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Paddy Dehusking Roller
 
Paddy has to be dehusked and polished to convert it into the marketable form of white rice. In the past, dehusking of the paddy was done manually by pounding the paddy in a receptacle with a wooden stump. This was later replaced by mechanical dehusking with the use of rubber rollers, which gave a greater dehusking efficiency and less grain breakage. When Malaysia started her factories in the 1970's, the rollers produced were based on silica-filled styrene-butadiene rubber (SBR) compounds because of their longer service life and better thermal stability compared to natural rubber (NR) rollers. However, lately many alternative materials such as acrylonitrile rubber especially the carboxylated grades, polychloroprene rubbers, EPDM, etc. were being used to give even longer service life than the SBR rollers.

In a modern rice miller, a pair of rubber-lined rollers are mounted in an enclosed chamber and driven at a friction ratio (figure 1). A typical unit using 254 mm nominal width x 254 mm outer diameter rollers run at 1.28 : 1 friction ratio with the faster roller at 900 revolutions per minute. The nip of the rollers is manually adjusted during dehusking to compensate for the gradual abrasion of the rubber surface and its thermal softening. The paddy in passing through the rollers nip is subjected to both compressive and shearing forces. When these forces are correctly predetermined for a particular grade of paddy (by rubber hardness, resilience, rollers nip, speed and friction ratio) the paddy will remain substantially intact after going through, while the husk is broken off. In practice, the dehusking efficiency can be as high as 90% per pass.

Paddy Dehusking Roller
Paddy dehusking Machine
Figure1: Paddy dehusking machine


Paddy dehusking Machine
Rice millers are often run between 15 to 24 hours a day non-stop and causes the roller temperature reaching up to 80C.

Taking into account the above factors, the compound design of a paddy-dehusking roller should satisfy the following criteria:
  • Low cost

  • Easy processing

  • Good service life

  • Good dehusking efficiency which determined by roller hardness usually 933 IRHD, etc.

  • Good final product i.e. white rice
Manufacturing Process

There are a number of possible routes to manufacture paddy dehusking rollers (see the flow chart). The most establish process is that of mixing (two-roll mill or internal mixer), pre-warming, calendaring, laminating to the metal core, preshaping, mould insertion and autoclave curing. Press moulding is commercially practiced but the economics of scale appear unfavourable. There are also possible alternative routes such as the use of wrapped laminated nylon tape thus dispensing with the use of moulds.

 
Other Info
  Processing Flow Chart
   
   
   
   
   
   
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