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Sunday 22 July 2012

Moulding of plastic pipe fittings


The manufacture of plastic fittings, from thermoplastic materials such as pvc, cpvc, abs, polyethylene, polypropylene and pvdf is carried out by injection moulding.

The injection moulding process involves a metal die, which is cut out to be the reverse image of the fitting. Molten plastic, maintained at the perfect temperature, is first filtered to get rid of any impurities, and then pumped at high pressure (as it is still quite viscous). The mould contains air release holes which must be strategically placed to ensure that no air pockets remain after all the plastic has been injected. The plastic fitting will have cavities and be not fit for purpose if all the air is not expelled. The air vents in the mould often leave a tail coming off the fitting which is cut off flat as it exits the mould. 

Before the plastic fitting can be released from the mould it must be cooled. If it is not cooled sufficiently then product will distort as it is automatically ejected from the machine. The die therefore has a complex structure of cooling tubes and vanes through which temperature controlled water is pumped prior to injection. The cooling process is critical to ensure that the quality of the plastic is uniform. If the plastic cools too quickly it can lead to weak areas and hairline cracks in the future. This is especially so with pvc and abs. If the fitting cools too slowly the machine cannot manufacture fittings fast enough.

The controlled measurements on a plastic pipe fitting are those that will marry up with the associated pipe. For socket fittings the internal diameter is critical whereas for spigots it is the external diameter. Some fittings have an integral spigot / socket and these are very difficult to manufacture accurately as the cooling rate must be very tightly controlled.

As there are different ranges of pipe, namely imperial and metric, used in different countries, most manufacturers have shims that they can add, or take away from their moulds to adapt to imperial or metric sizes. The cost of making the die is very high, especially for large plastic fittings where they can cost tens of thousands of pounds and so they are designed to double up as imperial and metric. manufacturers typically do not change the identification part of the fitting which leads to a lot of confusion, as. A pvc plastic pipe fitting, manufactured in a plant where they are using a die for both imperial and metric systems, may have a marking of 63mm/2” stamped on it. Users of plastic pipe who are not familiar with the different systems often assume that 63mm and 2” are the same whereas in fact a 2” fitting has been moulded with a die that has shims added and actually measures 60.3mm. This is the same for all sizes of pipe fitting. Polyethylene and polypropylene tend not to suffer these problems as they are generally only manufactured in metric and so do not need shims or a secondary sizes marked on them.

Pipe fittings are usually manufactured in just two colours; pvc and abs are made in grey or white, with the majority being grey. The manufacture of different colours is a costly process as it can take several hours, or even days to ensure a machine is totally empty of one colour, before starting to produce another. Polyethylene pipe is manufactured in black, yellow and blue. The three colours are cost effective due to the large quantities of these three colours used. Black is for general use, blue for potable water and yellow for gas supply.

2 comments:

Unknown said...

Thank you again for another” feel good, uplifting, that there are good people in this world story”. We read so many stories that are negavite about people and what they do to others. A random act of kindness is always such a great thing to hear about. Peace and blessings.


Plastic Pipe Fittings

Unknown said...
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