Aluminum casting & mold design services

Aluminum castings (aluminum alloy castings) are lightweight, high-performance parts materials that are used in a wide range of fields, including automobile parts, industrial machinery, and building materials. It has been attracting more and more attention in recent years due to its ability to deal with complex shapes and its processability. However, performance, quality and cost are actually judged at the design stage**.

Especially for beginners, designs progress without being vague about where to pay attention, and in many cases, problems arise during prototypes or mass production. Below is an example.

Many of these problems are “design-related defects,” and in other words, it can be avoided depending on the ingenuity of the initial design.

In this article, we will explain the concepts of design terms, principles, and schemes, as well as failure cases and checklists in stages so that even beginners in casting designs can understand. The purpose is to help people understand “why design is necessary” and to acquire the “designer’s perspective” that will help them communicate with the manufacturing site.

To properly design aluminum castings, the premise is that they are “material characteristics,” “differences in casting methods,” and “understanding basic terms.” This chapter will organize each one in a brief way.

Aluminum castings are parts that are formed by melting aluminum alloys and pouring them into a mold. It is lightweight and has excellent thermal conductivity and corrosion resistance, and is widely used in automobiles, industrial machinery, electronic equipment, etc.

Depending on the application, JIS materials are AC4C (Al-Si-Mg) and AC7A (Al-Mg) and are generally designed with surface treatment and machining assumptions.

The choice of casting method greatly affects design conditions. Here are some comparisons of the main casting methods:

Due to casting methods, wall thickness, gradient, cost and delivery time vary, so selection is essential during the design stage**.

Casting design requires an accurate understanding of the following terms:

All of these are directly related to casting quality, workability and cost, so it is the designer’s responsibility to clarify the design in the drawing instructions.

*Source: JIS H 5202:2010 Aluminum alloy casting

When designing aluminum castings, there are “5 principles to keep in mind” to prevent failure. These are “practical design guidelines” that take into account the physical constraints and material characteristics in the casting process, and are essential for both beginners and experienced people.

The thickness of a casting is an important factor that directly affects the cooling speed and strength. If the thickness is uneven, there will be differences in the cooling speed, and thick parts are more likely to develop “shrinkage cavities” due to delayed solidification. In addition, if the wall is too thin, the molten metal will not flow sufficiently, causing underfilling and defects.

The ideal countermeasure is to avoid extremely thick or thin walls and design to make the wall as uniform as possible. If strength is required locally, it is effective to use “ribs (reinforcement ribs)” instead of thickening the wall to avoid weight increase and poor cooling.

A draft is a slight inclination in the direction of the metal mold or sand mold to allow the product to be removed smoothly from the mold. If the draft is insufficient, the product may become stuck in the mold and become impossible to remove, or the mold itself may be damaged.

The angle of the draft varies depending on the casting method and product shape, but 1 to 3 degrees is a general guideline. If it is not specified at the design stage, it is left to the “on-site judgment” of the manufacturing site, so it is important to specify the drawing.

Castings generally have a rough surface called the “cast skin,” and machining such as cutting and polishing is required to finish the product. With this processing in mind, it is essential to provide a “machining allowance (finishing allowance)” on the surface in advance.

If the machining allowance is not set, there is a risk of dimensional insufficiency or areas that cannot be machined, and if recasting is required, costs and delivery times will increase significantly. On the other hand, excessive cutting allowances lead to material waste and increased processing time, so optimization is required for each part.

Aluminum alloys have the property of shrinking in volume during the solidification and cooling process. To reflect this in the design, it is necessary to consider the “shrinkage allowance” by making the mold or model larger than the actual size.

For example, it is common to set a shrinkage allowance of approximately 0.5 to 1.2% for aluminum alloys. It is recommended to determine the shrinkage coefficient for each material and casting method, referring to the JIS standard and past manufacturing results.

Although aluminum castings are generally referred to as such, there are many materials in the JIS standard that are used for different purposes. For example:

When designing, it is necessary to select the material according to the usage environment, required strength, and cost requirements, and adjust the wall thickness and rib design accordingly. In addition, depending on the material, the presence or absence of heat treatment and surface treatment conditions will change, so it is important to clearly state this on the drawing.

These five principles can be said to be the “common language with the site” in aluminum casting design. In the next chapter, we will take a closer look at actual melt flow design and casting methods (how to take the sprue).

*Source:Understand! Use! Introduction to Casting” Nikkan Kogyo Shimbun

In aluminum casting, the quality is greatly affected by how well the molten metal is poured into the mold. If the flow of the molten metal is disturbed or air or oxides are entrained, defects such as shrinkage cavities, bubbles, and rough surfaces will occur. Therefore, the design of the sprue and runner, and flow control within the mold = casting plan is extremely important.

A casting plan is a design of where and how the molten metal is poured. The typical methods and uses are as follows:

Aluminum alloys are prone to oxidation, so pouring quietly, quickly, and evenly is the key to selecting a method.

In casting design, the following three phenomena must be coordinated on a time axis.

To comprehensively design these, CAE solidification analysis and flow simulation are also being used. By utilizing the analysis results, it is possible to reduce the risk of defects even before prototyping.

Source: Light alloy casting and die casting production technology, Materials Processing Center
Source: Small and Medium Enterprise Agency, “Matters related to casting technology”

When designing aluminum castings, it is essential to understand the structure based on the “material properties x physical laws at the casting site”, rather than simply specifying the dimensions. Below are four typical mistakes beginners are likely to make and how to avoid them.

Mistake: Thickening the wall to increase strength, but delayed cooling causes shrinkage cavities (hollows), which has the opposite effect.
Countermeasure: Instead of increasing the wall thickness, increase rigidity with rib reinforcement.

Mistake: If the design is vertical without a gradient, damage or scoring occurs when removing from the mold.
Countermeasure: Specify a 1 to 2 degree draft angle on the drawing to avoid on-site judgment.

Error: If a model is made according to the dimensions in the drawing, due to cooling shrinkage will result in insufficient dimensions.
Countermeasures: Understand the shrinkage rate for each material and casting method and reflect it in the model design.

Error: As a result of selecting the general-purpose material AC2A, early deterioration occurs due to insufficient corrosion resistance.
Countermeasures: Depending on the usage environment and stress conditions, select an appropriate material such as AC4CH** to meet the performance requirements.

These failures occur due to the designer’s lack of structural understanding and on-site awareness. In the next chapter, we will delve deeper into practical perspectives through FAQs for questions that tend to arise during design.

In aluminum casting design, beginners and those who have experienced problems repeatedly ask similar questions. Below are 5 particularly common questions and their practical answers.

A: Less than 3 mm has the risk of not being filled, and more than 10 mm has the risk of shrinkage cavities.
The standard is 3.5 to 6 mm. Strength should be ensured not by wall thickness but by structural reinforcement such as ribs.

A: AC4CH has higher pressure resistance and toughness than AC4C.
AC4C is a general-purpose material, while AC4CH is ideal for applications requiring impact and sealing properties. The key to avoiding problems is to use them according to the application.

A: Yes. Design specifications must be adjusted to match the casting method.
Since draft angles, wall thicknesses, and dimensional tolerances vary, it is basic to optimize the design after the casting method is finalized.

A: It is calculated by comparing the prototype casting with the dimensions on the drawing.
Since there will be a difference from the theoretical value, the actual measured value obtained from the first prototype is the most reliable data.

A: There are five points to check: surface, dimensions, interior, material, and workability.
By having the designer present and checking together with the site, problems can be prevented when moving to mass production.

When designing aluminum castings, much of the quality and cost is determined at the time of drawing the drawing. If you do not have a good understanding of the physical phenomenon of casting, it is easy to cause defects such as shrinkage cavities and dimensional defects.

The main points of this article are as follows:

Especially when you are involved in casting design for the first time, it is important to understand that the completion of the drawing does not mean the end, but the starting point of communication.

Casting design begins with a dialogue with the site.
That first step is the key to reducing defects and improving quality.