Tablet Sticking, Picking, Chipping & Mottling: Causes & Fixes

If you've already ruled out capping and lamination — your tablets are staying in one piece, but something's still visibly wrong with the surface or edges — one of these four defects is almost certainly what you're looking at. Each one has a fairly narrow set of causes, so identifying which defect you actually have is most of the diagnostic work.

Sticking

What it looks like: Powder residue builds up on the punch face over successive tablets, eventually leaving a visible indentation, dull patch, or partial transfer of the tablet's surface back onto the punch.

Common causes:

• Insufficient lubricant. Magnesium stearate (or your chosen lubricant) isn't present in high enough concentration, or wasn't mixed in evenly, to prevent the compressed powder from adhering to the metal punch face.
• A tacky or hygroscopic formulation. Some actives and excipients are naturally prone to sticking, especially in humid conditions — the powder itself is picking up moisture and becoming sticky before it's even compressed.
• Excess compression force. Overcompressing can force powder into fine surface imperfections on the punch, increasing adhesion even with adequate lubricant.

Fixes: Increase lubricant slightly and confirm it's mixed in fully rather than clumped (see the mixing order in how to make a direct-compression powder). Check ambient humidity if the formulation is known to be moisture-sensitive. If force is on the high end of what the tablet needs, try easing it back before adding more lubricant, since over-lubrication introduces its own problems (see capping and lamination for what that looks like).

Picking

What it looks like: Small amounts of powder are pulled out of the tablet's surface and stay stuck in a specific spot on the punch face — commonly in an engraved logo or lettering, where fine detail creates more surface area for powder to catch on.

Common causes:

• Fine detail on the punch face (engraving, logos, score lines) that's harder for powder to release from cleanly than a flat or domed face.
• A punch face that's worn, pitted, or has residue buildup from a previous run — see how to clean and maintain a tablet press for a cleaning routine that prevents this from accumulating.
• The same lubrication and formulation issues that cause sticking, since picking is really a localized, detail-specific version of the same underlying problem.

Fixes: Clean punch faces thoroughly, including engraved detail, before a run. If picking is concentrated specifically in an engraved area, a formulation change (more lubricant, or a different lubricant that releases better from fine detail) usually resolves it faster than adjusting the press. Chronically bad picking on a specific punch set can also indicate it's due for replacement — see choosing punch and die shapes and sizes if you're evaluating whether a simpler face design would sidestep the issue entirely for a difficult formulation.

Chipping

What it looks like: A small piece breaks off a tablet's edge, either right at ejection or during subsequent handling — distinct from lamination, which splits the tablet into horizontal layers rather than losing a corner or edge fragment.

Common causes:

• Excess fines (very small particles) in the formulation that don't bind as strongly at the tablet edge as the bulk of the powder does.
• Low binder content, leaving the tablet's edges under-bonded relative to its center.
• A worn or damaged die bore, which can leave a rough edge on every tablet that's more prone to breaking off.
• Rough handling after ejection — a mechanically sound tablet can still chip if it's dropped, tumbled aggressively, or hits the take-off blade at an awkward angle.

Fixes: If fines are the likely cause, screening the powder before blending removes the smallest particles that contribute least to binding. If binder is on the low end for your formulation, a modest increase — the same MCC-heavy diluent/binder role described in how to make a direct-compression powder — often resolves it. Inspect the die bore for wear or damage if chipping is happening consistently at the same point in the tablet's edge rather than randomly, and see dies for replacement sets.

Mottling

What it looks like: Uneven color across the tablet's face — light and dark patches, speckling, or a visibly inconsistent shade from one tablet to the next in the same batch.

Common causes:

• Poor mixing. A colorant, a colored active ingredient, or an ingredient with a different particle size than the rest of the blend hasn't been distributed evenly through the powder — this is the cause behind the large majority of mottling cases.
• Ingredient particle size mismatch. Even a well-mixed blend can mottle if one ingredient's particles are significantly larger or smaller than the rest, since they segregate again during handling after mixing.
• Moisture migration, where localized dampness in the powder or a partially-dried granulation changes the local shade of the tablet during compression.

Fixes: Extend mixing time and confirm the mixer is appropriately sized for the batch — an underfilled or overfilled mixer blends less evenly than one loaded to its intended capacity (see how to use a powder mixer for load guidelines). If a specific ingredient is prone to segregating after mixing, consider a pre-blend step where that ingredient is diluted into a small portion of the bulk powder first, then blended into the rest. Mottling caused by particle size mismatch may require a differently-milled version of the problem ingredient rather than a mixing fix alone.