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NOTES
TECH N ICAL
FOREST
PRODUCTS
LABORATORY
U, S. FOREST SERVICE
MADTSON. WISCONSIN
t0-I1
343
^")
No. B-9 ,,
t
')
PBEPARATIOTI
O.A STOCi:FOR BENDINC
t d r e n d e r w o o c lp l i a b 1 e , b o t h m o l s t u r e a n d h e a t
are requirerl.
Neither of these factors is fu1ly effective
l n th e a b se i rce o f the other .
Ther e shoulr l be molstur e
e n o u g h , e rp e rL me n ts at the Sor est Pr od,ucts tr abor ator y
i nd ,i ca te , to sa tu ra te the cell walls, r ' ,' hich amount is equl v alent ordinarlly to a moisture content of 20 to 30 per cent.
No better effect is notice,J,where there is add.itional v,ater
i n 'wt hoeo dp. o r e s o r o p e n s l a c e s c f t h e s o o d . I h e p l i a b i l i t y
of
a p p a re n tl y i;r c:eascs quin,e r apidly vr lth incr eas e
o f te mp e ra tu re .
\ter y high t;enpe:- atur es, such as acconp eny
steam under pressure, honever, ere not adviaable, since
t he y re n d e r th e vi o od - 1..:' a,sh;
in fact, boiling for any le ngth
of tiroe perlrarr€!1.iJ.5'
1*ou,ce,: the strengih and toughness of
w o o d , to sn a l l fra cti ons
of tl,eir r :o:' na.^values.
Freshly cut stool: is, of co-arse, thoroughly
sa tu ra te d , a n d n e e d s no ad.dj- tj.onal moistur e.
Only heat i s
required to proCrrceflezib ii:.ty.
If tl:e stock has dried
f or a l cn g tj .n n e , i t vr i.r .:.hive a unifor ;oJ.y low m oistur e
c o n te n t th ro u g h o u t, p' r oba.bi;r about L?, to 15 per cent.
Suoh material requires the addition of a bonsiderable
amount of moistu::e beforo it is in proper condition for
bending, If the stock has o,ried f or a shorter period, tho
interlor part may be at a high noisture content while the
exterior srirfaces are quite dry.
Such stock needs the
addition of sufficient
noisture to bring each part of
it up to about 25 F,er cent.
C o n d e n s a t j . o no f t h e s t e a m m a y n o t f u r n i s h s u f ficient vrater if the stock ls very dry; ln this case the
edditlon of a hot-water spray to play on the stock in the
steam box vrould be advantageous. Admission of the steam
through water will insure its being thoroughly i'*et.
if any to the moisture
Pressur€ stean probably adds iittle
c ontent .
During steaning, the stooll should be so piled
stears'has access to ali sides of each stick.
B-e
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that
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i
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g+3
T h e fo l l o w i n g l s an extr act fr om an lnter esting letter recently re6eived, at the }orest Products Labo r d t o r y f r om a ch a i r ma n u fa c tur er ; r egar ding the pr eparation of stoek for bending in a hot press:
i l t r T eu se a h o rl zo n ta l retor t f or steaning our
wood before bencling. tT€prevlously had been.using
fairly
6 re e n l u mb e i w i th a nfgfr m oistur e content
s t e a n i n s i t xi th l i ve steam about 5O to 40 m inutes
at 5# t6 10# pressure and then bending ' Ou-r breakage averaged around 15 per cent on lurnber about
R e ce i rtly r ye elper imented along
t i / t O i lcE th i ck,
th e lbwer par t of the r etor t
t h b l i n es o f fl l l i n g
w i t h v , 'a te r, th ro u g h -w h l ch the steam had' to- pass , out of
a n d r a i s i n g o u r g i a te s to keop the nater ial
t h e w a te r." l Y e f5 u n d b y exper iment that vie got the
b e s t r e su l ts b y th i s me thod and by car r ying no pr ess u r e o n th e i n -si a e o f th e r etor t and steam ing about
4 hours before bending.
I'pending drytt, we reduced
p e r c e nt.
l .i e h a ve j u st b
( C u b a n ) o ve r tri ro ye a rs o 1
'
.I0 per cent breakage.tr
. ,
.
hotthe
A n o th e r ma n u fa ctu ?er states that in
.plate bending of red gum chair posts, he has recently
improved his output b! sprayir:,g-the stock with hot'
water whl}e in the steam box.
B-9
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