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.