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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
3 a5 ]! q8 v& R- J5 Y8 _% Y2 t* V) bthe casting and parts of the gating system. Because the polymer models are destroyed during$ ?/ E O z2 D/ T/ J' X$ u
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term, A& d' [3 p( R
"Vollformgießen".
# S+ z4 ~" w3 a! ?/ w8 a8 p; VWithin the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol
" `. J+ z! `$ K! [5 ^(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of3 t# k3 Y9 L9 z: X5 p% `
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,- p6 `$ s3 i2 e" @. n
with a predefined thickness. By glueing simple models together, model segments are then combined. }3 {. x4 Y& o$ |7 R5 F6 t/ H
to complex models. After that, several models are often combined to model trees. After attaching
/ {4 l. c7 R; `; C& d" Othe gating system, the model is coated by a fire resistant, anorganic coating and: c% Z4 _' M0 I$ | P+ w
embedded into compressed sand that is free from binding agents.( l( t/ ]! n2 R+ m' A9 D7 }
Within the actual casting process, the metal melt replaces the polymer model. The foam is being: a( ?6 M P2 f# {
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the$ M1 V- O, {+ z. T2 P
liquid metal fills the resulting cavity.4 Y: l+ D: J2 P m4 @; J
A couple of decomposition products, like CO2, CO and several organic compounds, are created
/ a7 N8 q% U3 C/ Gby the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or2 z, U$ `7 s. {' y) U- c
mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more6 U( o2 `/ y2 r+ Z
liquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,( H0 t: b+ a1 ~* ^0 @/ F6 R
where the pyrolysis reaction produces mainly gases. i+ J$ Y# k. h3 y; Y7 w
The emission of the pyrolysis products usually penetrates through the coating into the sand mold./ L9 @7 l$ h6 v; i5 f
The coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from
' U* V$ t; {7 R+ F4 {. K5 S3 @the tasks of enhancing the casting's surface quality and stabilizing the sand mold.
2 L- J$ k( ]" rLiquid decomposition products, which are created during the Lost Foam process of aluminium- H9 a& K( p1 b) ?4 S# P$ D6 N
casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
* l" Q5 L8 X" m! `decomposition phase (secondary pyrolysis), the liquid products are further decomposed into4 ^: t$ J- x' P9 ~
gas. These are then absorbed by the surrounding mold material.8 U6 ?$ |% S" A3 H# |. h
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
1 W' H) p- { o4 j6 h' z8 pproducts immediately develop due to the higher melt temperatures. The pyrolysis gases form a
% H4 f1 ]1 _ _* F, ^gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon( b+ C& h$ n2 x$ ]" u6 g6 \+ d. Y
the 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.% q' | I: n Q5 U, I
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the) I5 C( ]7 m+ _5 s4 {7 L! W
Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
* @) O( l5 B3 K# U2 y" b, `and porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold
/ K! _' l% z. q9 {% C& D% O) p6 \shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
# v' L7 ^% `% j" kthe liquid products do not penetrate deep into the mold material; due to their high viscosity, they
, B1 f6 [& p# b) Mconcentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer
( X0 m+ E# S2 A; z9 Q& D5 Wguaranteed.
. W" r' s. \. lDuring the compaction of the mold sand, one must ensure a compaction of the mold material as
, A( G/ r J8 H9 s. v, Qeven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
5 _/ F! ^% R, O. Efor a reproducible venting of the cavity during mold filling. Undercuts that appear during
9 p8 `; z ^# fthe embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly
& y4 p0 U8 _, q& l! }/ U+ O) mbe avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes- I+ h4 W6 m* W, u; u5 B
with a corresponding variation of the chosen frequencies.
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