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unneriess oulding
5 3 7
Summary
Insulated
runn
er too ls have largely been supe rseded.
Full ho t
runne
r tools kee p the mat
er
ial at melt tempera t
ure
and elim
inat
e the
prod ucti
on
of co ld runners and sp rucs.
Mo
st modern ho t ru nner too ls have a se
pa
r
ate
man ifold t
hat
maintains
mel t t
empera
ture .
T emperatu re con trollers and the
rmoco
uples must be used to con t ro l the
mel t co rrectly.
Op
en
ga t
es a re preferred wh erever possib le but whe re the melt tends to lea k with
l
ow
er viscosity ma terials like Ny lon valve o pera ted gates need to be used.
H ot runner m
ould
s shou ld al ways be used
wher
e hig h volume p
rodu
ct ion
IS
req uired w henever this is po ssib le.
5 4
e ting
T he co rrect hea ting o f ho t
runn
er
man
ifold systems is very imp ortan t and is crucial to t he
success o f t he tool in p roduction.
The
re mu st be
good
heat tra nsfer
betwe
en the hea ter
and the surface be ing heated or the heate r will fail prematurely. Hea te r channel s in the
mani fold s
hou
ld be s
moot
h in orde r to achieve this and must no t be left rough mac hined.
Ther
e are six types
of
hea ting sys
tems
that are co
mmo
n ly used in ho t runner too ling:
Band hea ters
Co il heaters
Ca rt ridge he
at
ers
T
ubul
a r heat ers
Integra l heaters
H ybrid heater/hea t pipe systems
5 4
and eaters
Thi
s is th e most co mmon met hod of heati ng external cylindrical sur faces such as barrels
nozzles and tubu la r manifolds. Band hea ters give good
unifor
m heating r ight acro ss the ir
widt h an d are avai lable in a wide range o f w
att
ages and sizes . Mica
and
cera mic
insula tion are usu ally used.
M ains voltage
ope
r
atio
n is most com mon although lo
w v
oltage syste ms a re ava ilable at
higher cost. Sizes up to
rum diameter by 150 mill long a re ava ilab le as standa rd bu t
specials are avai lable to o rde r.
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Mould esign uide
5 42Coil Heaters
These
are
no rmally used to h
eat
cylindrical sur faces where it is not neces
sar
y or
so
met
imes no t de
sirab
le to have uni
form heat
ing alo ng it s length. Coil he
at
ers ach ieve
th is by having a va rying he
atin
g capacity a
long
their length.
Dep
endin
g on
the actu
al heating requirements, t he co ils
may
be progressively spaced
f
urth
er apa rt to p rov ide a the rma l gradient. T his type o f heater is most frequently used
for
obtainin
g th
erm
al grad ients on nozzles.
Operati on at 240 vo lts is most usual. A rea sonable ra nge o f sizes is available but the ran ge
is m
or
e restr icted than tha t available for
band
hea
ter
s.
5 4 3 Cartridge Heaters
Th
ese a re mainly used to heat hot runner mani fold s and int ernally heated no zzles.
al t
h
oug
h they a re also used fo r hea
tin
g n
on
hot r
unn
er
tools when
pro
cessing
at
high tem
perature
s.
Car t ridge heaters are avai lable in a wide range o f d iam
ete
rs, lengths and
wa
ttages. Sizes
range up to 20
nun diam
eter by
aro
und 350
length. Special sizes are available
to
o rder.
It is impo
rtant
tha t ca rt ridge heaters are
not op
erated a t full powe r when heat ing up from
co ld as this can also lead to pr emature hea ter fa il
ure
. To avo id this problem a low voltage
o r tem pera t
ur
e shou ld be
app
lied fo r the first 15 minutes befo re switc
hing
over to full
working
tempe
rature.
As mentioned
above
. go
od
s
ur
face
contac
t b
etween
the heat er and the manifold is
essential to avo id pre
mat
ure
failure p
rob
lems. In
eff
icient heat transfer wil l also result
with po o r surface con
tact
.
Cart
ridge hea
ter
s are pa rticularl y p rone
to
th is p
roble
m.
W ith existing to ols tha t have poor surface contact pr
ob
lems. a heat tr
ans
fer pa ste
may
be
used .
Th
is wi ll grea t ly improve h
eat
tr an sfer
and
prolong ca r t ridge h
eat
er life but is
not
as effective as
prop
erly ma
chined
smo
ot
h c
hannels
.
5 4 4
ubul r
Heaters
T hese a re very sim ila r
to
cart r i
dg
e hea ter s except th
at
t hey are s
emif
lexi ble. T his allows
them to be ca r
ef
ully ben t i
nto
s
hap
e to fit into n
on
-st rai ght -l ine channels.
Th
ey are
ava ilable in
diam
ete rs
of 6
mill
and
in lengths lip
to
1000 mill
and
are usually 240 volts.
Th
e
same
precautions reg
ar
di ng surface cont ac t an d low
po
wer sta rt-up app ly to t
ubu
la r
heaters. If these precau tions
are
observed. heater life is qu ite
good
.
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unneriess oulding
15 4 5 ntegra Heating
T his is a differ
ent
meth
od
of h
eatin
g fro m
an
y of the methods prev
iou
sly descr ibed .
It
consis ts
of
heating elements made fro m hi
gh r
esistan ce
wir
e which is elect rica lly insula ted
on th e o utside. T he wire ca n be cast
into
hea ted manifo ld syste ms o r
in t
o to rpedoes to
form th e source o f hea ting in inte rn ally heated nozzles and bushes.
In tegral hea ting is less freq uently used t
han
t he previous
met
ho ds bu t can be usefu l fo r
mor
e spec ialist ap plications.
5 4 6 Heat Pipes
Th
e princip les o f hea t pipes and hea t rods have al ready been d iscussed in Cha
pte
r lI o n
mould temperature co ntro l.
Conventiona l hea ters
can
on ly be lo
cate
d in easily accessible a reas in a manifo ld resulting
in some areas of the manifo ld no t
being
heat ed sufficiently . T o co mba t th is heat pipes
can he used to t ransfer the heat to these less accessible areas.
Th
is has the d ist inct
adv
an t
age of greater simplicity o f heat er loc
at
ion and eas ier ma
inten
ance if the
co nventi on al he
at
ers fai l. Onc e again it is essent ial to have very good sur face co ntact
between th e hea t pipe and the sur face with which it is in co ntac t.
Heat p ipes arc very useful fo r reach ing mo re remo te areas o f the manifold. As they ca n be
o bta ined in sma ller di am eters t han ca rt ridge he
aters
they ca n also rea ch areas t
hat
w
ould
oth
erwise he inaccessible.
5 5 Temperature Control in nifolds
T emperat
ur
e cont rol in manifolds is very important fo r reliab le an d consist
en t
o peration
of ho t runner mou ld to o ls. Without ad
equ
ate he
atin
g co ntro l
and
hea t tra nsfer many
problems wi ll be ex perienced dur ing
pro
duction.
T here are tw o systems available fo r t
emp
era ture con trol of manifold assemblies:
Closed loop c
ont
ro l
O pen loo p con t rol
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