Field notes from the plant floor
PU troubleshooting guide
Thirty years of colour, release agents and in-mould coatings produce a short list of problems that keep coming back. Here is each one: what causes it, how to prove it, and how to fix it. Where the cause is your process and not our product, we say so.
9 guides
PU colorants
Shade drift, settling, specks, scorch, migration and dosing problems.
Why does the colour of my PU foam or moulded part vary from one block, shot or batch to the next?
In most plants shade drift is a process problem before it is a pigment problem. The usual causes are colorant dosing that is not held constant (pump calibration, temperature-driven viscosity change, settled colorant in the day tank), changes in polyol or catalyst, and different reaction temperatures. Measure first, then change one variable at a time.
Causes and fixesWhy has my PU colorant settled, separated or turned into a hard cake at the bottom of the drum?
Settling is physical separation of the pigment dispersion. Heavy inorganic pigments settle fastest. Soft sediment can be re-dispersed by thorough mixing, but a hard cake means the dispersion has changed and the batch should not be used for colour-critical work. Rotate stock first-in first-out, agitate before use and never thin the colorant to loosen it.
Causes and fixesWhy is the core of my slabstock foam block darker or browner than the outside, and can the colorant fix it?
This is scorch: the reaction heat in the middle of a large block is high enough to degrade the polymer and some pigments. A colorant cannot cure a formulation scorch problem, but the right pigment choice stops it being made worse or more visible. Pastels and whites show it first. Tell your supplier the block size and water level so heat-stable pigments are specified.
Causes and fixesWhy did my PU foam collapse, split or come out at a different density after I introduced a new colorant?
A colorant is not inert. It carries moisture, carrier and pigment that can interact with surfactant and catalysts. Moisture above about 0.2 % adds blowing water, an incompatible carrier upsets the silicone surfactant, and some pigments adsorb amine catalyst. Test any new colorant at its working dose against a carrier-only blank before scaling up.
Causes and fixesWhy do I see coloured specks, streaks or marbling in my polyurethane parts?
Specks are undispersed pigment agglomerates. Streaks and marbling are incomplete mixing of colorant into polyol. Check the colorant fineness of grind first, then the mixing and filtration on your machine, then colour changeover.
Causes and fixesWhy does my coloured polyurethane part yellow or fade when it is exposed to sunlight?
Aromatic polyurethanes (TDI and MDI based) yellow under UV whatever the pigment, and some organic pigments fade on their own. Whites and pastels show it most. Fix it with light-fast pigments, a UV-stable lacquer or in-mould coating carrying UV absorbers and HALS, or an aliphatic system for demanding exterior use.
Causes and fixesWhy is colour from my PU part bleeding or migrating into PVC, fabric, paint or adhesive it touches?
Migration happens when soluble dyes, or pigments with soluble fractions, move into the contacting material, helped by plasticisers, heat, humidity and pressure. Use insoluble pigments, avoid dyes where contact with PVC, fabric or white laminates is expected, and run a contact test under conditions agreed with your customer.
Causes and fixesWhy does my PU part match the approved swatch in the lab but not under shop, showroom or outdoor light?
This is metamerism: two colours made from different pigment combinations can match under one light source and differ under another. Approve against a physical standard under at least daylight (D65), a store light (TL84) and incandescent (A), and match on the real substrate and gloss, not on a different material.
Causes and fixesWhy is my colorant dosing inaccurate, and why are my dosing pumps wearing out?
Dosing error comes from viscosity swings, air in the line, abrasive pigments wearing soft parts, and very small volumetric doses. Check dosing by weight at working temperature, bleed air, use hardened wear parts for abrasive colours, and ask for a dosing colour engineered for your machine type.
Causes and fixes
4 guides
Mould release agents
Sticking, build-up, pinholes and parts that will not paint or bond.
Why are my PU parts sticking to the mould or tearing when I demould them?
Sticking means the release film is missing, too thin, still wet, or the wrong chemistry for your system and mould. Check mould temperature, flash-off time and the condition of the mould surface before changing product. Spraying more rarely fixes it and usually causes build-up.
Causes and fixesWhy is residue building up on my PU moulds and how often should I clean them?
Build-up is the non-volatile fraction of the release agent plus PU dust and flash. Over-application speeds it up. It dulls the surface, blurs texture and later causes sticking and paint defects. Spray thin and even, and clean on a cycle-count schedule with a cleaner suited to the mould material. Epoxy moulds can be attacked by aggressive solvents.
Causes and fixesWhy will paint, lacquer or adhesive not stick to my moulded polyurethane parts?
Almost always the cause is surface contamination from release agent, above all silicone, transferring to the part. Use paintable, non-silicone release grades, confirm contamination with a water-break or dyne test, and keep silicone sprays out of the production area altogether.
Causes and fixesWhy do I get pinholes, blisters or bubbles on the surface of integral skin or moulded PU?
Isocyanate reacts with water to make CO₂. If water sits at the skin surface, from a water-based release agent that has not flashed off, a damp or cold mould, or moist polyol and colorant, it forms gas bubbles right at the skin. Fully dry the mould, hold the correct mould temperature, apply a thin film and control raw-material moisture.
Causes and fixes
3 guides
In-mould coatings
Peeling, cracking, orange peel and spray defects in the coating film.
Why is my in-mould coating peeling or delaminating from the polyurethane part?
Adhesion fails when something stands between the coating and the PU. The usual culprits are release agent that is excessive or not coating-compatible, a coating film poured too early (still wet) or too late (fully dried), wrong mould temperature, silicone contamination, or a coating not matched to the PU. Validate the pairing of release agent, coating and PU first.
Causes and fixesWhy does my in-mould coating show orange peel, craters, wrinkles, pinholes or a white haze?
These are spray and solvent problems. Too thick or wet a film wrinkles, too little flash-off pops solvent into pinholes, oil or water in compressed air makes craters (fisheyes), poor atomisation gives orange peel, and a fast thinner in cool humid conditions causes blushing. Fix the spray set-up and match the thinner to conditions.
Causes and fixesWhy does the in-mould coating or lacquer crack when my PU part is flexed?
The coating is less flexible than the PU beneath it, or it is too thick, or the part is flexed cold. Match coating elongation and hardness to the flex demand of the part: footwear soles need a flexible film, while a steering wheel or armrest can take a harder one. Chemie coatings are adjustable for tensile strength and abrasion resistance.
Causes and fixes
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