The printing quality of carton printing machine is good or bad, people usually understand it as two aspects. On the one hand is the clarity of printing, including consistent color shades, non-blocking patterns, no ghosting, and no bottom leakage. On the other hand, the overprint accuracy of multicolor printing should generally be within ± 1mm, and a good printing machine can reach within ± 0.5mm or even ± 0.3mm. In fact, the printing machine also has a very important printing quality index --- overall printing positioning, that is, several colors are accurate, but they are inconsistent with the reference edge of the cardboard, and the error is large. Because this quality index of the general carton is not strict, it is easy to be ignored by people. If the overall positioning error exceeds 3mm or 5mm, the problem is more serious.
Regardless of the chain feed or automatic feed (backward feed or front edge feed), the reference edge of the overall printing position is perpendicular to the cardboard conveying direction, because the other direction (cardboard conveying direction) is not easy to produce overall positioning (unless The cardboard runs diagonally). This article will analyze the reasons for the overall position of the printing in the automatic paper feed printing press in the back-push mode.
The cardboard conveying of the automatic paper feed printing machine is to push the lowermost sheet of the aligned cardboard forward to the space between the upper and lower conveying rollers, and the forward conveying rollers are forwarded to the printing department. paper. Analyzing the conveying process of cardboard can help us find the reason for the overall printing position.
First of all, in the process of pushing paper, the drive chain of the pushing board must not have a large accumulation gap. The automatic paper feeding and printing machine pushes the cardboard into a reciprocating linear motion. Most manufacturers use a crank (slider) guide mechanism plus a rocker slider mechanism. In order to make the mechanism light and wear less, the slider of the crank slider guide rod mechanism is a bearing. If the gap between the bearing and the two sliding plates is too large, it will cause the uncertainty of the movement of the cardboard, which will cause paper feeding errors and cause the overall printing to move. Therefore, it is the key to ensure that the bearing performs pure rolling between the two sliding plates of the guide bar, and does not allow the bearing to have a large gap between the two sliders. The dual-bearing structure ensures that the bearing can be used for pure rolling without gaps between the two slides regardless of whether the bearings move down or up along the slide, thereby making the mechanism light and wear-free and eliminating gaps.
The connection between the guide rod and the rocker and the shaft is easy to loosen due to the alternating load, which is also the reason for the gap to cause the error of the paper pushing. The other mechanisms in the paperboard drive chain are gear drives, which improves the processing accuracy of the gears (such as grinding and honing), improves the center-to-center accuracy of each pair of gears (such as processing the wallboard using the machining center), and reduces the accumulation of transmission The gap improves the accuracy of pushing cardboard and pushing paper, thereby reducing the overall position of cardboard printing.
Second, the moment when the cardboard is pushed into the upper and lower paper feed rollers, it is actually an instantaneous speed-up process in which the cardboard speed is increased from the linear speed of the cardboard to the linear speed of the upper and lower paper feed rollers. The instantaneous linear speed of the cardboard must be lower than the linear speed of the upper and lower paper feed rollers (otherwise, the cardboard will be pushed and bowed). How much is smaller, the ratio of the speed of the two, the matching relationship is very important. It directly affects whether the cardboard slips at the moment of speeding up, and whether the paper feeding is accurate, thus affecting the overall printing position. And this is precisely not noticed by printing machine manufacturers.
When the speed of the host is constant, the linear speed of the upper and lower paper feed rollers is a fixed value, but the linear speed of the push board is a variable, from zero at the rear limit position to the maximum in the positive direction to zero at the front limit position, from the front limit position The zero then reaches the maximum in the reverse direction and reaches zero at the rear limit position, forming a cycle.
The picture on the right is a schematic diagram of the crank (slider) guide mechanism of the push board. The linear velocity formula of the push board can be derived by the analytical method.
V ï¼ nÏ€cos (α-α2) cosα2? Cosα1â‘
In the picture: c1 is the rear limit point of the rod end of the guide rod
c2 is the front limit point of the rod end of the guide rod
s is the stroke of the rod end of the guide rod (that is, the stroke of pushing the cardboard)
In the formula: V is the linear speed of the rod end of the guide rod (that is, the linear speed of the cardboard)
n is the rotation speed of crank OB, which is a known quantity
From formula (1), we can see that V is a variable, which varies with the position of point C (α2 is different). Through calculation, different speeds of the pushing board at different positions can be obtained. According to the proportional relationship between the linear speed of the paper pusher and the linear speed of the paper feed roller, after determining the instantaneous linear speed of the paper feeder pushing the paperboard into the upper and lower paper feed rollers, the best position for pushing the paperboard at this time can be determined.
Of course, the factors that affect the overall position of the cardboard printing are such as the curvature of the cardboard, the surface roughness of the cardboard, the moisture content of the cardboard, and whether the gap between the upper and lower paper feed rollers is appropriate, but the factors of the equipment are mainly reflected in the above two aspects.
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