Injection Moulding Principles

Welcome to class! Let's get you acquainted with one of the most popular production methods of the last 100 years!

Injection moulding is the process of squeezing molten plastic into a cavity, to be cooled and ejected as a solid object. For a process as straight-forward as this, there are quite a few fine details involved to make sure the object is made exactly as intended. Not to worry though, as in just a matter of steps, you can cover each key detail to the process and revisit anything you miss! You will cover aspects of injection moulding including what an injection gate is, the necessity of a parting line, draft angles and witness marks. All of these strange terms will become much clearer, so sit back, make sure you are comfortable and get stuck into it!


Disclaimer: LEGO®is a trademark of the LEGO Group of companies which does not sponsor, authorise or endorse this site.
Images are property of Bailey Fullarton and/or are used with permission of O. Blankenfeldt, and B. Roehl.
Mould diagram courtesy of Ariel Cornejo, Wikimedia commons, under CC BY-SA 4.0

Student: Bailey Fullarton
S-Number: s2958040
Contact: [email protected]
Topic: Approved personal choice - Injection Moulding Principles
Lecturer: Mr Dejan Stantic

Task 1 - Components to an injection mould

Components of an injection mould

There are 5 main design details to an injection mould:

  1. The runners
  2. The injection gate
  3. The A and B side of the mould
  4. The cavity, and
  5. The ejector pins

Think of this process like you would with a play-dough tool, pushing the dough into a narrow corridor, to be squeezed into a specific shape, ready to play with. Injection moulding is play-dough for big kids!

This is what the plastic looks like before being melted down. These granules can either come straight from the supplier or recycled.

When the hot, viscous plastic is at the right temperature and ready to go, it is thrust into the mould firstly into the sprue, usually with a gradually narrower passage to speed up the injection process. Once the plastic has made its journey, it must then travel through the runners and into the injection gate. This is the point of no return, once past the gate, the part is being created in the blink of an eye! Some moulded objects will leave the sprue attached to the part, which needs to be twisted off manually.

Remember the steps! 

Injection nozzle, Sprue, Runners, Gate

Going beyond the gate leads us to the actual mould cavity. This is negative space between the A and B side of the mould that once filled, creates the shape of the object intended. The A side is generally the side that is attached to the area of the machine that houses the injector, so an easy way to remember this fact, is that side A is on the same side as the gate. There's an A in gate, too, handy!

(Have a careful read through this passage, and remember key phrases to do with position and size, they'll come in handy in this next quiz.)

Match-making quiz

  • Runner
  • Sprue
  • Gate
  • Nozzle entry

Looking closer . . .

Now we're getting to the small stuff!

So when the A and B side of the mould close, the seam they create on the object is known as the parting line, much like if you were to part something with a knife. The parting line is essentially a side-effect of the injection moulding process, but it is good to be able to visualise where the parting line would be, so it is not obvious on the object when creating the mould. Sometimes if the mould isn't properly aligned, the object is left with a thin membrane of excess plastic along the parting line called flashing. Here is an example of flashing on a small lego plate. Notice the position of the flashing in relation to the rest of the part:

Another side-effect of the process is the necessity for draft, or a 'draft angle'. To be able to get the part out of the mould without creating a vacuum, there can't be any surfaces that create a seal between the part itself and the mould. An angle of 3 degrees is enough to stop the vacuum from being created, so you'll usually find a slightly beveled edge on at least some part of an injection moulded near the parting line. When the outside face of an object being at right angles is critical to its design, the draft angle may be factored into the design of the inside. The same can be said for the parting line.

To help automate the process of removing the object from the mould, ejector pins stick out from the mould and pry the object away. Each of these pins creates a small depression on the surface of the part, and also leaves its own slight seam where the pin is located on the object. These can be called an ejector pin witness or witness mark.

(Have a careful read through this passage, and remember key phrases to do with surface details, they'll come in handy in this next quiz.)

What is the last part of the mould that plastic will pass before entering the cavity?

  • Injection Nozzle
  • Injection Gate
  • Injection Flyscreen

True or False?

  • There is no need for a draft angle, as the ejector pins will be able to push the object off the mould.
  • Parting lines will always be present even on the highest quality moulds.
  • Runners come before the sprue.
  • Sprue is a funny word.

What sort of mark is left on an object when it is pushed out of the mould?

  • Injector pin witness
  • Ejector pin witness
  • Jehovah's witness

Task 2 - Identifying moulding artefacts on a LEGO brick

A bit of history

Believe it or not, The LEGO Group produce the largest amount of injection moulded parts of any company on earth. Boasting tolerances greater than NASA work towards and almost 70 years of experience with injection moulding, when it comes to precision and quality, they are king!

The humble LEGO 2x4 brick has been a staple from 1949, right up to modern times. It has seen its fair share of design changes and innovation, being refined over the lifetime of the brand's affinity for plastic.

With all of these design changes, there are certainly some different characteristics to a LEGO mould that aren't found on the typical production. For one, many bricks nowadays don't even come on a sprue, due to hot runners being used, wherein there is a continuous area of hot plastic in the mould to reduce wastage. The only parts that still have sprues attached are created on purpose, for convenience of transport, holding small parts neatly together.

What stays the same?

There are obviously key similarities that all injection moulded products still need to have. Firstly, there is still an A and B side to the mould (though for more complicated shaped there may even be a C and D portion!). In fact, due to the way the bricks are meant to butt up next to each other, and all the different combinations and geometry a LEGO brick must adhere to, designers created moulds that hid the parting line, placing it around the bottom edge of the brick so it wasn't obvious. Even then, this parting line is only truly visible if there is too much pressure in the injector, causing this excess flashing seen on these test bricks. 

 There is still a network of channels for the plastic to pass through, leading to the inevitable injection gate. This gate can be seen much clearer, when highlighted by another colour plastic, like these error bricks show. 

What follows involves you matching these artefacts to a brick. Watch out though, some artefacts may not be visible to you! Do your best to work out what you can't see, from what you can.

Choose the image that shows the injection gate artefact!

What artefacts are you seeing on the bottom edge of this brick?

  • Injection gates
  • Runner marks
  • Ejector pin witness marks
  • A side ribs

Using what you've learned, identify the correct layout of the mould.

  • B Side
  • Injection Gate
  • A Side
  • Parting Line

Task 3 - Identifying moulding artefacts on a complex LEGO brick

Expanding on what you've learned.

Here is a slightly more complex part, employing the same principles and showing the same artefacts of production, but in some more interesting positions. This 2x4x4 unit sized half-cylinder seems quite complex, but if you break down the geometry, it's really just a funny shaped rectangular prism, at least as far as the mould is concerned. See if you can point out some key details in these next questions.

Which surface contains an artefact of the injection gate?

Which surface contains an artefact of the parting line?

Task 4 - Identifying moulding artefacts on a foreign object

Take the leap!

Now comes the task of identifying these peculiar artefacts of production on objects that are under your very nose! Every household will have a handful of products produced in part, or solely made using injection moulding processes. The following common kitchen utensils and implements are obscured in some way, whether by position, or cropping, but can you work out what artefacts are showing?

Artefact Identification #1

Though faint, what artefact is evident, viewing this kitchen utensil from this angle?

  • Injection gate
  • Parting line
  • Ejector pin witness mark

Artefact Identification #2

Though faint, what artefact is evident, viewing this kitchen utensil from this angle?

  • Injection gate
  • Parting line
  • Ejector pin witness mark

Artefact Identification #3

Though faint, what artefact is evident, viewing this kitchen utensil from this angle?

  • Injection gate
  • Ejector pin witness mark

Fill in the blanks and prove your new-found knowledge.

The process of injection moulding uses melted plastic  to inject into a , which once cooled, creates a finished part. The area that the plastic takes the form of the part is called the mould 

Early stages of moulding

To get the plastic from the injection nozzle to the cavity, it must go through a series of channels or , and down some tapering passages called . The point between the cavity and the inner mould; the , leaves a small area of plastic still attached between the two cooled elements produced. 

Moulding quirks

The seam between the two sides of a mould leaves a  which can result in some excess plastic or  around the edges. 

Final stages of production

Once the object has cooled, and is ready to exit the mould, it must be removed with  , each leaving a  . There is another factor that needs to be taken into consideration for the object to be able to be removed. In order to stop a vacuum from being formed between the object and the mould, a  of even a few degrees will need to be applied to the object when being designed.