Chapter_3 duc phun.pdf

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Bi ging Thit b CN polymer TS. L Minh c

CHNG 3

C PHUN

(INJECTION MOLDING)

c phun l mt qu trnh a nng, to ra sn phm t vi gam n 150kg.

Nha nng chy c a vo khun c, lm lnh cho n khi nha ng

rn. Sn phm ly ra v qu trnh c lp li.

c khun: nng sut cao, chnh xc cao, to c sn phm c hnh

dng phc tp. Vi nha nhit do, 90% s dng c phun. c phun lin

quan n 1/3 sn lng nha tiu th gia cng nha nhit do.

Cng ngh c phun gm: my c phun, khun c, thit b cp liu v

vn chuyn, sy, iu chnh nhit , lm lnh, thit b iu khin t ng.

1. Thit b c phun (injection molding) Gm c 3 phn chnh: cm phun, bn kp v h thng iu khin. Thit b

phun lm ho do v phun nha. Bn kp , n, m khun, tho sn phm.

Cm phun y u phun tip xc vo rnh rt ca khun, nng chy nha,

phun nha nng chy vo khun, to v duy tr p sut.

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Trn hnh 5.55 l thit b c phun n cp pittng (single stage plunger),

nng chy nha bng cch cp nhit t thnh, y nha bng pittong thu lc.

Loi ny c nhc im: lng nha phun nh (

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Dng trc vt nng chy nha, pittong thy lc ring bit phun nha

vo khun. Hai chc nng ho do v phun c tch bit c th ci thin qu

trnh nng chy v khuy trn polymer nng chy, lng nha phun ln, d

khng ch lng nha phun, tn tht p sut nh, thi gian cho mi chu k

ngn li. Loi ny p dng vi: sn phm ln, c chiu di chy di ln (long

flow length).

Thit b phun trc vt chuyn ng qua li mt giai on (single stage

reciprocating screw injection units), trc vt quay ho do (plasticate)

polymer, chuyn ng thng phun nha nng chy. (hnh 5.57)


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Trn hnh 5.58, thit b phun c hai b phn chnh: ho do v trt

B phn ho do gm: phu, hng cp liu, thn, trc vt, ng c v u

phun (nozzle). Khi ho do nha, thit b ny (trc vt chuyn ng qua li

n cp) hot ng nh my n. Ht nha rn a vo phu, qua hng cp

liu, vo trc vt. Tuy nhin, trc vt c phun khng ging nh trc vt my

n, u cui l van mt chiu (nonreturn valve). Khi nha nng chy i qua

valve, n khng quay tr li pha hng cp liu trong pha phun ca chu k

c. u phun tip xc vi khun c, u phun m, nha c y ra. V

vy, dng nha phun ra c rnh ca b phn lm lnh chn li, nha b gi

li phn gia van mt chiu v u phun. p sut to ra t nha nng chy


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y trc vt tr li n khong cch nh trc, n mt gii hn nht nh,

ng c trc vit s dng li. Mt lng nha phun c xc nh. Trong qu

trnh phun, trc vt b y ln pha trc nh pittong thu lc hay ng c

in. Nha nng chy s c y vo khun nh u phun.

Thn my c phun thng ngn hn so vi thn my n. L/D thng

thng t 18:1 n 24:1, 22:1 n 26:1 vi my hot ng nhanh, 28:1 vi

my c phun c thot kh.


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Hnh 5.60 l cc dng van mt chiu (nonreturn valve). Van vng trt

(slinding ring valve) c y ln pha trc trong qu trnh ho do, c


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y ngc tr li khi qu trnh phun bt u. Loi van ny to dng chy tt,

ph hp vi nhiu loi vt liu, tn tht p sut nh. Tuy nhin qu trnh

chuyn ng ca van gy ra mi mn, r vt liu c bit khi si thu tinh

chn vo vng. Loi ny s dng nhiu vi loi vt liu c nht cao, c

phun c thot kh.

Van bi, qu cu bi c y ln pha trc khi ho do nha, y lui pha

sau khi phun nha. Loi ny d iu khin lng nha phun, nhng dng

chy b hn ch nhiu, p sut tn tht nhiu, gy mi mn thnh xylanh

nhiu hn loi vng trt. S dng thch hp vi vt liu c nht thp,

khng phi trn.

Khi khng s dng van mt chiu, u nhn bi trn c lp vo cui

trc vt. Tuy khng hn ch dng chy nhng cho nha chy ngc pha

sau. c s dng vi vt liu c nht cao, nhy nhit nh PVC cng.

u phun (nozzle) a nha vo khun c vi tn tht p sut nh nht.

C 3 loi u phun:

Knh h (open chanel): khng c van c kh t gia thn my v khun

c, khong cch phun ngn nht, dng nha nng chy khng b cn tr. Vi

nha c linh ng ln, u phun nh hn v to dn trc khi vo u rt.

Internally actuated shutoff nozzle: c gi cht bi l xo. c m ra do

p lc phun nha.

Externally actuated shutoff nozzle: hot ng nh piston thu lc.

Trc vt ca c phun quay nh ng c in c ni vi b gim tc.

ng c truyn dn cho trc vt c th t gia piston thu lc v trc vt.

Cm phun c th trt trn rnh. D ra sch khi thay loi nha khc, hay

loi cc tp cht bn trong my. V tr ca cm phun c th thay i ph hp

vi v tr ca u rt vi cc loi khun v u phun khc nhau. Lc tip xc

ca cm phun ngn nha r r tip xc u phun v u rt. Bng di y,

lc tip xc tng vi kch thc ca ngm kp.

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Cm phun c c trng bi: lng nha phun (shot size), p lc phun

cc i, kh nng ho do v tc thu hi, vn tc phun cc i . . . Shot

size l khi lng hay th tch ca nha c th phun ra mi mt ln phun.

Kch thc ca cm phun c xc nh qua lng nha cc i c th cp

vi mt ln chuyn ng ln pha trc ca trc vt my phun. M, shot

size c xc nh (n v ounce) theo nha polystyrene (general purpose

polystyrene). Chu u, dung lng nha phun c xc nh l th tch

(cm3) nha di chuyn vi p sut phun la 100Mpa.

p lc phun cc i l p sut ln nht c th t c khi phun. p lc

trn nha nng chy tng v ng knh trc vt thng nh hn ng knh

ca xylanh phun. p sut phun Pinj c tnh nh sau

hydinj

hydinj PA

AP =

Ahyd din tch ca xylanh thu lc, Ainj din tch mt ct ca trc vt, Phyd p

sut thu lc.

Dung lng ho do v tc thu hi c p dng theo tiu chun ca

Hi Cng nghip nha (The Society of the plastics Industry). C hai tiu ch

u da trn kt qu thc nghim vi nha Polystyrenen mc 50% dung

tch cc i. Dung tch ho do l lng nha to ra tnh theo kg/h, tc thu

hi l th tch u ra tnh theo cm3/s. Vn tc phun cc i mm/s.


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Cn kp khun, gi cht khun khi phun, m ng khun ph hp, h

tr tho sn phm, bo v khun. C 4 dng kp: thu lc (hydraulic), n

khu kch hot ng bng thu lc (hydraulically actuated toggle), n khu

kich hot bng in v c-thu lc. H thng cm kp c m t cc hnh

di y


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Lc kp cc i (maximum clamp force) l lc cn thit gi kn khun

trong qu trnh phun v in y khun (packing). Nu lc kp khun khng


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ln hn lc phun v lc in y khun, khun s b h, nha r r. Lc kp

khun c tnh: Fc=fnA

Fn h s nht hay p sut thc nghim, A l din tch phun (l din tch

mt vung gc vi rnh rt). Cch tnh khc Fc=Pmax.A; Pmax l p sut

khoang rng cc i. Hay Fc t l vi L/H, vi L l chiu di ca dng chy,

H l chiu dy ca sn phm.

Cc c tnh khc ca cm kp c m t di y


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2. Khun Khun phn b nha nng chy, to hnh dng cho sn phm, lm lnh, ly

sn phm ra. Chu ng c: lc phun, lc y sn phm ra, chuyn ng

tnh tin, hng chuyn ng cho cc b phn khc ca khun.

Hnh di y l cu to ca khun hai na (two plate mold)

Phn A hay cn gi phn to hc ca khun (cavity half of the mold), gm

c tm nh ca cm kp, tm hc, u rt, vng nh v, cht dn. Phn B


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hay cn gi l phn li (core plate) gm: tm , tm p y, tm li, ng lt

nh v, h thng phun.

Khun m ra theo mt hng, sn phm ri ra do trng lc. Rnh rt, knh

dn nha (runner), v chi tit dnh lin nhau khi y tho sn phm. Loi ny

s dng i vi nhiu phng php c m khng c vt phng ln hay hm

xung khi tho khun, sn phm c nhiu c tnh tt.

Khung c ba tm (three plate mold) c hai ng phn chia. Cc phn

c ng m t ng . Khng p dng c vi tt c cc loi vt liu, vt

liu dn s v khi y sn phm ra. Hn ch khi chn la cng rt nha,

nhiu ph phm.


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Loi th 3 l khun xp chng (stacked injection mold) s dng khi c hai

lp ca sm phm m khng cn tng lc p. Loi ny c sai s ch nh

loi khun hai tm.

H thng khun cng c th phn loi da vo h thng knh dn nha

(runner). Vi khun c knh dn ngui (cold runner mold), rnh rt v knh

b ng rn, b loi b khi tho sn phm. Mt mt nhiu nguyn liu.


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Knh dn ngm (insulated runner) c ng knh ln hn so vi knh

chun.

Do vy, phn nha pha ngoi knh th ng rn trong khi pha trong

tm vn cn dng lng. iu ny gim tiu hao nguyn liu, d thay i loi

nha v mu sc. Knh cn phi lm ng cng trc khi tho khun. Tho

khun rt kh khn v cc cng gn nh b nha ng cng.

Loi knh dn c t nng (hot runner mold), knh dn lun trng thi

nng chy. Cu to ny gim ph liu nhng vn hnh kh hn. V yu cu

phi s dng thp tt hn, tm cch v h thng iu khin nn chi ph cao

hn khong 25% so vi loi knh ngui.


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C hai loi khun ny c hn ch nht nh i vi mt vi loi nha. Loi

knh nng, gp kh khn thay i mu. Khun c cn c th phn loi da

vo s lng khoang c (cavity) trong khun. Vi khun gia nh (family

mold), nhiu phn ca sn phm c c ng thi

Cc phn c bn ca khung c c m t hnh di y.


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Rnh rt nhn nha t u phun, phn chia v cc knh nha (runner).

Chiu di ca cc knh phi nh nht, p sut ri v nhit nh nht t rnh

rt n khoang (cavity) c. Knh nha c nhiu hnh dng nhng loi tit

din trn l hiu qu nht (loi ny chi ph ch to cao). ng knh ca knh

nha xc nh Dbranch=n1/3.D

vi D l ng knh ca knh nha, Dbranch ng knh ca nhnh ngc

dng. Nha c phn phi ng thi vo cc khoang c.

Hnh di l cc ca ni t knh nha n cc khoang c.


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3. Qu trnh c phun

Qu trnh c phun gm nhiu bc: ng khun, phun, in y khoang

c, lu, lm lnh, ho do (plastication), m khun, ly sn phm.

My c phun hot ng dng: thao tc bng tay, bn t ng, t ng.

Khi u phun v rnh rt tip xc nhau, giai on phun bt u. Trc vt

chuyn ng tnh tin, y nha vo khun qua u phun. Khoang c c

in y nha. im cui ca giai on ny c xc nh bng thi gian, v

tr piston hoc p sut.


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Giai on packing: dng nha thm vo b s co ngt ca nha khi gp

khun lnh.

Giai on lu: khi in y kt thc, to p sut cho n khi ng rn cng

khun. Giai on in y v lu, nha nng chy chuyn ng c xc

nh bng nht v p lc tc ng ln pittong. Thi gian lu c kim

tra.

Giai on lm lnh v do ho: Thi gian ca gian on lm lnh c

kim sot. Trong khi lm lnh, trc vt quay v nha c dn li, ch cho

ln c phun sau. V qu trnh ho do cn thc hin ngay trc khi kt thc

giai on lm lnh, trc vt cn mt thi gian tr nht nh (do vy trc vt

khng cn phi quay chm li). Nha nng chy c khuy o thm nh p

sut ngc. Khi lng nha cho ln k tip, trc vt chuyn ng ngc

tr li (gi l gim p hay ht ngc), lm gim p sut trong nha nng

chy, trnh c s r r nha. Khi lm lnh kt thc, u kp m ra, sn

phm tho ra.

Cc thng s ca qu trnh: lng nha phun (shot size), tc phun, p

sut phun, thi gian phun v cc tham s ca qu trnh in y khoang, lu.

Nha c nht cao nn chy vo khun c dng vi phun nc. Dng

nha nng chy chy vo khoang c theo cc sng c hc. Lp nha u

tin b ng rn thnh. Dng nha sau c nhit cao hn tip tc chy vo

khoang c. Chiu dy ca lp ng rn thay i theo nhit nng chy,

nhit khun, tc hay thi gian phun.


Chapter_3 duc phun.pdf

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