range (C) Compressive strength (MPa) Modulus elasticity (MPa) Polyurethane8287 Shore D95120010801001.603 Densifi ed woodDIN 53456 120N/mm21901370200+10028018.000 Fig. 1 View of polymeric tool and its active parts (left); view of stamped part (right); punch dimension of cross geometry is 487mm. The work presented herein is part of a European Craft project, called in brief “Smart Tooling”. The global objective of the project has been the development of a tooling deci- sion support system able to estimate an optimal selection of tool material and its optimal fabrication route according to a specifi c product, required lead time and expected production. One of the tasks in the specifi c objectives of this project was theinvestigationofselectivealternativetoolingmaterialscon- cerning their manufacture and its service behaviour, a subject which is treated in the present paper. 2.Tooling materials For the choice of alternative to metallic tooling material, the important variables for decision were: machinability, strength and accessibility. The wood material being already a tradi- tional alternative to this kind of applications has been one of the choices. The other choice has been a polymeric material due to its widespread applications and resistance. Therefore in the present study, the materials used for tool manufactur- ing were a densifi ed wood and a polyurethane based material (OBO-Werke). The fi rst one is a high-quality resin impreg- nated compressed wood material, which basic components are beech veneer and thermosetting phenolic resin that are compressed to produce an extremely hard synthetic-resin- based laminated wood. The second material a polyurethane based material that was originally developed for functional models making and manufacturing of checking fi xtures and test units. The main mechanical properties of these two mate- rials are listed in Table 2. 3.Tool geometry, blank material and experimental conditions In the research project, several experimental geometries were studied. The tool geometry representing the most severe con- ditions for production of parts is the one selected for this paper. Fig. 1 shows a view of the experimental tool used for this study. Its geometry is a cross shape with a main length of 487mm for punch. Draw beads positioned in the four quad- rants of the cross are used to stamp the parts, as it can be seen in Fig. 1. A stamped part using this tool is shown also in Fig. 1, as well as its main dimensions. The blank size is a 240mm240mm square and its material is a mild steel, Fig. 2 Profi les to be measured on die (left) and punch (right). 50journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 4753 Fig. 3 Measured section profi les on die for polyurethane base material. DC04, with 1mm thickness. This thickness corresponds to the most usual production of intended sheet metal parts, it is the thickness to be used for this part and no other thicknesses were considered in this study. Punch stroke corresponds to part height, which is 56mm. Testingswereperformedina1600kNtripleaction hydraulic press and the stamping speed was set to 30mm/s. Blank holder force was 100kN and a counter-punch was used, which actuates the blank against punch by using four gas springs, applying a total force of 60kN. These experi- mental conditions correspond to industrial production of the part. 4.Results and discussion Main results to test these alternative tool materials have been: measurement of section profi les on die and punch, mea- surement of roughness at defi ned regions of tool and visual inspection of evident damages of any tool part. Concerning the measurement of section profi les the strategy has been to understand any evolution or variation of such profi les with the number of stamped parts. For such purpose, a reference monitoring of tooling has been performed for the follow- ing stamping intervals: 1-2-5-10-20-50-100-200-500. When the number of stampings attained to an interval value, all the active parts of the tool were disassembled from the press and the defi ned measurement section profi les (Fig. 2) for both die and punch was performed by using a CNC CMM (Coordinate Measurement Machine). 4.1. Tool section profi les Fig. 3 shows the evolution of die profi les 1 and 3 vs. number of stampings for the polyurethane based material. It may be seen that profi le 1 shows a uniform tendency for changing the die curvature (into the radius center) with increasing number Fig. 4 Measured section profi les on die for densifi ed wood material. journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 475351 Fig. 5 Measured surface roughness on die for polyurethane base material. of stampings, reaching a mean total variation of 0.5mm. For profi le3asimilarvariationisobserved,butonlyonthetopsur- face of die radius curvature. A further analysis of this change, by using some painting on the die surface, show that such geometry change is not due to wear (as it would suggest) but mainlytononrecoverablecompressionofthe“soft”polymeric type material. That change is more evident in profi le 1 than in profi le 3, which may be in accordance with the different force distribution in those profi les. The same study of evolution of die profi les was per- formed for densifi ed wood material, as seen in Fig. 4. For this tool material the evolution of both profi les is very sim- ilar: the change of geometry is located in the top surface of radiusofcurvature(totalvariationof0.45mm).Thisbehaviour and corresponding analysis suggests that, in this case, wear is the main reason for such change. Also the compressive strength of this wood base material is almost three times the polyurethane base material, which may be in accordance with previously observed changes of geometry. Although section profi les for punch are not presented, corresponding results have shown no visible variation along the stamping of the 500 parts, both for the polyurethane based material as well as for the densifi ed wood mate- rial. Two reasons may explain this observation of additional punch geometric stability: sheet sliding over punch is almost inexistent when compared to die and compressive forces applied to punch are signifi cantly lower than those of the die. 4.2.Tool roughness The evolution of die surface roughness with the number of stampings is shown in Fig. 5 for the polyurethane based tool material. The measuring equipment has been a surface roughness measuring instrument Mitutoyo 178 series 921D. It may be seen that overall evolution shows a decrease of surface roughness as if stampings were having a polishing effect. Curves 8 and 9 are those that have a lower evolution, due to its location in the matrix vertical walls (less sliding with sheet). These curves also show differences in the ini- tial surface roughness level, which is related to their different location and the way those surfaces were machined. The larger decrease for curves 1 and 5 than for curve 2 suggests an asymmetric polishing action of the die surface, which is a behaviour that we would not expect. Concerning the den- sifi ed wood material, a similar overall tendency behaviour of polishing is observed as for polyurethane tooling material. 4.3.Extra wear of tools Besides the previous evolution of profi le geometry and rough- ness, a visual inspection of the tool active parts has been Fig. 6 Die draw bead part and the observed scratches polyurethane based material (left) and densifi ed wood (right). 52journal of materials processing technology 2 0 2 ( 2 0 0 8 ) 4753 performed regarding possible damages occurrence. The main observedseveretoolwearwaslocatedatthedrawbeadregion. Fig.6showsthediedrawbeadareaandtheobservedscratches and tool material loss, which are the result of the sliding of the blank corner over the draw bead. It must be remarked that blanks have not been deburred for these experiments. This conditionofblankmetalburrslidingovertoolmaterialintends to correspond to the most usual and realistic use of industrial tools. However, we may say that this problem of damage on stressed regions (e.g. draw beads), when using soft tools, may be easily overcome if harder metal inserts are used in such problematic regions. 5.Tooling costs comparison for different raw materials Fig. 7 depicts the tooling costs for the active parts (die, punch and blank holder) made out of polyurethane, densifi ed wood and, for comparison, a steel. As it can be seen, the largest variation corresponds to the tooling material costs, but tool manufacturing has also a signifi cant cost in the cas