Why are engineering drawings and mechanical drawings blue? These drawings are blue, which is related to their drawing methods. These drawings are not drawn or printed, but "exposed". Perhaps you can always see some printing houses with signs such as "exposing drawings" posted on the door in the streets and alleys, which refers to this.

Exposing drawings are commonly known as "blueprints", which are a kind of chemical coating processed paper, specially used for various engineering designs and mechanical manufacturing drawings, and are indispensable supplies in production, scientific research, and construction.

Blue architectural engineering drawings are a mass drawing method formed in the era without computer drawing.

The draftsman draws the drawings with a carbon pen on a transparent paper called "sulfuric acid paper". The purpose is to facilitate the use of "exposing machine" to produce a large number of identical drawings when the drawings are used in large quantities in the future.

The blue color on the drawings is due to the working principle of the "exposing machine" ammonia fixation photosensitive working principle, and has no other meaning.

At present, draftsmen can use CAD to complete the drawing of various structural drawings without hand-drawing. At the same time, when drawing, they can use a large plotter shaped like a printer to print the computer drawings on white drawings, or they can print them in color.

However, in order to copy the same drawing in large quantities, designers still use the "printing method" of the past. This method has a very low drawing cost. The cost of printing an A1-sized drawing into a blueprint is about 0.80 yuan. And it is very efficient. It only takes a few seconds to print an A1-sized drawing into a blueprint.

If you don't want the drawing to be very blue, you can slow down the printing time; on the contrary, the faster the printing time, the bluer it will be.

Blueprint paper (blueprint yaper) is also called printing paper, and diazonium salt is like printing paper or diazo sensitive paper. A processed paper made by using blueprint base paper as the paper base and then coating the dry base paper with a photosensitive agent is a roll paper.

The base paper is required to have good wet tensile strength and absorption performance, minimal wet deformation, and no chemicals that affect the performance of sensitive materials. It is usually made of cotton pulp or bleached wood pulp on a papermaking machine. Blueprint paper made of qualified base paper has the characteristics of smooth paper surface, uniform coating of photosensitive layer, no white spots, spots, drug stains and wrinkles.

The blueprint paper is divided into the following according to the method of use:

1. Wet blueprint paper, that is, water-washed blueprint paper, that is, iron salt blueprint paper, its photosensitive reduction is washed with water, and some units still use this method.

2. Dry blueprint paper, that is, diazo addition blueprint paper, or ammonia fumigation blueprint paper, its photosensitive reduction is ammonia gas, and this type of blueprint paper is supplied domestically.

3. There are two types of semi-wet blueprint paper. One is to apply both the photosensitive agent and the reducing agent on the paper, and couple the drawing while it is exposed to light. It is not available in China; the other is used for dry blueprinting. When the ammonia water is evaporated and misted during the blueprinting, it is formed while it is exposed.

According to the color of the lines, there are:

1. Blue lines on white background, white lines on blue background, brown lines on white background, which are wet-type printing paper.

2. Purple lines on blue background, brown lines on white background, black lines on white background, which are dry-type printing paper. At present, most of the domestic lines are purple lines on blue background, and some areas have blue lines or black lines on white background.

According to the coating surface, there are two types: single-sided coating and double-sided coating:

1. Single-sided coating has two meanings: one is to apply photosensitive liquid only on the paper in use; the other is to apply photosensitive agent on one side and boric acid water on the other side to eliminate the warping of the paper and increase the storage period. It is actually double-sided coating. Because only one side is photosensitive, it is also called single-sided coated shoes. Most areas in China are of this type.

2. Double-sided coating is to apply photosensitive agent on both sides, and both sides can be printed, mainly to reduce the volume of the data and save paper. The development direction in recent years is white background with blue lines and double-sided coating, because the photosensitivity of blue line printing paper is 1/5 faster than that of purple line while the storage time remains unchanged, and the color is brighter and the tolerance of photosensitivity speed is larger.