Understanding the Injection Molding Machine Process, Step by Step

The injection molding machine process consists of four core stages — clamping, injection, cooling, and ejection — that together typically complete in as little as 10 to 60 seconds per cycle, depending on part size and material. Understanding each stage in detail helps manufacturers troubleshoot defects, optimize cycle times, and improve part quality.

Step 1: Clamping the Mould

Before any material is injected, the machine's clamping unit closes and secures the two halves of the mould together. This step must generate enough force to keep the mould shut against the pressure of molten plastic being injected inside — otherwise the mould will "flash," allowing material to leak out along the parting line.

Clamping force typically ranges from 5 to 2,000+ tons depending on machine size, and is calculated based on the projected area of the part multiplied by the injection pressure. Undersized clamping force is one of the most common causes of flash defects in production.

Step 2: Plasticizing and Injecting the Material

Plasticizing (Melting the Resin)

Plastic pellets are fed from a hopper into a heated barrel, where a rotating screw both conveys and melts the material through a combination of heater bands and mechanical shear. Barrel temperatures vary by resin type, typically ranging from 180°C for polyethylene up to 400°C for high-performance engineering plastics like PEEK.

Injection

Once enough molten material has accumulated at the front of the screw, the screw moves forward like a plunger, forcing the melt through a nozzle, into the sprue, and through the runner system into the mould cavity. This phase happens very quickly — often in under 2 seconds for smaller parts — to prevent the material from cooling and solidifying prematurely.

Step 3: Holding Pressure

Immediately after injection, the machine switches to a lower, sustained "holding" pressure. This phase packs additional material into the cavity to compensate for shrinkage as the plastic begins to cool, ensuring the part maintains accurate dimensions and avoids sink marks.

Holding pressure is typically 30–70% of peak injection pressure and is maintained until the gate — the small channel connecting the runner to the part — solidifies and seals off further material flow.

Step 4: Cooling

The part remains in the closed mould while cooling channels — typically water-based — circulate coolant through the mould to draw heat out of the plastic. Cooling is the longest phase of the cycle, often accounting for 50–70% of total cycle time.

Cooling time depends on wall thickness, material thermal conductivity, and mould temperature. As a general rule, cooling time increases with the square of wall thickness — meaning a part twice as thick can take roughly four times as long to cool sufficiently for ejection.

Step 5: Mould Opening and Part Ejection

Once the part has cooled enough to hold its shape, the clamping unit opens the mould, and ejector pins push the finished part out of the cavity. For parts with undercuts or complex geometry, additional mechanisms like slides or lifters may be needed to release the part without damage.

Ejector pin placement is a common source of visible defects, such as witness marks or localized stress whitening, if pins are positioned on cosmetic surfaces or applied with excessive force.

Typical Cycle Time Breakdown

Key Parameters That Affect Process Quality

• Melt temperature: too low causes short shots; too high causes degradation or burning
• Injection speed: affects surface finish, weld line strength, and the risk of jetting defects
• Mould temperature: influences cooling rate, surface gloss, and warpage
• Holding pressure and time: controls shrinkage, sink marks, and dimensional accuracy

Roughly 70–80% of common injection molding defects can be traced back to incorrect process parameters rather than mould design issues, which is why process engineers often start troubleshooting by reviewing machine settings before considering mould modifications.

Final Takeaway

The injection molding process follows a consistent, repeatable sequence — clamp, inject, hold, cool, and eject — but small variations in timing, temperature, and pressure at each stage can significantly impact part quality and cycle efficiency. Understanding how each phase contributes to the final result gives manufacturers a stronger foundation for diagnosing defects, reducing cycle time, and improving overall production consistency.