Casting Evaluation - Wall Thickness

The success of a die-casting project, in many cases, depends on the structural design of the die-casting. The structure of die castings is a combination of common basic elements. These elements include bosses, ribs, holes, wall thickness, fillets, draft angles, threads, inserts, etc.

Here is how things fall in place in the industry;

Product designers pay more attention to the functionality and structure of die castings. On the other hand, die-casting companies pay more attention to product manufacturability, die-casting process, and production cost.

Through this piece, we will share how to evaluate the structural design of die castings from the producer’s perspective. And we will be placing a focus on the die-casting wall thickness.

This article will analyze the wall thickness of die castings. As a result, the article is divided into 4 parts:

  1. Three problems caused by uneven wall thickness
  2. Design criteria for wall thickness
  3. Optimized design of wall thickness
  4. Optimization design methods for five typical connections

Without much ado, let’s get right into it.

1.Three Problems Caused by Uneven Wall Thickness

First, let’s look at a kind of casting defects.

In simple terms, it is an irregularity that occurs during the casting process. You can also call it a shrinkage defect. But the question is, how does the shrinkage cavity cause the defect?

It happens when the wall thickness is too large and when the inside of the casting is solidified. As a result, it cannot be fed during the final pressurization stage of die casting.

When designing castings, one should pay good attention to the wall thickness. Because when the wall thickness of the casting is too thick or too thin, it results in an uneven wall thickness.

The 3 problems caused by uneven wall thickness are:

1) Filling problems: such as poor filling, air trap

2) Thermal problems: for example, shrinkage, porosity, leak, crack, etc

3) Deformation problems: For example, internal stress concentration

Filling Problem

An uneven wall thickness causes filling problems. We will discuss the filling problem using the following headings.

Cold pattern defects

The direct cause of cold pattern defects is that the partial temperature of the alloy liquid is too low, and the fluidity is lost. If the wall thickness of the casting is too thin, or the wall thickness of the casting is not uniform, the temperature of the alloy liquid will be too low.

Why does the filling performance have a great relationship with the wall thickness?

Two important parameters determine the filling performance: The first is the filling time, and the second is the filling speed.

Filling time refers to the time required for the molten metal to fill the cavity from the gate to the complete cavity. The appropriate filling time determines the quality of the product. The thicker the wall, the longer the filling time.

As shown in the figure, for example, when the wall thickness of AlSi9Cu3 is 2mm, the suitable filling time is between 20ms and 30ms. On the other hand, if the wall thickness is 3mm, the suitable filling time is between 0.50ms and 0.60ms.

The filling speed refers to the flow rate of the molten metal in the gate. The filling speed is also closely related to the wall thickness. As shown in the figure, when the filling length is 300mm, and the wall thickness is 3mm, the better filling speed is about 34m/s. When the filling length is 400mm, and the wall thickness is the same as 3mm, the optimal filling speed is 45m/s.

The relationship between wall thickness and filling speed

Thermal issues

Castings will form hot spots in areas where the wall thickness is too thick. Shrinkage porosities will occur in the hot spot area when the die-casting solidifies.

The principle of causing shrinkage is shown in the figure

NTA line Neutral thermal axis alloy liquid concentrates on flowing to the hot spot.

Three mechanisms of shrinkage cavity formation in castings:

Formation mechanism 1

a. The molten metal fills the mold cavity, and the pressure of the plunger tip feeds the die casting.

b. The surrounding molten metal begins to solidify, but the molten metal in the middle area has not yet solidified, especially the molten metal in the thick part.

c. Continue feeding, the surrounding molten metal solidifies, and the feeding channel will be closed soon.

d. The feeding channel is completely solidified, but there is still unsolidified molten metal in the thick part.

e. The internal molten metal can only depend on cooling and solidification to form shrinkage holes.

Formation mechanism 2

On the left is the cold mold, and on the right is the hot mold. During the solidification of molten metal, the cold mold solidifies first, and the hot mold hardens later.

a→d indicates that the molten metal gradually solidifies, the channels fed by b and c are solidified and closed, and there is still unsolidified molten metal on the right side, thus forming shrinkage cavities.

Formation mechanism 3

The upper part of the casting wraps the mold to hinder heat transfer, and the lower part is recessed to facilitate heat transfer. During the solidification process of the casting, the upper part solidifies first, and the lower half solidifies later.

When the feeding channel is closed, the molten metal in the lower half has not wholly solidified; it forms shrinkage holes on the surface of the lower half.

Locations where shrinkage holes often appear:

  • The solidification shrinkage of the alloy is large in the thick parts of the casting; that is, the volume shrinkage of the alloy is too large.
  • Areas with a massive difference in wall thickness on the casting, such as the thin wall directly transitioning to the thick deep cavity

Shrinkage cavities will lead to a substantial decrease in the strength of the casting, and it is also the leading cause of leakage of the casting.

Diagram showing casting leakage mechanism

Porosity inside the casting is difficult to avoid. When the shrinkage cavity inside the casting communicates with the inside and outside, it results in leakage.

Many die casting surfaces need to be machined, which are prone to leakage. Because the original surface layer of die casting is very dense, there is a thick layer of about 0.5mm thickness, but this dense layer is removed after machining.

Therefore, during the design evaluation and analysis of die castings, it is necessary to communicate with the designer to minimize the area for machining of the castings.

2.Design Principles for the Wall Thickness of Die Castings

Here are the crucial takeaways for a perfect die casting wall thickness.

  1. The wall thickness of die castings should not be too thin; because it will produce cold barriers, which will affect the casting performance
  2. The suitable wall thickness is 2.5 to 4mm, and parts with a wall thickness of more than 6mm are not suitable for die casting
  3. The wall thickness is designed to be as uniform as possible, which is conducive to solidification at the same time after filling, avoiding casting stress, shrinkage, and other defects.

Porosity inside the casting is difficult to avoid. When the shrinkage cavity inside the casting communicates with the inside and outside, it results in leakage.

Many die casting surfaces need to be machined, which are prone to leakage. Because the original surface layer of die casting is very dense, there is a thick layer of about 0.5mm thickness, but this dense layer is removed after machining.

Therefore, during the design evaluation and analysis of die castings, it is necessary to communicate with the designer to minimize the area for machining of the castings.

A Quick Question

But in the actual design of die-casting parts, the product’s wall thickness is often very thick. What is the reason?

See the answers below

1. The designer feels that the strength of the casting is insufficient and hopes to increase the strength of the casting by increasing the wall thickness.

But the actual situation, as shown in the figure, as the wall thickness increases, the tensile strength of the casting shows a downward trend—the thicker the wall, the more defects inside the casting and the decrease in mechanical properties. Therefore, the increase in the wall thickness of the casting does not necessarily increase the strength of the casting.

2. The wall thickness of some castings is very thick, which is required due to its structural characteristics, such as some base parts. But if we deeply analyze the structure and mechanical properties, we can often improve these thick parts.

3 Optimized Design of Wall Thickness

How to eliminate some wall thickness positions? It can be through ribs, inserts, holes, grooves, etc.

Below we look at some cases: