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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
S+ }3 c3 A( P I5 athe casting and parts of the gating system. Because the polymer models are destroyed during6 \2 W9 q6 y- _: U) y: j
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term
8 }& l% I$ O% m8 O"Vollformgießen".* i! x& F% C, G& @& q7 D4 o6 `
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol4 F. ~/ b" b( ~- H+ J: `
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of1 w* Z1 B( W7 D
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,5 L4 c3 J) |$ g
with a predefined thickness. By glueing simple models together, model segments are then combined9 l# W& [. H8 j* O
to complex models. After that, several models are often combined to model trees. After attaching
# D% r- T6 @0 T/ J+ x; Ethe gating system, the model is coated by a fire resistant, anorganic coating and8 m5 O4 [$ a: N; V+ _2 a
embedded into compressed sand that is free from binding agents.
3 ]. r3 |, n; e& m8 Q, sWithin the actual casting process, the metal melt replaces the polymer model. The foam is being) r! Y! ^2 Y: v" X7 N
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the% J* D& y0 x" |
liquid metal fills the resulting cavity.
4 B& O& W& m- D3 nA couple of decomposition products, like CO2, CO and several organic compounds, are created% L( `: M- z% J
by the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or2 ]+ B4 N+ M, O( |
mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more8 [' ^- f6 I0 ? P8 T
liquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,5 V, d$ q2 R2 k6 n3 `
where the pyrolysis reaction produces mainly gases." A$ P- |7 _4 I
The emission of the pyrolysis products usually penetrates through the coating into the sand mold.
2 K6 J6 i/ R8 U+ \6 a# N" CThe coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from. [, W4 K9 {3 g1 a+ o& G# b2 M
the tasks of enhancing the casting's surface quality and stabilizing the sand mold.
: I/ C$ T' M4 c) x4 F: NLiquid decomposition products, which are created during the Lost Foam process of aluminium/ Z: ^3 p* s$ G c
casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
9 K& b- G$ u9 S5 D% v6 W$ b |) Tdecomposition phase (secondary pyrolysis), the liquid products are further decomposed into
1 f! X! o3 u6 j# n( d' ^gas. These are then absorbed by the surrounding mold material.3 a, L/ w$ h" F# d
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition# w! ?- B. K* K8 M
products immediately develop due to the higher melt temperatures. The pyrolysis gases form a0 O+ y+ K% ^" ? }' X: m0 J8 ?
gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon
( d' f, g9 i7 t& W! P5 @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.
! r5 K) d& c0 u0 c" d# n8 p Obviously, a certain porosity and gas permeability of the coating is vital for the application in the. M1 ^( w b2 E2 j8 V2 B
Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments6 p7 J( p% E: C
and porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold
6 b# [% w' u+ \* {. H; [9 rshuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,! D+ X) s2 i# n2 |# I8 \
the liquid products do not penetrate deep into the mold material; due to their high viscosity, they" ]+ b% e9 I3 A C4 Y
concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer# u N# D; W" F( z/ q Y6 K
guaranteed.
+ h* q9 T9 R% S7 z. MDuring the compaction of the mold sand, one must ensure a compaction of the mold material as
6 d/ u4 l$ C! w; T# Leven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite; T' ~1 V& i# M g
for a reproducible venting of the cavity during mold filling. Undercuts that appear during3 F3 ?2 d& Y6 [( N( p) _
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly
& Q# u; k$ ^. Mbe avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes
6 R" ?: M x3 M- z6 Jwith a corresponding variation of the chosen frequencies./ d5 z9 _. O9 H; g/ M" t$ g! u
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