Casting raw materials for lost foam casting production are roughly divided into several aspects, such as model raw materials, dry sand raw materials, coating raw materials, and alloy smelting raw materials. Since the lost foam casting process is a systematic project, the selection of raw materials is particularly important. Therefore, controlling the quality and parameters of various raw materials has become the basis for the success or failure of lost foam casting.
1. Model material
Commonly referred to as beads, the beads used in casting are generally divided into two types, namely polystyrene (PS-Polystyrene) beads and polymethyl methacrylate (PMMA-Polymethyl Methacrylate) beads, both of which are polymer materials. There is also a PS+PMMA polymer. For low-carbon steel castings, the carbon in the model material is prone to carbonization on the surface of the casting, resulting in various carbon defects. Among them, PS (92% carbon content), PS+PMMA copolymer, and PMMA (60% carbon content) have a decreasing degree of influence on the carbonization of the casting. In addition, the density of the model is an important control parameter for its gas emission. The gas emission of the above three materials is PS, PS+PMMA copolymer, and PMMA from small to large. At the same time, the size of the beads should be selected according to the wall thickness of the casting to be produced. Generally, thick and large castings use beads with coarser particle sizes, while thin-walled castings use beads with finer particle sizes, so that the thinnest part of the casting should have more than three beads.
In addition, the pre-production and forming control of the model material is also a key to the success of the technology. Generally, the density of the pre-production beads is controlled at about 0.024-0.03g/cm3, and its volume is about 30 times the volume of the original beads. The density of the formed model is controlled at about 0.02-0.025g/cm3.
2. Dry sand
It is the modeling material for lost foam casting. Due to the characteristics of this process, the selection of dry sand should be related to the material of the casting produced. High-temperature alloys use dry sand with higher refractoriness and coarser particles. At present, dry sand mainly uses natural quartz sand. The iron slag, dust and moisture in the sand should be removed, and the use temperature should be kept no higher than 50℃.
3. Coating
It is an indispensable raw material in lost foam casting. Now many foundries use homemade coatings. The main function of the coating is to improve the strength and rigidity of the model, prevent damage or deformation; isolate the molten metal and the casting; remove the gasification products of the model; ensure the surface quality of the casting, etc. The refractory aggregates in the lost foam coating mainly include zircon powder, bauxite, brown corundum powder, quartz powder, talcum powder, mullite powder, mica powder, etc. Its particle size grading should take into account both the prevention of sand sticking and high-temperature air permeability. The particle shape is conducive to improving air permeability. Usually a certain number of spherical particles are selected, which is conducive to the escape of gas after the model is gasified or the removal of liquefied products of incomplete decomposition of the model.
Control of coating configuration
The carrier of the lost foam casting coating is mostly water-based, which is conducive to environmental protection. Its adhesives mainly include clay, water glass, syrup, pulp waste liquid, white latex, silica sol, etc. Consider the following factors when selecting adhesives: high-temperature gasification; coating hanging; coating strength and rigidity; erosion model, etc. Suspending agent is used to prevent the coating from deposition, stratification, and agglomeration, so that the coating has thixotropy. Generally, bentonite, attapulgite clay, organic polymer compounds and their complexes can be used. In addition, surfactants need to be added to the lost foam coating to increase the coating’s coating properties and improve the affinity and bonding strength between the coating and the model surface. In addition, other additives are often added, such as defoamers, water reducers, preservatives, pigments, etc.
For this reason, the coating is required to have good strength, air permeability, refractoriness, thermal insulation, resistance to rapid cooling and heating, hygroscopicity, cleaning, coating, suspension, etc. In summary, it mainly includes working performance and process performance.
The working performance of the coating includes strength, air permeability, refractoriness, thermal insulation, resistance to rapid cooling and heating, etc., which are mainly the properties that should be possessed during the pouring and cooling process, among which the most important are strength and air permeability. The process performance of the coating includes coating, suspension, etc., which are mainly the properties required in the coating operation.
Generally, water-based coatings are mostly used in lost foam casting, and the coating and the model are generally not wetted, so it is required to improve the coating of water-based coatings. Coating property means that the model needs to be hung and dried after coating. It is hoped that the coating will not drip or flow as soon as possible after coating, so as to ensure the uniformity of the coating layer and reduce environmental pollution. Suspension property means that the coating maintains uniform density and does not deposit during use.
The control of coating preparation process is a key link in coating technology. Domestic coatings are mostly made by rolling, roller or stirring. According to production practice, the quality of rolling and roller mixing is better than that of stirring. It is recommended that enterprises with conditions can use rolling or roller mixing to prepare coatings.
Because different alloys have different effects on coatings, it is recommended to develop corresponding coatings according to different types of alloys, such as cast iron coatings, cast steel coatings, non-ferrous alloy coatings, etc. In the process of coating configuration and mixing, reasonable aggregate grading should be used as much as possible to mix aggregates, adhesives and other additives evenly.
In addition to the coating performance meeting the requirements, the coating and drying process also has a certain impact on production. Dipping is mostly used in production, and it is best to complete it in one go. It can also be applied in two times, but it should be dried after each application. During drying, pay attention to the uniformity of the drying temperature and the drying time to ensure that the coating is completely dry and does not crack.
