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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
4 \& \4 Y7 N9 Q- x. K1 l7 Othe casting and parts of the gating system. Because the polymer models are destroyed during5 @. u" K* ]4 i. L; J
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term
2 E, a) ?7 T2 n( x; T! R"Vollformgießen". Z0 E' {2 I; o% ~# [8 _
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol
. f+ d$ ?9 E; v8 Y7 H# t(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of# P( R8 b* z' w9 Y; @4 P5 ^
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,
+ r6 q: y. S' c: Z) z( d% q1 b8 Twith a predefined thickness. By glueing simple models together, model segments are then combined9 C4 j% A) u( N% q( `/ u
to complex models. After that, several models are often combined to model trees. After attaching
; B4 @9 J; g& z1 Z( u* Uthe gating system, the model is coated by a fire resistant, anorganic coating and
; K% y# @, n5 o0 x! E4 Cembedded into compressed sand that is free from binding agents.8 n1 k1 h, R0 `# z" m+ M3 g: @
Within the actual casting process, the metal melt replaces the polymer model. The foam is being9 [# p% ?4 z% _2 ~
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the ?7 I3 B. t& A0 w" q! D9 _9 ^
liquid metal fills the resulting cavity.
4 o" q) g0 W; T7 L0 [; o3 Q, d a! pA couple of decomposition products, like CO2, CO and several organic compounds, are created4 |) a; Y4 r4 G; {, r
by the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or
8 G& P4 X( _ t5 rmostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more I! B3 R9 A) h+ { L2 F, m5 g
liquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,% S7 c) {7 O) M$ U
where the pyrolysis reaction produces mainly gases.( ]/ f! R" P) \
The emission of the pyrolysis products usually penetrates through the coating into the sand mold.
) E. w. _) O* i5 H8 LThe coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from3 a4 o3 \$ [. L" T$ X H
the tasks of enhancing the casting's surface quality and stabilizing the sand mold.% T1 s' L" B/ q3 D; Z; _
Liquid decomposition products, which are created during the Lost Foam process of aluminium
+ Z) K, M) l/ x: Qcasting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second# @# G- X7 }: ~8 \
decomposition phase (secondary pyrolysis), the liquid products are further decomposed into
7 I8 U& j3 W& Sgas. These are then absorbed by the surrounding mold material., C2 H, V9 ^5 Q
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
) p% Y2 C/ a7 \' \. Oproducts immediately develop due to the higher melt temperatures. The pyrolysis gases form a' j w1 y* q# _
gap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon
2 o7 F/ M4 B7 F+ P& gthe 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.6 Q! i3 Q- [& M
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the
) L z( A. L/ G( x( b+ p4 GLost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments
* e% `" z8 Z' J/ b: m! dand porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold
) F) r. q# a3 ]6 ^! p3 a' ~) ]shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,! z) @9 B1 M- y$ v4 D; j
the liquid products do not penetrate deep into the mold material; due to their high viscosity, they
# G/ ?( S0 p+ T2 y- ?concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer
* ?: q, S$ R$ p# x& aguaranteed.) M4 I5 {" e! z0 K
During the compaction of the mold sand, one must ensure a compaction of the mold material as8 q5 R2 c8 D1 h, w/ O& X
even as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
1 z0 Z9 k# _6 F8 W3 C5 O( f _ Tfor a reproducible venting of the cavity during mold filling. Undercuts that appear during. A# W# F h$ K
the embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly8 K: e# q- C+ ]! H; r, n
be avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes
1 ]) P/ ?8 ~; E/ ywith a corresponding variation of the chosen frequencies.
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