Factor affecting size reduction are:
1.Hardness: Soft material easy break than hard.
2.Toughness: Drug with fibrous nature or those having high moisture content are tough and hard to reduce in size.
3.Stickiness: Material adheres to the grinding surface or sieve surface of the mill. It is very difficult to powder a drug of having gummy or resinous material.
4.Material structure: Material with some special structure cause problem during size reduction e.g. Vegetable drug with cellular structure produce long fibrous particle on size reduction, similarly a mineral substance having lines of weakness, produce flake like particle on its size reduction.
5.Moisture The presence of moisture in the material influences a number of its properties such as hardness, toughness or stickiness. The material having 5% moisture in case of dry grinding and 50% in case of wet grinding is permissible.
6.Temperature: Waxy material such as stearic acid or drug containing oils or fat, become softened during the size reduction, due to heat. This can be avoided by cooling the mill.
7.Purity: In some mills during size reduction there is chances of addition of impurities. If high degree of purity is required avoid such mills or Mills should be cleaned thoroughly.
8.Physiological effect: Some drugs are very potent. During their size reduction in mill, dust is produced which may have effect on operator.
9.Ratio of feed size to product size: To get a fine powder in a mill, it is required that a fairly small feed size should be used. Hence to carry out size reduction in various stages e.g. preliminary crushing followed by coarse powder and then fine grinding.
10.Bulk density: The output of the size reduction of the material in a machine depends upon the bulk density of the substance.