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
Click to enlarge!
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
Click to enlarge!
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.