TA的每日心情 | 郁闷 2017-6-11 13:54 |
|---|
签到天数: 11 天 连续签到: 1 天 [LV.3]偶尔看看II
|
发表于 2009-11-4 00:11:03
|
显示全部楼层
The Lost Foam Casting process (LFC) is the process that uses foamed polymer models to represent' J x/ f* N$ r8 L8 ^: E% M
the casting and parts of the gating system. Because the polymer models are destroyed during7 u8 L; r/ x' |0 A+ p6 \
this process, you will also find the term "Expendable Pattern Casting (EPC)" or the German term
) \ V3 r2 ^/ m) }"Vollformgießen". [) I. _: p3 v8 ~5 D
Within the Lost Foam Casting process, the models are typically made of foamed expandable polystyrol% x [5 ?6 x- o8 u) g+ Z
(EPS), of foamed expandable polymethylmethacrylat (PMMA) or of a foamed co-polymer of* E0 L! `. ~) @. \: w
EPS and PMMA. The models are created in permanent molds on automatic foaming machines,7 x: F3 m, N" K0 s
with a predefined thickness. By glueing simple models together, model segments are then combined1 _: r! |4 p7 C8 g3 i
to complex models. After that, several models are often combined to model trees. After attaching
( Y0 d/ S* Z6 hthe gating system, the model is coated by a fire resistant, anorganic coating and. M& G) X* u2 w+ J5 Z; l8 V
embedded into compressed sand that is free from binding agents.( D. V. i* Z9 {
Within the actual casting process, the metal melt replaces the polymer model. The foam is being$ `* A$ m$ R1 X$ S9 \- q1 n8 x3 b
pyrolized by the high melt temperatures (exchanged mainly by accompanying radiation), and the e1 h; | r9 j% g- p3 u; L$ ]7 e
liquid metal fills the resulting cavity.# H% |4 |: B* V3 f: d! }/ R" {3 K
A couple of decomposition products, like CO2, CO and several organic compounds, are created
3 L# ?! V& [ Gby the pyrolysis reaction. Depending on the melt temperature, these are either mostly liquid or2 C+ k- q9 a0 H. g) V
mostly gaseous products. If aluminium alloys are casted with Lost Foam Casting, there are more
1 `) _6 @8 R3 ]* @0 _. A7 Pliquid pyrolysis products (due to the lower melt temperatures) than in the case of iron or steel castings,8 K. j. M# K+ ]# S& i4 S
where the pyrolysis reaction produces mainly gases.
* {1 P0 m( m8 `, z/ t. Q3 n1 U1 W2 Y5 MThe emission of the pyrolysis products usually penetrates through the coating into the sand mold.
2 q' ^1 _4 _) ~9 y5 R wThe coating consequently plays a vital role in controlling the filling pattern of mold filling, apart from
7 z( o8 o7 {# ?" [) z/ g/ D1 fthe tasks of enhancing the casting's surface quality and stabilizing the sand mold.
; r& [ |( _) [ |$ ZLiquid decomposition products, which are created during the Lost Foam process of aluminium3 A' W( u2 C5 Q; u+ [/ m
casting, are absorbed by the coating in a first decomposition phase (primary pyrolysis). In a second
( \. @9 I# j, S6 Rdecomposition phase (secondary pyrolysis), the liquid products are further decomposed into3 T# [1 }8 S# H# I) b% A
gas. These are then absorbed by the surrounding mold material.. M# S* k9 X; X( a+ r: a( K
On the other hand, during the Lost Foam casting of iron and steel, the gaseous decomposition
; L }# I4 V( L' b, D. Gproducts immediately develop due to the higher melt temperatures. The pyrolysis gases form a
# j' n$ j/ a9 |5 j3 L. C( Jgap between the melt surface and those parts of the foam model that has not yet collapsed. Thereupon
& L6 C5 t0 i% W" w, b; Bthe 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./ a& H' i! U+ w( t
Obviously, a certain porosity and gas permeability of the coating is vital for the application in the
0 C* J. @2 ?$ jLost Foam Casting process. Unsatisfactory gas permeability results in the formation of gas entrapments# j4 D$ V- r6 u+ E5 f0 I4 _1 f
and porosities. An unsatisfactory absorption of liquid products leads to wrinkles, cold4 Z+ D/ N1 A0 D" I7 X) P, Y
shuts and entrapment errors, if the liquid polymers remain in front of the metal front. Moreover,8 P- N* t% W0 L4 f j4 j4 s, T1 Y$ U
the liquid products do not penetrate deep into the mold material; due to their high viscosity, they3 J- T0 Z. j; R' m; X% q$ q2 N
concentrate on the surface. This can lead to a "closing" of the pores, so that the venting is no longer
1 ~3 s8 o% F1 {% hguaranteed.6 q1 N" I( R4 w: Y( p
During the compaction of the mold sand, one must ensure a compaction of the mold material as
) _; r$ f& @2 Leven as possible in order to avoid the corresponding casting errors. An even compaction is a prerequisite
( ]3 \: Z0 V! z Tfor a reproducible venting of the cavity during mold filling. Undercuts that appear during
$ W- w' G/ Q9 m% r' a( S4 j& f/ fthe embedding of the foam tree into the sand bed and a resulting uneven compaction must possibly4 g5 O7 n' v0 F% h) r
be avoided. The mold material's compaction is achieved by uni- oder multiaxial vibration processes4 {% B' k I/ i3 d/ |
with a corresponding variation of the chosen frequencies.
* z1 \% V( ?- k% H" ]& C如果还嫌不够,可以给我发站内消息。 |
|