Weld Seam Liquid Coating: Equipment, Processes, and Troubleshooting

(1) Roller Coating Equipment: Inner coating device for weld seams, with a coating box mounted on a coating arm connected to the welding arm. When the tank body is transported to the coating wheel after welding, the coating of the weld seam is performed. This method involves applying liquid coating to the weld seam on the tank body using a roller, driven by a motor. The coating amount on the roller is controlled by a scraper. The coating wheel surface is processed into different curves to control the thickness of the liquid coating. The coating wheel system is placed in a coating box, and the coating material is supplied to the coating box from a coating tank through pneumatic pressure. The coating box also has a coating liquid level detection device to control the amount of coating material supplied from the coating tank to the coating box. The coating wheel’s coating surface is designed with different curves, including convex curves, straight lines, and concave curves, to control the thickness of the coating. The coating box is mounted on a support, and the coating liquid control is simple, maintaining a constant coating liquid level using the principle of pressure balance inside and outside the coating bottle.

Most external weld seam coatings use roller coating devices, which work in conjunction with the oxygen-free protection process to enhance the aesthetic appearance and rust prevention of the tank body weld seam.

For external weld seam coating devices, refer to Figure 3-71, including the coating box, tank conveyor belt, and control system. Liquid coating accessories are shown in Figure 3-72.

(2) Spray Equipment: No-air spray system, commonly used for internal weld seam liquid coating equipment by most manufacturers. Generally, it is an improved system based on the one invented by the American company NORDSON. The main components include an air pressure coating pump (air pressure-to-hydraulic ratio of 1:16), filter, coating heater, coating hydraulic pressure regulating valve, spray gun, nozzle, and a circulation valve group forming a closed-loop circuit. An electrical control system controls the coating heater temperature and the spray switch time of the nozzle. This coating process involves atomizing the liquid coating through a special nozzle, spraying it onto the weld seam. Good coating results are achieved by adjusting nozzle hole size, coating viscosity, coating temperature, spray pressure, and diluents. There are air and non-air atomization nozzles. The principles of non-air spray are shown in Figure 3-73.

Coating viscosity should be determined based on data provided by the coating manufacturer or experimental results. DIN4 viscosity cup (volume 100 mL, hole diameter 4 mm, ambient temperature 20°C) is commonly used for viscosity measurement. The coating pump sucks the coating into the pipeline (see Figure 3-76), and the pressure can be increased to 2-4 MPa. The main components of the spray system are as follows:

1. Liquid Coating Boost Pump: Driven by compressed air, the piston cylinder moves up and down, pressurizing the liquid coating to the required pressure (usually 2-4 MPa) in a ratio of 16:1. The pressure is determined based on the coating width, selected nozzle, coating viscosity, and coating temperature.

2. Coating Heater (see Figure 3-77): Reduces the viscosity of the liquid coating to increase the solid content after spraying, enhances the evaporation rate of the solvent, reduces the surface tension of the coating, and improves the adhesion of the coating layer. The liquid coating pipeline system usually has a circulation valve.

Notably, in non-air spray nozzles (see Figure 3-81), when working, they are mounted on the spray gun. The head is made of hard alloy material, and the special-shaped small holes are processed by a special method, allowing the liquid coating to form a fan-shaped atomization under high pressure. These nozzles come in various styles and different aperture sizes to adapt to different coatings. There are three types of hole shapes:

1. Standard Type (S-standard): The nozzle has “cat-eye-shaped” small holes, producing a fan-shaped atomization, suitable for certain liquid coatings.
2. Cross-Cut Type (X-cross-cuts): The nozzle has a square hole, providing clear fan-shaped atomization and higher adaptability to various coatings, resulting in a more even distribution of the coating. The cross-cut type may perform better than the standard type for some coatings but similar for others.
3. Flow Coating Type (F-FLOW-COATING): This nozzle offers the best coating effect, with minimal overspray and excellent coverage. It is suitable for both traditional and some special liquid coatings. After prolonged use, nozzles may wear differently, affecting their performance.

When selecting the nozzle model, it is essential to conduct tests based on different liquid coatings or follow the recommendations from the liquid coating supplier.

Additionally, when changing coatings, it is crucial to clean the pipeline, use appropriate diluents, and avoid mixing different liquid coatings, which may result in adverse effects on the adjustment, debugging, and final coating results.

(3) Baking and Curing Process of Liquid Coatings: After applying liquid coatings to the weld seam area, a curing process is required to polymerize the coatings. The protective effect of the coatings applied to the weld seam also depends on the baking and curing temperature and time, especially for applications like high-temperature sterilization at 121-129°C for 90 minutes in the food industry.

The curing device consists of an oven and a conveyor belt. The conveyor belt transports the coated tank body through the oven at a specific speed and holds it for a certain period. The hot air in the oven cures the coatings, forming a chemically stable coating layer. The ends of the oven can extract volatile solvents, as shown in Figure 3-82. The baking and curing process of liquid coating generally consists of two stages:

1. Solvent Evaporation Stage: This is a physical reaction. During the initial stage of baking in the oven, the solvent in the wet coating layer needs to evaporate fully before the solid components of the coating cure. Otherwise, if the coating surface begins to cure while the solvent has not completely evaporated, continued solvent evaporation can create bubbles on the surface of the cured coating, damaging the protective layer.

2. Coating Curing Stage: This is a chemical reaction. After the solvent in the coating layer applied to the tank body weld seam has completely evaporated, the solid content undergoes a polymerization reaction at the high temperature of the oven for 10-20 seconds (different coatings have different properties). This process transforms low-molecular-weight compounds into high-molecular-weight compounds, giving the coating layer the characteristics of high-molecular-weight compounds and achieving rust resistance and corrosion resistance. The distribution of coating thickness after baking and curing is shown in Figure 3-83.

Experience shows that approximately 1 second of baking time is required for every 1 μm of cured liquid coating layer. By using an improved solvent formula that ensures the required solder melting temperature for the coating and without bubbles, it is possible to reduce baking time by 30%.

The curing curve of liquid coating is shown in Figures 3-84 and 3-85, with the initial stage being the solvent evaporation period at lower temperatures and the later stage being the curing period at higher temperatures.

During the baking process, temperature and time are crucial parameters. Higher baking temperatures may lead to the following results:

– Good curing of the coating

layer.
– The coating layer may become too thin.
– The external ink layer may change color, and the tank may be scratched during transportation.
– Shorter baking time for curing.

The impact of baking and curing time is as follows:

– Less prone to bubbling during baking and curing, resulting in a well-cured coating layer.
– Favorable for subsequent processes, such as high-temperature sterilization.
– Adequate coating film thickness. Lower baking temperatures are beneficial for the equipment’s lifespan.

The distribution of the coating belt during baking, the actual state of the liquid coating layer, and the ideal state may differ significantly, especially in thinner areas of the weld seam (see Figure 3-86).

(4) Common Issues in the Baking and Curing Process of Liquid Coating Belts:

1. Coating Belt Deviation: Poor adjustment of the tank conveyor belt or poor connection between two conveyor belts can cause the tank body to rotate on the conveyor belt, misaligning the nozzle or internal roller coating wheel with the weld seam, resulting in coating belt deviation (see Figure 3-87). This affects the protective effect of the coating layer. The solution is to patiently adjust the conveyor belt to ensure smooth movement of the tank body on the conveyor belt without rotation.

2. Anomalies in Four Scenarios:
a. Coating Layer Too Thin After Curing: Possible causes include low solid content in the liquid coating, requiring an increase in solid content or coating viscosity. For roller coating, choose a roller with a thick coating curve. For spraying, increase the coating pump pressure and coating heater temperature, or switch to a larger nozzle. Alternatively, slow down the conveyor belt speed.
b. Coating Layer Porous After Curing (Microbubbles): This may result from too fast curing, causing the coating to boil and form bubbles and blister-like micropores. The solution is to slow down the curing speed by reducing the conveyor belt speed or lowering the temperature of the heating oven in the early stage.
c. Coating Layer with Bubbles: Bubbles may form in the coating layer during solvent evaporation or spraying. If the curing speed is too fast and the surface coating begins to cure while the solvent continues to evaporate, or if bubbles from the coating are not eliminated during spraying, bubbles and small pores may be left on the surface of the cured coating. Using the wrong solvent, such as toluene or banana water commonly used for regular paints, can also lead to bubbles that are difficult to eliminate during the curing of the liquid coating belt, as shown in Figure 3-89. Sometimes, bubbles form on the weld seam, as shown in Figure 3-90. The cause may be excessive heat on the weld seam and slightly higher viscosity of the coating. The solution is to add a cooling device after welding or reduce the viscosity slightly.
d. Poor Coverage of the Mouth Iron Edge Weld Seam Area: The quality of welding has a significant impact on the protective coating layer. Reasons and solutions for this issue include: i. Weld seam splatter: Slightly increase the overlap amount of the weld seam during welding or reduce the welding current to smooth the weld seam, facilitating coating repair. ii. Insufficient coating viscosity: Increase coating viscosity and correspondingly increase coating temperature.

3. Overspray: Some coating material outside the welding seam repair belt causes an impact on aesthetics (see Figures 3-91 and 3-92). Sometimes, these coatings are difficult to cure. Causes and solutions include: i. Coating viscosity is too low, requiring an increase in viscosity. ii. Incorrect nozzle type, meaning the selected nozzle has a width that is too narrow, causing the nozzle-to-weld seam distance to be too far, resulting in atomized coating drifting to other parts of the tank body. The solution is to choose the correct nozzle type.

4. Splattering: The repair belt is too fine, accompanied by splattering at both ends (see Figure 3-93). Causes and solutions include: i. Low spray pressure or pipeline blockage, requiring an increase in pressure. Check the pipeline. ii. Coating viscosity is too high, requiring a reduction in viscosity and an increase in the coating heater temperature. iii. The nozzle is too close to the tank body, requiring adjustment.

Essential Welding Machine Operations and Quality Inspections

In the actual operation of welding machines, although different welding machines may have slight variations, the basic principles are the same. Therefore, it is crucial to adjust each part of the welding machine according to the welding machine manual, with special attention to the following key points:

(1) Sheet Metal Specifications and Dimensions, Right Angles, and Burr Compliance, Correct Adjustment of the Magazine:
Ensure that the sheet metal meets the specified size, right angles, and burr requirements, and adjust the magazine correctly.
(2) Correct Adjustment of Sucker, Pusher, Double Sheet Detector, Flexer, Pre-roll forming, and Roll-forming:
Ensure the proper adjustment of sucker, pusher, double sheet detector, flexer, pre-roll-forming, and roll-forming.
(3) Working Height Adjustment of Z-Bar and Lower Welding Wheel Welding Position:
Adjust the working height of the Z-bar and lower welding wheel welding position according to the correction amount of the lower welding wheel.
(4) Proper Adjustment of Roller Guider and Welding Calibration Crown:
Ensure the correct tension, up-down, and front-back adjustment of the roller guider and welding calibration crown. Ensure that the can body’s opening shape meets the requirements, and maintain a deviation of less than 0.10 mm in the overlapping amount between the front and rear ends.
(5) Correct Adjustment of Synchronization and over push Amount:
Ensure the correct adjustment of synchronization and over push amount.
(6) Copper Wire Size and Forming Conformance:
Ensure that the dimensions of the copper wire meet the requirements, and the entire process operates smoothly. The tin traces on the upper and lower welding wheels should be centered, with a gap of approximately 1.5 mm. After passing through the welding station, the copper wire elongation rate should be less than 2%-3%, and the temperature should feel normal.
(7) Adjustment of Welding Pressure Based on Plate Thickness and Quality:
Correctly adjust the welding pressure based on plate thickness and quality. Adjust the welding current to the upper limit until the weld seam splatters irregularly, Then adjust it to the lower limit until the weld seam opens or breaks mechanically. Finally, adjust the welding current to its optimal value, usually slightly above 1/3 between the upper and lower limits.

Welding Quality Inspection:
(1) Visual Inspection of Weld Seam Appearance:
During the production process, visually inspect the quality of weld seams inside and outside the can. Visual inspection can identify obvious defects such as weak weld points, virtual welding, burn-through, overheating, uneven weld points, missed welds, splatter points, lack or excess of overlapping amount at the can body’s start or end, etc.
(2) Weld Seam Strength Inspection (Spherical Tester):
Place the sample can on the spherical tester, align the ball with the weld seam to form a straight line, and move the ball along the weld seam of the can body. Expand the weld seam outward by 1-2 mm (adjustable). Remove the can body from the spherical tester and check for any cracks or fractures in the weld seam.
(3) Tear Test:
Take two sample cans, cut 5 mm at one end of a can using scissors or pliers, bend the cut surface upwards, insert the can body onto the test mandrel, clamp the cut surface with pliers, and continuously and uniformly tear the weld seam upwards (at a 45° angle to the seam). Repeat the process on the other end of the second can. Visually inspect the torn weld seam: if the entire weld seam comes off clearly, feels smooth when shaken between the index and middle fingers, the weld seam is of good quality. If the weld seam breaks at a certain point during separation, the welding current is too weak. If the torn edge is irregular and feels rough, it indicates splatter points and excessive welding current. A good-quality weld seam should be flexible and not brittle.
(4) Cone Test:
The main purpose is to test the ductility of the weld points at the front and rear. Place the test can on the cone tester, press the conical body proportionally into the welded can body at a constant speed until it expands into a cone shape. Remove the can body and observe the expansion of the can: if the expansion value is above 20%, the flanging performance is good; 10%-20% expansion is acceptable; if the expansion value is ≤10%, it is considered unacceptable.
(5) Weld Seam Flip Curl Test:
The main purpose is to test the ductility of the weld points at the front and rear. Extract one can body from the welding machine, select the corresponding mold, fix it on the bottom plate of the flip curl tester, and place the can body on the mold. Operate the piston valve, press the plate to contact the can body for flip curling, curling about 10 mm, remove the can body, and observe and identify whether there are cracks in the flipped part. Cracks indicate failure.
(6) Inspection of Welding Before Overlapping, Welding Width, and Thickness:
Generally, use a magnifying glass with a scale or a projector to measure the overlapping width before welding and the width of the weld seam after welding. The weld seam thickness is measured using a pointed micrometer.
(7) Inspection of Inner Diameter Using Internal Diameter Calipers:
Check the inner diameter using internal diameter calipers, ensuring that the difference between the front and rear can diameters is less than 0.10 mm.

Drying Oven Systems in Tinplate Printing: Structure, Functionality, and Optimization

Tinplate printing commonly adopts a continuous heating and drying method, with printing drying ovens typically spanning 24 to 27 meters, and coating solvent content being higher, the drying ovens are generally around 30 meters. These ovens consist mainly of the furnace body, transmission mechanism, heating device, and a heat circulation system.

Furnace Body
The furnace body typically incorporates a good thermal insulation layer, and materials with poor heat conductivity are usually chosen as fillers.

Transmission Mechanism
The drying oven conveyor chain employs a single-row chain with a pitch of 25.4mm, equipped with graphite rollers on the outer side. The oven usually features front-end linkage and is equipped with precise electromagnetic clutches to control synchronous operation and disengagement with the preceding machine.

The furnace frame is a critical part of the conveyor system, and the quality of its surface and the alloy composition are crucial factors determining the product’s quality, particularly in terms of avoiding scratches.

Heating Device
The heating device is determined by three parameters:
(1) The temperature set for the oven – known as the Peak Metal Temperature (PMT).
(2) The duration of maintaining the Peak Metal Temperature (PMT).
(3) Machine speed.

Tunnel-Type Drying Oven
A tunnel-type drying oven typically comprises three sections: the heating zone, constant temperature zone, and cooling zone.
(1) The primary function of the heating zone is to elevate the tinplate from room temperature to the set temperature.
(2) The constant temperature zone’s main role is to maintain the temperature achieved in the heating zone.
(3) The cooling zone aims to ensure sufficient cooling of the dried tinplate. Inadequate cooling can lead to scratches on the tinplate during collection, and sticking issues may occur after stacking.

Baking time is related to machine speed. If the machine runs too fast and the peak temperature is maintained for too short a duration, the paint or ink may not be adequately baked, resulting in softening and failure to fully exert its protective or other functions. Insufficient baking of internal coatings can cause changes in the taste of food or beverages in contact with them. High-temperature cooking tests often result in poor adhesion. If the machine speed is too slow, overbaking may occur, causing many coatings to become brittle, prone to breakage during processing, and overbaking can also lead to discoloration of paints and inks. To avoid inadequate or excessive baking, it is essential to regularly check the oven temperature curve to determine the PMT time at the standard machine speed. Modern temperature measurement instruments such as “DATAPAQ” serve as ideal means for determining furnace temperature curves.

Overview of Coating Equipment and Adjustment in Offset Printing

Tinplate coating equipment is mainly used for internal and external coating and, at times, can be directly attached behind a printing machine for immediate curing.

Main Structure of Coating Equipment

Adjustment of coating machine

1. Conveying Section: Refer to the adjustment of the printing machine conveying section.

2. Setting Rubber Roller and Coating Roller (Using CRABTREE as an Example):
①Set the pressure between the bottom roller and the rubber roller; the minimum pressure between the two is the correct pressure.
②Bring the rubber roller into contact with the bottom roller, use manual wheel “B” to bring the coating head into contact with the rubber roller, then use manual wheel “A” to balance and ensure proper contact.
③Set the pressure between the coating head and the rubber roller; remember the correct pressure between the coating roller and the rubber roller as the minimum pressure.
④ Operate “B” to bring the rubber roller into contact with the bottom roller, and the coating head into contact with the rubber roller. Use “A” to adjust the coating head until the coating roller is parallel to the rubber roller and makes proper contact.

3. Dry Film Quality Setting:
① Dry film quality is a critical parameter, and its control is crucial. The key setting principle is that the gap between the feed roller and the coating roller of the coating head is crucial in controlling film weight.
②Set the gap between the feed roller and the guide roller to ensure proper feeding.
③Changes in the size of the two gaps follow the rule that a wider gap results in higher film weight and more feed.

4. Major Adjustments:
①Rotate counterclockwise to increase film weight/coating feed.
②Rotate clockwise to decrease film weight/coating feed.
③Each rotation of the adjustment disc changes the roller gap by 0.025mm, as indicated by the scale.
④ Secure the screws after setting.

5. Squeegee Setting The bottom roller is made of cold cast iron with a very hard and wear-resistant surface and is finely polished. The surface of the bottom roller is cleaned with a special doctor blade which is fixed in the paint groove. The overflow line in the trough is used to ensure that the doctor blade edge is always below the level of the solution in the trough. In this way the doctor blade edge can be kept lubricated at all times, scraping the bottom rollers clean so that the iron is instead free of stains. Special note: The machine can be started at any time when the doctor blade is in contact with the bottom roll or when the solution or paint in the doctor blade groove is not full of the doctor blade edge.

6. Rubber Roller Information:
①Rubber roller diameter: The maximum diameter of CRABTREE’s rubber roller is 339mm, and the minimum allowable diameter is 319mm. When the roller diameter is less than 319mm, it is challenging to install the coating machine.
In handling horizontal weld seams (transverse gaps), for a one-to-one correspondence between the bottom roller and the rubber roller’s speed, the rubber roller must have the same diameter as the bottom roller, i.e., 329mm.
②hardness of the unit “SHORE”: the hardness of the rubber rubber unit is “DEGREE SHORE”, when selecting rubber for the rubber roller, in addition to selecting and coating compatible rubber, but also must control the hardness of the rubber.
Generally speaking, the harder the rubber. Paint on the roll on the mobility of the worse, on the contrary, the softer the rubber, when the construction of empty space is more difficult, because the rubber is too soft, cannot prevent the iron side of the deformation embedded in the rubber.

7. Air Clutch Setting:
The purpose of the air clutch is to provide a quick and efficient method to separate the rubber roller drive and the gear chain.To use this function, the air pressure regulator on the machine’s operating side needs to be correctly set.
For example, when coating the entire plate, set the pressure to 0.4MPa; for horizontal gap coating, set the pressure to 0.7MPa.
Note: When coating the entire plate, use 0.4MPa. After the selector moves to the horizontal gap setting, the pressure will automatically increase to 0.7MPa and completely lock the clutch.

Overview of Coating Types in Tinplate Production: Functions and Characteristics

Tinplate coatings are broadly classified into internal coatings, external primers (base oil coatings), undercoats (white rubber coatings), and topcoats.

Internal Coating:
(1) Primary Purpose: Prevents the corrosion of the can wall and contamination of the contents, ensuring prolonged storage.
(2) Key Characteristics: Corrosion resistance, excellent adhesion, flexibility, non-toxic, and odorless, meeting food safety standards. Must withstand localized high-temperature heating during subsequent processes like welding and internal touch-up. Additionally, should resist fading, loss of gloss, and peeling even after canning and exposure to 121°C high-temperature boiling.
(3) Common Choices: Typically includes phenolic resin coatings, epoxy-phenolic resin coatings, acrylic resin coatings, and aluminum paste. The selection depends on the characteristics of the contents and the desired coating thickness.
(4) Major Process Parameters:

① Viscosity: Generally in the range of 80–120 seconds (measured with a #4 Ford cup), specific values determined based on supplier parameters and actual production conditions.
② Baking Temperature: Typically around 200°C for 12 minutes, with specific values determined based on supplier parameters and actual production conditions.
③Dry Film Quality: Primarily determined based on the nature of the packaged contents, e.g., 7g/m² or above for items like protein drinks, tea beverages, fruit juices, congee, and over 14g/m² for highly acidic products like tomato sauce, yangmei, and asparagus..
Primer Coating (Base Oil)
(1) Primary Purpose: Ensures secure adhesion of ink or white coating to the tinplate.
(2) main Characteristics:
①Transparency is essential, with minimal yellowing upon film drying, preserving the metal texture.
②Exhibits excellent leveling, appropriate heat-curing properties, and flexibility to facilitate subsequent processing.
③Possesses sufficient affinity for inks or white coatings, ensuring strong adhesion to various types of tinplate.
④After film formation, maintains good water resistance to facilitate printing.
(3)Common primer coatings include ethylene-based, modified ethylene-based, and epoxy-amine-based types. Since primers are mostly low-molecular-weight compounds, their film formation is susceptible to the influence of additives in other coatings applied over them, impacting their physicochemical properties (adhesion, yellowing). Therefore, when selecting a primer, subsequent processes involving white coatings and gloss oils must be compatible.
Primer (White Coating) – Process Parameters

(4) Key Process Parameters:
① Viscosity: Generally in the range of 30 to 50 seconds (measured with a 4F Ford Cup), specific values need to be determined based on supplier specifications and actual production conditions.
② Baking Temperature: Typically around 200°C for approximately 12 minutes, specific values need to be determined based on supplier specifications and actual production conditions.
③ Dry Film Mass: Typically around 1.5g/m², specific values need to be determined based on supplier specifications and actual production conditions.

Overview of Coating Types in Tinplate Production

Primer (White Coating) – Introduction

(1) Primary Purpose: According to principles of colorimetry in printing, a white base is essential to fully reflect the various spectra of white light, allowing accurate color development for inks applied to its surface. The silver-gray surface of tinplate exhibits metallic luster, and certain inks directly applied to its surface may display colors differently than when applied to a white surface. White coating with kaolin is mainly used for tinplate printing where the entire layout has a white base.

(2) Key Characteristics:

①Resistant to baking, avoiding yellowing.
② Exhibits good whiteness and fullness.
③Typically, the film-forming substance in white coating is a high-molecular-weight compound, offering resistance to various overprinting oils.
④Good resistance to processing, with favorable bending characteristics.
3) Common Recommendations:
Acrylic and polyester types are generally recommended. Polyester types exhibit good resistance to yellowing and can be directly used for various can bodies. Acrylic types, resistant to high-temperature cooking, are typically employed in deep-drawing processes. To enhance adhesion between the coating and tinplate, a primer is often applied before the application of white coating.

(4) Key Process Parameters:
① Viscosity: Typically in the range of 120 to 160 seconds (measured with a 4# Ford Cup), specific values need to be determined based on supplier specifications and actual production conditions.
②Baking Temperature: Usually around 180°C for approximately 12 minutes, specific values need to be determined based on supplier specifications and actual production conditions.
③Dry Film Mass: Typically around 15g/m², specific values need to be determined based on supplier specifications, layout color, and actual production conditions.

Overprint Varnish – Introduction

(1) Primary Purpose: To enhance the surface gloss and hardness of printed materials, providing a certain degree of flexibility and corrosion resistance to the coated surface.

(2) Key Characteristics:

Excellent color retention; solvents in the varnish should not cause ink bleeding or fading.
Sufficient hardness and firmness to withstand post-processing deformation.
(3) Common Recommendations:
Common types of overprint varnish include epoxy resin, alkyd resin, acrylic resin, and acrylic amine. Different compositions result in varied performances and applications. The selection depends on factors such as whether there is deep-drawing deformation in subsequent processes, localized high-temperature heating during welding, and the need for high-pressure cooking.
(4) Key Process Parameters:
① Viscosity: Generally in the range of 80 to 120 seconds (measured with a 4° Ford Cup), specific values need to be determined based on supplier specifications and actual production conditions.
② Baking Temperature: Typically around 180°C for approximately 12 minutes, specific values need to be determined based on supplier specifications and actual production conditions.
③ Dry Film Mass: Typically around 8g/m², specific values need to be determined based on supplier specifications, mechanical processing, and actual production conditions.

(5) Special Effects Overprint Varnish:
Besides gloss varnish, there are matte varnish, wrinkle varnish, and pearl varnish.
① Gloss Varnish: Achieves a high gloss effect, aligning with traditional aesthetic preferences.
② Matte Varnish: Conversely, it imparts a non-glossy surface to tinplate products, creating a paper-like texture and an elegant decorative effect.
③Wrinkle Varnish: When applied and shaped, it forms crystal-like transparent flakes. Light reflection enhances the three-dimensional effect of exquisite patterns on tinplate, providing high artistic appreciation value from different angles.
④ Pearl Varnish: Applied to large exposed areas, it exhibits a pearl-like luster, highlighting the brightness of printed graphics and text, offering extremely high aesthetic value.

Aluminum Material Varieties, Properties, And Applications in Can Manufacturing

Aluminum cans, also known as two-piece cans, consist of a can body and a can lid, both manufactured on different production lines using various aluminum alloys. In practice, the can lid is composed of two components: the lid body and the pull-tab, both produced with different alloys and thicknesses of aluminum strips on highly automated lines.

The can body typically employs Al-Mn series alloys, such as 3004, AA3004, AA3104, and AA3204, with a thickness ranging from 0.25 to 0.29 mm and a condition of H19. For can lids (cover, pull-tab components, etc.), the majority use 5182-H19 strips, while some may use 5052-H19 or H48. Cans with internal pressure, like beer and carbonated beverage cans, and those without internal pressure, like juice cans, utilize 5082 (or 5182) and 5052 alloy strips, respectively, in an H38 condition. The lid body (base lid) typically employs thin strips of approximately 0.27 mm thickness with 5082 or 5182 alloys. The pull-tab material is mostly 5052-H19, although Novelis Aluminum Sheet and Plate Company exclusively offers 5182-H19 and H48 coil strips. Material widths vary, including 45mm, 46mm, and 68mm.Materials for pull-tabs and sealing components are mostly 5052 alloy, occasionally with some instances of 5182 alloy. The combination of the pull-tab and sealing component is commonly referred to as the pull-tab.

Types of Coatings-Epoxy Ester Coatings

Currently, the main types of coatings used for canned goods include epoxy phenolic coatings, epoxy amine coatings, organic solvent coatings, ethylene-based coatings, polyester coatings, acrylic coatings, and epoxy ester coatings. Below is a brief introduction to the characteristics of various resins and the performance and applications of coatings made from them.

Epoxy Ester Coatings

Epoxy ester resin is a product obtained through the esterification process of plant oil reacting with epoxy resin. This resin imparts flexibility and color to coatings.

Epoxy ester coatings are primarily used for post-printing varnish, suitable for both mixed-can products and products resistant to boiling. Therefore, it is suitable for metal can bodies, deep-drawn cans, twist-off caps, crown caps, and all types of mixed-can products.

Most epoxy ester coatings are colorless, but the addition of dyes can produce golden epoxy ester coatings used for decorating the external surfaces of twist-off caps and crown caps. The key properties and advantages include:

(1) Excellent Gloss:

Exhibits a high level of gloss.
(2) Good Color Fastness:

Maintains good color stability, crucial for external designs.
(3) Combination of Flexibility and Hardness:

Combines flexibility with good hardness.
(4) Excellent Ink Compatibility:

Exceptionally compatible with inks, especially suitable for the “wet-on-wet” production process.
Compared to other synthetic coatings, epoxy ester coatings are one of the most versatile varieties. They can achieve the same level of gloss as oil resin varnish while overcoming the drawbacks of poor color fastness and susceptibility to yellowing.

Curing for epoxy ester coatings is achieved through oxidation and thermal polymerization. For mixed-can production, a peak temperature of 160-180°C is typically used, heated for up to 10 minutes when high ink color fastness is required. The difference in dry film color is minimal within this temperature range. When used for products resistant to boiling, heating with a peak temperature of up to 190°C for a maximum of 10 minutes is needed to maximize water and steam resistance performance.

Types of Coatings-Epoxy Amine Coatings

Currently, the main types of coatings used for canned goods include epoxy phenolic coatings, epoxy amine coatings, organic solvent coatings, ethylene-based coatings, polyester coatings, acrylic coatings, and epoxy ester coatings. The characteristics of various resins, as well as the performance and applications of coatings produced from them, are briefly outlined below.

Epoxy Amine Coatings:

Epoxy amine coatings are formulated with epoxy resin and amine resin as the primary components in specific proportions. Widely used as protective coatings for the external surfaces of can lids, can bodies, twist-off caps, and sealing caps, these coatings exhibit the following key properties and advantages:

(1) Colorless Coating:

The coating is transparent, providing a colorless appearance.

(2) Excellent Chemical Resistance and Processability:

Demonstrates good resistance to chemical agents and is easily processed.

(3) High Boil Resistance:

Exhibits strong resistance to boiling conditions.

(4) Good Color Fastness Upon Re-Baking:

Maintains good color stability when subjected to additional baking.

Types of Coatings-Epoxy Amine Coatings

Epoxy amine coatings are highly sterilization-resistant, particularly in strongly alkaline water with a pH range of 9 to 10, making them the preferred choice for protecting the external surfaces of can lids. This property is also utilized in packaging latex paint. The resistance to water absorption and resistance to whitening during the cooking process are significantly better than other modified epoxy products. Their excellent heat resistance prevents mechanical abrasions during heat treatment.

The color fastness and resistance to yellowing of epoxy amine coatings during re-baking are noteworthy, making them widely applied as protective coatings for the external surfaces of three-piece beverage and food cans, including can bodies, can lids, twist-off caps, and crown caps. They are especially useful when the final baking of internal coatings may affect the performance of external coatings.

Epoxy amine coatings maintain their performance well even when exposed to contents that may spill and corrode the external coating of the can. Due to their excellent adhesion, they are also used as base coatings for printing.

Curing for epoxy amine coatings is achieved through thermal polymerization. For mixed-can products, they are typically heated to a peak temperature of 170-180°C for up to 10 minutes. When higher boil resistance and chemical resistance are required, a peak temperature of 200°C is recommended.

Epoxy amine coatings are also frequently used as internal coatings, especially in beverage cans with low corrosive contents such as dairy products and fruit juice beverages.

Epoxy Phenolic Coatings Characteristics Performance and Applications in Canned Food Packaging

Currently, the main types of coatings used for canned goods include epoxy phenolic coatings, epoxy amine coatings, organic solvent coatings, ethylene-based coatings, polyester coatings, acrylic coatings, and epoxy ester coatings. The characteristics of various resins, as well as the performance and applications of coatings produced from them, are briefly outlined below.

Epoxy Phenolic Coatings:

Epoxy phenolic coatings are formulated using epoxy resin and phenolic resin in specific proportions. This coating system finds wide application in the field of metal packaging for cans, particularly as internal coatings for beverage and food cans.

Epoxy Resin:

Epoxy resin refers to a polymer with two or more epoxy groups, with a main chain composed of aliphatic, cycloaliphatic, or aromatic segments. The epoxy group content is a crucial indicator, and in beverage and food cans, high molecular weight epoxy resins, such as Type 7 and Type 9 epoxy resins, or even higher molecular weight variants, are often required. These resins exhibit excellent impact strength and toughness due to a high concentration of oxygen-containing groups in their molecular chains, providing strong bonding capabilities.

Epoxy Phenolic Coatings Characteristics Performance and Applications in Canned Food Packaging

Phenolic Resin:

Phenolic resin is obtained through condensation reactions of phenolic and aldehyde monomers. It consists of numerous methylene (—CH2—) and rigid phenol linkages, featuring a structure with abundant polar hydroxyl groups. The molecular structure is rigid, lacking flexibility, and further curing of hydroxyl groups forms a three-dimensional structure consisting of C-C bonds. This close-knit structure imparts stability against various chemical substances, particularly notable for its corrosion resistance, especially in acidic environments. Epoxy phenolic coatings combine the advantages of both resins:

(1) Excellent adhesion to metal substrates;

(2) Superior processability;

(3) Outstanding chemical resistance (especially against sulfur and acids in food packaging);

(4) Good heat resistance and wear resistance;

(5) Low shrinkage and low porosity.

Due to its excellent corrosion resistance, epoxy phenolic coatings play a crucial role in three-piece beverage and food cans, as well as in two-piece drawn food cans. They are extensively used as internal coatings for various fruit juices, herbal teas, and cans containing highly acidic or sulfur-rich foods. Typically, epoxy phenolic coatings exhibit a golden color with vibrant and full hue, while also demonstrating outstanding physical properties (wear resistance and flexibility) and resistance to boiling. Therefore, epoxy phenolic coatings are frequently employed for interior coatings in mixed cans and external coatings for the tops and bottoms of beverage and food cans.

Epoxy phenolic coatings require baking at 200–205°C for 10 minutes for complete curing in a single application. Many epoxy phenolic coatings can achieve full curing at lower temperatures through multiple baking cycles. However, repeated baking of epoxy phenolic coatings can lead to a significant decrease in adhesion, severely impacting the quality of the coating. Additionally, the color of the coating darkens with each baking cycle. Therefore, attention must be paid to curing temperature and coating processes to minimize the number of baking cycles while ensuring complete curing.

Epoxy phenolic coatings exhibit good wetting and leveling properties on substrates such as tinplate, chrome-plated iron, and aluminum sheets, minimizing common surface defects. The surface treatment of the substrate also plays a significant role in this performance. During application, attention should be given to the viscosity; excessively high viscosity can impact the flow leveling effect and lead to surface defects, while too low viscosity can result in sagging. Dilution with specialized solvents should be considered in practical operations. Moreover, thorough stirring before use is essential to ensure the uniformity of the coating due to the presence of additives in the coating system.

Aluminum paste or zinc oxide is often added to epoxy phenolic coatings for packaging certain sulfur-rich foods, such as meats, seafood, and asparagus. The addition of mold release wax in such systems can be used for packaging canned luncheon meats. Aluminum paste primarily serves to block hydrogen sulfide generated during food sterilization from penetrating through the coating and to cover potential sulfide spots. Zinc oxide’s main function is to absorb hydrogen sulfide produced during food sterilization, preventing sulfide corrosion. However, the inclusion of aluminum paste or zinc oxide in epoxy phenolic coatings results in softer coatings, leading to decreased processability.

Global Trends in Tinplate Development: From Production Strategies to Environmental Innovations

(1) In major tinplate-producing nations worldwide, there is a general trend of no longer establishing new tinplate production units, including those for Tin-Free Steel (TFS). However, developing countries, particularly China, are still in the process of constructing new production lines, experiencing rapid growth. TFS products offer lower production costs and contribute to the conservation of scarce tin resources. Over the past 40 years, significant progress has been made, and the industry has now stabilized, with ongoing development in China.

(2) In addition to promoting TFS for tin resource conservation, another trend is the thinning of tinplate thickness. Secondary cold-rolled tinplate, including TFS, is increasingly being applied. Three-piece can bodies widely adopt tinplate with a thickness of 0.14 to 0.17mm. Two-piece steel cans, particularly in Europe, commonly use DI material with a thickness of 0.235mm (for 330mL cans, equivalent to 0.245mm for 355mL cans). Some are already using 0.205mm DI material (330mL), and 0.19mm DI material (330mL) is in the experimental phase. In Japan, TULC material has been reduced to a thickness of 0.18mm (though the can body’s thin wall thickness is 0.08mm, thicker than traditional DI cans).

Global Trends in Tinplate Development

(3) Clean Production, Reducing Environmental Pollution.

In North America and Europe, research on hexavalent chromium-free passivation is currently underway. Traditional PSA plating solutions, due to their high toxicity and substantial pollution, are gradually being replaced by MSA plating solutions. The passivation film on tinplate is composed of metal chromium, hydrated chromium oxide, tin oxide, and other components. The most commonly used passivation treatment for food and beverage cans is cathodic heavy sodium dichromate passivation. However, chromium salts cause severe environmental pollution, prompting countries to develop passivation techniques with minimal environmental impact.

In the UK, a working group consisting of tinplate producers, coating manufacturers, can makers, and chemical companies has been established to assess passivation techniques involving zirconium, titanium, cerium phosphates, silicates, and some organic coatings. Clariant, a leading electrochemical company in the UK, has developed a basic chemical substance, phenol sulfonic acid (PSA), adopted by American tinplate manufacturers. Its fundamental formula contains 2% free phenol, considered to be the most environmentally friendly. The improved Clariant formula (LPSR) reduces free phenol to 1% and diphenyl sulfone (DDS) from 1% to 0.7%. This formula is not only environmentally friendly but also has the lowest cost.

In other European countries, the focus is on evaluating chemicals such as polyacrylic salts and rosin acid siloxanes. In the United States, it is proposed that zirconium sulfate (ZOS) and fluorinated zirconium acid (F-Zr) could substitute for chromic acid.

In the Asia-Pacific region, experiments involving mixed metals such as cerium oxide, silicon oxide, and titanium salts are ongoing, with fluorinated zirconium acid proving to be the most successful.

In the field of tinplate printing, traditional solvent-based inks are being replaced by eco-friendly inks. In the iron printing sector, 70% of tinplate lines are using environmentally friendly inks, with the UK already reaching 90%. Coating applications are also actively promoting the use of healthy and safe coatings.

(4) Recycling of Metal Cans

Developed countries place significant emphasis on the recycling and reuse of post-consumer metal cans. The European Union and others have enacted legislation specifying clear targets for recycling and recycling rates. Germany, for instance, mandates recycling rates of 70% for tinplate cans and 50% for aluminum cans. In the United Kingdom, the 2008 recycling targets were set at 61.5% for steel cans and 35.5% for aluminum cans. In the United States, the recycling rates in 1998 had already reached 56% for both steel and aluminum cans. Japan achieved recycling rates of over 80% for both steel and aluminum cans in the year 2000.

(5) Co-Extruded Composite Coating

ToyoKohan, a Japanese company, has pioneered the development of co-extruded composite coating for tinplate. ToyoKohan’s strategic approach involves collaborative research with their partners in can manufacturing, leveraging their expertise in thin iron production. In 1997, ToyoKohan successfully produced the first TULC can (Toyo Ultimate Can). Between 1998 and 2001, the company achieved successful development of non-oriented pressure-sensitive film and double-sided co-extruded composite coating for tinplate. The introduction of the dry forming process in 2006 reduced the cost of TULC cans, improved production efficiency, and further minimized environmental impact.

The internal non-oriented pressure-sensitive film enhanced the tensile properties of the iron, achieving optimal can forming effects. This thin film requires lower preheating temperatures during the production process, proving effective in coating adhesion and reducing production costs.

Double-ended co-extrusion stretching (DEC) represents an advanced forming process, allowing the creation of stable films as thin as 10 micrometers or even thinner on the can wall. However, the production cycle for cans has been shortened, and the speed of production lines is limited. It is anticipated that by 2008, DEC will reach a certain speed threshold.

Cans made from polymeric-coated iron, such as those utilizing Protact technology from Corus Packaging Plus, demonstrate resilience against impacts and friction encountered throughout the entire transportation chain. The continuous growth of the recycling rate for lightweight and sturdy canned packaging is evident, with the recycling rate in the EU-15 countries reaching 63%. Iron packaging, through Protact’s polymeric-coated technology, aligns with EU regulations, contributing to reduced raw material consumption, energy usage, and environmental impact.

Protact, utilizing Corus Packaging Plus’ multi-polymer coated iron (Protact) rolling technology and its advancements, combines the characteristics of polymers and iron, offering a secure, versatile, and high-performance packaging material. Two processes are employed in Protact iron production: the film-coating process has limited production speeds, approximately 50–80m/min, while the extrusion process operates at speeds of 100m/min, reaching up to 300m/min. Protact iron can feature various interchangeable composite layers, with options for single or double-sided coating, different thicknesses of polymers (transparent or colored), and customization based on specific requirements, such as tension during production, polymer adhesion, printability, and lubrication.

According to ICI Coatings’ Global Research and Investigation Division, achieving optimal corrosion resistance and food safety for iron cans depends on an ideal coating system formulation. The coating should adapt to the entire can production chain, ensuring the quality of the empty cans. In comparison to traditional three-piece cans and draw-redraw cans (DWI), the development of draw-and-iron cans requires specialized polymer coatings to withstand strong stretching forces. As forming capabilities increase, it becomes crucial to enhance the density of polymer cross-links to improve the corrosion resistance of the can’s interior.

Different steel bases will impact the surface tension of the coating, the interaction between the coating and the steel base, and the adhesion and corrosion resistance of the coating. Temperature is a significant factor during bending, as higher temperatures may lead to coating shrinkage and loss of adhesion. The more uniform the coating surface humidity, the fewer humidifying additives are needed, reducing the solvent requirements for water-based coatings.

(6) Technistan

Technistan is a novel electrolytic process for the production of tinplate, developed by the American company Tehic. This work is currently underway in the United States. The formulation of this process consists of four components: a tin sulfate solution containing approximately 20g of tin metal per liter; 5% concentrated sulfuric acid; 5% additive Techristan TP; and an antioxidant (to prevent excessive deposits on the surface of iron due to the use of sulfuric acid). Tehic is pursuing this new process in response to the rising prices of cold-rolled coils, seeking lower production costs. However, for this process to be widely accepted in the market, an extended period of time is required, along with the validation of its environmental impact.