Essential Spuitgieten[^1] Process Knowledge Every Newcomer Should Know**

The injection molding process for plastic parts consists of four core stages: filling, packing (holding pressure), cooling, and ejection. These stages are tightly interlinked and collectively determine the final quality of the molded part. Importantly, they form a single, continuous cycle—each phase influencing the next.

1. Filling Stage

The filling stage is the first step in the injection cycle, beginning when the mold closes and injection starts, and ending when approximately 95% of the cavity is filled with molten plastic.

While theoretically shorter filling times improve productivity, real-world constraints—including material properties, part geometry, and machine capabilities—limit how fast injection can be performed.

High-Speed Filling

At high injection speeds, shear rates increase significantly. Due to shear-thinning behavior (a non-Newtonian property of most thermoplastics), viscosity drops, reducing overall flow resistance. Additionally, localized viscous heating thins the solidified skin layer near the mold wall. In thin-walled sections, cooling effects are relatively weak, so the benefits of high speed dominate.

Low-Speed Filling

In contrast, low-speed filling results in lower shear rates, higher local viscosity, and greater flow resistance. Heat transfer to the cold mold walls becomes dominant, rapidly solidifying the melt surface. With minimal viscous heating, a thicker frozen layer forms—further increasing resistance, especially in thinner regions.

Weld Lines (Knit Lines)

Due to fountain flow, polymer chains at the advancing melt front align parallel to the flow direction. When two melt fronts meet (e.g., around inserts or from multiple gates), their aligned chains cannot fully entangle—especially if the meeting zones differ in temperature, pressure, or residence time. This creates microstructural weakness, visible as a weld line under proper lighting.

Weld line strength depends heavily on temperature:

• High-temperature welds: Better chain mobility allows deeper interdiffusion and stronger bonding.
• Low-temperature welds: Poor chain entanglement leads to weak, brittle joints—prone to cracking under stress.

2. Packing (Holding Pressure) Stage

After cavity filling, the packing stage applies sustained pressure to compact the melt, compensate for volumetric shrinkage, and increase part density.

Key characteristics:

• The cavity is already full; back pressure is high.
• Screw movement is minimal and slow—flow is negligible compared to filling.
• As the melt cools and solidifies rapidly, viscosity rises sharply, making cavity resistance very high.
• Packing continues until the gate freezes shut—the point at which the highest cavity pressure is reached.

During packing:

• Plastic exhibits slight compressibility under high pressure → denser regions form where pressure is highest.
• The solidifying melt acts as a pressure-transmitting medium, generating mold-opening force (also called bulging force).
• Adequate clamp force is essential to counteract this force and prevent flash, part deformation, or even mold separation.

💡 Practical Tip: Always select an injection molding machine with sufficient clamping force—not just to hold the mold closed, but to enable effective packing without compromising part integrity.

3. Cooling Stage

Cooling is arguably the most critical—and time-consuming—phase, accounting for 70–80% of the total cycle time. Efficient cooling directly impacts productivity, cost, and part quality.

Why Cooling Matters

• Parts must solidify sufficiently to retain shape upon ejection.
• Inadequate or uneven cooling causes warpage, sink marks, internal stresses, and dimensional instability.

Heat Removal Pathway

• ~95% of heat is conducted from the melt into the mold.
• From the mold, heat transfers via conduction through the mold steel to cooling channels, then removed by coolant via convection.
• Only ~5% dissipates directly to ambient air (via radiation/convection).

Factors Affecting Cooling Rate

Cooling System Design Principles

• Ensure uniform, rapid cooling across the entire part.
• Use standard-sized cooling holes for ease of machining and maintenance.
• Optimize channel layout based on part geometry: wall thickness, volume, and critical features.
• Balance flow rate, pressure drop, and thermal performance.

⚠️ Poor cooling design = longer cycles, higher costs, and defective parts. Invest time upfront—it pays dividends in production efficiency.

4. Ejection Stage

Ejection is the final step—but far from trivial. Even after full solidification, improper ejection can cause distortion, surface marks, or breakage.

Common Ejection Methods

1. Ejector Pins (Pushpins)

   • Best for parts with robust structure and non-cosmetic surfaces.
   • Pins should be placed evenly, preferably at high-resistance areas (e.g., ribs, bosses) and where part stiffness is greatest.
   • Avoid placing pins on thin or delicate features.

2. Stripper Plate (Ejection Plate)

   • Ideal for deep-draw, thin-walled containers or transparent parts where pin marks are unacceptable.
   • Provides large, uniform ejection force with smooth motion and no visible marks.

Design tip: Always simulate ejection forces during mold design. Consider draft angles, undercuts, and part flexibility to minimize sticking and ensure clean release.

Final Thoughts

Mastering the four-stage injection molding cycle—filling, packing, cooling, and ejection—is foundational for anyone entering plastics manufacturing. Each stage involves complex physics (rheology, heat transfer, mechanics), and small changes in parameters can have outsized effects on quality and efficiency.

For newcomers: Start by observing real cycles, analyzing part defects (e.g., weld lines, sink marks, warpage), and correlating them back to process settings. Over time, you’ll develop intuition—and that’s when true process mastery begins.