Mold shell

1) Optimized control of raw materials

The raw materials required for lost foam casting production are roughly divided into model raw materials, dry sand raw materials, coating raw materials, and alloy melting raw materials. Since the lost foam casting process is a system engineering, 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.

Model materials are often referred to as beads, and the beads used in casting are generally classified into two types, namely, EPS Polystyrene beads and polymethyl methacrylate (PMMA-PolymethyI Methacrylate beads, both Belongs to polymer materials. There is another
EPS+PMMA polymer. For low carbon steel castings, the carbon in the model material tends to cause carbonization on the surface of the casting, resulting in various carbon defects. Among them, EPS (containing 92% carbon), EPS+PMMA copolymer, and PMMA (60% carbon) have a decreasing effect on the carbonation of castings. In addition, the density of the model is an important control parameter for its gas generation. The gas generation of the above three materials is EPS, PS+PMMA copolymer and PMMA. At the same time, the size of the beads should be selected according to the wall thickness of the castings produced. In general, the thick castings use beads of coarser diameter, whereas the thin-walled castings use beads of finer size to make the castings the thinnest. It is preferred to keep more than three beads in the part.

In addition, the pre-fabrication and forming control of the model material is also a key to technical success. In general, the pre-beads have a density controlled at about 0.024 to 0.03 g/cm3 and a volume of about 30 times the original bead volume. The density of the formed model is controlled to be about 0.02 to 0.025 g/cm3.

Dry sand is a modeling material for lost foam casting. Due to the characteristics of the process, the choice of dry sand should be related to the material of the casting produced. The high-temperature alloy is made of dry sand with high refractoriness and coarse particles. At present, the dry sand mainly uses natural quartz sand, and the iron slag, dust and moisture in the sand should be removed, and the use temperature is not higher than 50C.

Coatings are an indispensable ingredient in lost foam casting, and many foundries now use homemade coatings. The main role of the coating is to improve the strength and rigidity of the model, to prevent damage or deformation; to isolate the molten metal and mold; to exclude the model gasification products; to ensure the surface quality of the casting. The refractory aggregates in the lost foam coating mainly include zircon powder, bauxite, brown corundum powder, quartz powder, talc powder, mullite powder, mica powder and the like. The particle size distribution should take into account the prevention of sand sticking and high temperature gas permeability. The grain shape is beneficial to improve the gas permeability. Usually, a certain amount of spherical particles is selected, which is beneficial to the gas escape after the model gasification or the liquefied product whose model is not completely decomposed. exclude.

2) Control of paint preparation

The carrier of the lost foam casting coating is mostly water-based to facilitate environmental protection. The binder mainly includes clay, water glass, syrup, pulp waste liquid, white latex, silica sol and the like. In the selection of adhesives, the following factors are considered: high temperature gassing: coating properties; coating strength and stiffness; etching modelability. Suspending agents are used to prevent deposition, delamination, and agglomeration of the coating, making the coating thixotropy. Bentonite, attapulgite clay, organic polymer compounds, and composites thereof are generally used. In addition, surfactants need to be added to the lost foam coating to increase the coating properties of the coating and improve the affinity and bonding strength of the coating with the surface of the model. In addition, other additives such as defoamers, water reducing agents, preservatives, pigments and the like are often added.

For this reason, the coating is required to have good strength, gas permeability, refractoriness, heat insulation, rapid cooling resistance, hygroscopicity, cleanability, coating property, suspension property and the like. The combination mainly includes work performance and process performance.

The working properties of the coating include strength, gas permeability, refractoriness, thermal insulation, rapid cooling resistance, etc., mainly in the casting and cooling process, the most important of which is strength and gas permeability. The process properties of the coating include coating properties, suspension properties, etc., mainly in the performance required in the coating operation.

Generally, the water-based paint is mostly used for lost foam casting, and the paint and the model are generally not wetted, thereby requiring improvement of the coating property of the water-based paint. The coating property refers to the suspension drying after the coating is applied to the model. It is hoped that the coating will not drip as soon as possible after coating, ensuring the uniformity of the coating layer and reducing environmental pollution. Suspension refers to the uniformity of the density of the coating during the use of the coating, and no deposition occurs.

Coating process control is a key part of coating technology. Domestic coatings are mostly milled, roller mixed or stirred. According to the production practice, the mixing and mixing of the rolls is better than the mixing. It is recommended that qualified companies use the mixing or roll mixing method to make coatings.

Due to the different effects of different alloys on the coating, it is recommended to develop corresponding coatings according to the different types of alloys, such as cast iron coatings, cast steel coatings, non-ferrous alloy coatings. In the coating configuration and mixing process, reasonable aggregate grading should be used as much as possible to make the aggregate and binder and other additives evenly mixed.

In addition to the coating properties that meet the requirements, the coating and drying process also has an impact on production. Dip coating is used in production, preferably once. It can also be applied in two portions, but it should be dried after each application. Pay attention to the uniformity of drying temperature and drying time during drying to ensure that the coating is completely dry without cracking.

3) Control of dry sand modeling process

Dry sand molding is to embed the model into the sand box, and perform vibration compaction on the vibration table to ensure that the dry sand around the model is filled in place and obtain a certain degree of compactness, so that the molding sand has sufficient strength to resist the impact and pressure of the molten metal. .

The first step of dry sand molding is to add dry sand to the sand box. In order to ensure the filling of the dry sand in the sanding, first add a certain thickness of the bottom sand in the sand box and vibrate it tightly, then put it into the model cluster and then add it. A certain thickness of dry sand, the model cluster is buried in one-third to one-half, and then appropriate vibration to promote dry sand filling the model cavity. Finally, fill the sand box for vibration, the vibration time should not be too long, to ensure that the model does not appear damage and deformation, and at the same time ensure that the paint layer does not fall off and crack.

The vibration parameters should be selected according to the structure of the casting and the form of the model cluster. For most castings, vertical one-way vibration should be used. For castings with complex structures, one-way horizontal vibration or two-dimensional and three-dimensional vibration can be considered. The magnitude of the vibration intensity has a great influence on the shape of the dry sand, and the vibration intensity is expressed by the vibration acceleration. For castings and model clusters of general complexity, the vibration acceleration is between 10 and 20 m/s2. The amplitude is an important vibration parameter that affects the model to maintain a certain stiffness. The amplitude of the lost foam casting is generally 0.5~1mm [4]. The choice of vibration time is subtle and should be selected in conjunction with the casting and model cluster structure. However, the overall vibration time is about 1~5min. At the same time, the vibration time of the bottom sand and the model cluster buried in half should be as short as possible. It can be selected for 1~2min. The vibration time after the model cluster is fully embedded is generally controlled at 2~3min.

4) Control of casting process

The lost foam casting process includes the design of the riser system, the pouring temperature control, the pouring operation control, and the negative pressure control.

The gating system plays an important role in the lost foam casting process and is a key to the success of casting production. In the design of the gating system, the particularity of this process should be taken into consideration. Due to the existence of the model cluster, the behavior of the molten metal after pouring is very different from that of the sand casting. Therefore, the design of the gating system must be different from the sand casting. When designing the cross-sectional dimensions of each part of the gating system, the resistance due to the existence of the model during the casting of the lost foam casting metal solution should be considered, and the minimum flow blocking area should be slightly larger than the sand casting.

Due to the wide variety of castings and the different shapes, the specific production processes of each casting have their own characteristics and vary widely. These factors directly affect the accuracy of the design results of the gating system. For this purpose, the castings can be sorted in some way. The combination of model clusters can basically reflect the characteristics of the casting and the form of the filling of the casting. The cross-sectional dimensions of each part of the gating system are related to the size of the casting, the combination of the model clusters, and the number of pieces per box. For this reason, in the process of designing new castings, the calculation should be carried out in a targeted manner according to the characteristics of the castings and with reference to the characteristics of the casting system of the same type.

Casting classification

Model and model cluster combination Application range Feeding method
One box Larger casting Riser feeding
Combined on the sprue (without sprue) Small casting Spurt (or riser) feeding
Combined on the runner Small casting Sprue (or riser) feed
Combined on the riser Small casting Riser feeding

Because of the existence of the model, the model gasification needs to absorb heat during the casting process, so the casting temperature of the lost foam casting should be slightly higher than that of the sand casting. For different alloy materials, the casting temperature of the lost foam casting is generally controlled to be 30~50 higher than that of the sand casting compared with the sand casting. This is 30~50 higher than the heat of the molten metal to meet the heat required for model gasification. If the casting temperature is too low, the casting is prone to defects such as insufficient pouring, cold separation, and wrinkle. Casting temperatures are too high, castings are prone to defects such as sand.

The most taboo of the lost foam casting casting operation is intermittent casting, which is easy to cause cold insulation defects in the casting, that is, the temperature of the molten metal poured first decreases, resulting in a cold separation between the molten metal and the post-casting metal. In addition, the lost foam casting system mostly uses a closed casting system to maintain the smoothness of the casting. In this regard, the form of the gate cup is closely related to whether the pouring operation is stable. When pouring, keep the liquid level in the pouring cup stable and make the pouring dynamic head stable.

Negative pressure is a necessary measure for lost foam casting of black alloys. The role of negative pressure is an important guarantee for increasing the strength and stiffness of sand, and it is also the main measure to eliminate the gasification products of the model. The size and retention time of the negative pressure is related to the material of the casting and the structure of the model cluster and the coating. For coatings with good gas permeability and coating thickness less than 1mm, the negative pressure on cast iron parts is generally 0.04~0.06MPa, which is the upper limit for steel castings. For the cast aluminum parts, the negative pressure is generally controlled at 0.02~0.03MPa. The negative pressure holding time depends on the model cluster structure. When the number of model clusters in each box is large, the negative pressure holding time can be appropriately extended. Generally, the solidified crust of the surface of the casting reaches a certain thickness, but the negative pressure is removed. For thicker coatings and poorer gas permeability of the coating, the negative pressure and holding time can be appropriately increased.