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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
' T5 R- l& f0 c/ g: B, n7 i+ qthe casting and parts of the gating system. Because the polymer models are destroyed during4 y& R4 |2 z( B' F' s! Y
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term& E( C( N4 I& p9 D
"Vollformgießen"./ s Z7 d' r: L8 n
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol5 w) g# j4 w5 n L$ X9 Q% K
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of+ M5 d& U) s v5 ?: r# S* ]
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,* ~2 y1 ]2 L( J' I& U
with a predefined thickness. By glueing simple models together, model segments are then combined4 E- s+ M, w* r" E
to complex models. After that, several models are often combined to model trees. After attaching0 r3 A! i: F3 L
the gating system, the model is coated by a fire resistant, anorganic coating and, x* }: Q. U! U7 Y9 X, v
embedded into compressed sand that is free from binding agents.
+ n- L8 Q' f1 [ Y6 h0 J. x9 }Within the actual casting process, the metal melt replaces the polymer model. The foam is being6 Q, ~1 x% k: |/ u; `
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the, R# V3 t- D3 y0 W: N
liquid metal fills the resulting cavity.) ~. B: s! m. C$ p n! }- v! G4 T
A couple of decomposition products, like CO2, CO and several organic compounds, are created
+ j4 e. W n) q0 B. D3 K2 Nby the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or
% n M. e' O5 a2 j, vmostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more
1 Z, t: b1 {: k( V" gliquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings, m8 W P+ h" @; a" [1 w
where the pyrolysis reaction produces mainly gases.
8 k( O- h2 h" v7 y% @The emission of the pyrolysis products usually penetrates through the coating into the sand mold.
5 P7 J# ?5 G# `. Y4 o- a. }- iThe coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from3 p/ Q0 I& R8 g: m) K
the tasks of enhancing the casting's surface quality and stabilizing the sand mold.; y; j% p" Y/ t9 `8 d0 [7 b
Liquid decomposition products, which are created during the Lost Foam process of aluminium) |4 s/ E5 d# m
casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second) U# h, v: d# n w! B
decomposition phase (secondary pyrolysis), the liquid products are further decomposed into
% ?1 K+ {4 C1 P% O. p- `gas. These are then absorbed by the surrounding mold material.1 d3 I9 R* {, T* o% [
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
- u, p+ B" R( [ T2 ` ]; vproducts immediately develop due to the higher melt temperatures. The pyrolysis gases form a
/ g* f, ]8 G) P9 W* \gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon
, Z4 y" z9 V. J" g0 m9 y1 i' xthe 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.. z, B1 ^/ k) E$ K1 q) w
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the7 x. w3 c$ g* o) i9 H7 \9 k3 j
Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
) Z5 e9 k0 {& i& S( n( L, f Band porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold
$ U* `9 f: U/ @4 {1 s- kshuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
# B# r7 t9 @6 _) Ethe liquid products do not penetrate deep into the mold material; due to their high viscosity, they
; _; M6 n9 a! v/ T- L: Rconcentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer
3 v5 U2 n: K% G9 g+ Rguaranteed.2 a" w/ p0 ^# h5 c+ u u$ H
During the compaction of the mold sand, one must ensure a compaction of the mold material as
) F1 G0 d* p! P( ?5 N7 Oeven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
( y& M }1 J0 @' ?) d/ gfor a reproducible venting of the cavity during mold filling. Undercuts that appear during: Y5 e+ s% z& S* m |- G
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly
. o/ y6 _2 a5 {* R* Abe avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes
: w2 m9 }9 u: n+ E% X. |9 E9 n6 xwith a corresponding variation of the chosen frequencies.
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