T-shirt bleach stenciling

Problem

A t-shirt is sometimes needed, with a pithy phrase. While there are vendors that provide printing, the printing operation is usually more expensive than the t-shirt itself.

Method

Stencil making

A stencil was designed in OpenSCAD; version 2015.03 or later is needed to provide support for the text() function.

The "Army" font, with stenciling feature, was used.

The design was exported as a DXF file, converted to polylines using sw_DXFlaserutils. The DXF file was then used with MoshiDraw to cut the design on the K40-III "Blue Horror" machine.

For the material, a discarded cardboard box of the cereal-box grade was chosen, on the basis of availability and mechanical properties.

Stencil image

• tshirt1-laser-raw.dxf - DXF file, as of from OpenSCAD
• tshirt1-laser-cooked-fv.dxf - DXF file, processed, zeroed and flipped vertically

Problem encountered

The software used is processing the elements in what seems to be random order. The y-axis seems to be skipping steps during rapid acceleration/deceleration, which leads to increased cumulative error, so stencil elements end up out of place.

Failed stencil

Failed stencil

Failed stencil

Failed stencil

The problem was alleviated by cutting in several passes. The file was loaded to the software, then two of the three lines were marked and deleted. The line was cut, then the operation was repeated on two other lines.

On the middle line the software seized at second-to-last curve and stopped processing. The line was burned once again, with the same result. Therefore it was decided to delete all its elements except the two unfinished ones.

The repeated tracing widened the pattern slightly. This was not perceptible on the result, but led to the middle of the "H" letter falling out. The element was lost. It was replaced by taking it from the other stencil that failed due to cumulative errors.

The line-by-line pass technique proved to be successful. The distances traversed by the head were not so bad so the total error, if present, was not perceptible.

Stencil preparation

The finished stencil was spray-painted with black car-grade spray paint from the rough, exposed carton side, for a degree of waterproofing.

The shirt was stretched on a seat. A layer of plastic foil was placed between the front and back layer of the fabric, to prevent penetration of the bleach. The stencil was positioned and attached to the fabric with masking tape. Plastic boards and sheets were lined around the mask to prevent sprayover of the bleach.

A missing segment of the rightmost letter H, salvaged from a miscut stencil, was manually put in its place.

Stencil taped to cloth

Stencil taped, mask added

Added missing segment

Bleach application

Undiluted sodium hypochlorite based bleach was put into a small mechanical spray bottle.

The spray was evenly applied on the stencil, carefully wetting the fabric only on the exposed areas. After application it was let for several tens of seconds until the color changed sufficiently and evenly.

The mask was then removed, with care to not drip the excess solution from the stencil surface on the now exposed fabric.

Problems encountered

More bleach was added in a misguided attempt to achieve brighter hue. This resulted in some penetration under the stencil in the middle area.

A discolored spot was found on the front part, away from the stenciled area. This was caused by a stray drop of the bleaching solution.

Bleach applied

Mask removed, stray drop visible near edge

Finished pattern

Finished pattern, defect detail

Final processing

The t-shirt was immersed in warm water and thoroughly washed. Then it was predried with hot air, followed by ironing in order to remove residual moisture and allow immediate wearing.

Finished pattern

Notes

• The laser cutter needs better control software. Multiline texts have to be manually cut at multiple passes, one line each.
• The letters have to maintain certain minimum size - the spacing between the segments has to be wide enough for the stencil material to hold the inner segments in place. These parts can get strained, especially during removal of the "hanging chaff" after cutting.
• The proper amount of bleach applied is crucial. Too much and it will soak under the stencil.
• Avoid dripping the bleach.
• Use of manually positioned stencils made from multiple segments is possible. Multiple application of bleach through several different stencils is also possible for more complex patterns.
• With bleach, the resulting color depends on the dye and character of the fabric. This has to be determined experimentally, case by case.
• Tampon application of the bleach could be tried to suppress soaking. Tap with a wetted pad.
• 30% hydrogen peroxide could be substituted for hypochlorite (todo: try).
• A fabric paint can be optionally used instead of bleach. With multiple stencils, multicolored patterns are possible. The fabric has to be prevented from moving and stretching differently, and the stencils have to be reproducibly positioned. Some registration aids, e.g. holes for pins, are suggested.

TODO

• Clean up and publish the SCAD file
• Do more designs
• Try multicolored designs, possibly try "pixel art" using sw_bmp2laser.
• Find a way to preprocess the DXF files to enforce minimal width of the inter-segment spacing, allowing use of smaller font sizes; without it the kerf width will join the segments. Alternatively, uniformly shrink every polyline segment by the kerf width; this is simpler but suboptimal.
• Get rid of the MoshiDraw disaster; upgrade laser cutter to use G-code, control it via LinuxCNC or other G-code player