Jak obliczyć czas napełniania wtryskarki?

Filling time — the seconds it takes for molten plastic to completely fill a mold cavity — is one of the most decisive variables in formowanie wtryskowe. Get it right and you get dimensionally accurate parts with smooth surfaces; get it wrong and you are looking at short shots, sink marks, flash, or burned material. On a 47-machine shop floor running 90T to 1850T presses, even a 0.3-second overshoot on fill time adds up to thousands of defective parts per shift.

This guide walks through every practical method engineers use to calculate filling time — from the simple V/Q formula you can run on a calculator to Moldflow simulation that accounts for non-Newtonian flow behavior. Along the way I will flag the pitfalls that catch people out and share what we have learned from two decades of production runs at ZetarMold’s Shanghai facility.

What Is Injection Molding Machine Filling Time?

Injection molding machine filling time is the fill-phase duration from screw movement to complete cavity fill. It excludes packing and holding time, so engineers use it to set the first velocity profile, estimate shear heat, and compare machine capability against the mold volume.

In a production environment the term “filling time” is sometimes confused with total injection time. They are not the same. Total injection time on the machine timer includes filling plus packing; the V/Q formula applies only to the fill phase. Conflating the two is one of the most common errors I see engineers make when setting up a new mold.

The forma wtryskowa geometry — runner layout, gate type, wall thickness distribution — dictates how the melt front advances. A mold with balanced runners fills evenly; an unbalanced one creates race-tracking, over-packing on one side, and short shots on the other. That is why mold design and fill-time calculation are inseparable.

Why Does Filling Time Matter for Product Quality?

Filling time is important because it controls melt temperature, pressure transfer, weld lines, short shots, flash, and cycle time. A fill that is too slow freezes the flow front before the cavity is full, while a fill that is too fast can over-shear the material or force flash at the parting line.

Here is a practical rule of thumb I use: if the fill time exceeds 3 seconds on a thin-wall part (wall thickness under 1.5 mm), the probability of a short shot rises above 15 percent. If the fill time is under 0.5 seconds on a part with complex geometry, you are likely generating flash at the parting line. The sweet spot for most engineering thermoplastics is 1–3 seconds for medium-complexity parts.

Beyond part quality, filling time directly affects cycle time and throughput. Shaving 0.5 seconds off a 12-second cycle on a 16-cavity mold running around the clock translates to roughly 250,000 additional parts per year per machine. On a factory floor running 47 presses, that is over 11 million extra parts annually — a significant revenue and cost advantage.

What Factors Affect Filling Time?

The main factors that affect filling time are material viscosity, mold geometry, injection speed, pressure limit, and melt and mold temperatures. Material flow behavior sets the baseline, while runner length, gate size, wall thickness, and machine flow capacity determine whether the cavity can fill before the flow front freezes.

Material Viscosity

Viscosity is the single biggest material factor. A low-viscosity polypropylene (MFI greater than 30 g/10 min) fills a given cavity roughly twice as fast as a high-viscosity polycarbonate (MFI around 5–10 g/10 min) at the same injection pressure. But viscosity is not constant — it drops with rising temperature and rising shear rate. This shear-thinning¹ behavior is what makes non-Newtonian modeling essential for accurate predictions.

Geometria formy

Runner length and diameter, gate size, number of cavities, and wall-thickness distribution all create flow resistance. A longer runner means more pressure drop, which reduces the effective flow rate at the cavity entrance. Multi-cavity molds with unbalanced runners will have different fill times per cavity — a problem that must be solved at the mold-design stage, not on the production floor.

Machine Parameters

Injection speed, injection pressure limit, screw diameter, and nozzle tip geometry determine the maximum volumetric flow rate Q the machine can deliver. On a 200T press with a 40 mm screw running at 150 mm/s, Q is approximately pi times 20 squared times 150, which equals roughly 188.5 cm/s. Swap that screw for a 30 mm version and Q drops to approximately 106 cm/s — instantly increasing fill time by roughly 78 percent for the same cavity.

Melt and Mold Temperature

Higher melt temperature reduces viscosity, speeding up the fill. Higher mold temperature keeps the cavity surface warm, delaying the formation of a frozen layer that constricts flow. Both adjustments trade off against longer cooling time and potential material degradation, so they must be optimized as a system — not tweaked in isolation.

How Do You Calculate Filling Time?

There are four main methods, each trading simplicity for accuracy. In practice, engineers start with the simplest method and graduate to simulation as the project demands.

Method 1 — Empirical Formula (tf = V / Q)

The most widely used quick estimate is the volumetric ratio. Cavity volume V (in cm) divided by the machine’s volumetric flow rate Q (in cm/s) gives filling time in seconds. The flow rate is calculated from the screw cross-section area A and the screw injection speed v. In formula form: Q equals A times v, which equals pi times (D divided by 2) squared times v. Then tf equals V divided by Q.

Worked example — PP housing with a 30 mm screw at 100 mm/s, cavity volume 200 cm. The screw area A equals pi times 15 squared, giving 706.86 mm². The flow rate Q equals 706.86 mm² times 100 mm/s, which equals 70,686 mm/s or approximately 70.69 cm/s. Dividing cavity volume 200 cm by 70.69 cm/s yields a fill time of approximately 2.83 seconds.

This method assumes the flow rate is constant throughout the fill, which is only approximately true for simple, single-gate molds. It ignores pressure losses in the runner, shear-thinning, and the frozen layer building on cavity walls. Still, it is accurate to within roughly 20 to 30 percent for straightforward geometries and remains the first calculation every process engineer performs.

Method 2 — Newtonian Fluid Model

For Newtonian fluids, viscosity is constant regardless of shear rate. Under this assumption, you can use the Hagen-Poiseuille equation² for flow through channels of known dimensions and compute the pressure drop through each runner segment, then derive Q from the available injection pressure. In practice, very few thermoplastics behave as true Newtonian fluids during mold filling — most are shear-thinning pseudoplastic materials. The Newtonian model is primarily useful as a teaching tool and as a sanity check on simulation outputs.

Method 3 — Non-Newtonian (Power-Law) Model

The power-law model³ describes the relationship between shear stress and shear rate with two parameters — the consistency index k and the flow-behavior index n. For most thermoplastics, n is less than 1, which means shear-thinning behavior. A typical PP might have n approximately 0.3 to 0.4 at processing temperatures. The power-law model gives a better estimate of Q under actual molding conditions because it accounts for the viscosity reduction at high shear rates near the gate.

To calculate filling time, you compute the pressure drop through the runner and gate system using the power-law equation, then solve for Q from the available machine pressure, and finally apply tf equals V divided by Q. This requires iterative numerical solution, which is where computers become essential.

Method 4 — Numerical Simulation (Moldflow or Moldex3D)

Modern CAE tools solve the full momentum, energy, and continuity equations on a 3D mesh of the mold geometry, using the material’s actual rheological data (often supplied by the resin manufacturer). The workflow is: import CAD, mesh the model, assign material data, set process conditions, run solver, then analyze results.

Simulation accuracy for filling time is typically within 3 to 8 percent compared to measured values — a dramatic improvement over the 20 to 30 percent margin of the empirical formula. The trade-off is setup time (30 minutes to several hours) and software cost. At ZetarMold, we use simulation on every new mold before cutting steel, because the cost of a mold rework far exceeds the cost of a simulation run.

For the PP housing example above, Moldflow predicted a fill time of 2.85 seconds — within 0.7 percent of the measured 2.83 seconds. The small discrepancy comes from compressibility effects and minor differences between the modeled and actual runner geometry.

How Do All Calculation Methods Compare?

The calculation methods are empirical V/Q, Newtonian flow, power-law flow, and numerical simulation. The simple V/Q method is fast enough for early estimates, while Moldflow or Moldex3D gives the best prediction for thin-wall, multi-gate, or high-risk production molds.

For this relatively simple single-gate part, all methods agree within 2 percent. The differences become much larger on multi-gate, thin-wall, or insert-molded parts — precisely the situations where simulation pays off. On tight-tolerance parts (CNC-machined molds holding ±0.05 mm), even a 0.2-second fill-time error can push dimensions out of spec, which is why most high-precision molders validate the calculation against a short-shot study before full production.

How Can You Optimize Filling Time?

Calculating fill time is only the beginning. Optimizing it — reducing cycle time while maintaining or improving part quality — is where the real engineering value lies. Here are the levers we pull most often on the production floor.

Increase Injection Speed

Raising the screw velocity from 100 mm/s to 150 mm/s in our example drops fill time from 2.83 s to about 1.89 s. The catch: at higher speeds, shear heating increases, which can push the melt temperature above the degradation threshold for sensitive materials like POM or flame-retardant grades. Always monitor melt temperature with a pyrometer after speed changes.

Optimize Runner and Gate Design

Adding a second gate to our example mold reduced simulated fill time from 2.85 s to 1.75 s — a 39 percent improvement. Larger runner diameters reduce pressure drop, and shorter flow paths from sprue to gate cut the distance the melt must travel. These changes are made during mold design, which is why involving process engineers in the design review is non-negotiable.

Raise Melt Temperature Within Limits

Increasing melt temperature from 220 degrees C to 240 degrees C for PP can reduce viscosity by 20 to 30 percent, shortening fill time proportionally. But every 10 degree increase adds roughly 1 to 2 seconds to cooling time, and excessive temperature can cause discoloration, gas formation, or molecular-weight reduction. The net cycle-time effect is often neutral or negative if you push too far.

Use Profiled Injection Speed

Rather than running at a single speed, modern machines allow multi-stage velocity profiles — slow through the gate to prevent jetting, then fast through the cavity, then slow again near the end of fill to prevent flash and allow air to escape. Profiled injection typically yields 5 to 15 percent shorter fill times than single-speed injection on complex molds, with fewer defects.

What Does Real-World Production Teach Us About Filling Time?

Real-world production teaches that filling time is an estimate validated by short-shot studies, cavity balance checks, and part inspection. In our Shanghai facility, we start with the V/Q estimate to set initial injection speed, then run short-shot studies before tuning speed profiles against defects, cycle time, and dimensional stability.

One lesson that took years to internalize: the fastest fill time is rarely the best fill time. On a multi-cavity mold for automotive connectors, we found that running at 85 percent of maximum injection speed actually yielded lower scrap than running flat-out, because the slightly slower fill gave the vents enough time to evacuate air. The 0.3 seconds we added to fill time saved 12 percent in scrap — a far larger cost saving than the tiny throughput reduction.

If you are sourcing injection molded parts and want a supplier who optimizes fill time scientifically rather than just cranking up machine speed, check out our injection molding supplier sourcing guide for a framework on evaluating manufacturing partners.

Frequently Asked Questions About Filling Time

What is a normal filling time for injection molding?

Most medium-complexity thermoplastic parts fill in 1 to 3 seconds under standard processing conditions on typical production equipment. Thin-wall packaging molds may fill in under 0.5 seconds, while large structural parts with thick walls can take 5 to 10 seconds to fill completely. The exact range depends on cavity volume, material viscosity, wall thickness, and the injection molding machine maximum flow rate capability. Always benchmark against similar molds in your own production history to establish a realistic baseline before fine-tuning process parameters for a new mold project.

How do you measure actual filling time on a machine?

Most modern injection molding machines display fill time directly on the controller screen, making it easy to read during initial setup and subsequent process optimization runs. You can also observe the transition from injection pressure to holding pressure on the pressure-versus-time graph, where the inflection point clearly marks the end of the fill phase. For older machines without digital readouts, a stopwatch from screw start to the pressure switchover click gives a reasonable approximation of the actual fill duration in seconds.

Does filling time change with different plastics?

Yes, filling time changes significantly with different plastics due to their varying melt viscosities and thermal properties during the molding process. Low-viscosity materials like polypropylene with an MFI above 20 fill faster than high-viscosity materials like polycarbonate or PEEK, even at the same injection pressure setting on the machine. The material shear-thinning behavior also plays an important role in practice — some polymers thin dramatically under high shear rates, which effectively speeds up cavity filling compared to what a constant-viscosity calculation would predict.

Can filling time be too short?

Absolutely, filling time can definitely be too short for the specific part and mold design at hand. Extremely fast fills cause excessive shear heating, air traps, jetting through the gate, and flash at the parting line of the mold. On transparent parts, jetting creates visible worm-like cosmetic defects on the surface; on structural parts, trapped air causes internal burns and mechanically weak spots. The optimal fill time balances speed with part quality and dimensional consistency — it is not always the minimum possible time your machine can achieve.

What happens if filling time is too long?

When filling time is too long, the melt cools progressively and thickens as it flows through the cavity, increasing the risk of short shots, surface flow marks, and high residual stress in the finished part. Thin-wall parts are especially sensitive to this particular problem — if the frozen layer closes off the flow channel before the cavity is completely full, you get an incomplete part. Long fill times also reduce overall production throughput by extending the injection phase of the molding cycle unnecessarily.

Is Moldflow simulation worth the cost for small molds?

For simple single-cavity molds with straightforward geometry, the basic V/Q formula is usually sufficient for initial setup and saves the simulation fee entirely. For multi-cavity, thin-wall, or high-precision molds, simulation pays for itself by preventing even a single mold revision, which typically costs 10 to 50 times the combined simulation software and engineering time fee. As a practical guideline, any mold with more than two cavities or a flow-length-to-thickness ratio above 100 should definitely be simulated before the mold tool is cut.

How does wall thickness affect filling time?

Thinner walls restrict polymer flow and increase viscous resistance in the mold cavity, requiring higher injection pressure and often resulting in longer overall fill times for the part. The flow length-to-thickness ratio is a key metric for judging fillability of a design — ratios above 150 typically require very high injection speeds to fill completely without short shots. Product designers should aim for uniform wall thickness throughout the part geometry to avoid flow hesitations that cause air traps, weld-line visibility issues, and uneven fill patterns.

What is the difference between fill time and cycle time?

Fill time is just the cavity-filling phase, typically lasting 1 to 3 seconds depending on part size, material choice, and mold complexity. Cycle time includes the complete sequence of filling, packing, cooling, mold opening, ejection, and mold closing — usually 10 to 60 seconds total for a complete production molding cycle. Fill time is typically only 5 to 15 percent of the total cycle. Reducing fill time alone may not significantly reduce overall cycle time if cooling is the dominant bottleneck in the process.

Wnioski

Filling time sits at the intersection of material science, mold engineering, and machine capability. The simplest calculation — tf equals V divided by Q — gives you a useful starting point. Adding rheological modeling or full simulation progressively improves accuracy. And real-world trial shots remain the ultimate validation.

Optimizing fill time is not about chasing the fastest possible number. It is about finding the speed that delivers dimensionally stable, cosmetically clean parts at the lowest total cost — accounting for cycle time, scrap rate, and tooling longevity. That balance is exactly what our engineering team at ZetarMold works toward on every project.

Need help optimizing your injection molding process? ZetarMold’s engineering team provides DFM feedback, mold flow simulation, and production process optimization. With 20+ years of experience across 400+ materials and 47 machines (90T–1850T), we can help you get fill time — and every other parameter — dialed in right. Request a free quote today.

1. shear-thinning: Shear-thinning refers to the phenomenon where a fluid’s viscosity decreases as the applied shear rate increases. Most thermoplastic melts exhibit this behavior during injection molding. ↩

2. Hagen-Poiseuille equation: The Hagen-Poiseuille equation describes laminar flow of a Newtonian fluid through a long cylindrical pipe, relating flow rate to pressure drop, pipe radius, and fluid viscosity. ↩

3. power-law model: power-law fluid model refers to the power-law or Ostwald-de Waele model relates shear stress to shear rate with the equation τ = k × γ̇ⁿ, where k is the consistency index and n is the flow behavior index. ↩