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 influence 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 there size reduction in mill, dust is produced which may have effect on operator.
9. Ration 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.