Introduction
If you’ve ever held a plastic bottle cap, a car dashboard component, or a medical syringe, you’ve held the result of screw injection moulding. This process shapes more plastic parts than any other manufacturing method on earth.
But what actually happens inside those massive machines? How does a pile of plastic pellets become a precision component—often in under a minute?
Screw injection moulding combines heat, pressure, and precise mechanical action to melt plastic and force it into a mold cavity. The rotating screw does double duty: it melts the material and then acts as a plunger to inject it. The result is a process that delivers speed, consistency, and the ability to create incredibly complex shapes.
At Yigu Technology, we run these machines daily, producing custom parts for industries ranging from medical devices to automotive. In this guide, we’ll walk you through each stage of the process, explain what can go wrong, and show you how to get the best results.
What Happens Inside the Machine?
The Three Main Stages
Every cycle follows the same rhythm. Understanding these stages helps you make better decisions about design, material selection, and quality control.
Stage 1: Plasticization
This is where solid plastic becomes liquid. Plastic pellets—called resin—pour from a hopper into a heated barrel. Inside, a rotating screw pushes them forward. The screw doesn’t just move material. It also generates heat through shear friction. External heating bands add more heat. Together, they melt the plastic into a uniform, viscous fluid.
For example, when processing polypropylene (PP), barrel temperatures typically range from 200°C to 270°C. Set it too low, and you get unmelted chunks that ruin the part. Set it too high, and the plastic degrades—turning yellow, losing strength, or releasing fumes.
Screw speed also matters. Most standard plastics run well at 100 to 150 RPM. Faster speeds melt material quicker but can overheat heat-sensitive plastics like PVC or certain nylons.
Stage 2: Injection
Once the plastic is fully melted, the screw stops rotating. Now it acts like a plunger. It moves forward with force, pushing the molten plastic through a nozzle and into the closed mold cavity.
Injection pressure typically ranges from 50 to 200 MPa (megapascals). High pressure is essential for filling thin walls or intricate details. Without enough pressure, the plastic cools before reaching the end of the cavity—a defect called short shot.
Injection speed—measured in millimeters per second—also matters. Fast speeds fill complex molds quickly. For small electronic components, speeds of 50 to 100 mm/s are common. But too fast, and air gets trapped, creating bubbles.
Stage 3: Cooling and Ejection
After the cavity fills, the machine holds pressure for a moment—this is called packing. It compensates for shrinkage as the plastic cools. Then cooling begins.
Cooling takes the longest. In fact, it accounts for 70 to 80% of the total cycle time. For a simple polyethylene part, cooling might take 10 to 20 seconds. For thick-walled parts, it can stretch to several minutes.
The mold itself has cooling channels running through it. Water or oil circulates to pull heat away. Mold temperatures are usually held between 30°C and 80°C, depending on the material.
Once the part is solid, the mold opens. Ejector pins push the part out. The cycle then repeats—sometimes every 15 to 60 seconds, thousands of times a day.
What Can Go Wrong?
Even with well-tuned machines, defects happen. Knowing what causes them helps you prevent them.
Common Quality Defects
| Defect | What It Looks Like | Common Causes |
|---|---|---|
| Flash | Thin excess material along parting lines | Injection pressure too high; mold not clamping fully |
| Sink Marks | Small depressions on thick sections | Insufficient packing; uneven cooling |
| Short Shots | Part doesn’t fully fill | Low injection pressure; cold material; blocked nozzle |
| Bubbles | Voids inside the part | Air entrapment; moisture in pellets; improper venting |
| Warpage | Part twists or bends after ejection | Uneven cooling; internal stresses |
Real-world example: A Yigu Technology client producing medical device housings encountered sink marks on thick wall sections. We traced the issue to cooling channel placement. The original mold design had channels only on one side. By adding conformal cooling channels—which follow the part shape—we reduced sink marks by 90% and cut cycle time by 15 seconds.
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