Managing Color Inconsistencies and Pigment Bleed in Silicone Cavity Batches

The second cavity is a shade darker than the first, and the third has a pink halo at the gate. That is not one problem. Shade drift across a shot, drift across a lot, and true pigment migration are different mechanisms. If you chase them with a single “add more color,” you will stain the next customer’s natural run.

This covers pigmented HCR and LSR in multi-cavity tools, plus RTV that was tinted on the bench. Food and medical colors have extra rules; those sit on top of the same physics.

Name the defect before you open a mill

Shot-to-shot or cavity-to-cavity shade mismatch. Same lot, same hour, different cavities or different shots look like different colors. This is fill, temperature, or dispersion, not a new pigment.

Lot-to-lot drift. Monday’s red does not match Thursday’s red. This is weigh-up, masterbatch lot, or a hopper that was not emptied.

Bleed / migration / halo. Color walks to the surface, onto a mating plastic, into chocolate, or into a solvent wipe. A halo around a gate or a bond line is often inhibition or oil carrying pigment, not “too much dye.”

Staining of the mold. The steel or the RTV cavity takes on the color and reprints it. That is deposit, not the part’s pigment load alone.

Walk the parts in that order. Mixing the names mixes the fixes.

Dispersion and loading (the usual cavity batch fault)

Pigment that is not fully broken down acts like specks and like local loading swings. HCR needs mill or mix time that actually distributes the masterbatch; a short mix to “save heat” leaves streaks that the press then orients along flow. LSR needs a pigment paste compatible with the A/B streams, dosed by a pump you calibrate, not a cup at the mixer.

Loading. Every gum has a pigment loading above which you buy opacity and lose physicals, and sometimes cure. Chasing a deep black by dumping more carbon into platinum LSR is a classic inhibition and set-up problem: sticky surface, incomplete cure, then a surface that rubs off as grey. Stay inside the gum supplier’s food or healthcare loading if those apply. For industrial, stay inside the masterbatch sheet.

Masterbatch carrier. A carrier that is not the same polymer family can bloom and take color with it. That bloom reads as bleed. For food, the carrier has to belong on the 21 CFR / BfR list. A cheap color paste that worked in a toy HCR can fail a chocolate sensory test.

Weigh-up. Color at 0.5-2 percent is unforgiving. A scale that is fine for gum is not fine for paste. Dedicated cups, tare discipline, and a retained plaque from every lot.

Heat and cure that shift shade

LSR color can shift with cavity temperature and cure time because the base yellows slightly or because the pigment’s undertone moves. Cavity 1 that runs hotter than cavity 16 will not match. A cold deck that drools and then scorches at the gate can darken a vestige halo.

HCR post-cure often deepens or settles a color. If QC inspects as-molded and the customer inspects after post-cure, you will fight a ghost delta E. Inspect at the shipped state, under a defined illuminant. Do not match a baked part to a raw plaque.

Undercure leaves a tacky face that picks up dirt and looks like mottling. That is not pigment.

Bleed that is actually chemistry

Some organic reds and yellows migrate in silicone, especially with oil-extending fillers or with a plasticizer that was never supposed to be there. The test is a contact stack: press the part against white polyethylene or a painted plaque at temperature and time that match service (or a chocolate plaque for culinary). If color transfers, you have migration. Changing hardness will not fix it. Changing pigment chemistry will.

Platinum inhibition around a gate, a bond, or a contaminated insert leaves a sticky, often lighter or pinkish ring that then picks up dirt. That ring is unreacted gum plus pigment sitting in oil. Clean the inhibitor (sulfur, tin, amine, some blacks, some tapes), do not add white to “correct” it.

Food contact: a pigment that bleeds into fat is a failed article, even if the gum passed 177.2600 as a natural slab. Test the colored article.

Mold and hopper hygiene

A 16-cavity tool that just ran carbon will ghost into a translucent optical lot. Dry-ice the face, ultrasonic the inserts if the texture holds pigment, purge the mixer until the purge is visually clean, then run natural start-up shots you will scrap. RTV cups that were used for red will tint the next natural pour. Throw the cup.

Steel that is porous or rusted holds color. That is a polish and plating problem.

A shop sequence that isolates the cause

1. Compare cavities in one shot. If they differ, look at temperature, fill, and gate balance first, not the formula.
2. Pull the retained plaque from the mill or the LSR log. If the plaque already mismatches the standard, stop the press. You are not going to mix your way out in the cavity.
3. Wipe the part with a white cloth, dry, then with IPA. If color comes off dry, you have a surface cure or bloom problem. If only IPA takes it, you may have migration; run the contact stack.
4. Check loading and lot of masterbatch. Two drums of “the same red” are two lots.
5. Only then change pigment. Keep a frozen or retained color standard and a written recipe. Do not match by memory under warehouse sodium lights.

Put on the RFQ: masterbatch type, loading, color standard (plaque, not a printout), illuminant, inspect-after-post-cure, and a no-bleed contact test if the part meets food, skin, or painted housings. A cavity batch is a controlled mix. Treat it that way and the steel will stop taking the blame.

FAQ

Why is cavity 2 a shade darker not the same problem as a pink halo at the gate?

Shade mismatch in one shot is fill, temperature, or dispersion. Lot-to-lot drift is weigh-up, masterbatch lot, or a dirty hopper. A halo at a gate or bond is often inhibition or oil carrying pigment, not "too much dye." Staining of the steel is deposit. Name the defect before you open a mill.

Why not dump more carbon into platinum LSR to get a deeper black?

Loading past the gum supplier's sheet buys opacity and loses physicals, and carbon can inhibit platinum: sticky surface, incomplete cure, grey rub-off. Stay inside food or healthcare loading when those apply. A carrier that is not the same polymer family can bloom and take color with it.

Why inspect color after post-cure under a defined illuminant?

HCR post-cure often deepens or settles a shade. QC as-molded versus customer after the oven is a ghost delta E. LSR color also shifts with cavity temperature; a hotter cavity will not match a cooler one. Undercure leaves tack that picks up dirt and looks like mottling.

How do I tell true pigment migration from an inhibition ring?

Press the part against white polyethylene, paint, or chocolate at service temperature and time. Color transfer is migration; change pigment chemistry, not hardness. A sticky lighter ring around a contaminated insert is unreacted gum plus pigment in oil; clean sulfur, tin, amine, or the offending black, do not add white.

Why does a tool that just ran carbon ghost a translucent optical lot?

Pigment packed in texture reprints. Dry-ice the face, ultrasonic inserts if needed, purge until visually clean, then scrap natural start-up shots. RTV cups that mixed red will tint the next natural pour; throw the cup. Porous or rusted steel holds color as a polish problem. Compare cavities in one shot before you change the formula.