Milling

Question 1

What is the purpose of milling? What can it affect?

Answer 1

reduces particle size
- process is known as comminution

affects

• mean particle size
• distribution

Question 2

What is crack propagation?

Answer 2

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)

Question 3

What properties must material have to allow crack propagation?

Answer 3

materials must already be brittle

Question 4

How does Brinell’s method measure surface hardness?

Answer 4

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.

Question 5

Which materials are difficult to comminute?

Answer 5

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

Question 6

What are the different methods of size reduction?

Answer 6

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

Question 7

How does cutter mill work?

Answer 7

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

Question 8

How does hammer mill work?

Answer 8

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

Question 9

How does vibration mill work?

Answer 9

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

Question 10

How does roller mill work?

Answer 10

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

Question 11

How does ball mill?

Answer 11

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

Question 12

What factors affect the ball mill method?

Answer 12

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

Question 13

How does fluid energy mill work?

Answer 13

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

Question 14

How does runner mill work?

Answer 14

compression method

• rotates around the axis
• the stainless steel balls move over the particles and break it down

Question 15

How does roller mill work?

Answer 15

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.