When a batch of parts comes out with colors that do not match, the reflex is to start adjusting the machine. In our experience the cause sits in one of three places: whether the variation between parts follows a pattern, whether the masterbatch ratio or batch changed, and how long the material sat in the barrel. Work through them in that order and the fault is usually bracketed within half an hour, which is far cheaper than repeated trial shots.
Why designers should care: color is a specification, not a machine setting, and the tolerance has to be agreed in writing before mass production rather than argued about after shipment.
The order: distribution pattern, then masterbatch (ratio, carrier, batch), then base resin grade and regrind ratio, then thermal history (melt temperature, back pressure, screw speed). The distribution pattern alone points the direction in nine cases out of ten.
Step 1: Classify the Variation - Where It Differs Matters More Than How Much
The core answer: the first move is not to measure with a spectrophotometer but to ask about distribution. Is one of four cavities consistently darker? Does the same cavity run progressively yellower from start-up to the afternoon? Is the whole batch a step away from the previous batch? Is there a gradient across a single part from gate to end of fill? These four patterns point to four completely different places.
Cavity-to-cavity difference points to the tool. The usual cause is a difference in runner length or gate size between cavities, which changes shear rate and therefore the actual melt temperature; the cavity with the smaller gate sees more shear heating, disperses masterbatch better and often comes out darker. Mold temperature plays a part too: if the cooling circuits are unbalanced and cavities differ by more than 5 degrees Celsius, crystalline resins such as PP, POM and PA develop different degrees of crystallinity and the color follows.
Drift along the time axis points to thermal history. Parts running light for the first few dozen shots and settling afterwards usually mean residual material from the previous batch has not been purged, or that the residence time from hopper to screw is too long. A gradient within one part points to fill and shear: near the gate the shear is strong and the temperature high, at the far end both are low, so masterbatch dispersion differs, which is most visible on light-colored parts.
Step 2: Masterbatch - Check Ratio, Carrier and Batch Separately
The core answer: masterbatch takes the blame most often, and it is also the easiest link to verify. Typical addition rates run 1 to 4 percent, and white or light-colored parts sometimes go to 4 to 6 percent. A few tenths of a percent matter: on a light part a 0.3 percent difference is visible to the eye. That is why we require gravimetric dosing units in the shop rather than hand blending, where uniformity depends on the operator and one drum differs from the next.
Carrier resin has to match. A PE-carrier masterbatch used with ABS or PC disperses poorly, producing color streaks, specks or an overall mottled look. When selecting masterbatch, ask what the carrier is and keep it in the same family as the base resin. Damp masterbatch also causes variation; it is normally dried with the base resin under the same conditions, which means roughly 2 to 4 hours at 80 degrees Celsius for ABS and about 4 hours at 120 degrees Celsius for PC.
Batch-to-batch variation is the most concealed trap. For the same color code, a difference of a few tenths to one ΔE between production batches is normal. Our practice is to require a color plaque and a retained sample with every batch, to check color on arrival, and to reject a batch that fails. When switching masterbatch batches on the same machine, a purge shot goes in between so two batches never mix inside one shipment.
Step 3: Thermal History - Three Extra Minutes in the Barrel Yellows a Light Part
The core answer: we had a run of PP housings that started normally and drifted yellow through the afternoon, getting worse as the shift went on. The masterbatch checked out and the base resin had not changed. The cause was regrind: the ratio of sprue and runner material had been raised on that shift. Every thermal pass raises the yellowness index of PP a little, so the more regrind, the more visible the shift.
We normally hold regrind below 15 to 20 percent, and more conservatively still on light-colored and cosmetic parts, where the preference is to avoid it altogether.
Residence time is the most overlooked item. Shot size should sit between 20 and 75 percent of the machine's maximum shot capacity. A small tool on a large machine means the material sits in the screw for a long time and degrades into yellow; we have seen more than once a color problem that disappeared when the same material was run on a smaller machine.
Melt temperature, back pressure and screw speed all change shear heat, and with it both masterbatch dispersion and thermal degradation of the resin. When adjusting, change one parameter at a time and run at least ten shots before judging. After a stoppage, discard the first dozen shots: their thermal history is not the same as the steady state. Color changes are their own discipline, and black to white is the hardest combination; it takes a purging compound, and plain resin often will not clear it.
Step 4: Root-Cause Criteria - Decided in Minutes
The core answer: the criteria collapse into a short table that can be read on the shop floor. If the cavity-to-cavity difference clearly exceeds the time-to-time difference within one cavity, look at the tool: runners, gates, mold temperature distribution. If the color is off at start-up and recovers, look at barrel residue and residence time. If it jumps when a material batch is changed and recovers when it is changed back, look at the masterbatch batch, the base resin batch and the regrind ratio. If the gradient runs along the flow direction within a single part, look at fill and shear. If none of the four applies, look at the measurement itself.
Numerical criteria belong in the contract before production rather than in an argument after shipment. The industry normally works in ΔE: up to 1.0 is accepted by most customers, 1.0 to 2.0 is a negotiation zone, and above 2.0 the difference is obvious to the eye. Measurement conditions must be fixed: a D65 light source with a 10 degree observer, consistent plaque thickness, and no direct comparison between a textured surface and a polished one, which produces wildly different readings.
We normally seal three plaques at each limit, upper and lower, together with a visual criterion: no perceptible difference under a D65 standard light booth. On a blood pressure monitor with off-white ABS cosmetic parts, that was the specification we agreed with the customer, ΔE at or below 1.0 plus no visual difference under D65, with a part pulled every two hours during production and the reading logged on the inspection sheet so any problem can be traced to a specific time window and machine.
What Designers Can Take Away
Ask about distribution before measuring numbers. The pattern of variation points at tool, process or incoming material.
Masterbatch is usually the easiest link to prove or clear. Ask for carrier, ratio and batch, with a plaque and retained sample per batch.
Thermal history is where light parts go yellow. Keep regrind below 15 to 20 percent and shot size between 20 and 75 percent of barrel capacity.
Agree the ΔE criteria in writing before mass production. Fixed light source, fixed observer angle, sealed limit plaques and a visual criterion.
FAQ: Injection Molding Color Questions
Q1: What ΔE counts as acceptable for injection molded color?
A: Most projects work to ΔE of 1.0 or less, 1.0 to 2.0 requires customer agreement, and above 2.0 the difference is clearly visible. Light and white cosmetic parts are usually held tighter. What matters is written confirmation before production, sealed upper and lower limit plaques, and an agreed visual criterion.
Q2: A single batch has mismatched colors. What do you check first?
A: The distribution pattern, not the numbers. Cavity-to-cavity differences point to runners and mold temperature; drift over time points to barrel residue and residence time; a jump when the batch changes points to masterbatch and base resin batches; a gradient within one part points to fill and shear. Only if none applies do you suspect the measurement.
Q3: The masterbatch ratio has been adjusted and the color is still unstable.
A: Look at the blending method and the carrier first, then the regrind ratio and residence time. Replace hand blending with a gravimetric dosing unit, keep the carrier in the same family as the base resin, hold regrind low on cosmetic parts, and remember that a shot size below 20 percent of barrel capacity means long residence and drifting color.
Q4: How do you purge cleanly when changing from black to white?
A: Use a purging compound rather than expecting plain resin to clear it, then run a dozen transition shots. After the compound, continue with the natural-color material about to be produced and check every hot runner nozzle and barrel dead spot. The first dozen shots after start-up have inconsistent thermal history and should be discarded.
Hertz Industrial Design (Dongguan, China) works across product appearance design, structural design and CMF, and more than 320 of its designs have reached mass production. Website: www.hezidesign.com.
Originally published on the Hertz Industrial Design website (hezidesign.com), Materials and Manufacturing column.
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