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Top 5 Investment Casting Design Considerations

Table of Contents

Whats investment casting

Investment casting belongs to precision casting. And precision casting is the general term for the process of obtaining accurate dimensional castings. Precision casting can result in castings with more accurate dimensions and a better surface finish than the traditional sand casting process. It includes investment casting, ceramic casting, metal casting, pressure casting and disappearing mould casting.

Precision casting is also called lost wax casting, its products are precise, complex, close to the final shape of the part, can be used directly without processing or little processing, is a near net shape of the advanced technology.

One of the more commonly used is investment casting, also known as lost wax casting: choose a suitable investment mould material (such as paraffin) to make the investment mould; repeat the refractory coating and refractory sand process on the investment mould to harden the shell and dry; then melt away the internal investment mould to obtain the cavity; bake the shell to obtain sufficient strength, and burn away the residual investment mould material; pour the required metal material; solidify and cool, remove the shell and clear the sand, thus obtaining a high precision finished product. The result is a high-precision finished product. Depending on the requirements of the product, heat treatment and cold working and surface treatment can be carried out.

Casting design

investment casting products
investment casting products

Precision casting is a high-precision, high-quality metal forming process that is widely used in the automotive, aerospace, electronics and other industries. In the design and verification of precision cast parts, the following principles and methods need to be noted:

1.Material selection

The material selection of precision cast parts should consider the feasibility of the casting process and the characteristics of the material, such as hot and cold fatigue, heat resistance, corrosion resistance, etc.. The cost and availability of materials also need to be considered.

2.Structure design

The structural design of the precision casting part should take into account factors such as functional requirements, process conditions and costs, while meeting the requirements of strength, rigidity and stability. The design should try to simplify the structure and avoid excessive complexity to facilitate processing and testing.

3.Mould design

The success of precision casting depends to a large extent on the design of the mould. The design of the mould should minimise the moulding error and maintain a stable temperature and pressure in the casting process. At the same time, attention should be paid to the manufacturability and service life of the mould.

4.Inspection methods

The inspection methods of precision casting parts include appearance inspection, dimensional measurement, tensile, hardness, metallographic organization and many other aspects. The inspection methods and standards should be determined according to the characteristics of the precision casting parts in order to ensure the quality and performance of the products.

5.Numerical analysis

Numerical analysis is one of the common methods used in the design and verification of precision castings. Using finite element analysis, mathematical models and computer simulation, the thermal characteristics, stress distribution and deformation of the casting process can be predicted to guide the mould design and the optimisation of the casting process.

Casting tolerance

Generally, we divide casting tolerances into two parts: linear tolerances and geometric tolerances

Linear tolerances

refers to the distance between two points, e.g. diameter, radius, width, depth, height, centre distance, etc. Generic standards use the millimetre (mm) as the basic unit for dimensions. Generally, when the default unit of measurement is mm. Bayside Casting can meet your fine casting needs according to your actual situation.

There are two popular linear tolerances in China casting foundry

The USA Investment Casting Linear Tolerance (ICI, USA)
The USA Investment Casting Linear Tolerance (ICI, USA)

Normal tolerances can be expected for production repeatability of all casting dimensions. As a general rule, normal linear tolerance on an investment casting can be +0.01” for up to 1”, and +0.005” for each additional inch thereafter.

Premium tolerance require additional operations at extra cost and achieve closer tolerances on selected dimensions only. Premium tolerances should be determined during consultation with our engineer.

ISO Linear dimension casting tolerances
ISO Linear dimension casting tolerances

The blue color is General Level, and the red color is Premium Level

Select tolerance level according to maximum outline dimension. The looser level should be selected for wall thickness.

Unless otherwise specified the chamfer and radius of corners needn’t inspect generally, Reference tolerance may be +/-0.6-1.0 for dimensions up to 6mm, and +/-0.2 for each additional 6mm.

 

Geometric tolerances

Geometric tolerances include shape tolerances and position tolerances. Any part is made up of points, lines and faces. These points, lines and faces are called elements. After machining, the actual elements of a part will always have errors relative to the ideal elements, including errors in shape and position. These errors affect the function of the mechanical product and should therefore be specified in the design with the appropriate tolerances and marked on the drawing according to the specified standard symbols

 

Shape and position tolerances include shape tolerances and position tolerances, and position tolerances include orientation tolerances and positioning tolerances. The details and tolerance symbols are shown in the following diagrams:

 

Shape tolerances

    1. Straightness The symbol is a short horizontal line (-) which is an index limiting the amount of change from an actual straight line to an ideal straight line. This is a requirement for straight lines that are not straight.

 

  1. Flatness This symbol is a parallelogram which is an index which limits the amount of change from an actual plane to an ideal plane. This is a requirement for uneven surfaces.

 

  1. Roundness The symbol is a circle (○) which is an index that limits the amount of change from the actual circle to the ideal circle. For parts with cylindrical surfaces (including conical and spherical surfaces) in a regular cross-section (plane perpendicular to the axis), this is a circular profile requirement.

 

  1. The cylindricity symbol is the circle sandwiched between two diagonal lines (/○/). This index is an indicator of the amount of variation that limits the actual cylindrical body to the ideal cylindrical surface. It controls various shape errors in the cross-section and axial section of the cylinder, such as roundness, straightness, straightness of the axis, etc. Cylindricity is a comprehensive indicator of the shape error of the cylinder.

 

  1. line contour degree the symbol is convex curve (⌒), it is an indicator which limits the amount of change between the actual curve and the ideal curve. This is a requirement for the accuracy of non-circular curve shapes.

 

  1. Surface contour degree The symbol is a semicircle at the top and a horizontal line at the bottom. It is an indicator that limits the variation of the actual surface to the ideal surface and is a requirement for shape accuracy surface
was pattern injection
was pattern injection

Directional tolerances

  1. Parallelism (∥) is used to control the requirement that the measured element (plane or line) on the part deviates from the reference element (plane or line) by 0°, i.e., the measured element must be equidistant from the reference.

 

  1. Perpendicularity (⊥) is used to control the requirement that the measured element (plane or line) on the part deviates from the reference element (plane or line) by 90°, i.e., the measured element must be 90° from the reference.

 

  1. Inclination (∠) is used to control the extent to which the measured element (plane or line) on the part deviates from a given angle (0° to 90°) with respect to the reference element (plane or line), i.e., the measured element is required to be at a certain angle (other than 90°) from the reference to a certain angle.

 

Positioning tolerances

  1. Coaxiality (◎) is used to control the angle of the different axes of the measured axis and the reference axis that should theoretically be coaxial.

 

  1. The symmetry symbol is the three horizontal lines in the middle. It is usually used to control the co-planar measurement elements (centre plane, centre line or axis) and reference elements (centre plane, centre line or axis) that are theoretically required. do not overlap.

 

  1. Position degree This symbol is a circle with two lines perpendicular to each other. It is used to control the variation of the actual element being measured with respect to its ideal position. The ideal position is determined by the reference value and the theoretically correct dimension.

 

Runout tolerance

  • The circular runout symbol is a diagonal line with an arrow. The circular runout is the maximum and minimum reading measured in a given direction by means of an indicator in a fixed position in a given direction. The measured value is the absence of axial movement or rotation difference during one revolution of the actual element around the reference axis.

 

  • The full-jump symbol is two diagonal lines with arrows. The full jump is the continuous rotation of the actual element measured around the reference axis without axial movement and the continuous movement of the indicator along the ideal origin line. The difference between the maximum and minimum reading of the indicator measured in a given direction.
steel liquid pouring
steel liquid pouring

Investment Casting Tips

  1. Keep the shell mould dry: check whether the tooling is dry before casting to avoid defects such as bubbles and shrinkage.
  2. Preheat the shell mould: Preheat the mould before casting, so that its temperature rises gradually to avoid damage caused by sudden contact with high-temperature metal.
  3. Pay attention to the atmosphere control during the casting process: avoid oxygen and moisture in the air from entering the metal melt and affecting the casting quality.
  4. Clean up the mould in time after the casting is finished: when cleaning the mould, be careful not to scratch the surface of the mould to avoid affecting the effect of the next use.
  5. Strictly follow the specifications: the casting should be operated in strict accordance with the relevant specifications, such as the control of casting temperature, pressure, speed and other parameters to ensure that the quality of the product meets the standard requirements.

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