Overview
Process
Imaging
Objects to be imaged are taken by photographing. Images are developed and output on negative film. However, images prepared digitally become more popular nowadays.
- If image is a real object, it is taken by digital camera. Image is then transferred to computer in digital format.
- If image is a photo or picture, it is digitally scanned and is saved as a digitally file in computer.
- If image is an idea, it could be designed by any graphic program softwares such as Photoshop, Corel Draw, AutoCAD, 3D Max and Solid Works, etc. Image is then saved digitally in computer.
When an image is designed by either one of above methods, it is then digitally printed on, instead of ordinary paper, transparent film by inkjet printer or plotter. The printing process is as simple and identical to office document inkjet or laser printing. Selected ink should be black enough suitable for subsequent UV exposure. Ready prepared film with images would be exposed under UV. Images are then transferred from film to stencil. This is to be described more detailed in UV exposure topic.
Frames Working
Frames for stencil are usually made of aluminum, steel or stainless steel profile. Frames are available in many profile cross sections for certain requests and applications. Straight bars are carefully corners joined and connected to form strong frames, usually rectangular. Fabric screens are stretched and stick on frames edges. Fabrics tend to pull the frames inwards. Hence, frames should be strong enough to resist tensile strength, bending strength and mechanical strength. It should be also anti-corrosive.
Screen Fabric Stretching
Silk screen fabric is the fundamental material of stencil. It is usually nylon or polyester available in various meshes. During screen printing, ink or paint is printed on glass through tiny mesh holes. The smaller is the holes, the finer is the printed images. While it is soft enough for handling, fabric should be anti-abrasive to resist erosion when physically contact with ink squeegee bar during silk screen printing. Proper width of fabric is selected and is unrolled and cut in requested size.
Four edges of fabric are usually fixed in place by clamp, jig or fixture with well prepared framework underneath. Fabric is stretched and reinforced under tension with fabric stretching and tensioning equipments. The stretching could be achieved mechanically, electrically or pneumatically. Manual stretch is not recommended because consistent tension strength across fabric is very difficult to obtain. A tension measuring gauge is usually put on fabric to monitor in real time. When requested tension strength is obtained, glue is applied on fabric edges. During glue drying, fabric stretching is maintained under tension until glue is thoroughly dried and fabric is permanently adhered on frameworks.
Emulsion Coating
Emulsion is usually applied on both front and rear fabric surfaces with framework placed vertically until surface is fully covered. Proper emulsion is a king of chemical which should be sensitive to UV. This UV sensitive emulsion will be cured when exposed under UV lamp in subsequent process.
Emulsion Drying
Fabrics are then put inside drying cabinet. Emulsion is then heated and dried until it is ready for UV exposure.
UV Exposure
Well dried fabric then lay up with the film with image described previously. It is then put in equipment with UV lamp. Film is put in front of fabric. During UV exposing, emulsion right behind images is not UV cured because images block UV. All other emulsion are cured because they are not UV blocked by images. Make sure no air is trap in between film and fabric.
High Pressure Washing
The fabric is then washed under high pressure water. Those un-cured emuslive are washed and removed from fabric's tiny holes leaving them "open". After thoroughly washing, those mesh holes with images are "opened" while remaining tiny holes are "closed". If necessary, fabrics are dried until all water vapor are gone. This fabric now becomes stencil and is ready for silk screen printing.
