Aluminum casting & mold design services

The big picture: two mold halves and three functional groups

A pressure die-casting mold consists of two main halves: the fixed half, attached to the shot-chamber side of the machine where molten aluminum enters, and the moving half, attached to the machine’s opening-and-closing mechanism, where the part is ejected. The two halves close together at the parting line, and the part’s shape is formed in the cavity between them.

To make it easier to remember, a mold’s components can be divided into three functional groups:

• Forming group: the cavity and cores, which determine the part’s shape.
• Metal-delivery group: the gating system, overflows, and air vents, which determine how aluminum enters and fills the mold.
• Support group: the cooling system, ejection system, guide pillars, and mold base, which keep the mold working stably across many cycles.

Aluminum pressure die-casting molds are typically used with cold-chamber machines, in which molten aluminum is poured into a separate shot chamber and then pushed into the mold by a piston. This detail explains why the “metal-receiving” side of the mold has a distinctive structure, as described in the next section. If you want to learn more about the pressure die-casting technology itself, see the article What Is Aluminum Die-Casting? Features, Advantages and Disadvantages Explained.

Following the metal: from the shot chamber to the cavity

The gating system: sprue, runner, and gate

Molten aluminum passes through three segments before touching the part. The first is the sprue (which carries metal from the shot chamber into the mold), followed by the runner (a channel that distributes metal to where it’s needed), and finally the gate, the point where metal enters the cavity.

The gate is the component with the greatest influence on fill quality. Its location, size, and direction determine where the metal stream hits, which path it takes, and in what order it fills the cavity. A poorly placed gate can cause uneven filling, air entrainment, or erosion of the mold surface at the impact point (see also the article Materials for Aluminum Molds: Key Points for Choosing the Optimal Aluminum Alloy).
Mold design principles, including gate placement, are covered further in the article Aluminum Mold Design: Key Points for High Precision and Long Life. The metal that solidifies in the gating system is later cut off the part and is typically remelted.

The cavity and cores: where the part takes shape

The cavity is the void that creates the part’s external shape. For parts with internal holes, pockets, or grooves, the mold uses additional cores to form these features. Cores can be fixed or movable.

When a part has features that prevent it from separating from the mold along the normal opening direction (for example a side hole or side groove), the mold needs a slide: a mechanism that pulls the core out along a different axis before the mold opens fully. Slides make it possible to form complex shapes, but also make the mold more complex, adding sliding surfaces subject to wear and requiring precise alignment.

Overflows and air vents

As molten aluminum fills the cavity, the air originally inside the cavity must have somewhere to escape. Otherwise the air gets compressed or entrained in the metal, creating gas porosity in the part. That’s why the mold has air vents, very thin slots that let air pass but don’t let liquid metal escape.

In addition, overflows are small pockets located at the end of the flow path. The metal at the front of the flow is usually cooler and carries more air than the metal behind it, so overflows catch this metal and keep it from remaining in the part. Overflows and vents are usually placed together, and whether their placement is well-founded significantly affects the part’s gas porosity (the causes and remedies of porosity issues are analyzed further in the article Causes and Solutions for Porosity, Cracks and Dimensional Deviation).

Systems running in parallel: cooling, ejection, and guidance

The cooling system

The mold absorbs a large amount of heat every time molten aluminum enters, and this heat must be removed so the part solidifies properly and the mold doesn’t overheat. That’s why the mold body contains cooling channels through which a coolant (usually water or oil) circulates, combined with a temperature controller.

The goal isn’t just “cooling fast” but balanced cooling: areas that accumulate more heat need more heat removed. Poorly balanced cooling can cause warping, localized shrinkage, and worsen thermal-fatigue cracking and soldering on the mold surface. This is why cooling-channel design is typically considered together with gate design.
If you’d like to explore the science behind cooling, see the article The Science of Cooling Rate in Aluminum Casting.

The ejection system

After the part solidifies and the mold opens, the part tends to stay on the moving half. Ejector pins mounted on an ejector plate move forward to push the part out of the mold, and the plate is then pulled back to its original position before the next cycle.

The number, position, and shape of ejector pins need to be considered so the pushing force is distributed evenly and doesn’t deform the part. Ejector pins usually leave small round marks on the part surface, which is a simple way to identify pin locations when looking at a casting. Ejector pins are also subject to repeated wear, so the clearance between pin and pin bore grows with cycle count.
Monitoring such wear-prone components is part of mold maintenance planning in general, covered in the article Aluminum Mold Maintenance: Tips for Extending Mold Life.

Guide pillars, bushings, and mold base

The two mold halves must close with high accuracy every cycle. Guide pillars and guide bushings keep the two halves aligned in the correct relative position when closing, while the mold base and support plates transmit clamping force and keep the whole mold assembly rigid under large pressure. If the two halves are misaligned, the parting line doesn’t seal evenly, metal can escape and form flash, and the part may deviate in dimensions.

One casting cycle: how the mold works

The mold repeats a continuous cycle throughout production. The main steps, in order, are:

Each step requires different parts of the mold to work well, and a small problem in one component repeats in every cycle. That’s why the structure and machining quality of each component affect the consistency of an entire production batch.

Reading the signs on the part to tell which component has a problem

Understanding the structure lets you “read backwards” from the part to the mold. The table below summarizes the relationship at a general level, as a starting point for discussion, not a definitive diagnosis for a specific case.

Using structural knowledge to talk with your mold maker

When reviewing the mold layout before the real mold is made, three structure-related questions usually reveal a lot: Where is the gate placed, and on what basis regarding the metal flow path? What principle are the vents and overflows laid out on, and have they been checked by simulation or by experience with similar parts? Which area of the part accumulates the most heat, and how do the cooling channels handle that area?
For the mold-making process and the factors affecting cost, see the article Aluminum Mold Manufacturing: A Complete Guide to Process, Techniques and Cost.