1. The main factors affecting the quality of welds
Resistance welding uses the thermal effect of current, that is, when the current passes through two metal plates to be welded, they are melted due to the high heat generated by the resistance on the welding circuit, and combined by pressure and cooling. Welding resistance consists of two parts: the contact resistance between the metal plates and the bulk resistance of the metal plates themselves. Therefore, in order to obtain a good weld, the contact resistance should be reduced and the bulk resistance of the material should be increased.
To control the weld quality, the following five basic parameters should be adjusted: welding resistance, welding pressure, overlap amount, welding speed, and another variable factor-tinplate. These factors will determine the pitch of the weld, the degree of melting, the shape and microstructure of the weld. These parameters are related to each other. When a parameter changes, the welding conditions must be reset.
(1) The relationship between welding speed and welding current
When other conditions remain unchanged, in order to obtain a good weld, the set welding speed and welding current should make the iron sheet melt well and the weld joints are connected. When the welding speed increases, the current must be relatively increased. When the welding speed is too low, it is easy to cause the iron sheet to overheat. At this time, the cooling speed of the solder joint is slower than the shrinkage speed of the iron sheet, so a large hole will be formed at the welding joint. On the contrary, when the welding speed is too high, it is easy to form a phenomenon that the solder joints are not connected. And because the iron sheet is not heated enough, elongated holes or soldering are formed between the iron sheets.
(2) The relationship between welding pressure and welding current
The surface of the tinplate is a good conductive metal with low resistance, and the tin has a very low hardness and is easily deformed under pressure, which greatly reduces the welding surface resistance and is easy to weld. The welding current increases with the increase of welding pressure. The reason is that the higher the welding pressure, the larger the contact area of tinplate and the smaller the contact resistance on the surface. The current required for welding should increase accordingly. The welding pressure should be adjusted in an appropriate range. When the pressure is too low, the fillet formed by welding will be higher, which will bring difficulties in repainting; on the contrary, when the welding pressure is high. It is easy to get a flat weld.
(3) The relationship between overlap and welding current
A large amount of overlap requires more welding heat, and the welding current increases as the amount of overlap increases. Under the setting welding conditions, when the overlap amount is larger than the normal value, the area covered by the same welding pressure increases, resulting in a decrease in welding current density and a slight increase in contact resistance. As a result, the welding heat is insufficient and the cold welding is caused. on the other hand, the decrease in the amount of overlap will easily cause the phenomenon of over welding and extrusion.
(4) The charateristic of tin plate on welding
1. The amount of tin plating
The amount of tinplate has an impact on the quality of the weld. Althoughit is a good conductor,if the contact resistance of layer of tinplate is smaller, but if the amount of tin plating is too small (the amount of tin plating <0.5g/m2), the alloy layer is relatively high because the surface contact resistance of the alloy layer is large , It is not conductive to welding quality. Especially for the same batch of iron, but the alloy layer has a large variation range and the amount of alloy tin is too high, cold welding is likely to occur under the same set conditions. For tinplate with high tin plating, the solder joint pitch obtained at the same welding current is smaller than that with low tin plating. Therefore, to achieve a good weld, the welding speed must be reduced. In addition, if the welding current is too large, when the iron sheet is welded and melted, tin will infiltrate along the grain boundaries of the iron, which may cause transcrystalline corrosion for some foods after canning.

2. Thickness
The thickness of the tinplate has an influence on the adjustment of welding parameters, especially when welding with a high-speed. When the thickness of the tinplate increases, the welding current required is higher, and the upper and lower limits of the welding conditions also decrease as the iron sheet increases.
3. Hardness
The setting of the welding current is related to the hardness of the tinplate. When the hardness of the tinplate increases, the welding current should be reduced accordingly.
Under the setting welding conditions, the variation of the thickness and hardness of the tinplate within the normal range will not affect the welding. However, if the thickness and hardness of the same batch of iron vary greatly, the welding quality will be unstable, resulting in cold welding or over welding. For example, under the setting pressure, when the tinplate hardness increases too much, the surface contact resistance between the two plates increases, and the welding current should be reduced.
4. Steel base material
When the carbon content of the steel base is high, the welder current needs to be increased. In addition, if there are many inclusions in the steel base, spattering points are likely to occur due to the increase in resistance during welding.

In a word, when producing different types of empty cans, or when changing the type of tinplate. It is necessary to reset the new welding conditions.
Second, the main factors affecting the quality of the coating film
After welding, the original protective tin layer of the tinplate has been completely removed, and only the iron of the substrate is stored in this part. Therefore, it must be covered with a polymer organic coating film to avoid corrosion and discoloration of contents after filling with the contact between the iron and the contents.
(1) Paint type
coatings can be divided into liquid coatings and powder coatings. Various types of coatings have their own characteristics due to their different compositions and coating baking processes,the coating film formed also have their own characteristics.
1. Liquid paint
There are mainly epoxy phenolic, acrylic, polyester, organosol and pigmented coatings, etc. This type of coating is suitable for the weld r coating for most food and beverage cans.
Epoxy phenolic coating has few micropores, excellent chemical resistance and sterilization resistance, but it requires high request for baking heat. When the baking heat is insufficient and , it will be nadequate curing . After sterilization, the coating film will become white, which not only affects the coating performance, but also affect Food safety ; the flexibility and adhesion of the coating film will deteriorate when over-baking, making the coating film intolerant to mechanical processing and easy to crack.
Acrylic and polyester coatings have excellent adhesion, flexibility, chemical resistance, and sterilization resistance. However, acrylic coatings are easy to absorb the color of the content and are not very resistant to vulcanization.
The biggest feature of organosol coatings is the high solid content, the ability to form a thicker film on the weld, it is not easy to produce bubbles, and it has good coverage. It has excellent flexibility and resistance to mechanical processing. In addition, it baking heat request is lower than other coatings; but organosol coatings have poor permeability resistance and are easy to vulcanization corrosion, so they are not suitable for sulfur-containing foods.

Pigment-containing coatings are usually made by adding titanium dioxide or aluminum powder to organosol, epoxy or polyester coatings. The purpose is to mask the corrosion spots under the film, so it is suitable for welding of sulfur-containing foods such as luncheon meat cans’ welding. coating etc.
2. Powder coating
The powder coating can form a thick and complete coating film to provide the most complete protection for the weld. At the same time, the coating has no problems of solvent escape and environmental pollution during processing. It has been widely used in food and beverage cans with high corrosion resistance requirements.
Powder coatings can be divided into two types: thermoplastic coatings and thermosetting coatings. The main components of thermoplastic coatings are polyester powder, titanium dioxide, barium sulfate and so on. Its film formation is only a simple melting process, so when the full tank is baked after spraying, when the temperature reaches the melting point of the powder coating, the coating film will melt again. The coating is characterized by excellent flexibility and resistance to various mechanical processing; but its chemical resistance is worse than that of thermosetting coatings, and it is easy to absorb the color of food. Its adhesion to the original color is lower than that to the weld. Therefore, the shape of the coating is like the arch of a bridge. The main component of thermosetting coatings is epoxy/polyester, which is different from thermoplastic powder coatings. The pre-polymer resin is heated and cured into a film. The thickness of the coating film is thinner than that of thermoplastics. It has excellent chemical resistance, but its processability is not as good as thermoplastic coatings. .

(2) coating film thickness
The protective characteristic of the coating film is related to its pore size. The number of micropores of the same type decreases with the increase of the film thickness. For liquid coatings, the thickness of the coating film depends on the solid content (viscosity) and coating amount during construction.
1. Viscosity
The same liquid coating, the higher the viscosity, the higher the solid content, and the better the coverage of the weld. However, the thickness of the coating film requires a long baking time. If the thickness of the coating film is increased per micron, the baking time needs to be increased by 1 second. At the same time, the coating film with high viscosity is easy to produce bubbles during baking, which will affect the micropores of the coating film; on the contrary, when the coating viscosity is low, the coating film is thin and has many micropores and poor protection performance. Therefore, it is necessary to adjust the appropriate construction viscosity to make the coating film have an appropriate thickness, and at the same time prevent the overspray and splash caused by the excessively low or excessively high viscosity, resulting in insufficient baking and curing.
2. Coating amount
In order to obtain a suitable coating film thickness, the coating amount should be controlled. In addition to adjusting the construction viscosity of the coating, attention should be paid to selecting the appropriate nozzle model, adjusting the pressure of the coating pump and the temperature of the coating when spraying. When the roller coating method is adopted, the coating wheels with different surface curves and the adjustment scraper can be selected to control.
Liquid coatings can never form a thick film in the areas where the weld seam needs the most protection, generally only 4-10 microns. This is due to the surface tension effect of the coating. Due to viscosity changes during baking, the paint will flow away from the edge of the weld (fillet) to both sides. The surface of differernt type coating.There is a difference in tension effect. The viscosity of organosol coatings will always increase with the increase of temperature, so it is not easy to cause poor coverage at the edge of the weld. With the increase of temperature, the viscosity of epoxy phenolic coatings first decreases, and then It rises straight again, so when the viscosity drops to the lowest point, the coating will flow to both sides, making the coating film thickness at the edge of the weld much lower than that on both sides.

The film thickness of powder coatings is much thicker than that of liquid coatings. Thermoplastic powder coatings should generally not be less than 40 microns, because only when the thickness of the coating film is greater than 40 microns, the number of micropores drop significantly, but when it is higher than 60 microns The mechanical properties will be worse, and the coating film will be easily broken during processing. The thermosetting powder coating has good compactness, and the thickness of the coating film is generally controlled at 20-60 microns. In addition to the thickness of the film porosity of the coating, it is also related to its density. Controlling the diameter distribution of powder particles can control the thickness and density of the coating film.
(3)Coating film integrity
1. Weld quality
The integrity of the liquid coating film depends largely on the geometry of the weld. For example, the weld has splash points or severe extrusion, and the surface is not smooth. It is impossible to completely cover it with liquid paint; in addition, the thickness of the weld also effect the coating film. If The welding is thicker ,the seam is not easy to cover. Generally, the thickness of the welding seam should be less than 1.5 times the thickness of the plate. For the second cold-rolled iron or iron with high hardness, the thickness of the welding seam is 1.5 to 1.8 times the thickness of the plate. .
Welds made without nitrogen protection will cause poor adhesion of the coating film after the oxide layer, which will cause the coating film to rupture during subsequent flanging, necking, and rolling, which will affect the integrity of the coating film .
Due to the sufficient thickness of powder coating, it can perfectly solve the problem of metal exposure caused by weld defects and provide excellent protection for the weld.
2. Bubble
The unreasonable solvent formulation of the liquid coating will affect the integrity of the coating film. When the liquid coating contains a lot of low-boiling solvents, or the temperature rises too fast during baking, or the welding seam temperature is too high, a large amount of solvent volatilizes during baking, leaving strings of bubbles or micropores in the coating film. This leads to poor coverage and reduces the protective effect of the repair coating on the weld.
(4) Baking and curing
1. the curing process of the coating
The baking and curing of liquid coatings can be roughly divided into the following stages: the coating is first leverl wetted and the empty parts (about 1 to 2 seconds), and then the solvent evaporates to form a gel (should be within 3~ Complete within 5 seconds, otherwise the paint will flow away from the weld), and finally polymerization. The paint must reach the required total heat. It has a great influence on the thickness and characteristic of the coat film. As mentioned earlier, if the bake temperature rises too quickly, bubbles are likely to occur, and if the temperature rises too slow, the peak temperature maintenance time is too short and will cause insufficient curing.
Various coatings have different peak times during baking. Epoxy phenolic coatings take longer than organosols, that is, more heat is required for baking.
The thermoplastic coating in the powder coating is simply melted to form a film during baking, and it has no polymerization reaction; while the thermosetting coating needs to undergo addition polymerization and crosslinking into a polymer compound after prepolymerization without melting, so the baking heat and supplement The characteristic of the coating film is closely related.
2. The effect of curing degree on coating film performance
coatings can show their characteristics only when they are fully baked and cured. Insufficient baking results in many micropores in the coating film, and poor processing resistance. For example, if the thermoplastic powder coating is not baked well, the coating film tends to wrinkle when flanging. Excessive baking will affect adhesion. For example, if epoxy phenolic paint is over-baked, the coating film will be scorched and brittle, and it will easily cause cracking of the coating film at flanging, necking, and rolling. In addition, sufficient cooling after baking also has an important effect on the performance of the coating film. If the thermoplastic powder coating is not quickly cooled to room temperature after the coating is baked, the coating film is likely to crack during flanging. If a cooling device can be added after the oven, the problem of flanging and cracking of the coating film can be avoided.
In one word, in order to ensure the quality of the coating film, that is, less porosity and good processing resistance, the thickness and curing degree of the film must be controlled.

Since 1978, with the development of electric resistance welding (Super-WIMA) in the can-making industry, the overlap of can welds has been reduced to 0.4-0.6mm, and lead-free resistance welding tin-plated cans have been widely used For food cans and beverage cans, this transformation greatly improves the overall quality of the can body weld lap. In order to make the can body weld perfect, it is an indispensable process to recoat the weld seam with a protective layer of non-destructive content. The protection and anti-corrosion process of the can body resistance welding seam is also widely used, so that The resistance welding can body is more safe and reliable application.
However, in different situations, different regions, different canned contents and their storage time, the protection and corrosion protection of can body welds, and the quality parameters are also different. Therefore, solving the corrosion of welds is a major issue. Corrosion occurs in different forms. In addition, the effects of the canned sterilization process and carbonated beverages, such as beer and soft drinks, have brought many technical problems to the weld coating process.
The welding seam protection and anti-corrosion process is mainly divided into two processes. The welding seam coating process and the coating drying curing process are divided into liquid coating technology and powder coating technology in the coating process, The coating thickness and protective effect are also different, see Figure 3-51, Figure 3-52 and Figure 3-53.

Figure 3-51 The coating of liquid paint, the coating is thin, especially in the area of welding seam

Figure 3-52 coating of thermosetting powder coating, thicker coating

Figure 3-53 The patch layer of thermoplastic powder coating, the thickest layer

1. Frequent questions about the drying and curing process of liquid coating strip

① Deviation of recoating strip. Due to the poor adjustment of the can conveyor belt or the poor connection between the two conveyor belts, the can body rotates on the conveyor belt, so that the nozzle or the inner roller coating wheel cannot be aligned with the welding seam, causing the recoating strip to deviate (see Figure 3-87), The coating protection effect is not good. Solution: patiently adjust the conveyor belt so that the can body moves smoothly on the conveyor belt without rotation.

Figure 3-87 Recoating strip is off-center
② There are 4 abnormal situations.

Figure 3-88 Abnormal coating film after curing
A. The coating is too thin after curing: The reason is, for example, the wet coating thickness is normal when recoating, the coating is thin after curing, and the solid content of the liquid coating is too low, so the solid content of the coating should be increased. Or increase paint viscosity. For roller coating, choose a thick repainting curve roller. For spraying, appropriately increase the pressure of the paint pump and the temperature of the paint heater, or change the nozzle to a larger size. Or slow down the speed of the can conveyor.

B The coating is porous after curing (micro-bubbles): The reason is that the drying and curing speed is too fast, the coating boils, and bubbles and blister-type micro-pores are generated. Solution: slow down the curing speed, that is, slow down the speed of the conveyor belt of the drying curing equipment, reduce the temperature of the heating oven in the front section of the heating curing device, so that the heating temperature gradually rises.

C The coating has bubbles: When the solvent has not evaporated, or when spraying, the bubbles generated by the coating, if the curing speed is too fast, the surface coating begins to cure, but the solvent continues to volatilize or the bubbles from the recoat are not eliminated. Air bubbles and small holes are left on the surface of the cured coating. In addition, if the wrong solvent is applied, such as thinner water or banana water for ordinary paint, since these solvents evaporate slowly, the liquid recoating strip will easily have bubbles when it is cured, and it is difficult to eliminate, as shown in Figure 3-89 .

Figure 3-89 Air bubbles formed on the coating surface
Sometimes bubbles are formed on the weld, as shown in Figure 3-90. The reason is that the welding seam is too hot and the viscosity of the paint is a bit high. Solution: add a cooling device after welding, or reduce the viscosity a little.

Figure 3-90 Bubbles formed on the weld

D. Poor coverage of the weld area at the edge of the tinplate: The quality of the welding has a great influence on the protection of the liquid coating. The cause and solution of this failure: i The welding seam is splashed, slightly increase the welding seam overlap amount, or reduce the welding current, so as to make the welding seam smooth, which is conducive to the welding seam coating; ii the paint is not enough in viscosity. Solution: increase the viscosity of the paint and increase the paint temperature accordingly.

③ Overspray. There are some oversprayed paints on the outside of the weld recoating strip, which will affect the appearance (see Figure 3-91 and Figure 3-92). Sometimes these coatings are not easy to cure. Reasons and solutions: i The viscosity of the liquid paint is too low, so the viscosity should be increased; ii The nozzle model is wrong, that is, the nozzle width of the selected nozzle model is too narrow, which causes the distance between the nozzle and the weld to be too far, so that the atomized paint Scattered to the rest of the tank. Choose the correct nozzle model.

Figure 3-91 Overspray

Figure 3-92 The paint has splash points in the tank

④ Splashing. The coating strip is too thin, and there are splashes at both ends (see Figure 3-93). Reasons and solutions: i The spraying pressure is too low or the pipeline is blocked, increase the pressure. Check the pipeline, ii The viscosity of the paint is too high. The viscosity should be reduced and the paint heater temperature should be increased i; ii The nozzle is too close to the can body. The nozzle should be raised.

Figure 3-93 Splashing

2. Re-coating of welded seam powder coating

To obtain the best microporous weld repair coating, a thick protective coating layer is required. However, during the drying and curing process of the liquid coating, the thick liquid coating layer is prone to air bubbles. This phenomenon can only be achieved by application Powder coating to solve.

To achieve the best quality effect of the welding seam paint protection layer, not only depends on the coating method of the paint, the welding effect also plays an important role, such as: when the welding frequency is low, or the application of low tin iron, chrome-plated iron, etc. splashing and rough surface on the weld, as well as the existence of the weld oxide layer, make the coating layer easy to be destroyed in the subsequent processes such as flanging, necking, and rolling. In addition, the thickness of the tinplate, the quality of the slitting, etc. will bring great problems to the liquid coating process.

Although the welding equipment has made many improvements to make the weld perfect, such as increasing the welding frequency, adding auxiliary welding wheels, etc., to reduce the spatter of the inner weld, and increase the weld anaerobic protection device, but the application of liquid coatings, you must think There are still some difficulties to achieve perfect protection, especially as the welding speed increases. In addition, the powder coating process has very little air pollution during the application process. Powder coatings can also cover welding spatter points (see Figure 3-94). During the application of powder coatings, the coatings between the cans can almost be completely recycled for reuse, with less waste. Therefore, in order to achieve perfect welding seam protection, the powder coating coating process has been applied by more and more Chinese manufacturers in recent years.

Figure 3-94 Welding spatter is covered by powder coating

The advantages of powder coating are well known, but it also has certain inconveniences. The powder inlet pipe and the powder return pipe of powder coating must be fixed on the welding arm of the welding machine, and the can body can still pass through the outside. Because the powder tube is relatively thick, the outer diameter is generally 6~8mm. When making D52 can type, a special welding arm is needed to fix the powder tube on the welding arm and let the D52 can type (202) pass. The cost of canning has increased.

3. Common problems of powder coating

(1) After the powder coating enters the full spraying process and curing in a large oven, the powder coating strip will be crinkle and uneven (see Figure 3-109). Generally, there is this phenomenon in thermoplastic powder coatings, that is, the melting point of thermoplastic powder coatings is low. During the curing process of full spray coatings, after the second melting, they are solidified. Because the thermal expansion coefficient is different from that of full spray coatings, When shrinking, crinkles are formed. The solution to this method can appropriately lower the oven temperature of the full spray coating, thereby reducing the secondary melting of the thermoplastic powder coating. If it still can’t be solved, only change to other types of powder coatings, that is, the coating contains thermosetting coating components to increase the melting point of the second melting of the coating.

Figure 3-109 crinkle of powder coating strip
(2) After the rolling and flanging, the powder coating strip is broken (see Figure 3-110). The reason is that the elasticity of the coating strip is not enough. The solution: If it is a thermoplastic coating, it may be over-dried. You can appropriately reduce the drying temperature or shorten the time, and accelerate the cooling after drying and curing to increase the elasticity of the coating strip. If it is a thermosetting powder coating or a coating containing more thermosetting components, the temperature should be appropriately increased to increase the adhesion of the coating strip.

Figure 3-110 The powder coating strip breaks after rolling
(3) After the thermoplastic powder coating is baked and cured, do a peeling test. If the coating peels off too much (see Figure 3-111), more than 5mm, or the adhesion test of the thermosetting coating does not work well, then Increase the baking curing temperature to improve the adhesion of the coating strip.

In addition, if after baking and curing, the adhesion of the powder coating layer is very poor, and large pieces fall off from the overlap with the primer coating (yellow printing iron), and the improvement of baking does not work, then it may be the primer (yellow iron printing) There is too much wax in the solvent of the paint. After the primer paint is baked, the wax precipitates and settles on the surface of the primer, which causes the adhesion of the powder coating to decrease. Solution: Improve the solvent of primer paint. Or before powder re-coating, use a strong organic solvent to scrub the weld re-coated area to remove the wax layer to improve adhesion.

Figure 3-111 Peel test, the coating is peeled off too much, exceeding 5mm, the curing temperature is not enough
(4) Before the powder coating is baked and cured, the powder coating strip is uneven and crinkled, especially the powder coating matched with a low-speed welding machine. This phenomenon is that the charging voltage is too high, and the powder forms a phenomenon of homosexual repulsion. It is necessary to appropriately reduce the electrostatic high voltage. In addition, this phenomenon may also occur if the powder has been stored for too long (far beyond the supplier’s shelf life).

(5) After powder coating, before baking and curing, the powder on the conveyor belt can easily fall onto the can wall. The electrostatic voltage can be appropriately adjusted to increase the powder’s adsorption capacity. At the same time, check whether the can body has collisions, jitters and jumps during the conveying process, try to make the can body move smoothly and reduce the falling of powder.

(6) After the powder is sprayed, the powder coating becomes thinner, sometimes accompanied by shaking, and the powder spraying pipe is blocked. Unblock the powder spray tube as soon as possible, or replace with a new tube. When installing the powder tube to the welding arm, pay special attention to the powder tube not to touch the copper wire guide wheel, and to keep the powder tube working at room temperature, otherwise, if the molten powder is blocked in the tube, it will be difficult to dredge.

(7) The powder coating on both ends of the can is too thick, which affects the sealing effect of the subsequent process. The thickness of the coating should be reduced, or the suction of the external powder suction port on the OHC should be increased to make both ends of the coating thinner.

(8) When powder coatings are stored for a long time or are repeatedly used during production, the performance of powder coatings will change, such as charging performance, fluidization fluidity, and some dirty dust mixed in it, so in production Attention should be paid. In addition, powder coatings generally contain a certain amount of moisture (0.6%~0.8%), so that the fluidity of powder coatings will be better. If it loses due moisture, the fluidity of the powder will become worse, and the powder coating will easily It clumps and looks like damp. Therefore, a moisture test must be carried out. If the difference is too large, add water appropriately to make it contain the normal amount of water. Then operate according to the methods and parameters provided by the powder coating supplier.

1. The name of each part of the first curling
1. The thickness of the first curling (Tc)
The thickness of the first curling refers to the thickness of the curling section after the can lid and the body are fitted, and the first rolling wheel is crimped to complete the first curling.
The thickness (Tc) of the first curling edge is affected by the size of the can lid’s round edge, the arc shape, the first curling roller groove and the pressure factors.
Tc=3tc+2tb+G′
tc: Thickness of iron for can lid
tb: Thickness of iron for can body
G’: The total thickness of the internal gaps of the first curling (usually 0.8-1.00mm, depending on the first curling groove shape, degree of newness or CH value)
Tc is usually controlled at 1.95-2.11mm

Figure 4-20 first step rolling
2. Width of first curling (Wc)
The width of the first channel curling refers to the width of the curling section after the completion of the curling
The curling width of the first track is affected by the size, shape, thickness of the iron sheet and the width of the first curling roller groove, etc., and it is usually controlled at 2.34 to 2.77mm. If it exceeds too much, the reason must be investigated.
Two
2. The name of each part of the second hemming

Figure 4-21 Schematic diagram of double crimping structure

1. Hemming thickness (T): the measured maximum size of each layer of iron in the hemming.
The curling thickness T consists of the thickness of the three-layer can lid and the two-layer can body. It is not parallel to the can body and forms an oblique angle of about 4°. It will be affected by the oblique angle of the outer edge of the indenter.
To
The curling thickness is usually affected by the groove shape of the second roller, the pressure of the second roller, the thickness and hardness of the tinned sheet, the type of sealing film and the amount of coating and other factors. The curling thickness T can be calculated by the following formula:
T=3tc+2tb + G (g1+g2+g3 + g4)
In the formula: tc is the thickness of the iron used for the can lid and Tb is the thickness of the iron used for the can body

G=g1+g2+g3 +g4 is the sum of the gaps of the iron sheets inside the crimping, which is approximately in the range of 0.10-0.40mm. Generally, G for φ63.5-φ98.9mm diameter tanks is 0.15mm. According to the actual situation, it is generally considered that G=0.15mm for water-soluble sealant and G=0.10mm for solvent-based sealant.

Regardless of the T value standard, the most important thing is that the thickness of the crimping in the same tank should be the same. No matter which type of tank is in the same tank, except for the joints (three-piece tank), the difference between the T values ​​of any two points must not be greater than 0.08 mm.

2. Hemming width W: parallel to the measured size of the body cover hook

The hemming width is the width of the hemming section after the second seal is completed, also known as the hemming length. There are also standards for the crimping width W, which is generally based on the shape of the can lid forming part, the first track, the second roller groove shape and the crimping pressure, the pressure of the supporting chassis, the body hook and the thickness of the can body and lid, and the wear of the roller groove And other major factors.
Reference formula: W=1.1tc+BH+Lc+1.5tc
Where: tc—the thickness of the iron used for the tank cover BH—the length of the body hook Lc—the bottom gap

Usually W is 0.3-0.4mm larger than Wc, for example WC=2.6mm, W=2.9mm (reference value)
Generally, the curling width>3.05mm (except for φ153 large cans) is considered abnormal. Except for the seams in the same tank, the curling width of any two points must not be greater than 0.13mm, and the curling width must be greater than the countersunk degree C is smaller than 0.13mm, otherwise, when the two rollers are sealed, the lower edge of the curling width will be deformed due to no indenter support and the lower edge part cannot be pressed tightly.

The curling width W is regarded as one of the indexes of the tightness of the curling during the operation. Under the same conditions of the can lid, the body, and the roller, the larger W indicates the tighter the curling. During the sealing process, the usual situation W will become smaller and smaller, that is, the tightness will become looser. Generally, the variation of W can be used to evaluate the change in the tightness of the hemming.

3. Countersunk degree C
The countersunk degree is the depth measured from the top of the crimping to the shoulder blade adjacent to the inner wall of the crimping after the second round of crimping. It is also the position where the can lid and the indenter are engaged during crimping. The countersunk degree is affected by the thickness of the flange of the indenter, the countersunk degree of the can lid, the relative height of the roller and the indenter, the distance between the indenter and the supporting chassis, and the pressure of the supporting chassis. Usually the thickness of the flange of the indenter is 0.1mm deeper than the countersunk of the can lid, and the countersink of the crimping edge after rolling is 0.1mm larger than the thickness of the flange of the indenter (0.13-0.20mm)

Example: The countersunk degree of the can lid is 3.30mm
The thickness of the indenter flange should be 3.40mm
The gap between the second curling roller and the indenter is controlled within 0.10mm
The countersunk degree of the curling edge after the second volume seal C=3.50mm
The countersunk degree of most food cans is between 3.05-3.30mm;
The iron cans for carbonated beverages and beer have a depth of 3.80-4.06mm;
The vast majority of 211-diameter aluminum cans have a countersunk degree between 6.35-6.60mm;
209 iron bottom cans, 209 aluminum bottom cans, C is between 3.05-3.33mm.

There is no absolute standard value for the countersunk degree C, which is determined by the thickness of the indenter and the roller gap, but it must be 0.13mm-0.20mm larger than the crimping width W, and the countersunk degree of any two points in the same tank should not be more than 0.13. mm. …

4. Body hook length (BH)

The body hook length BH comes from the flanging of the can body. It is the length of the can body flanging. It depends on the fit between the lid and the can body, the width of the can body flanging, the radius of the can body flanging R, the pushing force and pressure of the supporting chassis. The distance between the head and the support chassis will change; if BH is too small, tank leakage is likely to occur; if BH is too large, the cover hook will become too small.

There is no absolute standard for body hook length BH. Most tank types use 1.9-2.05mm as the standard. As long as the overlap length and overlap rate can meet the safety sealing requirements, it can be accepted. Therefore, BH needs to increase with the tank type, and W increases. Big and increase. Under good rolling conditions, BH and CH are almost equal.

In the same tank, the difference between the BH values ​​of any two points must not exceed 0.10mm, and the average value of the three tanks must not exceed 0.18mm. The greater the difference between this value, it indicates that the support plate is not parallel to the indenter or there is too much distance.

5. Cover hook length CH

The length of the lid hook CH refers to the bending length of the crimped part of the can lid inside the crimping, and the size of the crimping part of the can lid round edge can be changeable suject to the factor:the first track roller groove type, the first track roller pressure, the round edge arc shape of the can lid, the thickness of the indenter and other factors.

The cover hook length CH is the same as BH. It has different requirements according to the can diameter and varies from the width of the crimping. Generally, CH is 1.80-2.00mm, and usually the cover hook length CH must not be less than 1.65mm.

6. Cover hook gap (Upper gap) Uc, body hook gap (lower gap) Lc

The cover hook gap Uc and the body hook gap Lc are expected to be smaller in general cans. This is because the sealing film located in the crimping part moves to the crimping gap due to the pressure during the crimping process. Therefore, if Uc and Lc are too large, the sealing film will move to the gap and gather, instead of being firm, compressed, and isolated between the plates, resulting in poor sealing effect.

The Uc and Lc values ​​increase mainly due to insufficient curling of the first track, excessive pressure on the second roller, and the lower jaw of the roller to cause the CH curl to sag. Compared to the hook groove of the first roller, the second roller has passed the hook groove. High, also due to the second curling rollers to make the CH curling droop, BH or CH is smaller.

7. Overlapping length OL

Crimping length refers to the length of the overlapping part of BH and CH inside the crimping
It can be approximated by the following formula:
OL=BH+CH+(1.1)tc-W
The splicing length is one of the criteria for judging the sealing performance of the can crimping. Usually, the splicing length <1.00mm is considered unacceptable (micro-sealing<0.76mm is unacceptable).

8. Overlap rate OL％

The overlap rate OL% indicates the degree of overlap between the BH and CH inside the crimp, that is, the percentage of the actual overlap length (a) to the theoretical overlap length (b). It is the most correct to calculate based on the actual measured values ​​of the two parts a and b, and it can also be calculated approximately with the following formula

The overlap rate is also one of the criteria for judging the sealing performance of the can crimping. When the overlap rate is measured with a projector, the OL% must be >55%, and the formula calculation method must be >50%.

9. Tightness TR％
Lid hook wrinkle degree WR% refers to the degree of wrinkles on the inner edge of the lid hook after the crimping is disintegrated. It is visually graded according to the proportion of the wrinkled part to the length of the entire lid hook after the crimping is disintegrated. The number has increased. Tightness TR% and wrinkle WR% are in a corresponding relationship. The smaller the wrinkle, the higher the tightness. The wrinkle adopts ten-level method, and no wrinkle is 0. The tightness TR%=100%, and the wrinkle extends to the full cover. The elder is 10, and the tightness TR%=0%. It is usually judged by combining the flatness of the cover hook section as shown in Figure 4-22.

Figure 4-22 Tightness representation

Usually the tightness can be used to judge the sealing degree of the crimping or the adjustment of the sealing machine. When judging, the residual sealant on the lid hook should be removed, and the dent marks left by the removal should be calculated by the degree of wrinkle.
Several expression methods of crimp tightness are shown in Table 4-1.

Table 4-1 Tightness and Wrinkle Degree

10. The complete rate of inner lip, joint cover hook Jr%

For the part pressed out due to poor involvement of the cover hook, the cover hook becomes shorter and droops, which is called the inner drooping lip, as shown in Figure 4-23

Figure 4-23 – visual inspection of hook integrity of seam cover
The inner drop lip usually occurs at the joint, because the thick sheet iron at the joint often prevents the curling edge of the can cover from being drawn into the corner radius of the body hook like other parts, resulting in the drop lip. Serious external sagging or too thick welding seam and poor lap joint are enough to cause internal sagging, which will lead to leakage of the tank. After the disassembly of the curling edge, the proportion of the effective cover hook of the inner lip sagging of the cover hook to the width of the whole cover hook is expressed by naked eye or magnifying glass, and each drop is 25%. Due to the reduction of the thickness of the joint, the integrity rate of the joint cover hook is not the main problem Question.
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11. Indentation
After the edge is disassembled, there is a mark of indenter line (angle change) around the inner edge of the can body (contact part with the cover), which is called pressure mark, and appropriate indentation must be formed. Figure 4-24

Figure 4-24 body indentation