6.5.2 Forw ard tube extrusion
Fo rward t u be ext ru sio n in vo lves t h e p ro d u ct io n o f a t u bu lar p art wit h a red u ced wall t h ickn ess st art in g cf.
Fig. 6.1.1 fro m a cu p o r cylin d er.
Th e d ie o p en in g, wh ich d et erm in es t h e sh ap e o f t h e p ro d u ced p art , is fo rm ed by t h e p ress co n t ain er an d t h e p u n ch . If t h e m an d rel is p erm a-
n en t ly m o u n t ed in t h e p u n ch , it is called a m o vin g m an d rel: As a resu lt o f m at erial flo w, ad d it io n al frict io n al fo rces o ccu r wh ich can lead t o
crit ical levels o f t en sile st ress in t h e m an d rel. Th is can be avo id ed by u sin g a m o vin g m an d rel, t h at is m o ved , wit h in cert ain lim it s, by t h e
d efo rm in g m at erial. Frict io n -relat ed t en sile st resses o ccu r d u rin g fo r- ward ext ru sio n wit h t h e m o vin g m an d rel an d d u rin g st rip p in g o f t h e
wo rkp iece in bo t h fo rward t u be ext ru sio n m et h o d s. Th erefo re, geo m et - rical co n d it io n s m u st be set su ch t h at m an d rel lo ad d o es n o t exceed
1,800 N m m
2
. Th is is gen erally t h e case wh en t h e rat io o f h eigh t t o d iam et er in t h e st art in g m at erial h
d d o es n o t exceed 10 t o 15. Th e
ach ievable levels o f d efo rm at io n d ep en d o n t h e m at erial an d co rre- sp o n d t o t h o se ach ievable in fo rward ro d ext ru sio n . Th u s, co m p arable
sh o u ld er geo m et ries can also be fo rm ed Table 6.5.1
.
6.5.3 Backw ard cup extrusion and centering
A t h in -walled t u bu lar p art is p ro d u ced fro m a so lid billet . Th e d ie o p en in g wh ich d et erm in es t h e sh ap e o f t h e p art is fo rm ed by t h e
p u n ch an d co n t ain er, o r t h e d ie cf. Fig. 6.1.1
. In t h e backward cu p ext ru sio n o f st eel, a cert ain lim it in g rat io o f p en et rat io n d ep t h t o p u n ch
d iam et er h
2
d
1
= 3 t o 3.5 can n o t be exceed ed . Dep en d in g o n t h e m at e- rial, t h e m axim u m ach ievable d egree o f t ru e st rain is
w
= 0.9 t o 1.6
«
A
= 60 t o 80 . Ho wever, eco n o m ical t o o l life is o n ly ach ieved wit h so m ewh at lo wer levels o f t ru e st rain
Table 6.5.1 .
Du e to a m axim u m ad m issible p u n ch stress of p
St m ax
2,300 N m m
2
wh en p rocessin g steel, a m in im u m level of tru e strain of at least
w
= 0.16 to 0.43
«
A
= 15 to 35 m u st be en su red , i. e. it is n ot p ossible to back- ward ext ru d e a cu p wit h an y o p t io n al in t ern al d iam et er relat ive t o t h e
o u t sid e d iam et er. It m ay be p o ssible t o p ro d u ce sm all bo res wit h ad d i- t io n al seco n d ary geo m et ries e. g. a p ro t ru sio n fo r ext ru sio n p iercin g,
p ro vid ed t h ese relieve t h e m at erial flo w in t o t h e cu p wall.
474
Solid forming Forging
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
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
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