Why are engineering drawings always blueprints?
Engineering and mechanical drawings are traditionally blue because they are produced using the blueprinting method. This process, which you may see advertised at printing shops, gives the drawings their distinctive blue color.
Blueprint paper is a chemically coated material specifically used for blueprinting in engineering and mechanical manufacturing. It is an essential tool in production, research, and construction, representing a mass-production method that emerged before computer-aided drafting.
Drafters used carbon pens to draw on transparent paper called “sulfuric acid paper.” The purpose was to facilitate the mass production of identical drawings using a blueprinting machine when needed. The blue color on the paper is due to the ammonia-fixing and photosensitive working principles of the blueprinting machine and has no other significance.
Currently, drafters can completely avoid hand-drawing and instead use CAD to create various structural diagrams. Furthermore, when outputting drawings, large plotters, similar to printers, can print the computer-generated images onto white paper, or even in color.
However, to mass-produce the same drawing, designers still use the traditional “blueprinting method.” This method has very low production costs; printing an A1-sized drawing into a blueprint costs approximately 0.80 yuan. It is also highly efficient, taking only a few seconds to print an A1-sized drawing into a blueprint.
To reduce the blueprint’s intensity, the printing time can be slowed; conversely, a faster printing time results in a bluer color.
Blueprint paper, also known as diazo paper, is a type of diazo photosensitive paper. It is a processed paper made by coating dry base paper with a photosensitive agent, and is called roll paper.
The base paper must have good strength, absorbency, and minimal deformation. It’s usually made from cotton or bleached wood pulp. A quality blueprint paper has a smooth surface, an even coating, and no visible defects.
Blueprint paper can be categorized by its application method:
1. Wet blueprint paper, also known as water-washed blueprint paper or iron salt paper, uses water for photosensitive reduction. This method is still used by some units.
2. Dry blueprint paper, also known as diazo addition blueprint paper or ammonia-fumigated blueprint paper, uses ammonia gas for photosensitive reduction. This is the type of blueprint paper supplied domestically.
3. Semi-wet blueprint paper can be coated for image formation during exposure. In dry blueprinting, ammonia mist is used during the process.
Based on line color, paper types include:
1. White background with blue lines, blue background with white lines, white background with brown lines: these are wet-process printing papers.
2. Blue background with purple lines, white background with brown lines, white background with black lines: these are dry-process printing papers. Currently, a blue background with purple lines is most common in China, with some regions using a white background with blue or black lines.
Based on the coating surface, there are two types: single-sided coating and double-sided coating.
1. Single-sided coating has two meanings: one is applying photosensitive liquid only to the paper being used; the other is applying photosensitive liquid to one side and boric acid solution to the other to eliminate paper warping and improve shelf life. It is actually a two-sided coating, but because only one side is photosensitive, it is conventionally called single-sided coated paper. This type is common in most parts of China.
2. Double-sided coating applies photosensitive liquid to both sides, allowing printing from both sides. This mainly reduces document volume and saves paper. The recent trend is white background with blue lines and double-sided coating, because blue line blueprint paper has a light sensitivity speed 1/5 faster than purple line blueprint paper while keeping storage time the same, and the color is bright, and the tolerance for light sensitivity speed is large.