1. Actual case
In the process of producing a kraft paper sealing label, the company frequently encountered the problem of waste breakage when using the machine to discharge waste. In desperation, it can only be discharged manually. On-site observation of the broken waste found that the waste that is prone to breakage usually has a thick and sticky glue layer. The label is back-bonded to the waste after die-cutting, and the waste is adhered to the finished label and cannot be pulled apart, resulting in the breakage of the waste.
2. Analysis and processing
After observing this phenomenon, we tried to analyze the causes of waste breakage from the following aspects. First, check the die-cutting pressure of the equipment. Test with the ink test method. The specific method is: use ink to apply to the knife print position of the base paper to observe whether the knife print is complete and uniform. Appropriate die-cutting pressure can effectively ensure the cutting of glue. After testing, it was found that the die-cutting knife print was complete and uniform, which shows that the die-cutting pressure of the equipment is appropriate.
Figure 1 After the label was die-cut and left for a period of time, it showed obvious stringing phenomenon.
Secondly, check the material property table (TDS). After inspection, it was found that since the material of the label is kraft paper, the adhesive on it is reinforced hot melt adhesive with a viscosity of 14N/25mm. The adhesive layer used is thicker and flows actively at high temperatures. After observing for a period of time, it was preliminarily confirmed that the adhesive was the main cause of the waste breakage. Because the initial viscosity of hot melt adhesive is relatively high, there will be obvious back-adhesion and drawing phenomenon after being placed for a period of time after die-cutting, as shown in Figure 1, and this phenomenon is also common in the die-cutting process.
3. Improvement method
Afterwards, we tried to die-cut a label alone and found that the waste could be peeled off normally by tearing it open quickly, which was significantly different from the drawing adhesion at the waste discharge. In this case, our solution is to minimize the degree of glue re-adhesion, mainly from the following two aspects: First, reduce the force extrusion of glue after die-cutting, and find unnecessary force points throughout the process; second, after die-cutting and before waste discharge, minimize the stagnation time of the label to reduce the possibility of glue re-adhesion. The following changes were made specifically. First, the knife pad foam on the die is only padded inside the knife cavity, because the foam that exceeds the die jump length will also be squeezed and re-adhesive when die-cutting, so the excess part needs to be torn off;
Figure 2 The feed roller of the die-cutting material is changed from a long roller to two small rollers
Second, the feed roller of die-cutting material is changed from a long roller to two small rollers (as shown in Figure 2), and only press the edge of the material to avoid the main part of the finished product;
Figure 3 The suction tension at the feeding position of the waste discharge machine replaces the roller tension.
Third, use the suction feed belt to apply tension at the feeding position of the waste discharge machine (as shown in Figure 3), and all the rollers before waste discharge are beaten up to avoid squeezing the material with the rubber roller;
Figure 4 The PET waste discharge fixture is attached to the stripping knife of the waste discharge machine
Fourth, attach the waste discharge jig to the stripping knife of the waste discharge machine (as shown in Figure 4). Use 0.5mm thick PET to cut into protruding rounded corners according to the product layout width, with 2-3mm exposed compared to the stripping knife, and fix it with double-sided tape. Press it on the product surface at an angle to ensure that the waste is separated from the product smoothly.
After making the above improvements, if the die-cutting waste discharge is still intermittent and not smooth, you can consider not die-cutting the material at the die, and directly feed the material through the waste discharge fixture to the winding core, and then start the die-cutting machine to reduce the retention time from die-cutting to the waste discharge end.
After a whole set of improvement actions, normal machine waste discharge is achieved, avoiding the cost waste caused by manual waste discharge. In addition to the above improvement methods, there are some tips listed below for everyone to share.
(1) If the drawing has right angles or sharp corners, it will easily tear and break due to tearing force during waste discharge. Once this happens, it is recommended to change the right angle or sharp corner of the die to an R angle, and the size of the R angle>0.3mm, which is not much different from a right angle visually, and can reduce the risk of waste breakage.
(2) Avoid horizontal layout when laying out the die. Since the edge of the waste will be subjected to a relatively large force when it is pulled up, horizontal layout will be more difficult to discharge waste than vertical layout. Therefore, horizontal layout should be avoided when laying out.
Figure 5: Make a small cut on the waste area of the die to reduce the force in the depth direction.
(3) If the customer does not accept vertical layout, and the waste is always broken during waste discharge, you can make a small knife on the waste area of the die as shown in Figure 5 to reduce the force in the vertical direction, and the waste is collected by two receiving shafts.
(4) When the material encountered during waste discharge is brittle and the waste edge is small, you can consider adding a small transparent tape to strengthen the support of the waste edge and assist in waste discharge.
(5) If the order volume is large, you can also use a circular knife mold or a magnetic sheet to discharge waste directly with the die, which can effectively discharge waste.
