mold design - kocwelearning.kocw.net/contents4/document/lec/2013/chonnam/... · 2013-07-19 · mold...

Bong-Kee LeeSchool of Mechanical Engineering

Chonnam National University

Mold Design

10. Cooling Systems

School of Mechanical EngineeringMold Design

Design of Cooling System

Mold Cooling System– a mold is a highly stressed heat exchanger and the

cooling phase of the injection molding cycle is extremely important

• to cool the polymer as quickly as possible to a temperature at which the molded part can be safely ejected

– the cooling phase can be up to 80% of the overall cycle and often the most expensive cost component of the molding

• to preserve the physical properties of the material and the required quality of the molded part

– especially, crystalline materials will warp and distort and suffer dimensional changes over a period of time because of the formation of undesirable structures (,such as random orientations) in the frozen layers



Mold Cooling System– rate of cooling is most important rather than the

cooling itself

– the temperature of the mold should be:• high enough to allow the cavities to fill without premature

freezing of the material• as uniform as possible to ensure the molded part is cooled

equally in all areas• high enough with crystalline materials to avoid an

unsatisfactory structure



Mold Cooling System– external systems

• chillers– air cooled, water cooled– to circulate the coolant into the cooling channels in the mold

• temperature controllers



Cooling Channels– the simplest method of supplying cooling fluid to the

mold• to drill holes in the mold plate around the cavities• layout, diameter, distance from the mold surface, and so on, of

the cooling channels ~ uniformity, efficiency

basic cooling circuit more efficient cooling circuit



Cooling Channels– positioning

• if channels are too far apart– danger of an uneven thermal gradient developing between them

• if channels are too close together– the thickness of steel between them may become too low

• if channels are too close to the cavity surface– localized over-cooling, resulting in irregular temperature control

part thickness W channel diameter d

2mm 8-10mm

4mm 10-12mm

6mm 12-15mm

c=2d~3d, b=3d~4d

guide to cooling channel positioning



Core Cooling– in many cases, core pins will get very hot if no cooling

is incorporated in them, resulting in prolonged cooling cycles → provide cooling inside core pins

– useful methods (for cooling cores and cavities)• baffle system• fountain system• angled hole• stepped hole• spiral cooling• heat rods• heat pipes• beryllium copper cores and cavities



Core Cooling– baffle system

• cooling of smaller cores, (or larger cores with array-form)• a hole is bored into the core and a strip of copper is inserted

into it



Core Cooling– fountain system

• more efficient than baffle systems (because of the parallel cooling circuitry)

• more uniform temperature control over the entire area• a tube is fitted into the center of a hole in the punch or core

pin

single fountain design multiple fountain design



Core Cooling– angled hole design

• drilled holes at angles in larger punches for them to interact each other to create a path for the coolant to flow through

• difficulty in manufacturing the holes to interact each other on a full diameter → restricted maximum length of the holes ~ 150mm

• obstructions inside the holes, expensive EDM technique



Core Cooling– stepped hole

• easier mold-making works than the angled hole design• plugs are prone to leak due to the injection pressure and

continued cyclic expansion and contraction• witness mark of ring shape or blush mark owing to a

differential cooling effect through the plug to the mold surface



Core Cooling– spiral cooling

• for lager cylindrical cores above 50mm diameter• basic design ~ a channel that is machined down the outside of

a centrally inserted tapered diameter and follows the path of a helix

• seals have to be used to prevent leakage



Core Cooling– heat rods

• for smaller punches and core pins• high thermal conductivity materials that are inserted into the

core pin• copper ~ ductile and excellent conductor of heat



Core Cooling– heat pipes

• for small core pins• copper tube filled either with water or with a low-boiling-point

alcohol• efficiency of heat transfer depends on the contact between the

outside of the heat pipe and the inner wall of the core pin



Cavity Cooling– to introduce cooling into cavity inserts

• on many jobs, cooling the cores without cooling the cavities can lead to non-uniform cooling of the component

– sealing to prevent cooling fluid leaking• O-rings or gaskets• copper pipe with a BSP taper thread



Cavity Cooling

cooling system for circular cavities cooling arrangement for rectangular cavities



Cavity Cooling

annular groove cooling



Cooling Circuitry– series cooling

• all coolant flows through one connected circuit• there is a high temperature differential between inlet and outlet• there is a high-pressure drop between inlet and outlet• any blockage in the circuit is easilyidentified

baffle system in series



Cooling Circuitry– parallel cooling

• all circuits are fed by a common supply at a similar temperature• there is a lower temperature differential across the mold• there is a lower pressure drop between inlet and outlet• the mold temperature is more uniform• circuit blockage are less easily detected

fountain system in parallel



Analytical Thermal Calculation– design steps

• computation of cooling time ~ minimum cooling time down to demolding temperature

• balance of heat flow ~ required heat flow through coolant• flow rate of coolant ~ uniform temperature along cooling line• diameter of cooling line ~ turbulent flow• position of cooling line ~ heat flow uniformity• computation of pressure drop ~ selection of heat exchanger,

modification of diameter or flow rate



Calculation of Cooling Time– until the mold opens and the molded part is ejected– permissible demolding temperature, TE

– 1-dimensional analysis ~ heat conduction• in the direction of part thickness• heat exchange between plastic part and coolant

ydiffusivit thermal:

2

2

pC

k

x

T

t

T



Calculation of Cooling Time– assuming that

• immediately after injection, the melt temperature in the cavity has a uniform constant value of TM

• the temperature of the cavity wall jumps abruptly to the constant value TW and remains constant

s

xe

TT

TT

s

xne

nTT

TT

ts

WM

WE

n

ts

n

WM

WE

sin4

12sin

12

14

2

2

2

22

0

12

nesspart thick :s



Calculation of Cooling Time

18ln

1

numberFourier : & rate cooling :

8ln

1or

8ln

8

re, temperatudemoldingmean :

222

2

22222

2

2

0

0

2

2

s

tFo

Fos

t

TT

TT

TT

TT

s

t

TT

TTst

eTT

TT

dx

dxTTT

c

c

WM

WE

WE

WMc

WE

WMc

ts

WM

WE

s

s

E

EE1

tc

α

1

Fo



Calculation of Cooling Time

– from the experimental works, demolding usually takes place at the same dimensionless temperature

WM

WE

E

TT

TT

T

ˆ

ˆ

part ofcenter demolding,at re temperatu:ˆ

TW

x

TM

t=0 TW

x

TE

t=tc

TE

part theof re temperatuaverage on the based :16.0

or center, in the re temperatumaximum on the based :25.0ˆ



Cooling Time of Thermoplastics– estimation

– cooling time diagram

nesspart thick :

][s/mm 3 to2 ~constant :

timecooling :2

2

s

c

t

sct

c

c

cc



Cooling Time of Thermoplastics– nomogram

parts lcylindricafor 7.0ln8.5

parts planefor 8

ln

2

22

2

WE

WM

effc

WE

WM

effc

TT

TTRt

TT

TTst



Cooling Time of Thermoplastics– typical material data

melt temp.(°C)

wall temp.(°C)

demoldingtemp. (°C)

avg. density(g/cc)

ABSHDPELDPEPA 6

PA 6.6PBTPPC

PMMAPOMPPPS

PVC rigidPVC soft

SAN

200-270200-300170-245235-275260-300230-270270-320180-260190-230200-300160-280150-210120-190200-270

50-8040-6020-6060-9560-9030-9085-12010-8040-12020-10010-8020-7020-5540-80

60-10060-11050-9070-11080-14080-14090-14070-11090-15060-10060-10060-10060-10060-110

1.030.820.791.051.051.051.141.141.300.831.011.351.231.05



Cooling Time of Thermoplastics– for other geometries

WE

WM

effc

WE

WM

effc

TT

TTst

TT

TTst

ˆ4

lnor 8

ln2

2

22

2

WE

WM

effc

WE

WM

effc

TT

TTDt

TT

TTDt

ˆ602.1ln

14.23or 692.0ln

14.23

22

WE

WM

eff

cWE

WM

eff

cTT

TT

LD

tTT

TT

LD

tˆ

040.2ln14.23

1or 561.0ln

14.23

12

2

2

2

s Qy

yD L>>DQr

yDL~D

Qr

Qz



Cooling Time of Thermoplastics– for other geometries

WE

WM

effc

WE

WM

effc

TT

TTht

TT

TTht

ˆ064.2ln

3or 533.0ln

3 2

2

2

2

WE

WM

effc

TT

TTDt

ˆ2ln

4 2

2

D

h

h h



Heat Flux Balance– mold for processing thermoplastics has to extract fast

and uniformly, so much heat from the melt injected into the cavity

ksQ

adQ

cQ

cvQ

radQ

cdQ

platens machine into conductionby exchangeheat :

mold theof side at theradiation by exchangeheat :

mold theof side at the convectionby exchangeheat :

coolant with exchangeheat :

runner)hot from (e.g.flux heat additional :

part molded thefromflux heat :

0

balance)flux (heat

cd

rad

cv

c

ad

ks

cdradcvcadks

Q

Q

Q

Q

Q

Q

QQQQQQ



Heat Flux Balance– heat flux from the molded part

• enthalpy difference between injection and demoling– related to the mass (with the average density and the volume) can

be converted to the amount of heat (which has to be extracted from the molding and conveyed to the mold during the cooling stage)

pVUH (enthalpy) timecooling :

part theof volume:

mold theinto injected mass :

temp.demolding andinjection between

differencedensity average :

differenceenthalpy specific :

c

ks

ks

c

ks

c

ksks

t

V

m

h

t

Vh

t

mhQ



Heat Flux Balance– heat exchange by convection (at the side of the mold)

– heat exchange by radiation (at the side of the mold)

mperatureambient te :

re temperatumold external :

K)8W/m ~motion slight (in t coefficienfer heat trans :

faces side mold theof area :

0

2

0

amb

m

s

s

ambmsscv

T

T

h

A

TThAQ

0.85)~rustedheavily

0.6,~rustedslightly 0.25,~clean 0.1, ~ polished :steel(for

surface mold theof emissivity :

KW/m105.670 ~constant Boltzmann -Stefan : 428-

440

ambmsrad TTAQ



Heat Flux Balance– heat exchange by conduction into machine platens

80)~steelalloy -high 100,~steelalloy -low 100, ~ steelcarbon (for

K][W/mt coefficienfer heat trans :

faces clamping mold theof area :2

0

c

c

ambmcccd

h

A

TThAQ



Heat Flux Balance– determination of external mold temperature, Tm0

channel cooling of etemperatur

coolant of etemperatur

molds unheated smallonly with epermissibl

0

0

0

ccm

cm

ambm

TT

TT

TT

material mold theofty conductivi thermal:

surface mold external and channel coolingbetween distance :

0

m

mcs

cdradcvccm

k

l

kAA

lQQQTT



Flow Rate of Coolant– coolant throughput

• permissible temperature rise of coolant due to the cooling of the molded part ~ 3 to 5°C

→ adequate homogeneous cooling along the cooling circuit

coolant of rise re temperatuepermissibl :

coolant ofheat specific :

coolant ofdensity :

coolant of rate flow :

c

pc

c

cpccc

T

C

Q

TCQQ



Diameter of Cooling Line– turbulent flow for efficient heat transfer– pressure drops inside the cooling line

cc

cc

cccc

c

cc

ccc

cc

Qd

d

Q

d

Q

d

Qd

vd

vAQdv

2300

4300,2

4Re

drops) pressure gconsiderinby 10,000(or 2,300Re flow,ent for turbul

44Re

4Re

2

2



Diameter of Cooling Line– based on fluid mechanics

tcoefficien resistance :

turnsofnumber :

line cooling theoflength :

pipesin factor friction :

36002

164

2

2

c

c

c

c

ccc

cc

cc

K

n

L

f

Knd

Lf

p

Qd

bending round 90for 4.0

bending sharp 90for 9.1

law) (Blasius' Re

3164.04/1

c

c

K

f



Diameter of Cooling Line– determination of heat transfer coefficient– based on convective heat transfer

factor correction :

number Prandtl :Pr

number Reynolds :Re

number Nusselt :Nu

8.0Pr8.1230Re0235.0Nu 3.08.0

f

fc

ccv

K

vL

k

hL

Kk

dh



Positioning of Cooling Line– layout of cooling line ~ uniform heat flow– distance from the mold wall

Tw

Tc

dc

l

1q

Tcc

2q

21

122

1

qq

TThTThql

TTkTkq

cccc

ccwm

22

itypenetrabilheat :

contact before re temperatu wall:

contact before re temperatumaterial :

mperaturecontact te :

,

max,min,max,

max,

max,

coolantcwww

w

p

c

pwp

wwppc

TTTTT

b

T

T

T

kCbbb

TbTbT



Numerical Analysis– three-dimensional heat transfer

• for both mold base and plastic part• proper boundary conditions, including convection heat transfer

of coolant

• coupled analysis of heat transfer and fluid flows of plastic melt and/or coolant

rz

T

y

T

x

Tk

t

TCp

for

2

2

2

2

2

2

mold design - kocwelearning.kocw.net/contents4/document/lec/2013/chonnam/... · 2013-07-19 · mold...

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