Steam Boarding Knitted Bags: The Heat Setting Checklist That Holds 3% Shrinkage and 3 mm Tolerance
What does steam boarding actually do to a knitted bag? It relaxes the loops, locks the silhouette and stabilises the dimensions, so that the 3 mm tolerance and the 3% shrinkage limit mean anything at all. Applied wrong, it glazes the surface, flattens the stitch texture and sets a body 4 mm under size. This article is a working checklist: twelve items to confirm before the run, the temperature and dwell bands for polyester and nylon, the board geometry rules, and the measurement sequence that has to follow the setting step rather than precede it.
What Steam Boarding Does to a Knitted Bag, and What It Cannot Do
Steam boarding and heat setting do three things — they relax the loop structure, lock the silhouette against later shrinkage, and stabilise the dimensions that the tolerance table refers to — and they do not do the two things buyers often assume, which are adding stiffness and correcting a knitting fault.
A knitted body leaves the needle bed under tension, with the loops stretched in the direction of knitting and compressed across it. Left unrestrained, that stored strain releases slowly and unpredictably: the bag changes shape between the bed and the carton, and two pieces from the same run measure differently depending on how long they have been lying in a pile. Applying heat and moisture above the glass transition temperature of the polymer lets the chains rearrange into a lower-energy configuration, and cooling in that new configuration locks it. For polyester the glass transition sits near 70 to 80 °C and the melting point near 250 to 260 °C, which is why a dry setting band of 160 to 180 °C is effective without risking fusion.
What setting cannot do is equally important. A bag that must stand rigidly empty needs a bonded interlining or a moulded base, because heat setting adds no structural stiffness to a loop structure. And a knitting fault is a knitting fault: setting will not close a dropped stitch or hide a barre, and in practice it often makes both more visible, because relaxing the fabric opens the loop and the fault reads more clearly against the surrounding surface.
The Heat Setting Checklist: Twelve Items Before the Run
Twelve items should be confirmed before a setting run starts, and the first six are the ones that decide whether the rest matter: the polymer and its set temperature, the board reference, the dwell time, the steam pressure, the cooling method and the conditioning period.
- Confirm the polymer: polyester sets at 160 to 180 °C, nylon 6 at a lower band near 150 to 170 °C, and a blend follows the lower component.
- Confirm the board reference against the approved sample, not against a drawing, because a former wears and a drawing does not.
- Set the dwell: 30 to 90 seconds for dry heat, 8 to 12 minutes for a full steam boarding cycle including the cooling step.
- Set the steam pressure to 2 to 3 bar, giving saturated steam near 120 to 133 °C at the fabric surface.
- Plan the cooling: forced air to below 40 °C before the piece is handled, because handling warm fabric reintroduces distortion.
- Allow conditioning: 24 hours at 20 °C and 65% relative humidity under ISO 139 before the final measurement.
- Check the surface: no contact glazing, no flattening of the intended stitch texture, no watermark from uneven steam.
- Check the aperture: the handle opening must still measure to specification after setting, since this is where a board is most likely to distort.
- Check the base transition: the corner radius should be formed, not crushed.
- Log the temperature at the fabric surface rather than at the chamber, because the two differ by more than the tolerance.
- Log the piece count at every parameter change, so a drift can be traced to a run segment.
- Hold the first piece off the run and re-measure it 24 hours later, which is the only real check on whether the setting held.
The last item is the one that gets skipped and the one that matters most. A piece measured immediately after setting will read correctly and may still move overnight. Holding a first piece back and re-measuring it after conditioning catches a setting cycle that was too short or a cooling step that was rushed, before several hundred pieces have been through it.
Temperature and Dwell: 160 to 180 °C for 30 to 90 Seconds
For a polyester knit bag body the working band is 160 to 180 °C held for 30 to 90 seconds, and the correct pairing is the lower temperature with the longer dwell for a heavy body and the higher temperature with the shorter dwell for a light one.
The pairing follows heat transfer rather than chemistry. A 520 g/m² body at E7 in 600D needs time for the core of the fabric to reach temperature, so 160 °C for 90 seconds is safer than 180 °C for 30 seconds, which would set the surface and leave the interior under-set. A 240 g/m² body at E14 in 150D reaches temperature almost immediately and is better served by 180 °C for 30 to 45 seconds, because a long dwell at that weight begins to glaze the surface and flatten the stitch definition. Over-setting shows as a shiny surface, a board hand, and a small but consistent loss of thickness.
| Body specification | Set temperature | Dwell | Steam pressure | Risk if pushed |
|---|---|---|---|---|
| E7 in 600D, about 520 g/m² | 160 °C | 90 seconds | 2 bar | Under-set core, shrinkage on the first wash |
| E12 in 300D, about 320 g/m² | 170 °C | 60 seconds | 2 to 3 bar | Slight glazing above 90 seconds |
| E14 in 150D, about 240 g/m² | 180 °C | 30 to 45 seconds | 2 bar | Surface glazing and flattened stitch texture |
| Nylon 6 body, any gauge | 150 to 170 °C | 45 to 60 seconds | 2 bar | Yellowing and hand loss above 180 °C |
| Polyester and cotton blend | 160 °C | 60 to 75 seconds | 3 bar | Scorching of the natural component |
Steam Pressure, Moisture and the Cooling Step
Steam at 2 to 3 bar carries enough moisture to plasticise the fibre and speed up relaxation, and the step that follows it — cooling to below 40 °C before the piece is handled — is what actually fixes the new configuration.
Moisture acts as a plasticiser. It lowers the temperature at which the polymer chains can move, so a steamed body sets faster and at a lower temperature than a dry one at the same result, which is why steam boarding is the default for a shaped bag and dry heat setting is the default for a flat panel. The risk with steam is uneven application, which produces watermarks that are permanent on a piece-dyed body and less visible on a dope-dyed one. The cooling step is where most programmes lose the tolerance they had just achieved. A body taken off the board while still warm has not finished fixing; it continues to relax in the pile, and the direction it relaxes depends on how it happens to be lying.
The cooling and conditioning sequence
- Steam or dry heat to the specified band, held for the specified dwell
- Hold on the board through the first part of cooling, so the silhouette is fixed while the polymer is still mobile
- Forced air to below 40 °C before any handling
- Stack flat, not folded, and never compress a warm pile
- Condition for 24 hours at 20 °C and 65% relative humidity under ISO 139
- Measure the held-back first piece again after conditioning, before releasing the run
Board Geometry: Matching the Former to the Silhouette
The former decides the finished silhouette far more than the steam does, and the two rules that matter are a running gap of about 2 mm between board and body, and a corner radius on the board that matches the radius the programmer knitted into the base transition.
A board cut to the exact finished dimension will fight the fabric on loading and stretch the aperture on unloading. About 2 mm of running gap lets the body slide on without strain and still be held firmly enough to define the shape. The base corner is where geometry errors concentrate: a board with a sharp corner will crush a knitted radius that the bed spent effort forming, and the result is a base that looks creased rather than shaped. Aluminium is the usual former material because it conducts heat quickly and holds a machined profile, but it expands measurably at setting temperature, so the gap should be specified at working temperature rather than at room temperature.
- Running gap between board and body: about 2 mm, specified at working temperature rather than cold.
- Corner radius on the former: matched to the radius knitted into the base transition, not to a drawing default.
- Aperture post: sized so the handle opening is held without being stretched, since this is the most commonly distorted feature.
- Ventilation: a perforated or slotted former lets steam reach the inside of the body and avoids a dry patch on the panel.
- Former reference number: stamped on the board and recorded on the run sheet, so a worn former can be traced to the pieces it produced.
Measuring After Setting: Twelve Points and the 3% Rule
Measurement belongs after setting and conditioning, never at the machine, and the specification it is measured against is plus or minus 3 mm on a 380 mm body dimension, plus or minus 3 mm on handle drop, plus or minus 2% on finished GSM, and within 3% dimensional change after the agreed wash cycle.
Twelve points is a workable set for a tote: length and width at two heights, base depth at each end, handle drop on both handles, aperture width and height, and GSM taken on a conditioned sample. The reason for measuring at two heights rather than one is that a body can be correct at the base and wrong at the mouth, which is exactly the distortion a badly fitted former produces. Dimensional change after washing is a separate test with a separate method, AATCC 135, and it should be run on a piece that has already been through the setting cycle, because a body that has not been set will fail it regardless of how well it was knitted.
- Body length and width
- plus or minus 3 mm on a 380 mm dimension
- Handle drop
- plus or minus 3 mm, critical on shoulder-carry styles
- Finished GSM
- plus or minus 2% on a conditioned sample
- Dimensional change after wash
- within 3% in length and width, AATCC 135
- Conditioning before measurement
- 24 hours at 20 °C and 65% relative humidity, ISO 139
- Colour assessment
- against the signed-off standard under D65 lighting
Defects the Setting Step Creates When It Is Rushed
Rushed setting produces five recognisable faults — glazing, watermarks, crushed base corners, stretched handle apertures and residual shrinkage — and all five are created in this station rather than inherited from the bed, which is why they are worth naming separately from knitting defects.
Surface glazing
Dwell too long or temperature too high for the GSM. The surface goes shiny and the stitch texture flattens. Permanent on the piece.
Watermarks
Uneven steam distribution or a solid former blocking steam from the inside. More visible on piece-dyed bodies than on dope-dyed ones.
Crushed base corner
Former radius sharper than the knitted radius. The base looks creased rather than shaped and will not recover.
Stretched aperture
Board gap negative at working temperature, so the body is forced on and the handle opening is pulled on unloading.
Residual shrinkage
Under-set core, usually a heavy body given too short a dwell. Passes inspection and fails on the first wash.
The commercial difference between these faults and knitting faults is where they appear. A knitting fault shows at the bed or at the inline check, while a setting fault frequently passes every check in the workflow and appears at the customer's first wash. That is the argument for the held-back first piece: it is the only control that tests whether the cycle actually held, rather than whether the bag measured correctly on the day it was packed.
Where Setting Sits in the Sampling and Bulk Windows
Setting occupies about 2.5 minutes of touch time per piece and sits at station 4 of the workflow, which places it on day three of the 7-working-day sampling window and between days 18 and 26 of the 30-day bulk window.
In sampling, the setting step is what makes the fitting on day three meaningful: until the body has been set and conditioned, a fit comment is being made on a shape that has not stabilised. Programmes that skip setting on the first sample routinely approve a silhouette and then watch it change. In bulk, setting runs concurrently with the tail of knitting, which means a setting parameter error is discovered against a queue of several hundred pieces rather than against one. MOQ is 100 pcs per style or colourway, sampling runs to 7 working days, bulk to 30 days, and the release is against AQL 2.5 under an ISO 2859-1 sampling plan. FOB Xiamen is the standard term, and the reference unit cost for a standard E12 tote at specification is USD 4.80.
Get a setting cycle specified for your body
Send the silhouette, the gauge, the yarn count and the target GSM. Our knit team will return a setting band, a dwell, a former specification and a measurement sequence, quoted against MOQ 100 pcs per style, 7 working days of sampling and 30 days of bulk production. Start on the contact page.
Frequently Asked Questions
What is steam boarding on a knitted bag?
Placing the knitted body on a heated shaped former and applying steam so the loop structure relaxes into the finished silhouette and the dimensions lock. It is the step that makes the 3 mm tolerance meaningful, because an unset body keeps relaxing after it leaves the bed and two pieces from one run will measure differently.
How is steam boarding different from heat setting?
Steam boarding uses moisture as a plasticiser on a shaped former and suits a three-dimensional bag body. Dry heat setting uses hot air or a heated press and suits a flat panel. Steam reaches temperature faster and at a lower dry equivalent, which is why it is the default for a shaped bag.
What temperature should a polyester knit bag be set at?
160 to 180 °C, paired with the dwell: 160 °C for 90 seconds on a heavy 520 g/m² body, 170 °C for 60 seconds on a 320 g/m² body, and 180 °C for 30 to 45 seconds on a light 240 g/m² body. Polyester has a glass transition near 70 to 80 °C and melts near 250 to 260 °C.
Can nylon knit bags be set at the same temperature as polyester?
No. Nylon 6 should be set at 150 to 170 °C, because it softens lower than polyester and yellows above about 180 °C. On a blend, the setting band follows the lower-melting component, so a polyester and cotton blend sits near 160 °C to avoid scorching the natural fibre.
How long should the dwell be?
Thirty to ninety seconds for dry heat setting, matched inversely to the GSM: heavier bodies need longer at a lower temperature so the core of the fabric reaches temperature. A full steam boarding cycle including cooling runs 8 to 12 minutes. Over-dwelling glazes the surface.
What steam pressure is used?
Two to three bar, which delivers saturated steam near 120 to 133 °C at the fabric surface. Pressure above that band adds little and increases the risk of watermarks, particularly on a piece-dyed body where an uneven moisture mark is permanent.
Why does the bag need to be cooled before handling?
Because the polymer has not finished fixing until it is cool. A body taken off the board while still warm continues to relax in the pile, and the direction it relaxes depends on how it happens to be lying. Cooling to below 40 °C before any handling is what preserves the tolerance just achieved.
How long should a knitted bag be conditioned before measuring?
Twenty-four hours at 20 °C and 65% relative humidity, the standard atmosphere defined in ISO 139. Measuring before conditioning gives a reading that will not match the reading the customer takes, and the difference is frequently larger than the whole tolerance band.
What tolerance can be held after setting?
Plus or minus 3 mm on a 380 mm body dimension, plus or minus 3 mm on handle drop, plus or minus 2% on finished GSM measured on a conditioned sample, and within 3% dimensional change after the agreed wash cycle when tested to AATCC 135.
Does heat setting make a knitted bag stiffer?
No. Setting stabilises dimensions; it does not add structural stiffness. A bag that must stand rigidly empty needs a bonded interlining or a moulded base, which is a hybrid route with an added component and two added finishing steps rather than a setting adjustment.
Can setting hide a knitting fault?
Generally it makes faults more visible rather than less. Relaxing the fabric opens the loop, so a barre or a needle line reads more clearly against the surrounding surface. A dropped stitch is not closed by setting and should be corrected at the bed, where the correction is a re-knit of one piece.
What is surface glazing and how is it avoided?
A shiny, flattened surface caused by too high a temperature or too long a dwell for the GSM. It is permanent on the piece. Avoid it by pairing a lower temperature with a longer dwell on heavy bodies and a higher temperature with a shorter dwell on light ones.
How much gap should there be between board and body?
About 2 mm of running gap, specified at working temperature rather than cold, because an aluminium former expands measurably at setting temperature. A board cut to the exact finished dimension fights the fabric on loading and stretches the handle aperture on unloading.
Why hold a first piece back for 24 hours?
Because it is the only control that tests whether the cycle held rather than whether the bag measured correctly on the day it was packed. Under-set cores pass inspection and then fail on the customer's first wash, which is the most expensive place to discover a setting error.
Where does setting sit in the sampling window?
On day three of the 7-working-day window, ahead of the fit assessment. Until the body has been set and conditioned, a fit comment is being made on a shape that has not stabilised, and programmes that skip setting on the first sample routinely approve a silhouette and then watch it change.
Where does setting sit in the bulk window?
Between days 18 and 26 of the 30-day window, running concurrently with the tail of knitting. That concurrency is why a setting parameter error is discovered against a queue of several hundred pieces, and why the held-back first piece matters more in bulk than in sampling.
People Also Ask
Do knitted bags need heat setting?
Yes. Without it the loop structure keeps relaxing after it leaves the bed, so dimensions drift between production and delivery and the 3 mm tolerance has no meaning. Setting is what converts a knitted shape into a stable one.
Can you steam a knitted bag at home?
A light steam can relax creases, but without a former and a controlled cooling step it will not hold a silhouette. The board geometry and the cooling to below 40 °C are what fix the shape, not the steam alone.
Why does my knitted bag shrink after washing?
Usually an under-set core: the dwell was too short for the GSM, so the interior of the fabric never reached setting temperature. It passes inspection and fails on the first wash. Longer dwell at a lower temperature is the correction.
What is the standard atmosphere for textile testing?
Twenty degrees Celsius and 65% relative humidity, defined in ISO 139. Knitted bodies are conditioned for 24 hours under those conditions before dimensions and GSM are recorded, so the reading is repeatable.
Does dope-dyed yarn set differently?
The setting band is the same, but dope-dyed bodies show watermarks less than piece-dyed ones, because the pigment is distributed through the fibre rather than applied to the surface. Shade also holds more tightly between lots.
Sources and further reading
Every figure on this page is either our own programme data or attributed below. Open the originals before you write them into a specification.
- ISO 2859-1:2026 — Sampling procedures for inspection by attributes, Part 1: AQL-indexed sampling schemesISO Defines the sample size and accept or reject thresholds behind the AQL 2.5 release after the setting station.
- ISO 139:2005 — Textiles, standard atmospheres for conditioning and testingISO Defines the 20 °C and 65% relative humidity conditioning atmosphere used before measurement.
- AATCC — American Association of Textile Chemists and ColoristsAATCC Source of AATCC 135, the dimensional change method behind the 3% post-wash limit quoted here.
- OEKO-TEX STANDARD 100OEKO-TEX Finished-textile testing for harmful substances, requested alongside REACH and Prop 65 on knit bag programmes.
- Understanding REACHECHA The EU chemical regulation referenced for restricted-substance compliance on finished knit bag shipments.
The One Control That Pays for Itself
Of the twelve checklist items, only one costs anything to implement: holding back the first piece and re-measuring it after 24 hours of conditioning. Every setting defect described above passes a same-day inspection. None of them pass a next-day one. A programme that adopts this single control will catch under-set cores, rushed cooling and worn formers before they reach a carton, and it costs one piece and one day.
For the variables that feed into the setting step, see yarn tension control and the defects it produces. For the gauge and yarn decisions that set the GSM band, see knit bag materials and yarns.