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An Introduction Guide to Investment Casting

Views:69     Author:Site Editor     Publish Time: 2020-12-11      Origin:Site

The Characteristics of Investment Casting


Investment casting is also called precision casting or lost wax casting. It uses fusible materials (wax and plastics, etc.) to make precise fusible models. The models are coated with several layers of refractory paint, which are dried and hardened to form an integral shell. Then the mold shell is heated to melt and lose the mold and then baked at high temperature to become a refractory mold shell. The liquid metal is poured into the mold shell and becomes a casting after cooling.


Investment Casting Steps


Mold material-wax mold-group mold-mold repair-coating-sanding-demolding-roasting-pouring-cooling-sand falling-cleaning.


Compared with Other Casting Methods, The Main Advantages of Investment Casting Are As Follows:


  • Wax mold castings have high dimensional accuracy and low surface roughness and can cast complex shaped castings. Generally, the accuracy can reach 5~7, and the roughness can reach two Ra25-6.3μm.

  • Precision investment casting can cast thin-walled castings and very small weight castings. The minimum wall thickness of investment castings can reach 0.5mm, and the minimum weight can reach several grams.

  • Lost wax investment casting can cast fine patterns, characters, castings with fine grooves, and curved pores.

  • There are almost no restrictions on the types of casting alloys. They are commonly used to cast alloy steel, carbon steel, and heat-resistant alloy castings.

  • There is no limit to the production batch, and it can be mass-produced from a single piece to a batch.

  • China investment casting can reduce production costs. The shape of the casting and the shape of the cavity are almost unlimited, and parts with complex shapes that are difficult to manufacture by sand casting, forging, and stamping. Moreover, some assembly parts and welded parts can be directly cast into integral parts after slight structural improvement, thereby reducing the weight of the casting parts and reducing the production cost.


The disadvantage of investment precision casting is that the process is complicated, the production cycle is long, and it is not suitable for the production of castings with large outline dimensions.


precision investment casting


Mold Material Types And Performance Requirements


Classification of mold materials


With the development of investment casting technology, there are more and more types of mold materials with different compositions. Mold materials are usually divided into high temperature, medium-temperature, and low-temperature mold materials according to their melting point.


The melting point of the low-temperature molding compound is lower than 60°C. Paraffin wax and stearic acid each account for 50% of the mold materials belong to this category.


The melting point of the high-temperature mold material is higher than 120°C, and the typical high-temperature mold material is composed of 50% rosin, 20% ozokerite, and 30% polystyrene.


The melting point of the medium temperature mold material is between the above two types of mold materials. The currently used medium temperature mold materials can be divided into two types: rosin-based and wax-based mold materials.


Basic requirements for mold material performance


Thermal physical properties

Appropriate melting temperature and solidification interval, small thermal expansion and contraction, high heat resistance (softening point), and the mold material should have no precipitates in the liquid state and no phase change in the solid-state.


Mechanical properties

Mainly include strength, hardness, plasticity, flexibility, and so on.


Process performance

Mainly include viscosity (or fluidity), ash content, coatability, etc.


casting process discussion


Molding Process


According to the specified composition and ratio of the mold material, the various raw materials are melted into a liquid state, mixed and stirred uniformly, and impurities are filtered out and poured into a paste mold material, that is, the mold can be pressed. Compression molding generally adopts the compression molding method, which can use liquid, semi-liquid, solid, and semi-solid molding materials. Liquid and semi-liquid molding materials are pressed under low pressure, called injection molding; semi-solid or solid molding materials are pressed under high pressure, called extrusion molding. Whether it is injection molding or extrusion molding, we should consider the advantages and disadvantages of filling and solidification.


Injection molding


The wax injection temperature of pressure injection molding is mostly below the melting point. At this time, the mold material is a slurry or paste with liquid and solid phases coexist. In the slurry-like mold material, the amount of liquid phase significantly exceeds the amount of solid phase, so the fluidity of the liquid is still retained. Under pressure injection in this state, the surface of the investment mold has low roughness, and surface defects caused by turbulence and splashing are not easy to appear. The temperature of the paste mold material is lower than that of the paste mold material, and it has lost fluidity. Although there are few surface defects, it has a higher surface roughness.


When molding material is injection molded, we should adopt the lowest molding material temperature, and pressing working temperature as far as possible under the condition of ensuring good filling. The choice of pressure is not as large as possible. Although the pressure is high, the shrinkage of the investment is small, but the pressure and injection speed are too large, which will make the surface of the investment not smooth and produce "bubbles" (bubbles under the surface of the investment). At the same time, the mold material splashes and appears cold barrier defects. In the molding process, to avoid the adhesion of the molding material and improve the surface finish of the investment casting mold, a parting agent should be used, especially for the rosin-based molding material.


Extrusion


Extrusion molding squeezes the mold material in the low-temperature plastic state into the cavity and forms it under high pressure to reduce and prevent the shrinkage of the investment mold. The mold material during extrusion molding is in a semi-solid or solid-state. The mold material is relatively hard under normal conditions, but can flow under high pressure, and is characterized by high viscosity. Therefore, the pressure during extrusion depends on the viscosity of the mold material and the flow resistance in the injection hole and cavity. The greater the viscosity of the mold material, the smaller the injection hole diameter, the larger the cavity size, the smaller the cross-sectional area, and the longer the mold material stroke, the greater the resistance of the mold material to flow, and the higher the extrusion pressure is required. The semi-solid mold material is used for extrusion molding, and the solidification time of the investment mold is shortened, so the productivity is increased, and it is especially suitable for the production of thick and large section castings.


Shell Making Process


Shell making includes two processes: coating and sanding. Before coating, the investment mold needs to be degreasing treatment. The dip-coating method should be used when the coating and the surface of the investment mold should be evenly coated with the coating to avoid blanks and accumulation. Welding joints, rounded corners, corners, and grooves should be evenly painted with a brush or special tools to avoid blistering. Before coating each reinforcement layer, the floating sand on the previous layer should be cleaned. During the coating process, the coating should be stirred regularly to control and adjust the viscosity of the coating.


Sprinkle sand after coating. The most commonly used methods of sanding are fluidized sanding and rain-drenching sanding. Generally, after the investment mold is taken out from the paint tank when the remaining paint on it flows evenly and no longer drips continuously, it means that the paint flow is terminated and freezing begins, and sand can be sprinkled. Spreading sand too early can easily cause paint accumulation; spreading sand too late will cause sand particles to not adhere or adhere firmly. When sanding, the investment mold should be continuously rotated and upside down. The purpose of sanding is to fix the coating layer with sand particles; increase the thickness of the shell to obtain the necessary strength; improve the air permeability and concession of the shell; prevent cracks when the shell is hardened. The particle size of the sand is selected according to the coating level and is compatible with the viscosity of the coating. The viscosity of the surface coating is small and the sand particle size must be fine to obtain a smooth surface cavity. Generally, the surface layer sanding particle size can be selected as 30 or 21 sand. The reinforcement layer shall be sprinkled with coarser sand grains, which are best to be thickened layer by layer. When making the shell, after each layer of coating and sanding, it must be fully dried and hardened.


precision investment casting process


Defects And Prevention Methods


The defects of investment castings include surface defects and internal defects, as well as excessive size and roughness.


Surface and internal defects refer to under-casting, cold isolation, shrinkage porosity, pores, slag inclusion, hot cracking, cold cracking, etc. The size and roughness tolerances mainly include the elongation and deformation of the casting.


The surface and internal defects are mainly related to the pouring temperature of the alloy liquid, the baking temperature of the shell, and the preparation process, the pouring system, and the design of the casting structure.


The main reason for the size and roughness of the casting parts is the casting designing, the casting structure, the firing and strength of the shell, and the cleaning of the casting.


For example, when undercasting of investment castings occurs, the reason may be that the low pouring temperature and mold shell temperature reduce the fluidity of the molten metal. There are also problems such as too thin casting walls, unreasonable gating system design, insufficient shell baking or poor air permeability, and too slow pouring speed.


The above are the issues about precision investment casting. Do you get it? Hurry up and contact us to customize the casting design that suits your wishes! Yifei is a professional manufacturer and supplier for custom die castingprecision investment castingsand mold casting and custom forging in China.


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