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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
! i) {, ]0 Q' {# Ethe casting and parts of the gating system. Because the polymer models are destroyed during0 y9 I* T' {/ X: M, c
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term
0 \( j- w6 W B"Vollformgießen".
5 c3 s. W4 H" N( \- R$ Z; Q' R. ]Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol
+ R. A( G2 A* w(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of
% V* c; M" r$ A% EEPS and PMMA. The models are created in permanent molds on automatic foaming machines,! }# `& @+ `9 h3 e, G
with a predefined thickness. By glueing simple models together, model segments are then combined
$ M! }. c- `( Q& q- e: i$ Wto complex models. After that, several models are often combined to model trees. After attaching
4 a, }; _' [$ z- |the gating system, the model is coated by a fire resistant, anorganic coating and$ e1 z6 ?3 h7 U' B! P
embedded into compressed sand that is free from binding agents., N) T2 H" L& R+ G0 j: k9 a
Within the actual casting process, the metal melt replaces the polymer model. The foam is being
4 S5 v+ Y& V" x7 v" x1 |pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the
! t/ z8 }$ R7 J+ |5 O& f: u/ d0 [0 kliquid metal fills the resulting cavity.
9 g" w; _9 P% xA couple of decomposition products, like CO2, CO and several organic compounds, are created
8 {, s6 e: f! ]9 l* Jby the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or
$ r6 T1 R/ j9 Imostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more2 {- c( _- G7 x
liquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,% G. f$ _( m7 y
where the pyrolysis reaction produces mainly gases.6 C+ p6 V: _9 Z$ a! H
The emission of the pyrolysis products usually penetrates through the coating into the sand mold.
1 G! ^2 e% }2 E- F- lThe coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from
# C" Q/ j3 Z- N% [( dthe tasks of enhancing the casting's surface quality and stabilizing the sand mold.
' F! @) J: `4 H$ z" ^/ KLiquid decomposition products, which are created during the Lost Foam process of aluminium
0 K6 E8 M4 X) Y" t' }/ a! M: ^casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second; \. ~- A; ^0 K2 m
decomposition phase (secondary pyrolysis), the liquid products are further decomposed into; _* ^: ?( U6 ]* Y# c; X8 k
gas. These are then absorbed by the surrounding mold material.
9 v5 ]! r* r" x6 H' s. \4 AOn the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
" h* R; |( V) i8 z) n7 ]+ X0 M! `2 Tproducts immediately develop due to the higher melt temperatures. The pyrolysis gases form a0 `* F- N0 g$ L6 C4 b
gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon2 ]" P* K1 C) K8 a) c
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.( u) P, C7 @; |! E) B
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the
% b# J2 H9 k1 A7 |" `Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
4 w5 ]+ I$ U o3 {8 |; Oand porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold: {; f4 Y/ t; N# r* i3 h% b
shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
" c. l1 P5 L) m0 D; qthe liquid products do not penetrate deep into the mold material; due to their high viscosity, they2 z! g# H* u+ }
concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer5 c3 v5 F: D3 O2 ?& P( j
guaranteed.
5 j' L0 |& i- F YDuring the compaction of the mold sand, one must ensure a compaction of the mold material as
. Y; J: O* u! {. }even as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
0 H& l8 q3 X7 m; O( b; mfor a reproducible venting of the cavity during mold filling. Undercuts that appear during/ w2 r2 }/ a* `/ b" }
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly) D3 y( K8 @6 n- l7 }2 N
be avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes+ O* U" n! p. P% T( a1 }
with a corresponding variation of the chosen frequencies.2 N* a. z* e6 O1 z" S
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