Reducing open die forw ard extrusion

Th e m in im u m ach ievable wall t h ickn esses fo r st eel are aro u n d 1 m m , fo r alu m in iu m aro u n d 0.1 m m . Ext rem ely t h in walls can be p ro d u ced by a su bseq u en t iro n in g p ro cess fo llo win g backward cu p ext ru sio n . Fo r st eel, t h e m in im u m cu p bo t t o m t h ickn ess is 1 t o 2 m m , fo r NF m et als 0.1 t o 0.3 m m . Th e bo t t o m t h ickn ess sh o u ld always be great er t h an t h e wall t h ickn ess u n d erfillin g. In t h e case o f t h ick-walled cu p s wh ich are ad d it io n ally exp ect ed t o sat isfy h igh co n cen t ricit y req u irem en t s, it is ad visable t o carry o u t a cen t erin g o p erat io n p rio r t o cu p ext ru sio n . Cen t erin g is a p ro cess very sim ilar t o backward cu p ext ru sio n , alt h o u gh wit h o n ly a sm all d efo rm at io n st ro ke. Cen t erin g is gen erally p erfo rm ed u sin g gu id ed p u n ch es.

6.5.4 Reducing open die forw ard extrusion

Th e p rin cip le o f red u cin g is sim ilar t o t h at u sed fo r fo rward ro d ext ru - sio n as far as t h e d ie d esign is co n cern ed cf. Fig. 6.1.2 . Ho wever, in co n t rast t o fo rward ro d ext ru sio n , t h e billet m at erial is n o t p ressed again st t h e co n t ain er wall. Th u s, o n ly a lim it ed d egree o f d efo rm at io n is p o ssible wit h o u t bu cklin g. Du rin g t h e fo rm in g p ro cess, t h e m at erial m u st n eit h er be d efo rm ed u p set in t h e co n t ain er n o r bu ckle. In t h e case o f st art in g billet s wit h h d 10, red u ct io n s in cro ss sect io n can o n ly be ach ieved u sin g t h is p ro cess. Larger am o u n t s o f d efo rm at io n req u ire several su ccessive red u cin g o p erat io n s. Th e ach ievable level o f d efo rm at io n d ep en d s o n t h e m at erial, t h e p relim in ary st at e o f st rain h ard en in g an d t h e d ie o p en in g an gle. An o verall level o f d efo rm at io n calcu lat ed fro m t h e first t o t h e n t h red u cin g o p erat io n o f 50 sh o u ld n o t be exceed ed d u e t o t h e fo rm at io n o f cen t ral bu rst s ch evro n s. Red u cin g o r o p en d ie fo rward ro d ext ru sio n is a p ro cess wh ich is n o t u sed in p ract ical warm fo rm in g o p erat io n s, as t h e ach ievable d egree o f d efo rm at io n is t o o lo w d u e t o sign ifican t ly red u ced flo w st ress in warm fo rm in g. Wit h a d ie o p en in g an gle o f ap p ro x. 20°, it is p o ssible t o ach ieve a m axim u m level o f d efo rm at io n o f w = 0.3. If t h e m at erial u sed h as alread y been st rain h ard en ed , fo r exam p le as a resu lt o f a p revio u s red u cin g o p erat io n , a so m ewh at h igh er d egree o f d efo rm at io n is p o ssi- ble. Ho wever, wit h an in creasin g d ie o p en in g an gle, t h e ach ievable lev- el o f d efo rm at io n d ro p s su bst an t ially. 475 Force and w ork requirement Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Die an gles ran ge between 10 an d 45° an d gen erate tap ers with sh ou l- d ers between a = 5 an d 22.5° on th e p ressed p art cf. Fig. 6.1.2 . Th e p u n ch load is of m in or im p ortan ce, an d d ep en d s on th e billet m aterial. It sh ou ld n ot u p set th e billet p lastically, an d cau se bu cklin g stresses in slen d er billets. Neith er of th ese stress valu es sh ou ld be exceed ed . Th e p rocess is h igh ly sen sitive to flu ctu ation s in th e stren gth of th e startin g m aterials an d to th e q u ality of lu brication , i. e. p h osp h atin g an d soap in g.

6.5.5 Ironing