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1.3.1 Injection moulding

In this activity, you are asked questions on the following video, which shows how injection moulding of plastics to manufacture LEGO® pieces is done industrially.

Activity 1 Lego and injection moulding

Timing: Allow about 15 minutes

Consider the following questions as you watch the video and note down your answers.

  • a.How do the manufacturers ensure that the LEGO® brick is coloured throughout the whole piece?

  • b.What is the pressure of injection moulding in pascals?

    Use the conversion factor 1 psi = 6894.8 Pa (to 1 d.p.) and give your answer to one significant figure.

Download this video clip.Video player: t271_2018j_openlearn_vid045-640x360.mp4
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Transcript

[SOFT MUSIC]
NARRATOR
The LEGO® brick is an old invention by now. The first plastic brick was moulded in 1949. Since then, thousands of new and unique LEGO® elements have been developed. Pick a LEGO® brick from a box today, and it will fit with any brick that was moulded decades ago. This is the result of great precision and commitment for more than 50 years.
The LEGO® group has factories across the world. But how do you produce hundreds of different types of bricks with such precision and in a wealth of colours every single day? And how do the bricks find their way to the right boxes? This is the story about how small pieces of plastic granulate are transformed into creative playsets.
A truck arrives with raw material at the factory. A truck like this holds up to 28 tonnes of plastic granulate. The granulate is blasted from the truck into tall silos, where it is stored. Different types of plastic granulate are used, depending on the function of each LEGO® element. The granulate is sped down a labyrinth of long pipes into the factory. The pipes lead to the heart of the LEGO® factory, the moulding area.
High tech injection moulding machines produce LEGO® elements 24 hours a day, seven days a week. First, the raw granulate is mixed with the dye. Bricks are currently produced in over 50 different colours. The coloured granulate is then led into the moulding machine. Within no time, the granulate is heated to between 230 and 310 degrees Celsius. The plastic melts into a texture much like toothpaste. With great force, the paste is then fed into the mould. Great forces must be controlled during this process. The pressure can reach 29,000 PSI. In comparison, a car's tyre pressure is 29 to 43 PSI. In the mould, the material is cooled in a matter of seconds. And out comes a newborn LEGO® brick, as we know it.
[MUSIC PLAYING]
Plastic waste from the moulding machines is ground and recycled straight away. Each mould can make one shape of LEGO® element at a time. To make a different shape of element, the moulds must therefore be replaced. The unique moulds are part of the secret behind the success of the LEGO® group. In each factory, there's a department dedicated to regular cleaning and maintenance of the moulds. The moulds are made with great accuracy, ensuring that all LEGO® bricks always fit together perfectly. The moulds are therefore handled with greatest care.
Each mould has a specific set of instructions, which, among other things, cover pressure, time, and temperature. Temperature tests and moulding tests are carried out to ensure the machine is programmed to perfection at each replacement of the moulds. Samples are sent to the quality department, which measures such things as the durability and precision of the element. It ensured that the LEGO® element is perfect.
AGV stands for Automatic Guided Vehicle. Robots like these were introduced into the production as early as 1987. When the box by the moulding machine is full, these intelligent helpers replace it with a new empty box. The AGV then takes the full boxes to the conveyor system.
A unique barcode identifies the contents of each box.
The boxes are shaken to even out the contents, ensuring it takes up as little space as possible. Now, the lids can be closed.
This is where the journey of the boxes ends for now.
In high bay warehouses up to 37 metres high, the boxes are ready to be collected. When a specific LEGO® element is required, the box is collected. All registration and localization is entirely automated. The boxes are then transported onwards from here.
Many elements are taken directly to the packaging lines, but not these little guys. The mini figures must first get their own unique expression. A machine can produce more than 7,000 torsos per hour. Over half a billion mini figures are produced every year, making them one of the world's largest populations.
In the first part of the packaging process, counting machines ensure that elements are put in small production boxes. One by one, they're weighed and measured to secure the right numbers in each box. The bricks to be included in just one bag in a complete LEGO® set are placed in rows of counting machines. The contents of each production box is then automatically put in a plastic bag. The bags are dropped into open LEGO® boxes, along with the building instructions and large special elements. The most efficient packaging lines pack over 50,000 boxes every 24 hours. Now the boxes are closed and sealed.
Here, we have final LEGO® boxes as we know them. The LEGO® boxes are packed for shipping and stacked on to pallets. They are now ready for a long journey. The first stop of the journey is at one of the large distribution centres. From here, they are shipped to stores all over the world.
It is now up to children and adults to explore the fun and creative building experiences of the LEGO® set.
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Answer

  • a.Colour is added to the particulate material, mixed, and then heated to between 230 oC and 310 oC and ‘cooked’ in an oven to form a coloured paste.

  • b.In the video, you are told that the pressure of injection is 29 000 psi. This can be converted to pascals using the conversion factor 1 psi = 6894.8 Pa (to 1 d.p.), so

    29 000 prefix multiplication of 6894.8 equals 199 949 200 equals 0.1999 times ellipsis multiplication 10 super nine Pa equals 0.2 GPa left parenthesis to one s full stop f full stop right parenthesis full stop

    Therefore the pressure of injection is 200 000 000 Pa or 0.2 GPa (to 1 s.f.).

Note: As an engineer, you may be required to define or interpret values from the very small scale (individual atoms) to the very large (the Voyager 1 probe has covered billions of kilometres on its travels to the outer reaches of the Solar System). To accommodate this range of values, there is an additional unit extension, the SI prefix, which gives standard multipliers to the SI units. You may already be familiar with using prefixes like giga (G), mega (M), kilo (k), milli (m), micro (μ) and nano (n), but if not, here are a few examples:

1 Tm = 1012m (or 1 000 000 000 000 m)

1 GN = 109N (or 1 000 000 000 N)

1 MW = 106W (or 1 000 000 W)

1 ms = 10-3s (or 0.001s)

The SI prefixes allow you to present information in a format that is easier to interpret. So, at the small scale, the diameter of a hydrogen atom is 1.06 × 10 −10 m, or 106 pm (picometres) and, at the other extreme, Voyager 1 is approximately 20 859 million km from the sun, which is 20.859 × 10 12 m or 20.869 Tm (terametres).