blending

Imagine a pinch of strong red dye stirred into a tub of white flour. Until you stir well enough, some spoonfuls will be pink and others white. Blending is the manufacturing step that does this stirring for medicines: it works powders around until the active drug and the excipients are spread evenly, so that any small scoop of the mixture contains the same proportions as any other.

This matters because most solid medicines are made from a tiny amount of potent drug dispersed in a much larger bulk of fillers, binders and other excipients. If the blend is non-uniform, some tablets or capsules will be sub-potent and others super-potent — a direct patient-safety problem. Blending is usually done in tumbling blenders (V-shells, bins, double cones) or in high-shear mixers, and uniformity is confirmed by sampling several locations and assaying them.

A real subtlety is that mixing and un-mixing happen at the same time. Particles that differ in size, shape or density tend to separate again — by vibration during transfer, or by air during discharge — a phenomenon called segregation. So blending is not simply done for longer to be done better: over-blending can actually demix a previously uniform powder, and lubricant especially must be blended only briefly.