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Hot runner purging in plastic processing

Cleaning of the hot runner

The cleaning process for hot runners is always a major challenge in plastic processing. In the drilled hot runner, for example, combustion residues sediment in the dead zones. In these zones the plastic is exposed to high temperatures over a long time period, so that it burns and sediments at these places. Such combustion residues get carried along by the melt flow sometimes and end up in the moulded part. These so-called black specks are making the moulded part unusable.

Abrasive purging compounds do not help in these cases. The abrasive particles in the plastic melt cannot reach the dead zones of the melt stream and thereby cannot remove the combustion residues. The chemicals of the chemical purging compounds infiltrate the burned sediments, break them open and rinse them out completely (see graphic).


Sediments in a dead zone of the hot runner



Sediments in a dead zone of the hot runner

Chemical cleaning – Hot runner purging

Cleaning of the nozzle/ insulating cap

The same challenges exist in the sector of the nozzles tips (insulating caps). There are dead zones also filled with precursor material/color. The melt flow sometimes carries away the precursor material, which provides color streaks and thereby waste products.

Even here only a strong expanding chemical cleaner, which can infiltrate these zones, can help (see sketch). The nozzle temperatures need to be raised to a maximum. Depending on the cooling of the mould plate it should be turned off. By that the rigid insulating zone should be plastified. During the 5-minute down time the chemicals infiltrate deep into the insulating zone, due to its raising effect, solve the precursor material and rinse it out at the restart. These rinsed zones will be filled with the follow-up-material. On this way the precursor material gets isolated and cannot reach the moulded part.

Chemical Cleaning

Color change from red to blue

Chemical Cleaning - Nozzle / insulating cap cleaning

Abrasive Cleaning

Color change from red to blue

Abrasive Cleaning - Nozzle / insulating cap cleaning


On its way through the whole hot runner system (mainfold, nozzle, etc.) the plastic melt has multiple possibilities to sediment somewhere and get burned (see above).

Abrasive cleaner needs streaming to release sediments or precursor materials/ colors mechanically. They cannot reach the dead zones, whereby you produce waste when burned material gets carried out.

Chemical cleaner raises during their short resting time in the hot runner and reach into the dead zones. They dispense the burned material, precursor material / colors and rinse them out completely at the restart.

In addition to this advantage the chemical cleaner also preserves the metal surface. Abrasive cleaners damage the metal surface. Every damage is a spot where plastic can get stuck and burned over a long time. Burned material proliferates and get carried away by the stream, which causes black specks. The produced moulded parts are unusable.