Sealing Adhesive Application and Drying Protocol for Can Lids

Sealing Adhesive Application and Drying Protocol for Can Lids

The purpose of injecting adhesive into can lids is to provide a sealing function, which is a crucial step in cap manufacturing. Here are some important considerations regarding the injection of sealing adhesives for cans:

1. Preparation of Sealing Adhesive: Before use, bulk sealing adhesives need to be stirred to ensure uniformity. It is recommended to stir water-based adhesives for 20 to 30 minutes before use. Stirring should be vigorous enough to create surface movement without forming vortices. Fast stirring of water-based adhesives can introduce air, leading to instability and injection defects. Additionally, the storage temperature for water-based adhesives should be kept above freezing. Solvent-based adhesives should also be stirred before use. It is advisable to mix solvent-based adhesives in a mixer equipped with two sets of foldable mixing blades at high speed for 30 to 40 minutes. Extra caution is required when handling solvent-based adhesives to prevent static electricity, ignition sources, and solvent exposure. Use anti-static tools, ensure grounding and connectivity, and maintain proper ventilation according to fire safety regulations and occupational safety standards.

2. Transportation of Adhesives: There are various methods for transporting solvent-based adhesives to the injection machine, including troughs, circulating distribution systems, portable adhesive containers, or storage tanks. In all cases, ensure that all containers and equipment are well-grounded. Both water-based and solvent-based adhesives should avoid contact with copper, brass, zinc, and galvanized pipes or fittings to prevent premature aging or coagulation of the adhesive.

3. Filtration of Adhesives: Adhesives must be filtered before entering the injection machine. When transferring the adhesive from the storage tank or barrel to the injection machine, filtration is essential. A filter should be placed between the adhesive container and the nozzle, ideally using a 40-mesh (400 μm) screen. It is also best to install pressure gauges on both the input and output sides of the filter to monitor adhesive buildup. Regular maintenance of the filter is required to prevent soft particles or skins from passing through.

4. Adjustment of Adhesives Before Use:

- Solvent-based Adhesives: To ensure uniform distribution and control of adhesive volume, a Sealing Adhesive Adjustment Device (CCU) should be used. Most adjustment devices utilize gear pumps to shear the adhesive for uniformity while providing constant back pressure throughout the system. The adhesive should also be heated via the adjustment device, with an injection temperature requirement of 37 to 43°C. Injection should not occur at temperatures above 49°C to avoid bubble formation. The adhesive volume is controlled by adjusting the back pressure on the nozzle return side, which should be maintained within the range of 30 to 50 psi (206.84 to 344.74 kPa).

- Water-based Adhesives: A piston pump should be used to transfer water-based adhesives from barrels, intermediate containers, or storage tanks to the injection machine. Gear pumps or centrifugal pumps should not be used for water-based adhesives. A fluid pressure regulating valve should ensure stable injection pressure from the storage container to the nozzle. This pressure regulator should work in conjunction with an automatic shut-off valve. The recommended injection pressure is 15 to 30 psi (103.42 to 206.84 kPa). If the ambient temperature fluctuates more than 11°C between day and night, heated hoses are recommended.

Setting Up the Injection Machine

(5) Injection Machine Setup: To complete the injection process, the injection machine must be equipped with a lid feeding stacker, a clamping head, and a clamping plate that match the can lids. The machine must have a drive mechanism to achieve 2.1 turns of injection and at least 0.75 turns of reverse rotation, along with sufficient clamping head speed to meet the required adhesive volume and distribution. To help readers understand the injection process, we will introduce some key terms and adjustments related to the equipment.

(6) Clamping Head Speed: Before a new sealing adhesive is launched, its suitable can lid diameter and clamping head speed range must be tested to achieve the specified adhesive volume and film distribution. Given a specific can lid diameter, the desired adhesive film distribution depends on the clamping head speed, which is the product of the discharge speed and the gear ratio.

Clamping Head Speed=Discharge Speed×Gear Ratio

For example, if the discharge speed is 350 lids/min and the gear ratio is 7.5, then the clamping head speed is:

350X7.5=2520r/min

For reciprocating injection machines, the gear ratio can be understood as the total number of turns the clamping head makes per injection cycle. The gear ratio is the product of the external gear ratio and the internal gear ratio.

For instance, the gear ratios for the Graco Type 19 injection machine are calculated as follows:

64/28(2.28)X72/26(2.77)=6.32:1 gear ratio

This 6.32:1 gear ratio can provide the required 2.1 turns of injection and 0.75 turns of reverse rotation (for the lid in the raised position) for food can lids and beverage caps, making it the lower limit for the gear ratio.

If the gear engagement is unknown, the following method can be used to estimate the machine's gear ratio:

1. Remove all lids from the feeding stacker and rotate the handle to push the feeding rod to its frontmost position.

2. Mark the clamping head and the machine base with a marker.

3. Use the rotating handle to move the feeding rod through one complete cycle.

4. Count the number of rotations the clamping head makes during this cycle. This number indicates the machine's gear ratio. For example, if the clamping head rotates 6.3 times per feeding rod cycle, then the gear ratio is 6.3:1.

(7) Discharge Speed: The discharge speed, or the number of lids discharged per minute, corresponds to the number of cycles the injection machine's spindle completes in a minute. For reciprocating injection machines, this can be measured with a tachometer. For dual-channel injection machines, the discharge speed is twice the spindle's cycles per minute since the dual channel is controlled by the same spindle.

(8) Separation Blade and Feeding Rod: In injection machines equipped with standard separation blades, the blades are typically fixed to the feeding rod with four screws. The separation blade must be set to engage and move one lid from a stack in the feeding stacker. When the feeding rod reaches its end position, the blade removes a lid from the bottom of the stack. When the feeding rod is fully extended, the lid is placed into the clamping head, where it is held in place by the clamping head's position and the pressure from the upper pressing head. During injection, the feeding rod retracts to pick up another lid, and as the feeding rod moves forward, the top of the rod pushes the already injected lid from the clamping area onto the conveyor belt or other collection device.

The height of the separation blade must be set according to the height of the lid's curl. The correct height of the separation blade should equal the maximum curl height measured across several can lids, plus an additional 0.001 to 0.007 mm. If the gap of the separation blade is too small, it may damage the lid hooks or cause jams. Conversely, if the gap is too large, the blade may separate more than one lid at a time, also leading to jams.

In injection machines equipped with side-positioned separation blades, the blades are not fixed to the feeding rod but mounted on an eccentric cam structure.

Injection Machine Components and Operation

(9) Clamping Head Fit: The clamping head must match the structure of the lid. Its shape should correspond to the can lid and must fit fully into the lid's counterbore depth. A correct fit between the clamping head and the can lid ensures that the clamping head exerts a certain amount of force on the lid. The lid must not be too loose on the clamping head to prevent relative movement, and sufficient pressure must be applied to keep the lid securely in place.

A loose-fitting clamping head can lead to relative sliding of the lid during the injection process, which can result in insufficient adhesive overlap. Conversely, if the clamping fit is too tight, it may cause the lid to shake during rotation, leading to uneven adhesive width during injection. Excessive shaking of the lid while injecting can result in the injection nozzle contacting the adhesive surface, temporarily blocking adhesive flow and ultimately causing interruptions in the adhesive, which can easily be mistaken for a clogged nozzle.

(10) Upper Press Head Cushion: The upper press head cushion not only holds the can lid in place on the clamping head but also activates the mechanical structure of the nozzle. The contact surface of the clamping head should be large enough to maximize contact area with the lid surface, but it must fit within the groove of the clamping head. This design allows for the "no lid, no injection" feature. When the injection machine operates in lid feeding mode, the lid on the clamping head pushes the upper press head cushion upward, causing the ejector pin to move and allowing the sealing adhesive to flow through the nozzle. When the machine runs without feeding lids, the clamping head moves up and down, but since the upper press head cushion cannot align with the groove of the clamping head, the cushion's axis remains stationary, preventing any movement of the ejector pin.

During a lid feeding cycle, when the lid is fed to the clamping head, the upper press head cushion must exert sufficient pressure to prevent the lid from sliding off the clamping head. When the clamping head is at a lower position, the upper press head cushion should be high enough to allow the curled edge of the lid to pass underneath but low enough to make contact with the lid's shoulder. Depending on the lid's contour, the lid should be able to lift by 0.25 to 0.76 mm.

(11) Lid Feeding Time: To ensure the correct positioning of the can lid on the clamping head, the lid must be secured in place before the clamping head begins to lift. If a popping sound is heard when the lid moves toward the clamping head, this indicates that the feeding rod's timing is set correctly. If the feeding rod is pushed too far forward, the can lid will be forced past the correct position.

(12) Jamming due to Clamping Head or Lid Damage: If the feeding rod is pushed too far back, the raised clamping head may lift the front edge of the lid, causing it to strike the nozzle head. This can lead to machine jams or damage to the nozzle.

(13) Pneumatic Electronic Gun Structure: The pneumatic electronic gun operates using an air-operated electromagnetic valve system. The mechanical connection parts of the mechanical gun are replaced with electronic sensors and a timing control unit. A piston controlled by air pressure, located at the upper part of the gun body, is connected to the ejector pin. The air pressure on the piston keeps the ejector pin pressed against the nozzle opening, preventing adhesive from flowing out.

As the lid moves from the stacker to the clamping head, a probe installed on the lid feeder detects the lid. Once the electronic control system receives a signal indicating that a lid is in motion, a sensor mounted on the machine's spindle sends a signal to the nozzle control valve coil. This coil movement allows air to enter beneath the gun body piston while air above the piston is vented, raising the ejector pin and initiating adhesive injection. The injection time is set in milliseconds on the electronic timing unit (60,000 ms / clamping head speed = time for 1.0 turn of injection). The height of the ejector pin is controlled by a nut at the top of the gun body (one full rotation of the nut equals a 0.042 in increase in ejector pin height).

Nozzle Positioning and Drying of Sealed Can Lids

(14) Nozzle Positioning: The accurate positioning of the nozzle is determined by the required distribution of sealing adhesive. Generally, the adjustment of the nozzle should follow this sequence:

- Set the vertical position of the nozzle when the clamping head is fully raised to its highest point, adjusting the nozzle so that the nozzle tip is more than 0.8 mm above the lid’s adhesive application surface.

- The horizontal position of the nozzle should be as close as possible to the wall of the can lid clamping head without touching it. Special adhesive distribution requirements may necessitate positioning the nozzle even closer to the clamping head wall.

(14) Drying of Injected Can Lids: Injected can lids must be thoroughly dried before sealing, as inadequate drying may lead to adhesive oozing. In cold and high-humidity conditions, the sealing adhesive on the lids may not dry quickly. Good ventilation is crucial for adequate drying. Solvent-based sealing adhesives do not require heating for drying, while water-based adhesives must be dried using heat. Generally, can lids should reach a dryness level of 85% to 95% before being combined with paper tape or placed on trays. It is recommended to leave the can lids for at least 48 hours after injection before proceeding with shrink wrapping, transportation, or sealing to allow for full drying, evaporation of volatiles, and proper curing of the adhesive film.

For the use of solvent-based sealing adhesives, operational procedures must comply with fire safety regulations and labor protection laws. The residual solvent vapors emitted from the can lids being injected may accumulate at the injection site, creating a flammable, explosive, or oxygen-deficient environment, so it is essential to pay attention to the storage conditions of the can lids.