Milling Flashcards
What is the purpose of milling? What can it affect?
reduces particle size
- process is known as comminution
affects
- mean particle size
- distribution
What is crack propagation?
crack propagation occurs during size reduction
- highly localised stress produces strain in the particles
- strain causes bond rupture and propagation of crack through regions which have discontinuities (flaws)
What properties must material have to allow crack propagation?
materials must already be brittle
How does Brinell’s method measure surface hardness?
a known constant load applied on spherical indenter that in contact with the testing material
the permanent deformation formed is used to calculate the material hardness.
Which materials are difficult to comminute?
very hard materials
elastic materials
- lowering the temperature below glass transition temperature enables size reduction
= below glass transition temperature, they behave like glass and shatter when force/stress is applied
What are the different methods of size reduction?
cutting methods
- cutter mill
impact methods
- hammer mill
- vibrational mill
attrition methods
- roller mill
combined impact and attrition methods
- ball mill
- fluid energy mill
compression methods
- runner mill
- roller mill
How does cutter mill work?
cutting method
a series of knives are attached to a rotor and move against series of stationary knives attached to the casing
- applies cutting force
How does hammer mill work?
impact method
- made of four or more hammers attached to a central shaft. all are enclosed by a rigid metal case.
- while milling, the hammers move radially and its angular velocity produces a strain rate of 80s-1 which is high enough for most of the particles to undergo brittle fracture
How does vibration mill work?
impact method
- around 80% of the mill is filled with porcelain or stainless steel balls
= balls vibrate via springs and move up and down, hit the particles upon dropping causing them to break
How does roller mill work?
attrition method
- made of two or three porcelain/metal rollers mounted horizontally and separated by a gap
- when particles passes into the gaps, shearing takes place.
size of gap affects particle size
How does ball mill?
combined impact and attrition method
- consists of a hollow cylinder that can rotate around its horizontal longitudinal axis
= balls hit the particles - impact method
= particles hit each other - attrition method
What factors affect the ball mill method?
angular velocity/speed of rotation
- too high = balls are thrown out to the mill wall by centrifugal force
- too low = very little movement of the balls
ball size
- the bigger the balls, the higher the impact force and greater size reduction
amount of feed/pharmaceutical material
- too much can have a cushioning effect
= pharmaceutical substance cushions the balls and reduces impact
How does fluid energy mill work?
combined impact and attrition method
- made of hollow toroids of diameter of 20-200 mm.
- air is injected as high-pressure jet through nozzles resulting in forming zones of turbulence.
= particles hit each and the wall of the mill - coarse (large) particles are retained in the toroid by means of particle size classifier until sufficiently fine
How does runner mill work?
compression method
- rotates around the axis
- the stainless steel balls move over the particles and break it down
How does roller mill work?
compression method
- two cylindrical rollers that rotate around their long axis
= one of the rollers is driven directly
= introduction of particles between the two rollers drives the rotation of the second rollers by friction.
