The injection blasting pot is the oldest blasting device at all. It is an open vessel at the outlet of which an injector is mounted introducing the abrasive into the airstream. The compressed air flowing into the injector generates a vacuum at the abrasive inlet port with the help of which the abrasive is injected into the airstream.

Because a certain part of the existing pressure has to be converted into speed already at the injector in these systems the blasting performance is much lower when using such devices. Those devices had been developed at the beginning of the history of sand blasting technology, because at that time they didn’t use compressed air as an accelerating medium, but steam. Such devices will only be used if small roughness heights and low blasting performances are sufficient.

Difference is made between two systems, according to their type of construction each.

Injection blasting with suction system

A sectional drawing of an injector sandblasting pistol is shown as an example for the plants working according to the suction system.

The actual jet nozzle (pos. 5) is at the front. Here the abrasive exits at a speed of a maximum of 65 m/s.

All different models of this type of construction require, however, another additional second injector nozzle (pos. 4) which is installed in the inner part of the case (pos. 1) and which operates with compressed air (pos.2) in order to generate the required vacuum within the sandblasting pistol. This vacuum causes the abrasive to get drawn in via an intake line (pos. 3).

The model shown in the illustration operates according to the suction system. The intake line, mainly made of rubber, connects the pistol with the material collecting funnel of the blasting boot.

Sectional drawing of the injector sandblasting pistol operating according to the suction system


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The vacuum generated by the injector nozzle causes a suction draught acting as pneumatic transport that takes the abrasive from the material collecting funnel to the pistol where it is taken over by the compressed air jet leaving the injector nozzle and led through the jet nozzle.

Thus, a mixture of expanded compressed air, secondary air of the pneumatic delivery and abrasive is flowing through the jet nozzle.

In general, the mentioned nozzle proportion is 1 to 4. This is the proportion between the spaces of the borings of the injector nozzle, on the one hand, and of the actual jet nozzle, on the other hand.

Injection blasting with gravitation system

The acceleration of the blasting medium is also performed at the gravitation system by nozzle heads which are equipped with two nozzles and operate according to the injector principle.

Injector blasting with gravitation system

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A considerable part of the energy supplied from the compressed air is used for the pneumatic transport and the delivery of the blasting medium.
This disadvantage is avoided at plants being equipped with a bucket conveyor according to the gravitation system.

When assessing blasting plants which are operated with compressed air the jet speed of the blasting medium as well as the adjusting range play an important role.

The maximum value of the jet speed of the abrasive amounts to a maximum of about 80 m/sec for the injector gravitation system. The jet speed can be better regulated at this system than at the suction system. In many cases priority is given to the injector gravitation system for the plants operated with compressed air because of this considerably better opportunity of regulation, also compared to the suction system. This also applies towards the pressure system. A better adjustment is mainly of special importance as regards plants for the fine treatment of surfaces (fine and fine blasting plants), as well as for the equipment for the “shot-peening” method.

Furthermore, the devices for processing the abrasive (cascade collector) can be easily mounted at the bucket conveyor outlet in case of the gravitation injector principle; this also ensures convenient accessibility.

In such cases where these systems are mainly used those are marked by high efficiency because these systems are not operated with much air, high pressure and at high velocities.

2.2.3 Vacuum-compressed-air blasting and vacuum injection blasting

Compressed-air devices can be used as blasting devices to achieve high performances, and injection blasting devices can be used for more simple blasting work.

The blow head (outlet) the case of which seals completely the actual impact area from the surrounding is a typical feature of these systems. In general, the jet nozzle is in the centre of the blow head; this nozzle is provided with abrasive from a normal pressure pot with vacuum container placed on it as well as an integrated collector. After having impinged on the workpiece the abrasive is drawn back by the generated vacuum at the edge of the blow head and it is processed again.

The vacuum airstream for transporting the abrasive back can also be generated by separate vacuum facilities. This mainly applies to bigger plants where several blasting units work with that system at the same time.

It is, however, possible to connect more blasting units between blasting unit and vacuum facility according to the respective distance. The required vacuum is between 0.2 and 0.5 b.

Such systems can be used for any recyclable abrasive, e.g. chilled and steel casting granulate, electric-furnace corundum or glass beads, as well as for non-reusable abrasive.

Vacuum – injector head (principle)

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Cases of application could be reworking or smaller blasting processes where the used abrasive has to be collected immediately because the abrasive and the parts got off of the object to be blasted may contain toxic substances that have to be disposed separately.

The fact that this system operates under a closed cap (blow head) reduces decisively the performance. By this, the abrasive rebounding from the surface of the object to be blasted disturbs the abrasive flow leaving the nozzle. Part of the generated kinetic energy therefore neutralizes itself. It is also disadvantageous that it is not possible to monitor the just blasted surfaces.