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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.

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  • 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 horizontal 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 200000000000 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).