Injection mold explained, with components

The two halves

Every mold splits into two halves that meet at the parting line. The cavity half forms the outer surface of the part and stays fixed to the injection side of the machine. The core half forms the inner surface and moves with the machine platen. Because the part shrinks onto the core as it cools, it stays on the moving half when the tool opens, which is exactly where the ejection system is waiting for it.

The plate stack

A standard two-plate mold base is a stack of ground steel plates, each with a defined job.

Feed system

Molten polymer travels from the machine nozzle to the cavity through a feed system, and each part of it leaves a signature on the finished component.

• Sprue bushing. The first channel, tapered so the solidified sprue pulls free when the tool opens.
• Runner. The distribution channel feeding one or more cavities. In a cold runner it solidifies with the part and is trimmed off; in a hot runner it stays molten and produces no scrap.
• Gate. The restricted entry into the cavity. Its position sets fill pattern, weld line location, and packing, and it leaves a vestige on the part.
• Cold slug well. A short blind extension that catches the cooled front of the melt so it does not enter the cavity.

Ejection

When the tool opens, the part is gripped on the core by shrinkage and has to be pushed off without damage.

• Ejector pins. The standard method. They leave small circular witness marks, so they belong on non-visible surfaces and against stiff features such as ribs and bosses.
• Sleeve ejectors. Push around a core pin, used to eject bosses without marking the face.
• Stripper plate. Pushes on the whole rim of the part, used for thin-walled and cylindrical parts where pins would deform the wall.
• Air poppets. Break the vacuum on large flat parts that would otherwise cling to the core.

Cooling

Cooling is usually the longest phase of the cycle, so the cooling circuit is an economic component as much as a technical one. Channels are drilled through the plates and inserts to hold a uniform tool surface temperature, because uneven cooling produces warp. Cores are harder to cool than cavities since heat has nowhere to escape, which is why bubblers, baffles, and conductive inserts exist.

Actions for undercuts

Geometry that would trap the part on the tool needs a component that moves out of the way before ejection.

• Slides. Cam-driven blocks that retract sideways as the tool opens, forming external undercuts such as side holes and snap windows.
• Lifters. Angled components that move inward as they rise, forming internal undercuts.
• Collapsible cores. Segmented cores that shrink inward to release internal threads.
• Unscrewing units. Rotate a threaded core out of the part.

Why this matters at quotation stage

Nearly every line in a tooling quotation maps to one of these components. Cavitation multiplies the cavity and core inserts. An undercut adds a slide or a lifter, plus the cam and wear plates that drive it. A hot runner replaces the cold runner and adds a controller. Reading a quote against the component list is the fastest way to understand what you are paying for and which design change would remove a cost.