What Are the Four Stages of Injection Molding? A Technical Guide
Injection molding is a highly precise manufacturing process. Every cycle — from the moment molten plastic enters the mold to the ejection of a finished part — passes through four distinct and critical stages: filling, packing (holding), cooling, and ejection.
Further Reading
For neutral technical background, see injection molding background.
Understanding each stage is essential for engineers, mold designers, and buyers who want to optimize part quality, reduce cycle time, and prevent common defects. This guide breaks down each stage with process parameters, quality implications, and practical tips.
Overview: The Four Stages of Injection Molding
| Stage | What Happens | Key Parameter | Typical Duration |
|---|---|---|---|
| 1. Filling | Molten plastic injected into mold cavity | Injection speed & pressure | 0.5–5 seconds |
| 2. Packing (Holding) | Additional plastic packed to compensate shrinkage | Hold pressure & time | 2–15 seconds |
| 3. Cooling | Part solidifies in the mold under controlled temperature | Mold temperature & cooling time | 5–30 seconds |
| 4. Ejection | Ejector pins push the solidified part out of the mold | Ejection speed & force | 0.5–3 seconds |
Stage 1: Filling
What Happens During the Filling Stage?
The filling stage begins when the injection screw moves forward, pushing molten thermoplastic through the nozzle, sprue, runners, and gates into the mold cavity. The objective is to fill the cavity as completely and uniformly as possible — typically to 95–99% full — before transitioning to the packing stage.
Key Process Parameters
- Injection speed (mm/s or cm³/s): Controls the rate at which material enters the cavity. Higher speeds reduce fill time but can cause jetting, burning, or excessive shear stress.
- Injection pressure (MPa or bar): The force behind the screw. Typical values range from 70 to 140 MPa depending on material viscosity and part geometry.
- Melt temperature (°C): Must match the material’s processing window. Too cold = short shots; too hot = degradation or discoloration.
- Switchover point (V/P transfer): The position or pressure at which the machine transitions from velocity-controlled injection to pressure-controlled packing. This is a critical setting that directly affects part weight consistency.
Common Defects in the Filling Stage
- Short shots: Cavity not completely filled — caused by low pressure, too-low melt temperature, or blocked gates.
- Jetting: A snake-like surface defect caused by too-high injection speed through small gates.
- Weld lines: Visible seams where two flow fronts meet and fail to fully bond — a design or gating issue.
- Burn marks: Char or discoloration near the end of fill — caused by trapped air (inadequate venting) or excessive shear heat.
Stage 2: Packing (Holding)
What Happens During the Packing Stage?
After the cavity is filled, the machine switches to a lower, sustained hold pressure to compensate for the volumetric shrinkage that occurs as the plastic begins to cool. Additional material is continuously packed into the cavity until the gate freezes off — the point at which no more plastic can enter.
This stage is critical for dimensional accuracy, surface finish, and sink mark prevention.
Key Process Parameters
- Hold pressure (MPa): Typically 50–80% of peak injection pressure. Too low = sink marks and dimensional shrinkage; too high = overpacking, flash, or internal stress.
- Hold time (seconds): Must be long enough for the gate to freeze. Verified via a “gate freeze study” — increasing hold time until part weight plateaus.
- Pressure profile: Multi-stage hold profiles (decreasing pressure over time) can reduce residual stress in thick sections.
Common Defects in the Packing Stage
- Sink marks: Depressions on the surface over thick sections — caused by insufficient hold pressure or hold time.
- Flash: Thin plastic fins at the parting line — caused by excessive hold pressure exceeding the mold clamp force.
- Internal voids: Vacuum bubbles inside thick parts — caused by insufficient packing combined with rapid skin formation.
- Residual stress: Over-packing can freeze in stress that causes warping after ejection or during service.
Stage 3: Cooling
What Happens During the Cooling Stage?
Once the gate has frozen, the injection stage ends and the cooling stage begins. The part remains in the closed mold while heat is extracted through the mold’s cooling channels (water circuits). The part must cool sufficiently to be rigid enough to be ejected without distortion.
Cooling accounts for 50–80% of total cycle time, making it the single biggest lever for cycle time optimization.
Key Process Parameters
- Mold temperature (°C): Controlled by a mold temperature controller (MTC). Lower mold temperature accelerates cooling but can cause surface defects and brittleness in some materials.
- Cooling time (seconds): Determined by part wall thickness, material thermal conductivity, and required ejection temperature.
- Cooling channel layout: Channels should be positioned within 1.5–2× the channel diameter from the cavity wall. Conformal cooling (3D-printed channels) can dramatically improve uniformity.
- Coolant flow rate and temperature: Turbulent flow (Reynolds number > 10,000) is essential for efficient heat transfer.
Cooling Time Estimation Formula
A simplified formula for cooling time (t) estimation:
t = (s² / (π² × α)) × ln[(4/π) × ((Tmelt − Tmold) / (Teject − Tmold))]Where: s = wall thickness, α = thermal diffusivity of the plastic, Tmelt = melt temperature, Tmold = mold surface temperature, Teject = part ejection temperature.
Common Defects in the Cooling Stage
- Warping / distortion: Uneven cooling creates differential shrinkage, bending the part. Requires balanced cooling channel design.
- Sticking to the mold: Part is too hot at ejection — increase cooling time or lower mold temperature.
- Surface blemishes: Uneven gloss or cloudiness caused by temperature variations across the mold face.
Stage 4: Ejection
What Happens During the Ejection Stage?
Once the part has sufficiently cooled, the mold opens and ejector pins, sleeves, or stripper plates push the part off the core side of the mold. Simultaneously, the injection screw begins rotating to plasticize and meter the next shot of material (screw recovery) — this runs in parallel with ejection and cooling to minimize cycle time.
Key Process Parameters
- Ejection force and speed: Must be sufficient to overcome the adhesion between part and core, but not so aggressive as to deform or crack the part.
- Ejector pin layout: Pins must be positioned to apply force on stiff, thick sections — never on fragile or cosmetic surfaces.
- Draft angle: A minimum of 1–3° draft on all vertical walls is essential for smooth ejection. Insufficient draft is the leading cause of ejection damage.
- Mold release agents: Used sparingly for complex geometries or high-friction materials (like certain grades of TPE or glass-filled Nylon).
Common Defects in the Ejection Stage
- Ejector pin marks: Circular witness marks on the B-side of the part — unavoidable but can be minimized by enlarging pin diameter to reduce pressure.
- Part distortion / cracking: Caused by insufficient draft, undercuts, or ejecting the part too early (before adequate cooling).
- Sticking / galling: Part adheres to the core — address with polished steel, proper draft, or mold release.
How the Four Stages Connect: The Complete Injection Molding Cycle
In a typical production cycle, the four stages are orchestrated as follows:
- Mold closes → clamp force applied
- Filling: Screw advances; molten plastic fills cavity at high speed
- V/P switchover: Machine transitions to hold pressure
- Packing: Hold pressure maintained until gate freeze-off
- Cooling: Part solidifies; screw begins recovering next shot
- Ejection: Mold opens; ejector system activates; part falls or is removed by robot
- Cycle repeats
The total cycle time is the sum of fill + pack + cool + eject + mold open/close times. In high-volume production, optimizing each stage can save fractions of a second — which at millions of parts per year translates to significant cost savings.
Frequently Asked Questions
What is the most important stage of injection molding?
All four stages are interdependent, but the cooling stage is most critical for cycle time (it takes the longest) and the packing stage is most critical for dimensional accuracy. Most part quality issues trace back to improperly set packing pressure or cooling time.
What is the V/P switchover in injection molding?
V/P (Velocity-to-Pressure) switchover is the transition point between the filling and packing stages. It is triggered by a set position, pressure, or time threshold. Setting it too early causes short shots; too late causes overpacking and flash.
How do the four stages affect cycle time?
Cooling time dominates total cycle time (often 50–80%). Reducing wall thickness by 0.5 mm or optimizing cooling channel layout can reduce cooling time by 20–40%, significantly increasing output per machine hour.
What is gate freeze-off?
Gate freeze-off is the point at which the plastic at the gate solidifies and prevents any further material from entering the cavity. After freeze-off, hold pressure no longer has an effect — extending hold time beyond this point wastes energy without improving part quality.
What causes warping in injection molded parts?
Warping is primarily caused by uneven cooling (leading to differential shrinkage), over-packing (residual stress), or asymmetric wall thickness in the design. It is addressed through balanced cooling channel design, part redesign, and optimized packing profiles.
How do draft angles affect the ejection stage?
Draft angles of 1–3° on vertical walls allow the part to slide off the core without friction damage. Without sufficient draft, the part grips the steel, causing ejection marks, tearing, or stuck parts — a leading cause of production downtime.
Summary
The four stages of injection molding — filling, packing, cooling, and ejection — each play a distinct and irreplaceable role in determining part quality, cycle time, and manufacturing efficiency.
For product developers and procurement engineers, understanding these stages allows you to better communicate with your mold maker, interpret defect reports, and make informed decisions about tooling design and process parameters. For quality engineers, each stage maps directly to a set of controllable variables that can be systematically optimized using scientific injection molding (SIM) principles.
Whether you are designing your first plastic part or auditing an existing production line, mastering the four stages is foundational to injection molding success.