Yarn Lot Consistency and Colour Matching for Knitted Bags: A Buyer's Decision Sequence

Decision 1: One Lot or Several, Taken Before Yarn Is Booked

The first decision is how many dye lots the programme will use, and it has to be taken before the yarn is booked because it sets the quantity ordered per lot. A single dye lot for the whole programme is the safest choice and it is achievable for most knit bag runs, because a dye lot for polyester or polyamide yarn is typically several hundred kilograms, which is far more yarn than a 100 pcs bag programme consumes. The constraint is not capacity; it is planning.

Single-lot working removes an entire category of dispute. Panels knitted from one lot cannot show a lot line, so there is nothing to argue about at inspection, and the colour approval applies to the whole shipment rather than to a portion of it. The cost is that the yarn has to be ordered up front in one quantity, which requires the colourway split to be reasonably settled before booking. Where a buyer genuinely cannot commit to a split, the answer is to accept several lots and to control the difference between them, which is what decisions two to five are for.

Decision 2: Instrument Tolerance or Visual Standard

The second decision is what the standard actually is. A physical approved swatch is necessary but it is not sufficient, because a swatch fades, gets handled and moves physically between the buyer and the production floor. An instrument tolerance turns the standard into a number: state the reference, the instrument geometry, the colour difference formula and the limit. A typical line reads dE 1.0, CMC formula, measured with a d/8 spectrophotometer, calculated under ISO 105-J03.

There is a trade-off between tightness and yield, and it should be understood before the number is written. A tolerance of dE 0.8 is achievable on a single lot with careful process control, but it raises the rejection rate at dyeing and therefore the cost. A tolerance of dE 1.5 is comfortably achievable and is usually invisible on a bag, because bags are large objects seen at distance in variable light. Most programmes do best at dE 1.0 on primary colourways and dE 1.5 on secondary ones, which concentrates the process effort where the brand is most visible.

Decision 3: Which Illuminant Decides the Match

The third decision is the illuminant, and it is the one most often left out of a spec. Colour matching is meaningless without a light source, because two panels can match under one illuminant and diverge under another. That divergence is metamerism, and it is common when a yarn is dyed with a different dyestuff combination from the reference, which happens whenever a mill substitutes a dye for availability or cost reasons. Naming the illuminant prevents the substitution from being invisible.

The practical standard is to evaluate under D65 as the primary illuminant and to check under at least one store lighting condition as a secondary. Retail environments vary widely, and a secondary check under a fluorescent store illuminant catches the metameric pair that would otherwise show up on a shop floor rather than in a light booth. Write both into the spec: primary D65 for acceptance, secondary check for information, and require the mill to report both readings on the lab dip.

Primary illuminant

D65 for acceptance decisions

Secondary check

one store illuminant, reported for information

Instrument geometry

d/8 spectrophotometer, specular included or excluded stated

Colour difference formula

CMC, calculated under ISO 105-J03

Primary colourway tolerance

dE 1.0

Secondary colourway tolerance

dE 1.5

Grey scale assessment

ISO 105-A02 grade 4-5

Decision 4: What Counts as an Acceptable Lot-to-Lot Difference

If the programme will run more than one lot, the fourth decision is the inter-lot window. This is a different number from the tolerance against the reference, and conflating the two is a common error. A lot can sit at dE 0.9 from the reference and another lot can also sit at dE 0.9, while the two lots differ from each other by dE 1.8, because they sit on opposite sides of the standard. The inter-lot window has to be written separately and it is usually tighter than the reference tolerance.

A workable window is dE 1.0 between lots measured against each other, with the additional rule that consecutive lots must fall on the same side of the standard. That second rule sounds pedantic and it is the one that actually prevents the visible line, because it stops the programme from oscillating around the reference. Where a programme is long enough to need three or more lots, ask for a shade sequence plan up front, so each lot is dyed deliberately rather than matched to whatever came before it.

Decision 5: Where Panels From Different Lots May Touch

The fifth decision is geometric: where in the bag may two lots appear next to each other. The sensitivity is not uniform. Two panels meeting along a long straight seam on the front of a tote will show a line that a customer sees immediately. The same two lots used on a front panel and an interior lining will never be compared by anyone, because they are not seen together in the same glance. Sorting lots by visibility costs nothing and it is the single most effective control after single-lot working.

The rule to write is simple: no two lots may appear on the same visible surface, and no two lots may meet along a seam longer than a short distance. Interior linings, base panels and pocket bags are the natural places to absorb a second lot. Where the bag is small, with one dominant panel, there is nowhere to hide a second lot, and the honest answer is that the programme has to run on one lot or accept the risk. That is a decision worth taking at specification stage rather than at inspection.

Decision 6: What to Do When the Lot Runs Out Mid-Programme

The sixth decision is the contingency, and it should be written before it is needed. A lot running out mid-programme is not a rare event; it happens when a colourway sells faster than forecast or when a defect rate at knitting is higher than planned. Without a written contingency, the decision gets made under time pressure, which is when programmes accept a second lot and discover three weeks later that the two do not sit together.

A workable contingency has three parts. Reserve additional yarn from the original lot at booking, typically 10% over the calculated requirement, held for top-up. Set a rule that any second lot must be approved as a panel against the original panel, not as yarn against the reference. And state where the second lot may be used, using the visibility map from decision five. Those three clauses turn an emergency into a procedure, and they cost nothing at the point of writing.

How a Dye Lot Is Built and Why It Drifts

Understanding where drift comes from makes the controls obvious. A dye lot is a fixed quantity of yarn dyed in one bath or one continuous run, and the variables that move colour between lots are the dyestuff weighing, the liquor ratio, the temperature profile and the dwell time. Polyester dyeing with disperse dyes runs near 130°C and polyamide with acid dyes nearer 100°C, and a few degrees of difference in the ramp changes exhaustion enough to shift the shade outside a dE 1.0 tolerance.

Water quality and substrate variation are the two that catch people out. Dyehouse water chemistry changes seasonally, and a recycled yarn substrate varies in base tint between input batches, so even a perfectly repeated recipe produces a slightly different shade on a different substrate. That is why the control has to be measurement at the end rather than recipe repetition at the start, and why a recycled programme needs a tighter lot plan than a virgin one.

• Dyestuff weighing and standardisation: small weighing errors shift shade measurably at dE 1.0 tolerance.
• Temperature profile: a few degrees of change in the ramp alters exhaustion on both polyester and polyamide.
• Liquor ratio and dwell time: both affect levelness as well as depth of shade.
• Water chemistry: seasonal variation in dyehouse water is a real and frequently overlooked variable.
• Substrate base tint: recycled yarn varies between input batches, so the same recipe gives a different shade.

Measuring Colour: Metamerism and the Instrument Question

Metamerism is the reason a visual approval is not enough. Two panels match under the light booth's D65 and separate under store lighting because their reflectance curves differ even where their tristimulus values agree under one illuminant. The instrument does not solve this on its own; it solves it when the spec requires readings under two illuminants, because a metameric pair will show a much larger difference in the second reading than a non-metameric one.

The instrument question itself is about repeatability rather than brand. Any modern spectrophotometer with d/8 geometry and UV calibration is adequate; what matters is that both parties state whether specular is included or excluded, because including specular component changes the reading on a shiny filament surface considerably. A knitted bag panel made from filament yarn is shiny enough for this to matter, so the spec should state the setting along with the tolerance.

Fastness Checks That Belong With the Colour Decision

Colour and fastness are decided together, because a shade that cannot hold its rating is not a usable shade. The three that matter for a knitted bag are wash fastness by ISO 105-C06, light fastness by ISO 105-B02 and rubbing by AATCC 8, and each has a staining and a change component that should be read separately. A deep shade can hold grade 4 on change of shade and fall to grade 3 on staining of an adjacent light panel, and only one of those numbers is visible in a finished bag.

The check that is most often skipped is colour transfer within the bag itself. Where a bag pairs a dark panel with a light lining, or dark webbing against a pale body, the risk is transfer rather than loss. Add a transfer clause to the spec requiring ISO 105-C06 staining assessment on the actual combination rather than on the panel alone, because the combination is what the customer receives and the panel is not.

Lot Records, Inspection and the Document Trail

A lot plan is only as good as its records. Every lot number should appear on three documents: the yarn cone label, the knitting production record, and the inspection report. When those three agree, a query about a colour issue can be answered by identifying exactly which bags came from which lot, and the answer takes minutes. When they disagree, the only safe response is to hold the whole shipment, and the cost of that falls on the programme rather than on the yarn.

Inspection is where the plan is tested. Inspection runs to AQL 2.5 under ISO 2859-1, which sets how many units to pull and how many defects to accept, and colour is a defect class that has to be defined in advance. The inspector needs the reference, the tolerance and the illuminant, plus a rule for where a lot line is acceptable. Without those, the inspection report records an opinion about appearance, and an opinion is not something a buyer can act on or dispute.

Frequently Asked Questions

People Also Ask