Injection Mold Design Guide: What to Get Right Before You Cut Steel

Designing an injection mold is not complicated. But it is precise. Every part that comes out of an injection mold is determined by decisions made before the first piece of steel is cut. If the mold is designed wrong, the parts will be wrong. If the mold is designed right, the parts will come out consistent, cycle after cycle. This guide covers the basics of injection mold design. Whether you are working with an injection mold China supplier or designing in-house, the same rules apply.

The Basics of a Mold

Every mold has two halves

The mold splits into two sides. The stationary half stays attached to the injection molding machine. The moving half opens and closes. An injection mold China manufacturer designs the parting line, which determines where the two halves meet. The parting line affects how the part looks and how the mold works.

The cavity shapes the outside of the part

The cavity is the empty space in the mold. Injection mold China suppliers machine the cavity to the shape of the part. The plastic is injected into the cavity and takes the shape of the part.

The core shapes the inside of the part

The core is the insert that creates the interior shape of the part. Injection mold China manufacturers use the core to form internal features.

The ejection system pushes the part out

After the part cools, it needs to come out. Injection mold China suppliers design ejection pins and plates that push the part out without damaging it.

Hot or Cold Runner

The runner delivers plastic from the nozzle to the part

The runner is the channel that plastic flows through before it reaches the part. An injection mold China manufacturer designs the runner to deliver the right amount of plastic to each cavity.

Cold runner molds leave a sprue to be trimmed

Cold runner molds are simpler and cheaper to build. Injection mold China suppliers use cold runner molds for low to medium volume production. The leftover runner is scrap.

Hot runner molds eliminate waste

Hot runner molds keep the plastic molten in the runner. Injection mold China manufacturers design hot runner systems for high-volume parts. There is no waste, and the cycle time is faster.

Gate Placement Affects Part Quality

The gate is where plastic enters the cavity

The gate location affects the appearance and strength of the part. An injection mold China supplier will tell you that the gate should be placed in the thickest section of the part.

The gate leaves a mark on the part

No matter what, the gate will leave a mark. Injection mold China manufacturers design the gate location to minimize visibility.

Some gates are automatically trimmed

Hot runner molds can have gates that break cleanly when the part is ejected. Injection mold China suppliers offer valve gates and other automatic gate designs.

Common specs for injection mold design:

• Mold base: standard size
• Cavity material: hardened steel or pre-hardened
• Core material: hardened steel
• Runner type: cold or hot
• Gate type: edge, submarine, or valve gate
• Ejection type: pins or plate
• Cooling: water lines

Draft Angles and Undercuts

Draft angles help the part release from the mold

If the sides are straight, the part will stick. Injection mold China manufacturers add draft angles to the cavity and core walls so the part slides out easily. A minimum of one to two degrees of draft is recommended per side.

Undercuts require slides or lifters

An undercut is a feature that locks the part in the mold. Injection mold China suppliers use side actions, slides, or lifters to release undercuts. Slides are expensive, so avoid them if possible.

The ejector system pushes the part clear

Pins push the part out of the mold. Injection mold China manufacturers design the pin placement to avoid damaging the part.

Cooling and Thermal Control

Water lines regulate the mold temperature

Plastic cools as it sits in the mold. Injection mold China suppliers design water lines to remove heat evenly from the part.

Uneven cooling causes warpage

If one area cools faster than another, the part warps. Injection mold China manufacturers balance the cooling lines to prevent warpage.

The cycle time depends on cooling

The faster the mold cools, the faster the cycle. Injection mold China suppliers design cooling systems to minimize cycle time.

A well-designed injection mold produces consistent parts, cycle after cycle. The parting line needs to be in the right place. The gate needs to be located correctly. The cooling needs to be balanced. Every decision affects the quality of the part. If you are working with an injection mold China supplier, make sure they understand these basics. The design phase is where the success of the mold is decided. Get the design right, and the mold works. Get it wrong, and nothing works.