Injection Molding Basics

Injection molding is a manufacturing process for producing parts by injecting molten material into a mold. It is the most common method for mass-producing plastic parts with high precision and repeatability.

The Injection Molding Cycle

The complete injection molding cycle consists of four main stages:

1. Clamping

The mold closes and is clamped under high pressure. The clamping force must be sufficient to keep the mold closed against the injection pressure. Typical clamping forces range from 20 to 5,000 tons depending on part size.

2. Injection

Plastic pellets are fed through a hopper into the barrel, where they are heated and melted by heater bands and screw rotation. The molten plastic is then injected into the mold cavity under high pressure (500–2,500 bar).

3. Cooling & Packing

After injection, additional packing pressure is applied to compensate for material shrinkage as the plastic cools. The cooling stage typically accounts for 50–70% of the total cycle time.

4. Ejection

Once the part has solidified enough, the mold opens and ejector pins push the part out. The mold closes again to begin the next cycle.

Key Terminology

Term	Definition
Shot	One complete injection cycle
Runner	Channel that carries molten plastic from nozzle to cavity
Gate	Entry point where plastic enters the cavity
Draft angle	Taper on vertical walls to facilitate ejection
Weld line	Where two flow fronts meet; a potential weak point
Sprue	Main channel from nozzle to runner system

Equipment

An injection molding machine consists of two main units: the injection unit (plasticizing, melting, and injecting) and the clamping unit (holding the mold closed and ejecting parts).