the Mould Design Guide-modificated

7/25/2019 the Mould Design Guide-modificated

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unneriess oulding

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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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2/3

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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3/3

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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the Mould Design Guide-modificated