Knowing how to choose a filling machine starts with a question most buyers skip past too quickly: what, exactly, does the product do when it moves? Thin and free-flowing, thick and slow, or somewhere in between — the answer determines which filling technology will actually work, before speed, footprint or budget even enter the conversation. This guide compares the three most common filling technologies — volumetric, gravity and piston — to help match one to your product.
Start with Product Viscosity
Water-thin liquids, medium-viscosity products and thick or dense products each behave differently as they move through a filling head, and a technology optimised for one will underperform on another. Describing the product honestly — not aspirationally — is the single most useful input into choosing a filling machine, more useful at this stage than comparing brochures.
Gravity Filling
Gravity fillers let the product’s own weight do the work, flowing from a reservoir into the bottle through a valve. They are mechanically simple, cost-effective, and well suited to thin, free-flowing liquids such as water, still wine and light oils. Accuracy is typically in the range of ±1-2%, which is adequate for many products but not for those requiring tight fill-level tolerance. Gravity systems struggle as viscosity increases, since flow becomes slower and less predictable.
Volumetric Filling
Volumetric fillers dispense a fixed, pre-set volume using metering technology rather than relying on gravity alone, achieving accuracy around ±0.5% across a wider range of viscosities than gravity systems. This makes volumetric filling a strong general-purpose choice when a line needs to run several product variants without swapping the entire filling technology between them.
Piston Filling
Piston, or positive-displacement, fillers use a piston to draw product from a reservoir and dispense an exact volume into each container. This method handles medium-viscosity products — creams, sauces, some personal care formulations — that gravity and standard volumetric systems cannot move reliably, typically achieving ±0.5-1% accuracy. Piston fillers are usually the right answer whenever “thick” is an honest description of the product.
Quick Comparison
- Gravity: best for thin liquids, ±1-2% accuracy, lowest complexity and cost.
- Volumetric: best for a broad range of viscosities, around ±0.5% accuracy, strong general-purpose choice.
- Piston: best for medium-viscosity and dense products, ±0.5-1% accuracy, required where gravity and volumetric systems fail to fill reliably.
Beyond the Filling Technology Itself
Once the core technology is right, production speed, changeover time between formats, and integration with the rest of the line — capping, labelling, conveying — determine how well the filler performs in practice, not just on a spec sheet. AXTRA designs filling machines matched to product viscosity and required accuracy as part of a full modular line. Contact AXTRA to discuss which filling technology fits your product.
Product viscosity, the starting point for this decision, is explained in more depth on Wikipedia.