Short answer: Film coating is the practical, small-lab-accessible method — a thin polymer coating (commonly HPMC-based) is sprayed onto tumbling tablets in a coating pan until it builds an even layer, then dried. It's done to mask taste or odor, protect the tablet from moisture, make swallowing easier, or add a branded appearance. It requires dedicated equipment beyond a tablet press — a coating pan and spray setup — and isn't necessary for every product; plenty of tablets ship uncoated with no issue.

Coating is a separate process and a separate equipment investment from pressing — it's worth understanding before you decide whether your product actually needs it, rather than assuming every tablet should be coated by default.

Why Coat a Tablet

  • Taste and odor masking. An uncoated tablet's surface is exposed directly to the tongue; a coating can mask an unpleasant-tasting or -smelling active far more effectively than formulation tricks alone.
  • Moisture protection. A coating adds a barrier against ambient humidity, which matters for actives or excipients that are moisture-sensitive — see how to store homemade tablets for the storage side of the same concern.
  • Easier swallowing. A smooth coated surface goes down easier than an uncoated compressed surface for some users.
  • Appearance and branding. Coating allows color and a glossy or matte finish that an uncoated tablet can't achieve, on top of whatever shape and engraving the punch and die already provide.

Types of Coating

Film coating is the standard modern method and the practical choice for a small lab: a thin polymer layer, commonly built on hydroxypropyl methylcellulose (HPMC), sprayed on as a solution or suspension and dried in place. It's relatively fast, doesn't add much bulk to the tablet, and is what nearly all commercially coated tablets use today.

Sugar coating is the older method — building up layers of sugar syrup over many application-and-drying cycles — and is largely obsolete outside specific legacy products. It takes far longer, adds significant bulk and weight, and offers no real advantage over film coating for a new product. This guide focuses on film coating for that reason.

Basic Equipment Needed

Coating requires equipment beyond what pressing alone needs: a coating pan (a rotating drum that tumbles the tablets continuously so the coating applies evenly) and a spray system (an atomizing nozzle that applies the coating solution in a fine, even mist as the tablets tumble), plus warm air flow to dry the coating as it's applied. This is a genuine additional investment on top of a press, mixer, and dies — worth budgeting for separately rather than assuming it's included in typical press or mixer equipment.

The Film Coating Process

  1. Prepare the coating solution — typically an HPMC-based polymer dissolved or suspended in water, with color and plasticizer added as needed for the finished look.
  2. Load tablets into the coating pan and start it tumbling at a steady, consistent speed — even tumbling is what keeps the coating uniform tablet to tablet.
  3. Begin spraying the coating solution in a fine, controlled mist while warm air flows through the pan to dry each layer almost as it's applied.
  4. Build the coating gradually over multiple passes rather than trying to apply it all at once — a coating applied too quickly is prone to defects (see below).
  5. Dry fully once the target coating weight is reached, then cool before packaging.

Common Coating Problems

  • Picking — the coating tears and sticks to itself between tablets, usually from spraying too fast relative to the drying rate.
  • Twinning — two tablets stick together during tumbling, usually from an underpowered tumble speed or coating solution that's too wet.
  • Orange peel — a rough, uneven surface texture from the coating drying too fast before it has time to level out.
  • Uneven color — inconsistent tumbling or spray coverage, similar in root cause to the mottling defect seen in the tablet core itself.

Most of these trace back to the balance between spray rate and drying rate — too fast in either direction causes a different defect, and dialing it in takes a few test runs on a small batch.