The best-known toy brick maker is the LEGO company, which is based in Billund, Denmark (as of November 2026). Its brick has a documented but disputed lineage. Hilary Page, an English toy designer, patented hollow interlocking plastic bricks with 4 and 8 studs; he founded his company Kiddicraft in 1932, and the V&A says he patented the brick in 1939. The Christiansens came across Kiddicraft prototypes in 1947 and made their own version two years later. Histories disagree about when LEGO got the rights to Kiddicraft's design: the V&A museum says 1949, while Grace's Guide says LEGO acquired his intellectual property in the 1980s. In 1981 LEGO settled out of court with Page's estate, reportedly for about £45,000, to keep using the brick design.

What the Danish company contributed is clearer from its own patent. In 1958 the bricks were hollow underneath, so models were wobbly and customers complained, and Godtfred Kirk Christiansen sketched several fixes, including tubes. The idea was sketched at a meeting on January 23, 1958, and the patent was filed five days later on January 28. It covered a system of interlocking bricks, not only the studs and tubes, and the application included a hand-glued 2x4 mock-up. The plain hollow Kiddicraft-style brick gripped less well; the tubes beneath gave the studs more points of contact and stiffened the brick against warping.

Bricks stick together because the studs on top fit tightly between the tubes inside another brick. A brick one stud wide has no room for a tube, so it has a thin bar that the studs squeeze slightly. Clutch power is the force needed to pull two bricks apart; it is measured in newtons, and LEGO keeps the exact ideal number secret. LEGO has not changed the brick's basic connection since 1958, so a new brick still fits an old one, and the company calls the interlocking principle what makes it unique.

The material is ABS, and its toughness comes from a deliberate microstructure. ABS is styrene-acrylonitrile (SAN) plastic made tough by polymerizing it in the presence of rubbery polybutadiene, which leaves a long polybutadiene chain crisscrossed with shorter SAN chains. Rubber toughening works because fine rubber particles are spread through a rigid plastic: when the product is stressed, crazing that starts at the particles helps to increase the strength of the polymer. The same method has been used to strengthen other polymers such as PMMA and nylon. Each ABS ingredient has a job: acrylonitrile adds chemical resistance, hardness and rigidity, styrene gives a shiny surface and easier processing, and polybutadiene adds toughness.

ABS is an amorphous solid with no definite melting point, because its monomer chains vary in length and are cross-linked to different degrees, and its glass transition temperature is about 105 degrees C. Thermoplastics change their physical properties sharply above the glass transition temperature without a change of phase. For most uses ABS works between -20 and 80 degrees C, because its mechanical properties change with temperature. At LEGO the ABS is heated to 232 degrees C (450 degrees F) until it is dough-like and injected into the molds with forces of between 25 and 150 tonnes.

In a molding machine, pelletized plastic drops from a hopper into a heated barrel with a reciprocating screw. The screw mixes and homogenizes the melt and, by mechanically shearing the plastic, adds frictional heat that shortens the heating time needed. After the screw injects the plastic, a packing pressure is applied until the gate, the small entrance to the cavity, freezes solid; this compensates for thermal shrinkage, which is quite high for thermoplastics. Only when the gate has solidified can the screw refill for the next shot while the part cools. Cooling lines circulating water or oil from an external temperature controller cut the cooling time sharply, and then the mold opens and pins, sleeves or strippers push the part out.

The mold is the expensive object. Aluminum molds cost less and are made faster, but they wear, dent and deform more easily than steel, so they suit low volumes and loose tolerances, the amount a part's size is allowed to be off. Tool-steel molds are used for high volumes and narrow tolerances and can last hundreds of thousands of molding operations. At LEGO, molds are the most guarded secret of the factory, and a 2011 report said the average mold was worth $72,000 and the most valuable about $360,000.

A single big factory module in Billund had up to 64 molding machines in two aisles of 32, and there were 12 such modules in 2011. A 2016 report said an average LEGO plant keeps its ABS granules in about 14 silos, each holding 33 tons, and at Billund in 2008 the silos fed the molding lines about 60 tons of granules every 24 hours. When a collection bin under a molding machine fills, the machine signals a robot, which drives down the aisle on its own, takes the full box and leaves an empty one so production resumes; in 2016 eight robots moved 600 bins per hour.

Published numbers for precision and timing do not always agree. LEGO switched from cellulose acetate, which is based on wood, to ABS in 1963 after a series of tests, because ABS was more stable and color-fast and allowed greater precision in molding, which the company's history gives as an accuracy of 1/200 mm. A 2016 trade report quoted a tolerance of as little as 0.002 mm, so the figures should be read as rough and dated. Sources also disagree on how long a brick takes: one 2016 report says the new pieces cool and fall onto a conveyor after about seven seconds, while Wikipedia, citing a 2008 source, says the plastic takes approximately 15 seconds to cool.

Quality control combines human eyes with scales. Human inspectors look at the output of the molds to eliminate significant variations in color or thickness, and about eighteen bricks in every million fail to meet the standard, according to the company. A 2011 factory tour found that a bag of bricks had to weigh between 94.9 and 95.7 grams, and a display showed 5,354 bags in range and six out (five too light, one too heavy). Scales that notice a difference of a few micrograms set off an alarm so an operator can re-sort the box.

Engineers at the Open University tested a 2x2 ABS brick in a hydraulic testing machine until it failed. The average brick took about 432 kg (950 lb) before it deformed, and since one brick has a mass of 1.152 g that equals a tower of about 375,000 bricks, roughly 3.5 km high. The brick did not shatter: it failed by plastic deformation, the material flowing out of the way while the brick kept deforming without the load increasing. To work out a tower's limit you need the mass of the material and its yield strength, the load it takes before it begins to deform. A real tower would fail long before the bottom brick, because a tiny flaw in the structure would be massively amplified, and the researchers believe 1x2 bricks would likely withstand more than 2x2 bricks.

The material also has a footprint. ABS resin is derived from natural gas and petroleum, and PlasticsEurope's figure is that making 1 kg of it in Europe uses an average of about 95 megajoules of energy. LEGO's bricks are mostly ABS, an oil-based plastic. LEGO's rise to 52 percent renewable and recycled content in 2025 came mainly from large increases in buying certified materials, not from a new kind of brick (as of March 2026). LEGO also reported using less new fossil-based material to make bricks in 2025 than in 2022, even though its revenue grew 29 percent over those years.

The search for other materials has not been easy. In 2018 the New York Times reported that LEGO's footprint was about a million tons of carbon dioxide a year and that its researchers had tried around 200 alternative materials; by 2023 the company said it had tested more than 300, aiming to cut emissions 37 percent by 2032. A recycled-bottle brick prototype was dropped in 2023 after two years of testing because it would not reduce carbon emissions. LEGO is also moving its brick packing lines away from single-use plastic bags, and 56 percent of the lines have switched to recyclable paper bags (as of November 2026).

Plastic made in the world grew from about two million tonnes in 1950 to more than 400 million tonnes a year today (2026). Estimates of how much plastic enters the oceans have changed: earlier work said up to 8 million tonnes a year, while recent high-quality studies suggest 1 to 2 million tonnes, which shows how hard it is to measure. Much plastic waste comes from places with weak trash collection, so how trash is handled matters as much as how much there is.

Inside the factory, waste is small. A 2008 report said LEGO recycles all but about 1 percent of its manufacturing plastic waste, and in 2011 a visit found that almost all the plastic in a LEGO factory was reused. What cannot go back into bricks, such as faulty pieces and the clear plastic used to flush a machine between colors, is ground up and sold to other industries, which in 2008 made pipes and even heating oil from it. Plastic left in the channels of a mold falls down the machine, is ground up and goes back into the production cycle.

Outside the factory, waste lasts far longer. A thrown-away ABS toy is not easy to recycle at home: ABS can be recycled, but it is not accepted by all recycling facilities, and in the UK councils do not take it in household recycling. Plastic that is thrown away does not rot away like food or paper, and it can stay around for hundreds of years. Bricks are built to be taken apart and built again, and new bricks still fit bricks made since 1958.

Words to know

• polybutadiene: rubbery ingredient
• amorphous: no melting point
• crazing: starts at particles
• tolerance: allowed error
• cavity: hollow space