The spraying angle of the wet spraying trolley has a significant impact on the rebound rate of concrete. The following are the key conclusions and operating points, which are comprehensively compiled from professional literature and engineering practice:

   I. The direct relationship between the spraying angle and the rebound rate

   1. Vertical spraying (90°) is the best angle

   ① When the nozzle is perpendicular to the sprayed surface, the kinetic energy utilization rate of the concrete bundle is the highest, the aggregate is embedded densely, and the rebound rate can be reduced to 5%-12%.

   ② The greater the angle deviation, the easier it is for the concrete to roll on the sprayed surface instead of compacting, resulting in aggregate separation. When the angle is less than 70°, the rebound rate rises sharply to 30%-50%.

   2. Critical angle: 70°

   ① Engineering tests show that when the spraying angle is lower than 70°, the rebound amount increases significantly, and the density of the concrete decreases, affecting the support strength.

   ② The angle of the steel mesh area can be fine-tuned (≥70°), and a small angle tilt is used to ensure that the back of the steel bar is densely filled.

  II. Operation specifications for different construction scenarios

   Leveling rock surface: strictly keep the nozzle vertical (90°±5°), use spiral or S-shaped trajectory to spray evenly, rebound rate ≤15%;

   Steel mesh/grid area: nozzle tilt ≥70°, close to the mesh (distance 0.6-0.8m), low-speed spraying to ensure that the concrete penetrates the mesh, and the rebound rate increases by more than 20% when the angle is insufficient;

   At the junction of the arch and the side wall: the robot arm preferentially covers the side wall (from bottom to top), and the arch adopts short-distance, high-frequency point spraying, angle ≥75°, and the rebound rate is reduced by 10-15% compared with the traditional method.

   III. Collaborative optimization of related factors

   1. Coordination of spraying distance

   ① The optimal distance is 1.0-1.5m: too close (<1m) will disperse the concrete, too far (>2m) will weaken the impact force, and both will aggravate the rebound.

   ② When adjusting the angle, the distance needs to be corrected synchronously: for example, when the angle drops to 80°, the distance should be shortened to within 1.2m.

   2. Robot arm motion control

   A 9-DOF robot arm is used to reduce blind spots and avoid being forced to reduce the angle due to position limitations. The intelligent sensing system can calibrate the angle deviation in real time.

   3. The influence of the amount of accelerator

   When the angle is insufficient, the amount of accelerator needs to be increased (but not more than 5%) to accelerate the initial setting and reduce the shedding caused by flow.

  IV. Operation suggestions

   1. It is preferred to use an intelligent wet spraying trolley equipped with an angle sensor and automatic calibration function.

   2. Training points: Strengthen the operator's grasp of the angle-distance correlation and simulate the steel mesh area spraying training.

   3. Abnormal handling: When the angle is limited, the pumping speed can be temporarily reduced (to maintain the density of the material bundle), and the cleaning frequency can be increased to prevent pipe blockage.

   Tip: In actual construction, parameters need to be adjusted dynamically based on surrounding rock seepage and concrete mix ratio (such as sand ratio 55%-65%). See the test data for details.