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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. G, P r2 o) D( |$ f3 B
the casting and parts of the gating system. Because the polymer models are destroyed during# \: P) {9 \5 {5 @7 f" Y/ C
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term$ G4 ?: i/ D2 Y- Z* x3 }
"Vollformgießen".* {. b8 f$ f5 q" d
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol5 Y: n, L! \9 w+ a
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of) d9 ]' {; R' q) b7 M8 a
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,' m: [0 |# K0 N: E6 g
with a predefined thickness. By glueing simple models together, model segments are then combined# {7 I" F# a+ ^, |8 k
to complex models. After that, several models are often combined to model trees. After attaching
* M! @2 N/ r: u* m' F/ Vthe gating system, the model is coated by a fire resistant, anorganic coating and M1 \' Y# }6 E
embedded into compressed sand that is free from binding agents.% {" [6 `+ J5 v: W$ o* A+ j
Within the actual casting process, the metal melt replaces the polymer model. The foam is being$ i: J6 Z, i9 ~1 |' C( _, q
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the+ h, Y" F9 y, C
liquid metal fills the resulting cavity.: [# Y' N$ x& i+ ~5 @
A couple of decomposition products, like CO2, CO and several organic compounds, are created+ [; E( Y, m0 v& w- b8 t& Q# R
by the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or
8 B6 G; F; U! |mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more
; z# J9 F$ }) ?" s5 d p* v0 Bliquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,
9 q' W: x# K' U3 t$ Ywhere the pyrolysis reaction produces mainly gases." Q' g% K# f* q2 r) C
The emission of the pyrolysis products usually penetrates through the coating into the sand mold.# C: a& c; y5 x5 [2 W: c* Y
The coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from" B4 N0 B1 e& A' M" M
the tasks of enhancing the casting's surface quality and stabilizing the sand mold.; {$ w E6 `. b3 }7 ^# y& C( F1 ~
Liquid decomposition products, which are created during the Lost Foam process of aluminium
. x: V$ M7 S! O" c9 q) O4 ~9 k# ecasting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
9 E* ~5 c1 c( W+ G! g8 j* y$ P& edecomposition phase (secondary pyrolysis), the liquid products are further decomposed into
6 r$ d( U1 Z, O- Ogas. These are then absorbed by the surrounding mold material.5 _: s+ {0 M1 b
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition2 M" m: ?8 `" [, w, ^" n0 G- _# g
products immediately develop due to the higher melt temperatures. The pyrolysis gases form a5 P# E( ^# d4 s8 z, }
gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon8 d) o4 j0 {0 M
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.
/ J2 d. U7 j. G* W# d* [4 o2 y Obviously, a certain porosity and gas permeability of the coating is vital for the application in the# [2 C1 F' _+ W% ]: y) D! y
Lost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
# K \3 x, p- u& W; wand porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold9 w) o1 ~$ |9 s
shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,
, d5 B/ X, e+ Q2 N. q' {, Wthe liquid products do not penetrate deep into the mold material; due to their high viscosity, they
1 i7 _/ N/ f' j5 hconcentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer! Z% ?# M- `' l8 @ a9 Z2 g
guaranteed.. f/ @4 v0 E% _, E8 m- B% ^; K
During the compaction of the mold sand, one must ensure a compaction of the mold material as
G& y% y0 s1 E, m% o7 @8 Deven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite" q8 s: M2 s; @2 t
for a reproducible venting of the cavity during mold filling. Undercuts that appear during" l* W8 t& P+ X$ s0 K3 }: y
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly) Y3 k$ V& _9 m
be avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes
. d' v [2 n6 d% rwith a corresponding variation of the chosen frequencies.- |+ [1 N7 @- @! f' O
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