Infill refers to the internal structure or material used to fill a space, and the term shows up in three very different fields: 3D printing, construction and urban planning, and dentistry. The meaning you’re looking for depends on the context, but the 3D printing definition is by far the most commonly searched. Here’s what infill means in each case and why it matters.
Infill in 3D Printing
In 3D printing, infill is the internal structure inside a printed object. Rather than printing a completely solid part, most 3D printers fill the interior with a repeating geometric pattern at a specified density. This saves material, reduces print time, and still provides enough strength for the object’s intended use. Infill density is expressed as a percentage: 0% means the inside is completely hollow, and 100% means it’s completely solid.
Most everyday prints use somewhere between 10% and 30% infill. A print at 20% infill might use only about a third of the material a solid print would require. Low-density prints in the 10% to 20% range also finish significantly faster than prints at 50% or above. The tradeoff is straightforward: more infill means more strength and more material, less infill means lighter parts printed faster.
Common Infill Patterns
The shape of the internal pattern matters just as much as the density. Most 3D printing software (called slicers) offers a menu of infill patterns, each with different strengths:
- Grid: A simple crisscross pattern that prints quickly and works well for general-purpose objects.
- Honeycomb: Hexagonal cells that distribute force efficiently, making it a popular choice for parts that need to handle weight.
- Gyroid: A wavy, three-dimensional pattern with no flat internal surfaces. It distributes loads evenly in all directions, which makes it especially useful for parts that experience stress from multiple angles.
- Lines or rectilinear: Parallel lines that alternate direction between layers. Fast to print but weaker under certain loads.
- Triangles: Triangular cells that resist lateral forces well, useful for parts that need wall strength.
Research comparing these patterns has found that bending resistance increases in a roughly linear relationship with density, regardless of the pattern chosen. The differences between patterns are less dramatic than the difference between, say, 15% and 50% density. That said, for specialized applications like prosthetics or mechanical parts, pattern choice can matter. The gyroid pattern, for instance, has become a favorite in both engineering and medical applications because of its consistent performance under load from any direction.
How Infill Is Used in Medical Devices
Infill design plays a critical role in 3D-printed prosthetics. Researchers developing prosthetic limb liners have used gyroid infill at densities as low as 6% to create soft, cushioning structures, while 14% density produces a harder liner better suited for high-impact activities like running. A 10% infill liner hits a middle ground, reducing pressure on the residual limb while maintaining enough stability to keep the prosthetic securely in place. In this context, infill density directly translates to how the device feels and performs for the person wearing it.
Infill in Urban Planning
In real estate and city planning, infill development means building on vacant or underused land within an already developed area, rather than expanding outward into undeveloped land (which is called sprawl). An empty lot between two buildings in a downtown neighborhood, a demolished factory site being turned into apartments, or a parking lot converted into mixed-use housing are all examples of infill development.
Cities promote infill for practical reasons. Building within existing neighborhoods takes advantage of infrastructure that’s already in place: roads, water lines, sewers, and public transit. It tends to create more walkable neighborhoods, reduces commute distances, and puts less pressure on natural land at the city’s edges. Urban greenery incorporated into infill projects can help reduce air, water, and noise pollution, and offset some carbon emissions through tree canopy absorption. For residents, infill often means shorter distances to shops, schools, and workplaces.
The downsides are real, too. Infill projects can increase density in ways that strain existing parking and traffic. They sometimes face neighborhood opposition over building height, style, or the loss of open space. And because urban land is expensive, infill housing often costs more to build per unit than suburban development.
Infill in Dentistry
In dental work, an infill (more commonly called a filling or restoration) is the material placed into a tooth after decay has been removed. The goal is to restore the tooth’s shape, strength, and function while sealing out bacteria.
The two most common filling materials are amalgam (the silver-colored metal mix) and composite resin (tooth-colored). Modern composite fillings use a mixture of plastic resin and tiny glass or ceramic particles. These fillers typically make up over 82% of the material by weight, with particles ranging from 40 nanometers up to 10 micrometers. The resin binds everything together when hardened with a curing light.
How Long Dental Fillings Last
Amalgam fillings have a median lifespan exceeding 16 years, while composite resin fillings last a median of about 11 years. Amalgam’s durability advantage is most noticeable in larger restorations on back teeth, where chewing forces are strongest. Composite has largely replaced amalgam in many practices because it matches tooth color and bonds directly to the tooth structure, but the longevity gap is worth knowing about when you’re choosing between the two for a molar.
Sensitivity After a Filling
Some sensitivity to hot, cold, or sweet foods after getting a filling is normal. About 15% of patients report sensitivity two days after a composite filling, dropping to around 4% by one week. For most people, any lingering sensitivity resolves completely within 30 days. In cases where it persists longer, it typically fades by 90 days. Sensitivity that appears for the first time more than a week after the procedure, or that gets worse rather than better, is worth having your dentist evaluate.