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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 represent9 y5 I% l l$ W. S: M
the casting and parts of the gating system. Because the polymer models are destroyed during
( [0 }/ A8 [2 v1 [8 mthis process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term. N) S; o/ o; T$ i3 C, F# Z' ~1 q
"Vollformgießen".
( \) `- Q! _6 }$ V) k( vWithin the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol; a t) p: o) d& d8 X9 O
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of
6 r8 T" U( @) L6 fEPS and PMMA. The models are created in permanent molds on automatic foaming machines,
0 ^8 [; ?! d j6 H" A1 u6 Mwith a predefined thickness. By glueing simple models together, model segments are then combined
4 b5 [1 t3 c- ^ H' B- Vto complex models. After that, several models are often combined to model trees. After attaching
5 O% }# g6 g ]7 `the gating system, the model is coated by a fire resistant, anorganic coating and
" O) V N- W8 z# c7 K, Qembedded into compressed sand that is free from binding agents.
+ W7 d1 ?# d- f! S+ {2 WWithin the actual casting process, the metal melt replaces the polymer model. The foam is being
( N: ~! j1 @1 f" E# Q" jpyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the; \" R, m1 n6 u$ o( h$ ]; C
liquid metal fills the resulting cavity.0 g6 o6 W: {# a ]
A couple of decomposition products, like CO2, CO and several organic compounds, are created" M9 ~ |' N! f F# `3 a7 K3 T
by the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or
1 D" D3 c& c' J4 |0 K" Q* l; _! _mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more
0 Y( l6 N( z1 z5 ^$ N: K' aliquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,* \% `9 A; F1 H
where the pyrolysis reaction produces mainly gases.
' W8 Z/ M+ e% F7 B3 YThe emission of the pyrolysis products usually penetrates through the coating into the sand mold.2 y6 V/ w. l& _8 Z# ?
The coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from& W8 @, C1 @! ]( _$ e1 ^( x* d
the tasks of enhancing the casting's surface quality and stabilizing the sand mold.
4 t3 O4 I. ]0 p+ H- CLiquid decomposition products, which are created during the Lost Foam process of aluminium
$ C o4 q# Q+ F+ J9 ~casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
6 _& j3 c4 P9 \) ~, Sdecomposition phase (secondary pyrolysis), the liquid products are further decomposed into* c3 q6 l/ g" @+ m% v
gas. These are then absorbed by the surrounding mold material.
4 K# s6 Z- _* W$ b$ u% N) S5 KOn the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition- j8 K$ S% s3 e4 k5 D7 Z* H
products immediately develop due to the higher melt temperatures. The pyrolysis gases form a' {; E+ @' V+ a) p1 d1 e; V
gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon: g; \ G ?! E8 O a
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." l2 b7 p- `: t+ w
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the! q5 d! Q# c- C- O' `9 S
Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
$ g+ W: F% R& J+ s% W! j( Yand porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold( q# f/ t) U2 ~/ G* L& M& D( M% j5 F5 n
shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
, F- j, T8 X: Ythe liquid products do not penetrate deep into the mold material; due to their high viscosity, they6 C3 l$ ^# u1 z; B
concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer
+ N) I, h7 R4 x4 H4 Z7 @guaranteed.
" n; `- C7 ^" `5 T8 b X) u. DDuring the compaction of the mold sand, one must ensure a compaction of the mold material as
$ U3 u U ]% r" c( Neven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
4 Z, o2 e( @+ b& o2 \ D$ efor a reproducible venting of the cavity during mold filling. Undercuts that appear during; R) r" k% b6 w2 p$ T6 o
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly
1 E9 j# G4 c# E. ~; Bbe avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes
* s7 I; D% ^" E9 r" m% @$ ^ Z( w( Jwith a corresponding variation of the chosen frequencies.
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