Plastic bottles made of polypropylene materials through […]
Plastic bottles made of polypropylene materials through injection molding are basically outer packagings of light industrial products, such as cosmetic bottles, washing supplies bottles, drinking bottles, oil bottles, etc. These bottles have very good weather resistance, chemical resistance and mechanical strength, so they are widely used in the packaging industry. The smaller bottles are made into cylindrical, conical and oval shapes. These curved bottles are generally printed with domestic semi-automatic curved surface printing machines, and the overprint color is not more than 3 ~ 5 colors.
Curved body monochrome printing is based on 4 rollers to support the printed matter. Screen printing moves the printed matter to complete the printing. However, this method cannot be used for plastic bottle color printing. It uses a dynamic and static clamp to clamp the substrate, and through the meshing process of the rack and gear, it drives the screen to move and the substrate to rotate to complete the printing. The following is a brief description of the key points in the process of PP bottle overprinting.
1. All kinds of screens need regular plate making
The so-called regular platemaking includes two aspects: blank screens with the same number of meshes and uniform tension are overlapped at the same position on the platen; the positioning cross lines on several overprint negatives are overlapped at the same position on the platen on.
1. Positioning of the screen version: The positioning of the screen version is relatively simple, generally using a square or a three-point positioning block. The positioning block needs to have a certain thickness. It can be made of plexiglass with 3 ~ 5 mm and fixed on the platen with double-sided tape. If the screen frame is not at 90 angles, the longest side of the screen frame must be against the positioning block or square. To ensure the flatness of printed graphics.
2. Negative film positioning: There are many overlapping positioning methods for negative films, and the following two methods can be used. (1) Replacement method: When the first screen is finished, do not remove the negative from the plate, but place the second negative on top of the first negative and overlap the positioning crosslines. Secure one of the sides with scotch tape, and then remove the first negative from the platen. The replacement method is also called the left and right side shortage positioning method, that is, the same side of the two negatives should have a change in size. As shown in Figure 2, the size of the first film (A film) is slightly smaller than the size of the second film (B film) (5 ~ 10 mm), so that when the second screen is baked, the first A negative can be successfully removed from the plate. The positioning of several negatives is similar.
(2) Centerline extension method: When the original is separated by a computer, the color separation films do not use the cross-line overlapping positioning method, but use the centerline positioning method, that is, there are several center positions around the color negatives. 4 positioning short lines, the short line length is 5 ~ 10 mm, and the width is 0.3 ~ 0.4 mm. When making a plate, lay the screen printing side up on the desktop, draw the center line on the screen frame, and then place the negative on the silk screen. Use a ruler to string the four short and vertical lines together. It is not allowed. If there is a misalignment, after the correction is completed, fix the film on the screen with transparent tape, and then turn the screen over and fix it with the three-point positioning block or positioning square of the platen to inhale the plate.
Since the squeegee and the ink return knife of the curved surface printer occupy a large area of the screen (about 200 mm), the screen is generally rectangular. During the process of printing, only 4 points in the vertical direction are often connected accurately , And 4 points in the horizontal direction, 3 points or 2 points can be connected together (the squareness of the frame is biased). This will ensure that the printed graphics will not tilt.
In addition, the rulemaking plate also contains two factors that cannot be taken lightly, that is, the correct selection of the screen mesh number and the photosensitive film thickness. The silk screen generally uses a 300-500 mesh yellow polyester screen, the solvent-based ink uses a 300 mesh screen, and the light-curing ink should choose a 350 mesh screen. If there are longer lines and barcode lines in the printed graphics, a 22.5 ~ 30 oblique stretch net must be used. The smooth substrate photosensitive film can be thinner, and the matte and granular substrate photosensitive film should be slightly thicker to improve the coverage of the ink.
Reasonable preparation of gears and positioning pin sleeves
Surface chromatic printing is to complete the printing task by driving the substrate to rotate through the meshing process of gears and racks. The rack is an inherent part of the printing press, and the gear is an element that changes at any time according to the size of the substrate. The matching gear should follow the following principles: substrate diameter = gear diameter, that is, the rotation circumference of the gear should be consistent with the rotation circumference of the substrate, and the tooth profile of the gear should also completely match the rack. Screen printing factories generally do not have the ability to make gears by themselves. The solution is to remove the rack and gear holder from the printing machine and send them to the mold factory or gear processing factory to ask them to process a suitable gear. Since the ellipse is part of a perfect circle, when equipped with gears, be sure to find the center and radius of the real object and then customize the gear. Attention should be paid to two points in the production of positioning fixtures: (1) the location of the positioning pin should be consistent with the plastic positioning groove; (2) the fixtures are generally made in a circle. A shaft sleeve is provided at the center of the disc, and the shaft sleeve is fastened with a screw and a gear seat. The circumference of the disc is also welded with a stainless steel belt to form a retaining wall to prevent plastic bottles from escaping from the chuck. The height of the retaining wall is 5 ~ 10 mm. The oval plastic bottle should also be welded to the inner retaining wall. The arc of the inner retaining wall is the same as that of the plastic bottle.
For a large-opening conical bottle (with a very small taper), the conical jet head cannot directly bear against it, and the substrate cannot be rotated for printing. At this time, a T-shaped cap can be made of metal or nylon. The diameter of the circle in the lid is equal to the diameter of the circle in the bottle. At the same time, an air inlet of about 20 mm must be opened on the lid, so that the conical jet head can hold the bottle. And inject air into the bottle.
There are two methods for printing conical plastic bottles: (1) the screen is parallel, the table is tilted, and the rotation tangent of the bottle is always parallel to the screen; (2) the screen is tilted, and the squeegee and the ink returning knife are also Tilt and adjust the method by attaching a wooden strip at the bottom of the screen holder to the taper of the bottle so that the screen can be tilted to the angle we need. At the same time, loosen the doctor blade holder and use the horizontal screws on both sides to adjust the doctor blade. Angle with the ink return knife so that they are parallel to the screen.
In addition, the outline of the conical plastic bottle on the screen is flat, so the starting point mark must be marked on the multi-color screen to facilitate the smooth printing of each color sequence.
The rack is parallel to the ground during the scratching process, and when the table is lowered to a static state, the rack and the ground should present a certain angle. The formation of the angle is caused by the table rising or falling, regardless of the rack In what state, the nylon roller on the rack should always maintain a good contact state, otherwise overprinting will occur.
Plastic bottle printing is divided into full-circle printing and half-period printing. Cylindrical plastic bottles can be printed on the whole week, while oval plastic bottles can only be printed on the half-cycle.
The color printing of plastic bottles can be carried out according to the following steps: (1) Loosen the screen positioning stopper fastening screws on the optical axis, loosen the screen into the screen clamp, and tighten it. Move the screen slide rack left and right to scrape ink The knives are 15 ~ 30 mm beyond the printed text. The distance depends on the specific size of the printed text from the middle of the bottle. Finally, tighten the left and right positioning stoppers; (2) Align the bottom groove of the plastic bottle. Position the bumps in the fixture to make them contact tightly. The bottle mouth is supported by a support frame to make the bottle body parallel to the working platform. Rotate the bottle body so that the middle seam of the bottle faces up to become the center of rotation of the substrate. Put the rack down and contact the gear. At the same time, place the rack roller on the rack and loosen the fastening screws. When the contact between the pressure wheel and the rack is good, the screws can be tightened and the ink can be printed.
Third, the flame processor and color fastness
Plastic bottles made of polypropylene have low surface energy and poor affinity with inks. They can only be used for printing after surface treatment with a flame treatment machine. The flame processing machine is composed of a conveyor belt, a cone insert, a flame nozzle and a gas tank. When using, put the plastic bottle on the conical insert, and the conveyor moves forward at a uniform speed. Before the flame nozzle, the conical insert starts to rotate, and at the same time, the bottle is also rotated. Improve the surface energy of plastic bottles and increase the binding force with ink. And the intensity of the flame, the running speed of the conveyor belt, and the smoothness of the bottle when rotating all directly affect the treatment effect, of which the smoothness of the bottle when rotating is the most critical. The smoothness is directly proportional to the size of the bottle. The shorter the bottle, the smaller the radius and the better the result after processing.
For larger bottles, we must take measures to increase its smoothness. A common method is to add a nylon sleeve to the tapered pin, the inner diameter of the sleeve is also tapered, and the outer diameter is consistent with the size of the bottle mouth. Covered bottles cannot be dropped by themselves and must be pulled out manually.
Flame-treated plastic bottles should be printed immediately. Multi-color overprinting should choose light-curing inks, and monochrome printing can use ordinary inks. When printing with ordinary ink, the color sequence interval should not exceed 6 hours, otherwise the surface of the plastic bottle will be reduced, and it is easy to be dusted twice, which will affect the adhesion of the ink. For companies with smaller production units, the second or third color printing can use high-quality two-component inks with good results.
Working Process of Overprinting Plastic Bottles
The printing plate is loaded into the screen folder, the position is adjusted, and the ink is poured into the blank space of the starting point of the screen plate.
Switch on the circuit, start the air compressor and switch on the printer air pressure valve.
Place the plastic bottle on the work platform support frame, and the positioning groove on the bottom of the bottle is aligned with the positioning protrusion in the positioning fixture.
Press the "ICYCLE" key and "STOP / START" key on the operation panel by hand, the work platform rises, the substrate is clamped by the substrate, the plastic bottle is clamped, and the conical jet head injects air into the plastic bottle body.
The ink return knife rises, the scraper drops into the screen, and makes linear contact with the plastic bottle.
The racks and gears on the screen scooter fully mesh with each other, causing the screen to move to the left and driving the plastic bottle to rotate. The ink is transferred to the plastic bottle under the squeezing force of the squeegee.
The squeegee rises, the squeegee drops, the screen moves to the right and stops at the starting point, the table drops, the cylinder is released, and the plastic bottle is removed.
In short, with the development of society and the enhancement of people's packaging awareness, the market potential of curved color printing is great; at the same time, it is becoming more and more refined, which requires screen printing companies to continuously accumulate experience and improve in the production process. Technology level can keep pace with the development of the times
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