Why do we have a hole on the centre of plug of electrical appliances?

It’s a quick but longer solution. Please bear with me.

Let’s begin with the basics.

The plastic components are made through a process called Moulding/Molding. It involves placing molten plastic into the cavity of an assembly made of mechanical parts known as Mould/Mold.

In honor of its creators, I will make use of some images to help make the ideas clear

In above case the molten plastic is injected, hence it is called Injection Moulding.

Most of the machines that we see operate horizontally, to ensure that the product comes out of the mold due to gravity.

Here’s a shop for molding to show you every one of them in a horizontal position on your floor.

Now, Let’s learn the next level.

Do you find any similarity between these products ?

You are seeing a marriage between plastic and metal, you will find plenty of them around you. If you open the monitor or the Cabinet of your computer, you will find them too. Your automobile filter is insert molded too. Steering wheel has a metal frame inside the plastic covering.

This plate isn’t the only one. In this instance, a thin printed plastic sheet is then molded with a more robust backing.

Because there is the an insert of a second product the moulding process, this procedure is known as insert molding.

Oh, yes coming to the point wait.

Our power cord is also an insert that is molded using wire and metal. Then, before molding, it appears a something like the picture below.

Now, the problem is the mold is horizontal, complete long wire cannot get in to the mold, as ti takes bigger space. Bigger space means bigger machine, bigger machine means lower efficiency, means more cost.

Thus, the metal plug pins are soldered to the wire, and then placed inside the mold prior to injecting plastic to cover the mold. The pin’s end must be secured within the mold chamber ( Cavity is a opening cut out of a metal plate to form the space , which is in opposition to the desired shape)

The wire assembly is required to remain in place until the plastic that was injected is solidified.
Feel my sketching skills that aren’t great here.

The Dark blue is the plastic, light blue is metal pins, red one it the wire. In this arrangement inside the mould cavity there is pin to which the wire assembly can be securely hanged. Some what like this. When the insert is small and light it will sit there without much trouble.

Since there is a pin, the plastic does not flow. When the mold opens after few seconds the plastic is solid, but the pin leaves a hole. It is always a round hole, as the round pin is easier to manufacture. ( unless the designer is a fresher or eccentric !)

It finally comes out as this

That’s right, up until the time you realize that all holes are black !


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