Knives – the sharper, the better

Knife design and quality are directly related to noise, dust, vibrations and energy consumption. The blunter the knife, the more energy it will consume. And the more dust, noise and vibration it will generate. Well-maintained, sharp knives not only deliver better regrind quality, they also last longer.

The Rapid Constant Cutting Circle (CCC) guarantees the correct distance between screen and knives that is essential for high quality regrind, especially after multiple knife re-sharpenings.

At Rapid, you can pick from a range of knives tailored for different materials. And no matter which you pick, you can be sure you’ve invested in a quality knife that will be efficient and last longer between re-sharpenings.

In need of spare parts?

Order your Rapid original parts online for same-day delivery from our stock.

The correct knife gap

The gap between the rotating knives and the fixed knives is crucial for regrind quality and for minimizing dust and noise from the cutting process, as well as for energy consumption.

The general guidelines for the correct gap are (mm):

  Optimal gap         Maximum gap      Optimal gap film
   (mm)
(mm)
(mm)
150-Series       0.15-0.30 0.60
200-Series 0.15-0.25 0.60 0.10-0.15
300-Series 0.20-0.40 0.70 0.15-0.20
300-LBB 0.20-0.40 0.60 0.15-0.25
400-Series 0.20-0.40 0.70 0.15-0.20
500-Series 0.20-0.50 0.80 0.20-0.30
600-Series 0.20-0.60 0.90 0.30-0.40
Raptor
0.40-0.80  0.90 0.40-0.60


If running the granulator with a gap wider than maximum recommended, the stress put to the granulator is increasing exponentially to a level that can be considered as machine misuse, that might violate the warranty.

150-Series
Optimal gap: 0.15-0.30 
Maximum gap (mm): 0.60
Optimal gap film: -
200-Series 
Optimal gap: 0.15-0.25
Maximum gap (mm): 0.60
Optimal gap film: 0.10-0.15
300-Series
Optimal gap: 0.20-0.40
Maximum gap (mm): 0.70
Optimal gap film: 0.15-0.20
300-LBB
Optimal gap: 0.20-0.40
Maximum gap (mm): 0.60
Optimal gap film: 0.15-0.25
400-Series
Optimal gap: 0.20-0.40
Maximum gap (mm): 0.70
Optimal gap film: 0.15-0.20
500-Series
Optimal gap: 0.20-0.50
Maximum gap (mm): 0.80
Optimal gap film: 0.20-0.30
600-Series
Optimal gap: 0.40-0.60 
Maximum gap (mm): 0.90
Optimal gap film: 0.30-0.40
RAPTOR
Optimal gap: 0.40-0.80 
Maximum gap (mm): 0.90
Optimal gap film: 0.40-0.60

The correct torque
  • Rapid provide knife bolts in 12.9 quality only which is the only quality withstanding the forces applied in granulation.
  • The only way to know that the bolts are tightened to the correct torque is to use a torque wrench.
  Follow the specified torques below:

Granulator Series Bolt size, 12.9 quality         Rec. torque Nm
150 series, staggered rotor M8  37
200-Series, staggered rotor M10 75
200-Series, 3-blade open rotor        
M16
280
300-Series M16   280
300-LBB M16   280
400-Series   M16 280
500-Series  M20   600
600-Series  M20 600
RAPTOR, QuadCUT rotor M10 75
RAPTOR, Claw rotor M12 120
Rapid 150 series, staggered rotor
Bolt size, 12.9 quality: M8 
Rec. torque Nm: 37
200-Series, staggered rotor
Bolt size, 12.9 quality: M10
Rec. torque Nm: 75
200-Series, 3-blade open rotor   
Bolt size, 12.9 quality: M16
Rec. torque Nm: 280
300-Series
Bolt size, 12.9 quality: M16 
Rec. torque Nm: 280
300-LBB
Bolt size, 12.9 quality: M16 
Rec. torque Nm: 280
400-Series  
Bolt size, 12.9 quality: M16 
Rec. torque Nm: 280
500-Series
Bolt size, 12.9 quality: M20 
Rec. torque Nm: 600
600-Series 
Bolt size, 12.9 quality: M20
Rec. torque Nm: 600
RAPTOR , QuadCUT rotor
Bolt size, 12.9 quality: M10
Rec. torque Nm: 75
RAPTOR , Claw rotor
Bolt size, 12.9 quality: M12
Rec. torque Nm: 120

(for further information and older machines, please refer to the instruction manual)

Sharpening of knives
  • How long the knives remain sharp depends on type of material, contamination and naturally, throughput
  • Knives that are properly sharpened increase capacity, lower the amount of dust and fines, lower energy consumption and overall, minimize the stress to the granulator.
  • Always have at least one set of extra knives, sharpened and preset to assure minimum downtime when stopping for knife change
  • A set of rotating knives must be grinded to the same dimensions to keep the rotor balanced. Always grind the most worn knife first to set the level.
  • A set of knives lasts for 10-12 resharpenings with normal wear
  • Staggered rotors like in the 150- and 200-Series use small cassette knives that cannot be resharpened
  • Rapid provide professional re-sharpening of your knives, making the knife edges as sharp and lasting as new ones
The right knife quality

Rapid ProTech is Rapid’s standard material for knives and delivers excellent results in the majority of applications. The knives are produced from high alloy steel, with a unique heat treatment process that gives approximately 30% longer life for general plastic. 95% of all sizes are available from stock.

In addition to the standard material, a number of alternative materials are available and suitable for e.g. contaminated materials or for abrasive material containing e.g. glass fibre. Contact Rapid to discuss which knives suite your production.