The causes of skews and bows

Creasing in duvet covets, sheets and pillowcases is now top of the launderers’ problem hit list. Operators need to know how to prevent such pieces, duvet covers in particular, becoming creased during their passage through a multi-roll ironer.

Errors in feeding linen into the ironer will obviously give a poor result but generally the presentation of large flatwork should not be a problem as highly sophisticated equipment is now available. However, despite big improvements in ironer-feeder technology, creasing problems are growing rather than declining.

Laundry managers may wonder if excessive creasing results from operator error but often the problems result either from the way the ironer is set-up or even more frequently from the quality of the textiles.

Assume that the ironer has been set up correctly and the engineer has checked the following points.

The rolls have been girthed and have been correctly sized by using the ironer manufacturer’s girthing tapes.

The roll clothing is in good condition with plenty of resilience and there are no hard areas where the clothing has been allowed to get wet.

The spring padding is in good condition for the whole circumference of each roll and does not have any patches of rusty or collapsed springs.

All the steam traps are working effectively, opening to release condensate as it is formed and venting air continuously.

There are no “cool spots” on the ironer beds as a result of air pockets or of condensate backing-up.

The bed-roll interface pressures are correct and give uniform bed-contact over the entire surface.

The roll-to-roll differential speeds are correctly set to give a stretch of 0.1 to 0.2% between each roll.

The vacuum on each roll is working effectively across the roll’s full width.

The ironer beds are free from deposits. This applies especially to the leading edge of the first bed.

The beds are being regularly and correctly waxed with the right type of wax. The vacuum should be switched off while the beds are waxed to avoid surface blinding.

Bed temperatures/pressures are maintained throughout the day without any significant drop or variation.

The speed differentials between the feeding equipment, feed-bands, ironer rolls and folder take-off bands are all correctly adjusted.

The textiles have the correct and consistent moisture retention when they are fed into the ironer.

The distribution brushes and vacuum or tension rollers on the feeding equipment are all functioning correctly.

There are no sharp edges or partially opened crocodile clips that are catching and dragging on the items being fed into the ironer.

All the feed-tapes are fitted and correctly tensioned.

If these points are as described here the laundry manager can be confident that the ironer operators are feeding work correctly, that the engineer has done a good job and that the ironer has been correctly tuned and adjusted.

So if the laundry is still getting excessive creasing, especially on duvet covers, the next step must be to check the textiles.

Several factors in textile manufacture and in the way textiles are turned into finished goods can have a significant effect on the way a piece of bedlinen can be handled and also the results obtained after washing.

The mill will apply sizes at various stages in manufacture and these need to be removed in the laundry before the linen goes into circulation. The warp yarns are sized during beaming-up so that very high shuttle speeds can be used without the risk of burning the yarns. Failure to remove this size completely will result in a rough dried finish, often accompanied by cracked-ice creasing. These faults will appear for at least the first 15 – 25 wash and finishing processes.

In the final stage of manufacture, the textile finisher will apply a fabric size to ensure the cloth can be handled correctly by the automatic feeding, cutting and sewing machinery used to make-up the finish product – a process known as CMT (cut, make and trim).

This needs to be removed before the linen is issued to customers to avoid problems of set-stains and to allow the ironer to finish the fabric without any cracked-ice type creases.

Launderers that find their normal processes do not completely remove these sizes may need to seek advice as special processes are required.

The next problems to consider are the bowing and skewing that can occur during manufacture or CMT.

Bowing is the curvature of the warp or weft of woven fabrics. It usually affects the weft yarns that run from side to side of the roll of cloth. To gauge the amount of “bowing”, measure the greatest distance, parallel to the selvedges, between a weft yarn and a straight line drawn between the points at which this yarn meets the selvedges. The best way to describe this is as a percentage of the width of the fabric at that point. In the diagram on this page, the bow is calculated as follows:

(B ÷ A) x 100=%. So if B=20mm and A=1,500mm, then the bow is (20 ÷ 1,500) x 100 = 1.3%. Where possible, rental launderers need to aim for a bow of below 1% to avoid customers complaining about edge creases along the closed end of a duvet – in some cases, the creases will be down the sides, depending on the cut.

Skewing in the roll: If during manufacture, the roll of cloth is fed through the final finishing machine (usually the stenter where the fabric is heat set) with one edge moving ahead of the other, then although the weft yarns are straight they will be set at an acute angle to the warp yarns, instead of being exactly at right angles. The skew can be measured by determining the extent of misalignment of the weft yarns across the entire width.

Referring to the diagram (p18), the percentage skew is (C ÷ D) x 100 – if C=30mm and D=2,000mm, then the skew is (30 ÷ 2,000) x 100% = 1.5%.

Rental launderers need to aim for a maximum skew of 1% if at all possible, to avoid problems with twisted edges on duvets and pillowcases or with sheets that go off the square.

Bowing and skewing in the roll are usually caused during operations where there is a risk of uneven tension across the fabric width.

Cutting skew: This can be introduced during cut, make and trim if the cloth is not cut precisely in line with the warp and the weft. This is a particular problem with pillowcases.

In the diagram (p18), if E=18mm and F=360mm then the cutting skew is (18 ÷ 360) x 100 = 5.0%. This degree of cutting skew is not unusual on carelessly cut orders. It is best detected by tearing one pillow case or one duvet from the batch, from end to end and from side to side. If the tears are at right angles to each other but they are not parallel to the seams, then this has probably been caused by cutting skew and the items will never iron properly.

Bowing and skewing faults will look worse on coloured, patterned fabrics than they do on plain colours. This is because the contrast in the patterns makes the distortion more prominent.

These defects often cause sewing problems and they will also make draping the fabric, as part of a finished product design, more problematic.

However, in some cases, a specified amount of skew is needed, for example, to prevent twisting of trouser legs made from twill fabric.

Wavy or sharp breaks in the bow line are more detrimental to the appearance of small parts of a garment (such as collars or pockets) than a gradual slope from a straight line.

Both bowing and skewing often present the rental launderer with significant problems. Finishing a duvet cover or pillowslip on a multi-roll ironer becomes impossible without adding large creases.

Bowing will often cause pillowslips to twist so that the side-seams move 5 – 10cm into the main body of the linen. Skewing has a similar effect and can also give the pillowslip a trapezoid shape so that it is impossible to fold it without leaving large “dog-ears”.

Skewing can also make a duvet cover impossible to fit correctly or prevent a table cloth or sheet lying squarely.

Effects similar to bowing and skewing can be caused during cut make and trim – double-thickness pieces such as duvet covers or pillowslips are particularly prone to such problems. If the two faces are not perfectly aligned or if the row of stitching “runs-off”, the piece will become twisted as it goes through the ironer.

Fabric can become skewed during its manufacture if it is placed on the stenter guide pegs in such a way that the weft yarns are not perpendicular or at right-angles to the direction of travel.

The weft yarns need to be only a few millimetres out of alignment to set a slight twist throughout the whole roll of cloth. This twist will become clear in the first ironing cycles in the laundry. Even if the fabric is correctly aligned on the stenter, problems can still occur if the parallel stenter drive pegs do not run at precisely the same speed as each other. This difference in speeds will eventually cause the fabric to twist as it is fed into the machines.

The first few sections will be fine but the distortion will become greater as the roll progresses. So the laundry needs to check several duvet covers in a delivered batch and not just one.

Fabric will become bowed during manufacture if it is wound under high tension and the pull on the middle is greater than that at the outer edges. The problem will be worse if the selvedges are thicker than the main body of the cloth.

Launderers can identify bowing or skewing during manufacture or CMT by checking whether pillowslips or duvet covers have excessive creasing on the side seams and/or on the bag end and by checking whether such double thickness items have twisted so that the side seams have moved into the main body of the piece.

These faults cannot be corrected once they have developed but laundry managers need to act to prevent further problems. The first step is to confirm that the ironer is in perfect condition, using the checklist in this article.

Assuming this is correct, then discuss the problem with the textile supplier. Replacing the whole batch may be the only answer but this will usually be easier to negotiate if the laundry has processed a few sample pieces and identified the fault before the batch has gone into circulation.

Even if the laundry manager knows in advance that a batch is going to be problematic, he cannot prevent faults developing. It is therefore essential that the laundry can identify the source of the fault. One method of identifying the source and type of fault is to tear down the length, making sure the tear is next to a seam or hem. Then examine the line of the tear against the row of stitching to see if the line of stitching follows the threads in the weave.

Alternatively a pillowslip or similar can be split down both side seams along the line of stitching and opened out into a single long piece of material. Then manually re-align the two hems, smooth the material and measure the amount of skewing that has occurred during CMT.

If the fabric is then torn first along the warp and then across the weft, it is possible to determine the amount of bowing and skewing which has occurred during weaving or, as is more likely, when the cloth was finished on the roll.

There are other reasons for creasing and also several types of creasing. It is often possible to identify these by their shape and location and see how and when they occurred and decide what action to take.