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TIPS TO ENHANCE QUALITY IN ROTATIONAL MOULDING
Carbon Black is popular and cost effective UV stabilizer which shields plastics from
harmful UV rays avoiding loss of mechanical properties and/or discoloration,
cracking, fading, and chalking,
in outdoor (like water tanks) and other applications of plastics, It is experimentally
proven that 2.5% addition of carbon black gives best result.
How to achieve 2.5% Carbon Black in Roto moulding material:
All black master batches looks alike but carbon black loading normally varies from
30% to 60 %.
Find out from your master batch supplier how much is loading. If carbon loading in
master batch is X%, lets say 50%. For every 100 Kg of LLDPE you need to add 2.5
kg of raw carbon. So you will need to add - (2.5 x 100/X) Kg i.e. 5 kg of master batch
of 50 % to be added to 100 kg of LLDPE to get 2.5% loading.
Rotational moulding process does not contribute much to dispersion of carbon black
as rotational moulding is very low shear process and if poorly dispersed or diluted, its
full benefit will not be realized. It is highly advisable to achieve proper dispersion
during compounding. Dry blending of carbon black with LLDPE powder in high
speed mixture is not right way of dispersion and should be avoided.
Expected life of Single Layer all black tank with 2.5% carbon content is highest i.e.
more than 15 years.
Double Layer Tank with outer black layer will last more than 10 years only if carbon
content is 2.5% and black layer thickness in tank is more than 2.5 mm.
Triple Layer Tank with middle black layer will last more than 10 years only if carbon
content is 2.5% and black layer thickness is more than 2.0 mm.
In nutshell it is very important for rotational moulder maintain 2.5% carbon black
loading and of 2.5 mm thickness of black layer in multilayer tank, so to offer
maximum guarantee to customer and to gain, retain and enhance their market
share.
PULVERIZER TROUBLE SHOOTING
Rotational molding needs resins in a pulverised powder form which plays a very
important role. The particle size and particle-size uniformity are the measures of
powder quality. The powder must be smooth, near cube shaped particle and uniform
in size. Such powder allows uniform flow into sharp corners and angle of the mould.
It melts and fuses with minimum heat and minimum time.
The particle size mostly is specified as 35 mesh in which more than 95% of the
powder passes through 500 micron screens, however powder passing through 20-
25 meshes is good enough. The powder must be moisture and contamination free
before molding. A good pulveriser must have following features-
lUniform and consistent powder quality.
lHigh output to meet the requirement of rotational molding machine
lPower consumption to output ratio must be suitable
lReplacement of any bearing, part must be easy
lCooling must be sufficient and effective – to avoid mill jams

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