threaded plastic injection mouldings - the processes, advantages and disadvantages
Moulding threaded plastic injection mouldings is a specialised part of the plastic injection moulding process.
The main challenge with threaded mouldings is removing the moulding from the moulding tool.
Mouldings with thread on the outside
Mouldings with thread on the outside can generally be made by making a mould tool with half of the thread in the core and half of the thread in the cavity. That way, the moulding can be removed from the tool without problem. A very thin joining line will be visible on the thread where the two halves of the tool met.
Mouldings with thread on the inside
However, to obtain a plastic moulding with the thread on the inside, or a moulding with the thread on the outside where no join lines may be visible, a different process is needed. For these mouldings, a removable threaded core (or cavity, if the thread is on the outside of the moulding) is placed inside the mould tool. The polymer is injected into the mould tool and sets to create the thread. The core and the moulded part are then removed from the mould tool and the core must then be removed from the moulded part in one of the following ways:
In this process, the threaded core is manually placed inside the mould tool and once the part is moulded, the moulding with the core still inside is removed from the moulding machine. The operator then removes the core from the threaded moulding, often with a power tool. There will most likely be 2 threaded cores, so that the machine can carry on moulding with one core while the operator removes the other core from the part.
This technique is more labour intensive and therefore the cost per part will be high, but cost of the mould tool will be average. This method is therefore very suitable for low run products. Here's a video of the process:
In a fully automated process the unscrewing of the core from the moulding is completely automated and takes place within the machine. This is a fast way of making the parts and the price per part will be low, but the cost of the mould tool will be very high. This method is therefore most suitable for high & long run products. Here's a video of that process:
3. CNC - Machining
Another way of achieving a threaded moulding is to mould the part without any thread and CNC machine the thread afterwards. The advantage is that no threaded insert is needed at all; the same part can be machined with different sized threads; and it's possible to use the same mould for different polymers.
Finally, it's possible to overmould a metal threaded insert that stays inside the moulding – for more about this process, have a look at our insert moulding page.
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