The development and continuous improvement of the lost foam casting method is the evolution process from the foam model wet sand method (FM method) to the foam model thousand sand negative pressure method (FV method). This kind of evolution is not only the evolution of the modeling method, but the fundamental change of the process principle. The disappearance of the bubble model has undergone a fundamental shift.
The solid-cast foam model is violently burned away under open conditions.
Dry sand solid casting is mostly used for cast aluminum body, and the foam model disappears mainly in liquefaction mode.
The negative compaction type dry sand (foam mode) casting method, the foam model disappears in a gasification-based manner.
The three methods described above not only have different ways of disappearing the foam, but also the principles of the setting are different. Therefore, the requirements for the performance of the coatings are not the same.
The solid casting relies on the powdering agent to shape the sand and the open casting. The coating mainly acts as a partition between the molding sand and the liquid metal to prevent the sand from being difficult to remove.
The setting mechanism of the dry sand solid casting sand without vacuuming is relatively complicated. This method must be filled by the bottom of the model to the human metal liquid, and the liquid metal is filled by the static pressure, and the radiation heat conduction mode is adopted during the rising process of the liquid surface. The carried heat is transferred to the foam model, and the foam shrinks rapidly after being heated, liquefying and dripping on the surface of the liquid metal layer. The liquid foam dropped on the surface of the liquid metal is subjected to a higher temperature and rapidly vaporized to generate a high pressure gas. The sand in the air gap is the shape of the model cavity that is maintained by the high pressure of the gas.
The continuous convex form of the molten metal surface rises, and the liquid foam is instantly squeezed between the liquid metal and the coating. In order to avoid the occurrence of casting defects, this method requires the coating to have good wetting properties, absorb the liquefied material of the foam model, and remove the liquefied material of the foam model into the sand gap under the action of the liquid metal pressure.
Under the action of high temperature of liquid metal, the liquid foam continuously vaporizes into the peripheral sand gap during the discharge process. At this time, the vaporized foam is condensed into a liquid substance in the cold, so that the molding sand is bonded and shaped.
In the state of vacuuming, the pressure difference generated by the atmosphere makes the sand compact as hard as a stone. This paper discusses the problems related to the coating used in the lost foam casting method with foam model, dry sand and vacuum. .
There are three methods of using foam casting. Although the foam molds disappear, in China’s atmosphere, only the casting method with three types of foam, dry sand and vacuum should be called lost foam casting.
Lost foam casting currently has three processes
Since the entry into China in the early 1980s, lost foam casting has experienced a long period of chaos. It has been circulating in the industry – the words: seemingly simple lost foam casting, a look If you do it, it will be scrapped!
As an independent technical method, lost foam casting should have its own theoretical basis. It is precisely because we neglect the study of the unique theory of lost foam casting. For a long time, we borrowed the theory of traditional casting to explain the lost foam casting. We have entered a misunderstanding with the actual ambiguity.
When we asked ourselves what topics should be studied for lost foam casting, we discovered that the name of the lost mold is very appropriate! Very good! Three words set the theme!
Lost foam casting is to study: the mold disappears!
To put it simply, we need to study three problems in the disappearance of the mode: the way the mode disappears, the time the die disappears, and the amount of die disappearance.
(1) The disappearance mode of the lost foam casting mold
The lost foam casting mold is a compound mainly composed of carbon and hydrogen, and it disappears in two ways.
The burning disappeared.
In the process of casting in the lost foam casting, if the liquid metal can close the sprue and not let the air enter the cavity, the foam of the model will be cracked into small molecules by the macromolecule under the condition of high temperature and no oxygen, and the solid state will change into the gaseous state. The coating is vacuumed and discharged.
If air is drawn into the cavity during the casting process, the foam will burn violently. This common sense is shared by everyone. Foam burning produces a lot of free carbon and carbon bundles. This is because the concentration of oxygen in the air is insufficient, and the active hydrogen atoms combine with oxygen to free the carbon. When free carbon and carbon bundles are dissolved in liquid metal, diffuse carbonization occurs, and carbon segregates bright carbon, which changes the material quality, quality and processing properties of the casting.
The carbon concentration of gray iron and ductile iron is higher, the carbon adsorption tendency is not obvious, the carbon concentration of carbon steel is very low, the greedy absorption of carbon, the free carbon produced by foam combustion becomes a deadly killer of steel castings. Therefore, in the early days, few people were able to make casting molds for lost molds, especially small dry cast steel parts.
We know the principle of free carbon generation. We also know the danger of free carbon. We also know that the occurrence of free carbon is inevitable. As long as there is a way to make the free carbon produced by combustion out of the cavity, it is insoluble in liquid metal. It can prevent the occurrence of carbonization defects. Therefore, we tried to cast directly from the riser. The spacious riser channel allows the gas and free carbon produced by the combustion of the foam to be fully discharged, solving the serious carbonation problem of the casting. The open casting method is to solve the problem of carbon-discharging. Therefore, we call it the carbon-discharging method of lost-foam casting, which is mostly used for thick and cast steel parts that require Zengkou feeding. The closed casting method corresponding to this is to create a condition of high temperature and no oxygen, and to make the foam gasification disappear, so we call it the gasification method of lost foam casting.
(2) The disappearance time of the lost foam casting mold
a. The disappearance of the lost foam casting mold is actually the replacement of the liquid metal with the foam type. Since it is a replacement of two substances, there are three manifestations in the time relationship between one in and one out;
b. The filling speed of the liquid metal is faster than the disappearing speed of the foam model;
c. The filling speed of the liquid metal is synchronized with the disappearing speed of the foam model. If the filling speed of the liquid metal is faster than the disappearance of the foam model, the liquid metal will be buried in the foam before the foam cutting bottom disappears and fills the cavity. The gel, which causes the gas to continue to vaporize, cannot escape from the cavity and solidify in the casting, creating pore defects, which we do not want to happen. We hope that the rate of disappearance of the foam (whether gasification or combustion) will disappear with the filling of the liquid metal or before the filling of the cavity with the liquid metal. Thus, the disappearance of the foam type has no effect on the filling of the liquid metal. Therefore, when we summarize the design principles of the lost foam casting system, we propose to control the cross-sectional area of the sprue, balance the feed rate of the liquid metal and the gasification speed of the foam. The best way is to burn the foam model first or first.
(3) Loss of lost foam casting mold
In the lost foam casting process, we hope that the foam will disappear completely without leaving any traces. The special defects such as pore defects, carbonization defects, and wrinkle defects in lost foam casting are traces of the mold lost during the disappearance process. We are known as: gasification defects.
The measures to overcome the gasification defects are – in general terms, seven words: one low, two high, three breathable. Namely: use a lower density foam model, higher temperature molten iron and a coating with good permeability.
In this article we outline three process and theoretical foundations for the development of lost foam casting to date. The three methods are:
(1) “sealed pouring” of lost foam casting gasification method;
(2) Lost-die casting carbon discharge method “open pouring”;
(3) The lost shell casting method of empty shells is “first burned and then poured”.
The purpose of introducing the three methods is to show that the development of lost foam casting has not been a single method, and the requirements for coating performance are different.