Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Plastic injection molding is a widely used manufacturing process for producing high-volume, precise parts from thermoplastics, such as toys, automotive components, and medical devices. It involves melting plastic material and injecting it into a custom mold under high pressure, where it cools and solidifies into the desired shape. This method is efficient for mass production due to its speed and repeatability.

How Does It Work? A Step-by-Step Breakdown

The process typically occurs in a specialized machine called an injection molding machine, which includes a hopper, barrel with a helical screw, injection unit, clamping unit, and the mold itself. Here's how it unfolds:

1.Material Feeding: Plastic pellets (usually thermoplastics like ABS or polypropylene) are loaded into a hopper at the top of the machine. Gravity feeds them into a heated barrel. This step ensures a continuous supply of raw material.

2.Melting and Mixing: Inside the barrel, the pellets are heated (typically to 200–300°C, depending on the plastic type) and melted by friction and external heaters. A rotating helical screw mixes the molten plastic thoroughly, pushing it forward toward the nozzle while building pressure. This creates a uniform, viscous melt ready for injection.

3.Injection: The screw (or plunger in some machines) forces the molten plastic through a nozzle into the closed mold cavity at high pressure (up to 20,000 psi). The mold is a two-part steel or aluminum tool clamped shut by the machine's hydraulic or electric clamping unit. The plastic fills the cavity completely, taking on its precise shape, including fine details like threads or textures.

4.Packing and Holding: After injection, additional pressure (called "packing") is applied briefly to compensate for shrinkage as the plastic cools. The mold remains closed under clamping force to prevent defects. This holding phase ensures the part is fully dense and dimensionally stable.

5.Cooling: The mold is cooled using channels with circulating water or air, solidifying the plastic in seconds to minutes (cycle time is often 15–60 seconds total). The part must reach a temperature below its glass transition point to hold its shape without warping.

6.Ejection: Once cooled, the clamping unit opens the mold. Pins or air blasts eject the finished part from the cavity. The part is then trimmed of any excess material (like gates or runners) and inspected. Scrap from runners can often be reground and reused.