H THNG LM NGUI

1. Tm quan trng ca h thng knh ngui

Thi gian lm ngui chim khong 60% thi gian chu k khun, v th vic lm sao c th gim thi gian lm ngui nhng vn m bo tnh k thut v cht lng sn phm l rt quan trng. Nhit chy ca nguyn liu nha thng thng vo khong 150C 300C, khi nguyn liu nha c a vo khun nhit cao ny, mt lng nhit ln t nguyn liu nha truyn vo vt liu khun v thng qua h thng lm ngui gii nhit khun. Nu h thng lm ngui v mt nguyn nhn no , h thng lm ngui cha a kp nhit ra khun mt cch hu hiu, lm nhit trong khun khng ngng tng ln, lm tng chu k sn xut.

V vy, phi tnh ton, thit k c mt h thng gii nhit ph hp v m bo yu cu k thut.

Gi cho khun c nhit n nh nha c th gii nhit u

Gii nhit nhanh, trnh trng hp nhit gii khng kp, gy nn hin tng bin dng sn phm gy ra ph phm

Gim thi gian chu k, tng nng sut sn xut

2. Mt s cht lm ngui

Cht lm ngui thng dng

Cht lm ngui

Nhit lm vic (C)

Cht chng ng bng (glycol/nc)

-20 0

Nc chng lm lnh hoc nc gia nhit

0 90

Du truyn nhit

90 200

Gia nhit in

150 450

3. dn nhit ca kim loi

Gi tr dn nhit ca mt s kim loi lm khun thng dng

Vt liu khun

dn nhit (W/m.K)

Thp Ni Cr

30 60

Thp khng r (12 15%Cr)

13 18

Nhm

197

4. Cc thnh phn ca h thng lm ngui trong khun p nha

H thng lm ngui trn khun

Cch b tr

trong

Collection manifold: b cha dung dch lm ngui

Supply manifold: ng cung cp cht lm ngui

Pump: bm

Regular Cooling Channels: knh lm ngui

Hoses: ng dn

Baffle: vch lm ngui

Temperature controller: b iu khin nhit

5. Quy lut thit k knh dn ngui

a/ Cc quy lut thit k knh ngui

m bo ngui ng u ton sn phm. ch n nhng phn dy nht ca sn phm.

Knh dn ngui nn gn mt lng khun c th gii nhit tt hn.

ng knh ca rnh dn ngui nn khng i trn ton b chiu di knh dn trnh s ngt dng s lm trao i nhit khng tt.

Thit k ng nc sao cho c mt u vo v mt u ra

Nn chia knh lm ngui thnh nhiu vng lm ngui. Khng nn thit k chiu di knh dn ngui qu di dn n mt p v tng nhit chnh lch gia u vo v u ra tng qu 3C

Dng nc lm mt phi chy lin tc v mang nhit t ni c nhit cao sang ni c nhit thp.

B tr ng nc theo kiu ni tip hoc song song hay va ni tip va song song quan trng l phi ph hp vi quy lut dng chy.

Nhit u vo v u ra cng t chnh lch cng tt m bo trong khong

1 5C

Tnh ton tit din l nc sao cho ngun nc cung cp cho c h thng.

8 P.T 1/8 hoc 10 P.T 1/4

Cn u tin thit k ng nc cho nhng v tr trn sn phm c thnh dy t ngt

Cn lu lm mt cho c nhng tm khun quan trng, mc d ni khng c sn phm VD tm tch xng keo ca khun 3 tm.

Knh lm ngui phi c khoan c nhm to dng chy ri gip trao i nhit tt hn dng chy tng 3-5 ln.

Dng chy ri c t trng bi s Raynold (Re). Theo bng:

S Raynold

Trng thi dng chy

Re > 10000

Chy ri

2300 < Re < 10000

Chuyn tip

100 < Re < 2300

Chy tng

Re < 100

ng

S Raynold c th tnh theo cng thc sau:

: khi lng ring ca lu cht. (kg/m3)

d: ng knh knh lm ngui (m).

: vn tc c trng ca dng chy (m/s).

: h s nht knh lm ngui (m2/s).

Cn xem xt bn c hc ca tm khun khi khoan cc knh lm ngui.

b/ Ch khi thit th knh dn ngui

Cc knh lm ngui phi t cng gn b mt khun cng tt nhng cn ch n bn c hc ca vt liu khun

ng knh knh lm ngui phi ln hn 8mm (8 hoc 10)

Nn chia h thng lm ngui ra nhiu vng lm ngui trnh cc knh lm ngui qu di dn n s chnh lch nhit ln

c bit ch n vic lm ngui nhng phn dy ca sn phm

Tnh dn nhit ca vt liu lm khun cng rt quan trng

Lu n hin tng cong vnh do s co rt khc nhau khi b dy sn phm khc nhau

Lu rng cc knh lm ngui c thit k cch nhau t nht 3mm. Vi nhng knh di hn 150mm, th khong cch an ton gia cc knh l 5mm

6/ Gii nhit cho khun:

Lm ngui khun ch yu l lm ngui tm khun, lm ngui li, lm ngui cht v lm ngui lng khun

a. Gii nhit cho tm khun:

Yu cu k thut:

Cc knh lm ngui c thit k cch nhau t nht 3mm. Vi nhng knh di hn 150mm, th khong cch an ton gia cc knh l 5mm

Lm ngui trong tm khun l 1 trong nhng h thng lm ngui thng thng nht v ch yu c dng cho cc sn phm nh

b. Gii nhit li:

Cch n gin nht l lm li bng vt liu c dn nhit cao nh ng hoc ng berilium, nhng c nhc im l bn thp

Hoc dng h thng lm ngui c bit, kiu rut g y nc ln theo ng xon, khi ln ti nh th nc s chy xung. Dng cho nhng trng hp m lng khun cn gii nhit nh m cao, kh a nc ln

Rut g

Gii nhit li kiu rut g

Mt phng php gii nhit tt khc l t cc knh ngui trong li

u im ca phng php ny l nhit c th in khin c bng s tng gim dng cht lng lm ngui chy qua knh. Nu c s lp ghp trn phn th phi c 1 tm nh ti t trc, t trc nt chn mm trnh lm hng li

c. Gii nhit cho cht:

Cc cch thit k gii nhit cho cht:

Thit k 2 knh ngui gn cht

Gii nhit trc tip ln cht

Thm 1 cht ng bn trong cht

d. Gii nhit cho lng khun:

Ni chung lng khun c th lm ngui tt v c s truyn nhit n cc phn khc ca khun

7/ iu khin nhit khun:

a. Tnh ton lu lng nc cn thit gii nhit cho khun

Tiu chun xc nh trng thi dng chy l t s gia lc qun tnh v lc ma st nht ca lu cht v c gi l s Reynold (Re)

Trong :

: t trng ring ca cht lm ngui (kg/m3)

U: vn tc trung bnh ca dng cht lm ngui (m/s)

d: ng knh knh lm ngui (m)

: h s nht knh lm ngui (m2/s)

b. Xc nh ng knh knh dn cht lm ngui:

Da theo kinh nghim trong sn xut khun, ng knh knh lm ngui c s dng trong bn sau:

B dy thnh sn phm

ng knh knh lm ngui

Khong cch t tm knh lm ngui nh thnh sn phm

Khong cch gia 2 tm knh dn ngui

2

8 10

2 4

10 12

2 2.5d

2 3d

4 6

12 14

c. Tnh lu lng nc lm ngui:

Lu lng nc lm ngui c tnh theo cng thc sau:

Trong :

p: Tn tht p dc ng trong knh dn ngui

g: H s gia tc trng trng (g = 9.81 m/s2)

L: Tng chiu dai knh ngui

D: ng knh knh ngui

: H s tn tht

Tn tht p dc ng trong knh dn ngui (p) c nh ngha nh sau:

p = Pvo Pra

Pvo, Pra: p sut vo v ra khi h thng lm ngui

ng knh knh ngui D: vic tnh ton ng knh knh ngui rt quan trng. H thng gii nhit c lm vic hiu qu hay khng ph thuc rt nhiu vo ng knh knh ngui. T cng thc trn, vit li nh sau:

Theo cng thc trn, ng knh knh ngui t l nghch vi tn tht p trong knh ngui. Nu ng knh knh ngui gim th din tch trao i nhit ca knh ngui v khun gim lm cho tn tht p tng ln ng k. Nhng ng knh knh ngui qu ln th s nh hng n bn khun, cng nh cc b phn ca h thng y c b tr trn khun

Theo thc nghim, c th chn lu lng nc ti thiu yu cu tng ng vi ng knh knh ngui nh bng sau:

ng knh knh ngui (mm)

Lu lng nc ti thiu (lt/pht)

8

2,84

10

3,41

14

5,11

16

5,68

20

6,82

25

9,46

H s tn tht: i vi dng chy ri th h s tn tht c xc nh ch yu da vo thc nghim v ph thuc vo cc yu t sau:

Trong :

: nhm tuyt i

D: ng knh knh lm ngui

d. Tnh ton thi gian lm ngui:

Thi gian lm ngui ti thiu c tnh theo cng thc sau:

Vi dy thnh ca sn phm t 1-> 14 mm, nhit thnh khun di 600C:

Tc = Smax x (1 + 2 x Smax)

Smax: thnh dy nht ca sn phm

Trng hp nhit thnh khun cao hn 600C th thi gian lm ngui cng thm 30%:

Trong :

h: chiu dy sn phm

: khuch tn nhit

T = Tc + 30% * Tc

Trng hp tng qut:

tE: nhit sn phm v c chn theo bng sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

tE(0C)

82

118

93

116

170

tW: Nhit khun, i vi 1 s loi nha c th chn theo bng sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

tW(0C)

57

82

79

41

85

tM: Nhit chy do c chn theo bng sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

tM(0C)

238

302

260

243

260

khuch tn nhit c tnh theo cng thc sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

K (W/m.0K)

0.264

0.19

0.246

0.246

0.25

CP: nhit dung ring ca nha v c chn theo bng sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

CP (J/kg.0K)

1314

1298

1252

1741

4400

: khi lng ring ca nha v cho theo bng sau:

Loi nha

ABS

PC

PC/ABS

PBT

Nylon

(kg/m3)

1040

1200

1120

1310

961

8/ Kim sot nhit khun:

Nhit khun l thng s quan trng nht nh hng chu k nh hnh v cht lng sn phm

Thi gian lm ngui c xc nh ch yu da vo nhit khun

p ng c tnh cht sn phm, c bit l tnh nh hnh th cn phi xc nh nhit khun t c

9/ Cc loi h thng tn nhit

a) H thng lm ngui vi vch ngn thng (Baffle system)

Khoan nhng l pha mt sau ca tm core, trong l b tr mt vch ngn thng ng to nn mt dng chy trong tm core. y ca nhng l khoan ny c nt kn bng nhng nt plugging

b) H thng lm ngui kiu vi phun (Fountain system)

H thng kiu vi phun cho nng sut cao hn so vi kiu vch ngn v gip phn b gii nhit u khp khun

H thng lm ngui 1 vi phun

H thng lm ngui a vi phun

Vi h thng ny, nc lm ngui s i vo gia v r sang hai bn nn kh nng gii nhit rt u

c) Kiu angled hold system

Vi thit k ny, c mt nguy c l phoi t cc hot ng khoan c th b mc kt ti cc nt giao ca cc l, do hn ch dng chy ca nc lm mt thng qua li. Nn kim tra cc knh ngui m bo chng c thng sut khng b bt k chng ngi vt cn tr dng chy trc khi tin hnh p phun.

L su v nh th c th tin hnh gia cng bng phng php EDM. y l mt la chn t tin hn nhng cng c gi tr chi ph cho vic lp t cc knh lm mt nh hn, li phc tp hn.

d) H thng gii nhit dng l tng bc (Stepped hole)

H thng ny th d thit k hn so vi h thng Angle hole, nhng nhc im ca h thng ny l sau khi nhng l c khoan phi bt mt u li iu chnh dng chy; tuy nhin i khi, chng c th b hng dn n r r

e) H thng lm ngui dng xon c

H thng lm ngui c bit, kiu rut g y nc ln theo ng xon, khi ln ti nh th nc s chy xung. Dng cho nhng trng hp m Cavity cn gii nhit nh m cao, kh a nc ln.

f) Cc dng gii nhit khc

Dng thanh gia nhit Heat Rod, nc lm lnh s truyn nhit cho nh gii nhit cho khun v Heat rod cng ng vai tr l li khun

Tnh nng ca heat pipes cng ging nh thanh gia nhit heat rod nhng n khng ng vai tr ca li khun

Cch n gin nht l lm li bng vt liu c dn nhit cao nh ng hoc ng berilium, nhng n c nhc im l bn thp.

S tm tt thit k h thng lm ngui li khun (core)

Thit k kiu lm ngui cho khun

B rng li khun

Kiu lm ngui

3mm

Lm lnh bng kh

5 8mm

Thanh nhit, ng nhit, ng berry

8 40mm

Kiu vi phun

>40mm

Kiu xon c

Ln hn

Xon c i v vi phun

H THNG NH V - DN HNG

C rt nhiu phng php nh v hai tm khun. Phng php c chn ph thuc vo hnh dng chi tit, chnh xc ca sn phm, thm ch c tui th d kin ca khun. C vi cch chn sau y:

1. Khng s dng nh v trong khun

2. Cht dn hng bc dn hng

3. Kha cn gia lng khun v li khun

4. Kha cn gia nhm lng khun v li khun

5. Kha nm

6. Kt hp 2 vi 3,4,5 hoc 6

1. Cht dn hng v bc dn hng

Chc nng chnh ca cht dn hng v bc dn hng l a khun sau vo khun trc thng hng vi nhau. Cht dn hng nm khun trc v bc dn hng nm khun sau.

di ca cht dn hng phi di hn ming ghp cao nht trnh hng hc khi ng khun c bit l khi lp rp

Cc loi cht dn hng

Loi khng vai

Loi c vai

Cc loi bc dn hng

Loi trn

Loi c vai

Loi khng vai t bi trn

Loi c vai t bi trn

C cu nh v

Thng thng cc cht dn hng c th gi c mt thng hng s b, nhng vi khun chnh xc th dung sai ca cc cht dn hng v bc dn hng l qu ln v th cn b nh v

i vi loi sn phm ln, nht nh phi dng b nh v. Trong trng hp ny, khun phi chu lc p mt bn, c bit khi khun cha in y v cc cht dn hng khng th chu c cc lc p mt bn ny.

C cu nh v mt cn

a,b : bc nh v mt cn chun

c : c th tho v lp trn mt phn khun.

1 & 6:vt, 2 & 9 : bc nh v chnh xc mt cn

3 : tm khun c nh

4 : cht nh v chnh xc mt cn

5 : tm di ng

7 & 8 : vng m

C cu nh v chnh xc bng mt vt

V tr cht & bc dn hng :

Thng dng cc cht c ng knh khc nhau trong khun, vi khun cht dn hng th mt cht phi c ng knh khc hoc l x dch so vi cc cht ( l ) cn li.

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