The Lego bricks
It doesn’t require any introduction for this popular building brick toy adored by both kids and adults around the world. With a Lego brick, imagination is the limit. We can build the most complex structures with this small piece of plastic brick. Many studies show building structures with these interlocking bricks will enhance creativity and exercise the reasoning skills of the human brain.
Lego bricks had been in the market since 1949 and to this date, more than 600 billion lego bricks are produced. Lego group has cleverly marketed their brand, from toys to theme parks to Lego movies making it one of the most popular toys in this world.
Orgin of an iconic toy
The name Lego was derived from the Danish word “leg godth” which means “play well”. Founded in 1932 by Ole Kirk Christiansen in Billund, Denmark, who was a carpenter making wooden toys. He initially crafted wooden Yo-Yo toys which became popular among the kids at the time. But, later when demand for the Yo-Yos faded he started developing wooden interlocking bricks which eventually became the Lego bricks that we see today. Early Lego bricks were made with wood and were very popular among the kids in Billund. When the demand for Lego got higher the company decided to adopt plastic Injection moulding to manufacture Lego bricks in 1947. One interesting fact is that we can still interlock the Lego bricks which are currently in the market with the Lego bricks of 1958 which indicate dimensional control and precision in manufacturing Lego bricks.
What makes the Lego brick’s design clever
At a glance, we cannot find anything special with Lego bricks but if we start to examine Lego bricks with a curious mind, it will become clear about the fact that the Lego bricks were cleverly designed. To identify and understand these clever design aspects I will be categorizing each aspect into attributes that are usually applied to Injection moulded parts in general. We will go through the following design attributes:
- Plastic Material
- Wall thickness
- Draft angle
- Wall ribs
- Edge fillets
- Bosses
Plastic material
During the early 1960s, Lego bricks were manufactured using Cellulose Acetate material, a bioplastic that is biodegradable. After the introduction of ABS Material in the 1950s in the commercial market, Lego adopted ABS as its new material. To date, Lego has manufactured its bricks with ABS. Lego also uses various other plastic materials for other Lego products.
ABS (Acrylonitrile Butadiene Styrene) is a combination of three monomers Acrylonitrile, Butadiene & Styrene hence the name ABS. ABS is an amorphous plastic categorised as an engineering plastic. ABS is very popular for its high impact strength. The material does not have any known toxic contents hence making it safe for children. Material has less warpage & colour fading which makes it easy to manufacture and does not deteriorate with time. This clever selection of ABS as material for Lego bricks is helping to achieve stability in all the following attributes.
Wall thickness
The wall thickness of a Lego brick is 1.20mm which is slightly higher than one-tenth of a centimetre. For injection moulding parts this is a normally given dimension. An experiment done by United Kingdom’s Open University proved that a Lego brick can withstand an average of 4240N which is equivalent to a mass of 432kg. A single Lego brick weighs 1.152g if we divide 432kg with the weight of a Lego brick it would be 375,000 that means a single Lego brick can withstand the weight of whopping 375,000 Lego bricks mounted above it.
If we dwell into it further, 375,000 Lego bricks multiplied by the height of a Lego brick (9.60mm) which gives us 3.60km that would be almost 4 times the height of the world’s tallest building Burj Khalifa in Dubai, UAE.
The thickness of Lego bricks plays an important role in the cost, strength & cosmetic appearance of a Lego brick. From the cost perspective, If the wall thickness of a Lego brick is increased by 0.1mm which is one-tenth of a millimetre will cause an extra material cost of around 100 thousand USD ( 2×4 Brick as reference) for the Lego group assuming annual production of 100 million pieces with an average cost of 1.27USD/kg for ABS material in granules. Inappropriate thickness can cause shrinkage, warpage and residual stress which will adversely affect the cosmetic appearance and strength of the Lego bricks.
Walls are also important during the ejection of the Lego bricks from the mould. Generally, ejector pins are used to eject the parts from the mould after cooling down the injection moulded part. But Lego uses plates to eject the Lego bricks from the mould by applying force against the faces of vertical walls. This is a clever idea to prevent any damage on horizontal wall surfaces while ejection since the concentrated force from the ejection pin can cause damages on the thin-walled surfaces.
Draft angles
This might be new knowledge for some of you who are not familiar with injection moulding manufacturing. Plastic parts manufactured with the Injection moulding method does not have a perfectly vertical surface. This is done for the ease of manufacturing a methodology called DFM or Design for manufacturing.
Find out more from here: Design for Manufacturing (DFM) for Injection moulding
Lego bricks have a draft angle of 0.5 degrees on each face which makes them looks like perfectly vertical sides without hindering the easy ejection of Lego brick from mould.
Edge fillets
Edge fillets with a small radius are given to the edges of the Lego bricks for the proper flow of the molten plastic during injection moulding and for ergonomic reasons to avoid sharp corners preventing cuts on the skin. Another purpose of fillets are for cosmetic appearance, even though the draft angles are very small, still, when interlocking these bricks one above the other it would be visible that the bottom edge and the top edge of the bricks are not straight. To avoid this misalignment, Lego cleverly added a fillet at the bottom edge of the bricks to deceive our eyes to look like perfectly vertical sides.
Wall ribs
Ribs are features that support the walls to enhance their overall strength without increasing wall thickness. Ribs play an important role in strengthening and maintaining the shape of the Lego bricks. There are some interesting facts about the ribs. There are two types of ribs at the inner side of the Lego bricks, ribs sticking to the wall & ribs connected to the bosses. Ribs sticking on the wall spaced at equal intervals are thin yet provide support to the vertical walls. The selection of proper rib thickness avoids any cosmetic defects like sink marks on the Lego bricks. There is only one rib connected to the boss at the centre in a 2×4 Lego brick, this is to support the middle area of Lego bricks from the warpage during the injection moulding cooling process. The appropriate placement and thickness of the ribs give the shape, strength and cosmetic appearance of the Lego bricks.
Bosses
Bosses are injection moulding features extruded from the wall that generally helps in the assembly of parts. Bosses in the Lego bricks are thin-walled cylinder-shaped features that aid the interlocking of the Lego bricks. The corner studs on the Lego bricks contacts with two adjacent vertical walls and the bosses which creates three-point contact for the studs for interlocking.
It is a clever decision to select the design of thin-walled cylinder-shaped bosses instead of any other shaped features. This selection will help to avoid sink mark defects on top of the Lego bricks. The round shape of the bosses will aid in the proper filling of the molten plastic in the cavity reducing the defects called short shots. The round shape also helps to maintain the tightness while interlocking since it will not deform or bend due to the stress applied when interlocked.
Conclusion
Examining the above design attributes of Lego bricks shows the clever design ideas applied to it. These clever design ideas helped Lego to maintain its aesthetic appearance, strength, reducing the manufacturing cost, dimensional stability & interlocking/assembly life. This article stresses the importance of applying DFM or Design for manufacturing Injection moulding parts and its benefits. Lego brick is a perfect example to demonstrate how far clever design ideas can influence a product.