Die and punch design

6.7.2 Die and punch design

Dies are su bject ed t o h igh d egrees o f in t ern al p ressu re wh ich can q u ick- ly lead t o d ie failu re u sin g sin gle rin g ext ru sio n co n t ain ers m an u fac- t u red wit h d ie st eels available t o d ay. Th e ext rem ely h igh st resses, p re- sen t o n t h e in t ern al d ie wall o f t h e ext ru sio n co n t ain er cf. Sect . 6.5.1 can be red u ced by p re-st ressin g sh rin k-fit t h e co n t ain er wit h o n e o r m o re sh rin k rin gs. Pu n ch es are su bject ed in it ially t o p ressu re an d t h en ad d it io n ally, d ep en d in g o n t h e p ro cess, t o ben d in g st ress as a resu lt o f o ff-cen t er lo ad s. Less freq u en t ly, t h e p o ssibilit y o f p u n ch bu cklin g m u st be t aken in t o co n sid erat io n . Th e calcu lat io n m et h o d s o u t lin ed h ere fo r d ies an d p u n ch es are d escribed in d et ail in VDI Gu id elin es 3176, VDI 3186 an d ICFG Do c. No . 5 82. 491 Die design Fig. 6.7.5 Shearing die w ith w edge-action pusher, holding sleeve hydraulically clamped Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Die design In Fig. 6.7.6 , vario u s sh rin k rin g d esign s are illu st rat ed . Cylin d rical sh rin k fit s are sim p ler t o p ro d u ce, bu t can o n ly be u sed u p t o in t erfer- en ces wh ich can be ach ieved by warm sh rin k fit t in g, in o rd er t o avo id co ld weld s d u rin g assem bly. Tap ered sh rin k fit s p erm it sim p ler, scrat ch -free d isassem bly. A d raw- back o f t h is m et h o d are t h e h igh p ro d u ct io n co st s. Th e t ap ered sh rin k fit exh ibit s d ifferen t levels o f rad ial p re-st ress, wh ich can lead in case o f very lo n g co n t ain ers t o d im en sio n al d ifferen ces o n t h e fo rged p art . Usu al t ap er an gles are 0.5 an d 1°; if t h e h eigh t t o d iam et er rat io o f t h e co n t ain er is belo w 0.8, t ap er an gles o f 2 t o 3° are u sed in o rd er t o avo id d isassem bly o f t h e sh rin k rin gs d u rin g p art eject io n . An alyt ic ap p ro xi- m at e o p erat io n s are based o n t h e m et h o d by LAMÉ an d were p u blish ed by Ad ler Walt er. Th ese assu m e an id eal lo ad in g sit u at io n wh ere an in fi- n it ely lo n g t h ick-walled cylin d er is su bject ed t o a co n st an t h yd ro st at ic p ressu re over its en tire len gth . Usin g Tresca’s yield con d ition , wh ich can be ap p lied in th is case, th e stress d istribu tion can be calcu lated . It is fou n d th at th e m axim u m valu es are at th e in tern al wall of th e con tain - er an d corresp on d ap p roxim ately to th e valu e of th e in tern al p ressu re p i , 492 Solid forming Forging Fig. 6.7.6 Shrink fits w ith single and double rings – each w ith cylindrical and tapered fits cylindrical shrink fit surface cylindrical shrink fit surface tapered shrink fit surface tapered shrink fit surface single shrink ring double shrink ring Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 fo r rad ial st ress s r an d t an gen t ial st ress s t Fig. 6.7.7 , left. Th e in t ern al p ressu re can t h erefo re, d u e t o am o u n t t o n o m o re t h an , fo r t h e effect ive st ress s v = R p 0.2 . Th u s, fo r m at erial valu es o f R p 0.2 = 2,000 N m m 2 , m axim u m p ressu re levels o f 1,000 N m m 2 are p erm issible. If, in t h e case o f t h ick-walled p ip es su bject ed t o in t ern al p ressu re, t h e effect ive st ress exceed s t h e yield st ren gt h o f a m at erial wit h su fficien t t o u gh n ess, t h en p last ic flo w o ccu rs at t h e in sid e wall o f t h e co n t ain er. If t h e u lt im at e st ren gt h is reach ed in co n t ain ers o r in sert s fro m brit t le m at erials, cracks o ccu r. As a resu lt o f ad d in g co m p ressive st ress, t h e t an gen t ial st ress an d t h u s also t h e effect ive st ress st at u s at t h e in sid e wall o f t h e ext ru sio n co n t ain er o r in sert are red u ced Fig. 6.7.7 center, right. Th e rad ial p re-st ress is gen erat ed by m ean s o f sh rin k rin gs. A sh rin k rin g h as an in t ern al d iam et er t h at is sm aller t h an t h e o u t er d iam et er o f t h e co rresp o n d in g in n er rin g by a select ed d im en sio n in t erferen ce. By m ain t ain in g t h e o u t er d iam et er u n ch an ged , t h e p erm issible in t ern al p ressu re can be in creased by u p t o 100 as a resu lt o f t h e sh rin k rin g co m p ared t o a co n t ain er wit h o u t a sh rin k rin g. Fo r a given p erm issible in t ern al p ressu re, t h e o u t sid e d iam et er o f t h e co n t ain er, u sin g sh rin k 493 Die design Fig. 6.7.7 Theoretical stress distribution in a thick-w alled single-piece hollow cylinder left, a tw o-part center and three-part right shrink fit design red: w ithout internal pressure, blue: w ith internal pressure . . . . . - . - . d v i p t i p r i p p i a b p i r t p i t r + - σ σ σ v r t i p = + = 2 R p0 2 2 . Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 rin gs, can be red u ced by abo u t 60 Table 6.7.2 . In m u lt ip le-st at io n fo rm in g m ach in es t h is resu lt s in a su bst an t ial red u ct io n o f d ie d iam e- t ers, t h e d ist an ces bet ween st at io n s, t h e d im en sio n s o f t h e d ie h o ld er an d t h u s also t h e m ach in e bed size. Carbid e can n o t be su bject t o t en sile st resses. Th erefo re, wh en u sin g d ies o r in sert s fro m carbid e, t h e ext ru sio n co n t ain er m u st be p re- st ressed in su ch a way t h at n o t an gen t ial t en sile st ress o ccu rs u n d er in t ern al p ressu re. Acco rd in gly, t h e in t erferen ce d im en sio n s are co n sid - erably h igh er t h an t h o se u sed fo r t o o l st eels. Gu id elin es also h ave been est ablish ed fo r t h e calcu lat io n o f axial p re-st ress levels; in t h e t h in , rin g- sh ap ed lat eral sp lit , su rface p ressu res o f 700 t o 1,000 N m m 2 are as- su m ed as gu id elin e valu es. In ext ru sio n d ies, t h e act u al st ress co n d it io n s d iffer fro m t h e id eal assu m p t io n s m ad e h ere, h o wever. Th e ext ru sio n co n t ain ers h ave a fin it e len gt h , a lim it ed an d p o ssibly eccen t ric p ressu re area, n o u n ifo rm in t ern al p ressu re exist s, an d t h e co n t ain ers h ave o ft en o ff-cen t er d ie o p en in gs. Realist ic co n d it io n s in t h e co n figu rat io n o f sh rin k fit assem - blies can be d et erm in ed by u sin g d iscret e ap p ro xim at io n t ech n iq u es, fo r exam p le t h e fin it e elem en t m et h o d FEM. Fo r p ract ice-o rien t ed ap p licat io n , vario u s calcu lat io n s h ave been co n d u ct ed by m akin g a p aram et ric st u d y u sin g FEM. Th ese resu lt s h ave been m ad e accessible fo r p ract ical ap p licat io n in t h e fo rm o f No m o gram s VDI 3176, ICFG Do c. 5 82 an d co rresp o n d in g co m p u t er p ro gram s. Punch design Ext ru sio n p u n ch es are su bject ed t o an axial fo rce wit h an average co m - p ressive st ress. Th ey are calcu lat ed as fo llo ws: 494 Solid forming Forging Table 6.7.2: Increase of the allow able internal pressure p i zul. or reduction of the outside diameter d a through single and double shrink rings Characteristic value Press container without shrink ring with single shrink ring with double shrink ring p i zul. [N mm 2 ] 690 1,160 1,395 d a [mm] with d i = 20 mm 100 40 36.4 p F A St St St = Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Th e o ff-cen t er ap p lied fo rce cau ses ad d it io n al ben d in g st resses. Th e o verall st ress in t h e p u n ch t h en am o u n t s t o : wh ereby d [m m ] is t h e p u n ch d iam et er an d e [m m ] t h e o ff-cen t er p o si- t io n o f fo rce ap p licat io n . Valu es fo r crit ical bu cklin g st ress are p ro vid ed in Fig. 6.7.8 . Wit h backward cu p ext ru sio n , t h e eccen t ricit y gen erally am o u n t s t o e d = 0.01 t o 0.02. Th e calcu lat ed p u n ch st ress is co m p ared t o t h e co m p ressive yield st ren gt h o f t h e p u n ch m at erial in it s h ard en ed st at e. Th e cou n terp u n ch is su bjected to an average com p ressive stress of d u rin g backward cu p ext ru sio n . 495 Die design Fig. 6.7.8 Critical buckling stress in function of the slenderness ratio ld of a punch made of high-speed steel: 1 forw ard rod extrusion; 2 backw ard cup extrusion; d punch dia- meter; e off-center position of force application; l free punch length 1,000 2,000 3,000 2 4 6 8 10 12 0.1 0.05 0.01 ed = 0 forw ard rod extrusion backw ard cup e trusion md = 3,000 Nmm E = 225,000 Nmm d e σ St ges St p e d = + ⋅ 1 8 p F A G St = Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998

6.7.3 Die and punch materials