An Introduction to the Mold Tool: The Heart of Injection Molding

By Ichiplas Tooling Team · Oct 2025 · Updated Nov 2026 · Reviewed by the Ichiplas engineering team

If the injection molding machine is the engine, the mold tool is the heart of the operation.

It's not just two blocks of steel; it's a piece of precision capital equipment, custom-engineered and built to produce your specific part hundreds of thousands, or even millions, of times.

Understanding the basic anatomy of a mold is essential for appreciating why design details matter so much and why tooling represents the most significant upfront investment in your project. A typical production mold is made up of several key systems working in harmony.

The Two Halves: The A-Side and B-Side

Every standard mold is comprised of two halves that meet at what's called the parting line.

The A-Side (Cavity Side): This is typically the “show” side of your part—the exterior cosmetic surface. It's mounted on the fixed half of the molding machine, and it's where the nozzle injects plastic into the sprue.

The B-Side (Core Side): This is where the magic happens. The B-side is mounted on the moving half of the machine and usually forms the interior, functional geometry of your part (think ribs, bosses, and snaps). As plastic cools, it shrinks and grips tightly onto the B-side core, which is why this half contains the ejection system.

The Ejection System: Getting the Part Out

Once the part has solidified and the mold opens, it needs to be removed from the B-side core. This is the job of the ejection system. The most common method uses a series of hardened steel ejector pins.

After the mold opens, these pins push forward, applying a uniform force to push the part off the core, allowing it to fall into a collection bin or be removed by a robotic arm. These pins will leave small, circular witness marks on your final part. As with gates, a key part of the design process is ensuring these ejector pin marks are located on non-critical, interior surfaces.

The Cooling System: The Key to Efficiency and Quality

Injecting 200°C plastic into a steel tool generates a tremendous amount of heat. Removing that heat efficiently is arguably one of the most important functions of a mold. If a tool runs too hot, parts can warp, and the cycle time (the time it takes to make one part) gets longer, driving up your unit cost.

To manage this, every production mold has a network of cooling channels drilled through the steel blocks of both the A and B sides. Water or a specialized thermal fluid is continuously circulated through these channels to maintain a consistent operating temperature, ensuring parts cool evenly and quickly.

Putting It All Together

These systems—the two halves, the ejection mechanism, and the cooling network—all work together in a rapid, precise cycle. The quality and expense of this tool are directly tied to the complexity of your part. Features that prevent the part from being easily ejected, which we call undercuts, often require additional complex mechanisms that add to the cost.

Mold Components and What They Do

Main components of an injection mold and their functions
Component	Where it sits	What it does
Cavity	A-side (fixed half)	Forms the outside, usually cosmetic, surface of the part
Core	B-side (moving half)	Forms the inside geometry: ribs, bosses, snaps
Locating ring and sprue bushing	A-side, facing the machine nozzle	Aligns the mold to the machine and takes the melt in
Runners and gates	Parting line or hot runner manifold	Deliver plastic from the sprue into each cavity
Ejector pins, sleeves, blades or stripper plate	B-side	Push the part off the core after the mold opens
Cooling channels	Both halves	Carry water to remove heat and control cycle time
Guide pins and bushings	Between the halves	Keep the halves aligned every cycle
Slides and lifters	Either half	Form undercuts, then move clear before ejection
Mold base	The whole frame	Standard steel plates, commonly P20, that hold everything together

Frequently Asked Questions

What are the A-side and B-side of a mold?

The A-side (cavity side) is mounted on the fixed platen of the machine and usually forms the cosmetic outside of the part. The B-side (core side) is mounted on the moving platen, forms the inside features, and contains the ejection system, because the part shrinks onto the core and stays with it when the mold opens.

How long does an injection mold last?

It depends on the steel, the resin and how the mold is maintained. The SPI mold classification is a useful guide: Class 101 molds are built for more than a million cycles, Class 102 for up to about a million, Class 103 for under 500,000, Class 104 for under 100,000 and Class 105 for prototype quantities. Pre-hardened steels such as P20 or S50C suit lower volumes; hardened steels such as H13 suit high volumes and abrasive glass-filled resins.

What is a mold base?

The mold base is the set of steel plates, guide pins and hardware that holds the cavity and core inserts, ejection system and cooling circuits. Mold bases are commonly made from a pre-hardened steel such as P20. Using standard plate sizes and components saves cost and makes spare parts easy to source.

Why does my part have small circles on the inside?

Those are ejector pin marks, left where the pins push the part off the core. They are normal, and a good mold design places them on non-cosmetic interior faces, sizes them to avoid stress whitening, and sets them flush so they leave only a faint witness mark.

Who owns the injection mold?

In most custom molding arrangements, the customer pays for the tooling and owns the mold, while the molder stores, maintains and runs it. Ownership, maintenance responsibility and the right to transfer the mold should always be confirmed in writing in the tooling quotation or supply agreement.

What's Next?

Now that you understand the basic anatomy of a mold, we can begin to discuss the specific design principles you must incorporate into your part to ensure it works flawlessly with the tool. In our next guide, we tackle the most fundamental rule of all: What is Draft Angle and Why is it Critical for Your Part?

Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.