Investment casting tolerances refer to the permissible dimensional variation. Several investment casting processes include lost foam casting, solica sol casting, and water glass lost wax casting. Every casting method is unique in the shapes that it can create and the materials it can use to produce different products.
Tolerances can be affected by several factors like die temperature, firing temperature, plastic or wax temperature, mold or shell composition, the position of the part on the tree, backup sand, rate of cooling, and heat treat temperature. These factors have a direct effect on the tolerances required in investment casting.
There is a tolerance standard in the investment casting business, but individual foundries may deviate from it depending on their expertise. For example, some may be better at producing huge parts, while others excel at producing thin-walled, precision components. Casting tolerances could also be affected by the machinery used.
Investment casting tolerance standards can be divided into two broad categories: linear and geometric.
Linear Tolerance
Length, fillet radii, corner radii, curved holes, concentricity, holes, straightness, and flatness, are typical qualities subject to linear tolerancing when working with investment castings. Linear tolerances vary amongst casting processes.
The as-cast tolerances achieved through the investment casting technique are consistently high in quality. With normal as-cast capabilities of +/-.003 to +/-.004 per inch, or +/- 3 standard deviations when measuring a single location multiple times. Because the capability to hold tolerances depends heavily on the layout of the part, this can change.
There is far less fluctuation in the solidification process for symmetrical structures with uniform wall sections than for non-symmetrical shapes with non-uniform wall sections.
These three types of variance are reflected in the typical linear tolerances for standard dimensions:
- Estimating shrinkage and part factors (20%)
- Process variation (70% of linear tolerance)
- Tooling and diemaker tolerance (10%)
Tolerance variance arises from several sources, including part configuration effects leading to non-uniform shrinkage and every other process variation in making a ceramic mold, wax pattern, and the casting itself.
We listed the liner tolerance standards for casting below. Leisheng Machinery is a professional Investment Casting Manufacturer, our Linder Tolerance can reach from CT6 to CT7
| ISO Casting Dimensional Tolerance Standard ISO8062 (GB/T6414) CT4~CT8 | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Normal Dimension (mm) | Linear Dimension Tolerance | |||||||||||
| > | < | CT4 | CT5 | CT6 | CT7 | CT8 | ||||||
| 0 | 10 | 0.26 | ±0.13 | 0.36 | ±0.18 | 0.52 | ±0.26 | 0.74 | ±0.37 | 1 | ±0.5 | |
| 10 | 16 | 0.28 | ±0.14 | 0.38 | ±0.19 | 0.54 | ±0.27 | 0.78 | ±0.39 | 1.1 | ±0.55 | |
| 16 | 25 | 0.30 | ±0.15 | 0.42 | ±0.21 | 0.59 | ±0.29 | 0.82 | ±0.44 | 1.2 | ±0.6 | |
| 25 | 40 | 0.32 | ±0.16 | 0.46 | ±0.23 | 0.64 | ±0.32 | 0.90 | ±0.45 | 1.3 | ±0.65 | |
| 40 | 63 | 0.36 | ±0.18 | 0.50 | ±0.25 | 0.70 | ±0.35 | 1.00 | ±0.50 | 1.4 | ±0.70 | |
| 63 | 100 | 0.4 | ±0.20 | 0.56 | ±0.28 | 0.78 | ±0.39 | 1.10 | ±0.55 | 1.60 | ±0.80 | |
| 100 | 160 | 0.44 | ±0.22 | 0.62 | ±0.31 | 0.88 | ±0.44 | 1.20 | ±0.60 | 1.80 | ±0.90 | |
| 160 | 250 | 0.50 | ±0.25 | 0.70 | ±0.35 | 1.00 | ±0.50 | 1.40 | ±0.70 | 2.00 | ±1.00 | |
| 250 | 400 | 0.56 | ±0.28 | 0.78 | ±0.39 | 1.10 | ±0.55 | 1.60 | ±0.80 | 2.20 | ±1.10 | |
| 400 | 630 | 0.64 | ±0.32 | 0.90 | ±0.45 | 1.2 | ±0.60 | 1.80 | ±0.90 | 2.60 | ±1.30 | |
How to Achieve Premium Linear Tolerances
It is possible to minimize all three types of variation by:
- Modifications comprise a redesign, in part, with new features, including tie bars, gussets, and ribs
- Adjusting wax injection molds after the first sample is injected to achieve target dimensions.
- Straightening/coining
- Machining
- Additional inspection/gaging
It is possible to obtain premium tolerance capacity, but this capability must be evaluated on a component-by-component, dimensional-by-dimensional basis. Although the standard premium cast tolerance is +/-.004″ per inch, it is capable of holding +/-.002″ in some features.
Designating tight tolerances that are critical to part function is essential while leaving the rest open to standard linear tolerances.
Geometric Tolerances
Parallelism, flatness, roundness, straightness, curved holes, hole tolerance, internal radii and fillets, and so on are typical examples of investment casting features that benefit from geometric tolerancing.
Parallelism
Having parallel prongs on a blueprint that are only connected at one end presents a unique difficulty in achieving high precision in an investment casting. If your item has a parallelism requirement, the engineer will contact you before manufacturing to explore options.
Volumetric shrinking of the supporting structure makes it challenging to keep these prongs in parallel. As yoke holes warm up to room temperature, they can also undergo significant deformation. To produce a specific part, it is crucial to account for volumetric shrinkage in the design stage. A greater investment in tooling and finishing is warranted for these components.
Flatness
Tolerance for unevenness is the maximum distance from a perfectly flat plane that can be tolerated. The surface of the investment cast component must be in a plane that is less flat than the maximum allowable deviation and flatter than the flattest plane.
Investment casting flatness is always determined by the volumetric shrinkage of the wax and metal during cooling. This reduction in the mass’s center is known as a “dish” and is common in many astrophysical objects. There will always be some degree of this dish, but it can be managed with the right tools. Due to the large diversity of designs and alloys used in investment castings, standard flatness tolerances cannot be quoted.
Roundness
The degree of roundness can be measured by calculating the radial distance between a genuine circle and a line segment of a specified circumference. After rotating the part through 360 degrees, you can verify its sphericity by reading the total indicator. Another method for ensuring roundness is to divide the difference between the maximum and minimum values by two. The second approach is suggested because it saves time and effort.
The degree of roundness is quantified by comparing the smallest and largest radii of a segment of a circle. During the inspection procedure, it will strictly regulate a part’s roundness and identify any that are out of spec.
Straightness
Since the two concepts seem so similar to the untrained eye, they can cause confusion. The straightness tolerance describes the largest allowable deviation from a perfectly straight line that this part can have.
Certain investment castings may have serious issues with straightness. Since some of the thinner, shorter pieces can be bent with little effort. While we may use our knowledge of casting to determine if a designed part is likely to bend, it is often impossible to predict how much the part will actually deform before it is cast. An approximate axial bow of 0.024 inches per foot is seen in constant sections.
Curved Holes, Internal Radii, and Fillets
Curved holes can be made using preformed ceramic cores or soluble wax. To accommodate for the curvature, the tolerance on the typical hole will be twice, and the tolerance on all other dimensions will be multiplied by two as well. Considering that you could not precisely measure the diameter of these holes, engineers would suggest a tolerance of +/-.005″ Since it is difficult to manage and examine interior radii and fillets precisely, it is best to make them as large as possible during the design phase.
Tolerance for Holes
A cast hole must meet a roundness tolerance, and this is known as hole tolerance. Fit and/or clearance issues can be avoided if the metal around the hole is perfectly symmetrical. This is crucial for cast parts that need to fit together, such as wheel shafts, so they don’t vibrate. Internal concavity increases as hole length increase until the diameter of the interior hole is larger than that of the apertures. More effort is required to achieve the required hole tolerances.
Tolerance in the holes will be useful for internal shrinkage, and diameters up to 1/2 inch, clearance might be within +/-.003″, ensuring a precise form and fit.
Concentricity
Cylindrical surfaces that intersect at a common point or axis are used as a test for concentricity. Measured in terms of the distance between their centers, eccentricity is quantified in terms of its magnitude. Assuming a wall thickness of 1/2 inch, the bore center to outside diameter will be concentric to within .003 inch.
