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发表于 2009-11-4 00:11:03
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The Lost Foam Casting process (LFC) is the process that uses foamed polymer models to represent
( n% Y6 \* o+ L8 Uthe casting and parts of the gating system. Because the polymer models are destroyed during: ]4 j6 O- v* L8 S/ a
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term: ~0 K1 {! S) A, _
"Vollformgießen".! `) ^9 @5 u* e9 U
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol, ^0 f+ z& V$ W1 f7 w
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of* c: s' W1 f+ X0 ^8 r
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,& L) E4 v0 o- H
with a predefined thickness. By glueing simple models together, model segments are then combined
$ I0 E3 i1 G. O- w. R; l+ E7 oto complex models. After that, several models are often combined to model trees. After attaching
1 M0 ^$ a& J$ w: H& Athe gating system, the model is coated by a fire resistant, anorganic coating and
/ m5 d4 B* o: \ c$ N5 \7 Membedded into compressed sand that is free from binding agents.3 w' Y: z2 b- {: m. z+ O6 o
Within the actual casting process, the metal melt replaces the polymer model. The foam is being
' I" W2 U2 q5 t2 N* N3 rpyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the
- `% [1 _! `3 a* Oliquid metal fills the resulting cavity.
, x) F& Z& W2 l5 {' z" E" OA couple of decomposition products, like CO2, CO and several organic compounds, are created
. ?% y8 T+ f' `- s' X5 Sby the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or9 f u* L# x5 F" ^8 p' N/ R
mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more
' a2 r; b/ ^. `3 Y. v* E" yliquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,
. V" h% D, i0 M& B+ r( W: Mwhere the pyrolysis reaction produces mainly gases.
" l) H* f! [; h/ |The emission of the pyrolysis products usually penetrates through the coating into the sand mold.! e! }: N7 b- D! ~
The coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from
5 u, Z4 N( y, V0 Fthe tasks of enhancing the casting's surface quality and stabilizing the sand mold.
% N/ ]6 p, G. w5 I0 SLiquid decomposition products, which are created during the Lost Foam process of aluminium- |$ a& M2 g6 R! b0 I
casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
0 P# q& u* G4 n* a' cdecomposition phase (secondary pyrolysis), the liquid products are further decomposed into
# F x% F& W+ X, A0 ~7 B% Dgas. These are then absorbed by the surrounding mold material.
/ T {3 z- H' {& W; BOn the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
9 i" {5 A3 s& {6 @products immediately develop due to the higher melt temperatures. The pyrolysis gases form a
( f& R% e) U9 x6 g! Tgap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon
/ h/ b: W6 Y$ x$ T+ y7 U$ ]% J1 \. Fthe gases also pass through the coating into the surrounding mold material, where they finally condense. While the mold sand heats up more and more, a secondary pyrolysis starts here,too.
+ a. g, q, v! H5 X) W Obviously, a certain porosity and gas permeability of the coating is vital for the application in the
. c* z! D7 ~- {) d9 V3 l2 v0 TLost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
) G8 y' w& R' s0 ~and porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold- B3 Y" R& m2 {
shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
: L5 i, F" t' q, dthe liquid products do not penetrate deep into the mold material; due to their high viscosity, they
& z: f- V2 N0 W( x; x/ }( B9 j0 |concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer4 \0 c% z* e( p1 K. D
guaranteed.
( Z0 K' r/ c2 n; L E, mDuring the compaction of the mold sand, one must ensure a compaction of the mold material as
# B: k1 R0 @$ s8 B1 p" Y- y+ e! veven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite& k7 O. g8 D4 Y- m# O3 ~
for a reproducible venting of the cavity during mold filling. Undercuts that appear during+ U9 W: r2 X- ? _
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly- p; ^! [; I9 l' F2 R
be avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes |3 ]6 \$ a# e3 L
with a corresponding variation of the chosen frequencies.
, m4 W: [1 Z/ m8 v如果还嫌不够，可以给我发站内消息。 |
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