How does a 3D printer print on any surface?

On the right side of the photo, an inkjet printer prototype machine developed jointly by Mimaki Engineering and the Nagano Industrial Advanced School is located in a black background. This is not an ordinary printer, but a "3-D printer" that prints directly on arbitrary surfaces. On the upper left side of the photo are four print heads, and the silver-white round object diagonally below the right side is the work piece (printing object, hemispherical).

Originally when printing on the surface of a three-dimensional object, a method of printing in a flat unfolded state and then combining and establishing a body, or pasting a printed narrow rectangular sheet is generally used. In this regard, 3D printers can print hard materials that cannot be spread in two dimensions and do not require a paste process. For example, when a hemispherical resin workpiece is used to manufacture the northern hemisphere part of the globe, only one printing process is required. Three-dimensional printers can also be considered for use in areas such as hard hats and automotive interior parts.

So, how does such a 3D printer control the positional relationship between the print head and the workpiece surface? Ordinary ink jet printers usually use a single axis to move the paper and print head to achieve two-dimensional print back and forth. How does it move on a three-dimensional arbitrary surface?

Incidentally, the print head of the 3D printer in the photo uses the same product as the existing printer. The resolution on the globe is 600 dpi, and the printing time is about 20 minutes. Although the prototype machine in the photo is in the standby state, at the beginning of printing, the workpiece and the print head are close to each other, and both parties are in motion during printing. I hope readers can imagine how printers and artifacts act in order to increase accuracy and reduce printing time.

Let's take a look at axisymmetric workpieces. We will use the example of printing the globe's northern hemisphere as an example. Print from the equator to the North Pole in order.

The workpiece is fixed on the spindle so as to rotate about the ground axis. First, the attitude of the spindle is adjusted so that the vicinity of the equator is directly below the print head (ink ejection position). In this state, the workpiece is rotated, and a range of colors is printed in a band in a color. After the printing of one color is completed, the print head of the second color is moved directly above the workpiece and printed with this color. The same action is repeated for 4 colors.

After completing one stripe printing, tilt the spindle so that the latitude on the north side is moved just below the print head. In the same manner as above, the stripe printing is still performed in four colors. This repeats the operation of changing the spindle angle. Finally print the North Pole section *1.

*1 Since ribbon printing is performed, the ink discharge direction may not be perpendicular to the surface of the workpiece. Although the narrower the printing bandwidth, the higher the resolution can be, but the printing time will be longer.

The process of printing axisymmetric workpieces. Ribbon printing is carried out color by color while the workpiece is rotated, and the rotation axis of the workpiece is tilted after the printing of the four colors is completed to perform the adjacent ribbon printing. In addition, an exposure amplifier is provided with the same axis as the print head for each print.

In this way, the print head moves left and right on one axis, and the workpiece changes position with the position of the stripe print while maintaining the rotation. In order to maintain a uniform gap between the workpiece and the print head, the spindle support mechanism is controlled by the tilt of the spindle and the three axes of up and down and forward and backward parallel movement (Figure 2). That is, the printer is controlled by a total of five axes, one for the print head, and four for the workpiece (including the axis of rotation).

The spindle with the workpiece is moved in the up and down direction and in the front-rear direction simultaneously while tilting to maintain a uniform gap between the print head and the workpiece.

If the workpiece is axisymmetric, it is only necessary to rotate the spindle during ink ejection. Instead of an axisymmetric shape, the position of the print head and the spindle must be simultaneously controlled*2. Let's take a look at the example of printing on the side of an elliptical column.

*2 This prototype machine can print areas of a hemisphere with a diameter of 300 mm and a cylinder with a diameter of 300 mm and a height of 150 mm.

When printing on the side of the elliptical column, the print head is moved left and right according to the phase of the rotation, and the rotation axis is moved up and down. In this way, ink can be discharged from the side at an appropriate distance and in an appropriate direction.

First, the distance from the center of rotation to the side will change, so the height of the spindle needs to be adjusted according to the phase of the rotation. In addition, the print head also needs to move left and right because the portion where the side tangent is horizontal (printing position) is not necessarily directly above the rotation axis. By controlling this complex operation, printing on elliptical columns can be achieved, and it can also be applied to arbitrary surfaces.

Since it is necessary to calculate in advance how to make the various parts to be optimally moved, it is necessary to make preparations for reading the shape of the workpiece using a three-dimensional scanner and output the three-dimensional CAD data to the printer. In the future, Mimaki Engineering plans to improve the control software, and develop a device that can meet specific applications.