Key Points For Choosing A Coating Machine
1. Overview of Coating Machines
The coating process refers to the application of one or more layers of functional coating liquid onto one or both sides of a flexible, thin substrate to impart specific properties to the substrate.
The coating lamination process refers to the process of bonding two or more materials together to form a multi-layer material.
As process equipment, coating machines must be custom-designed and manufactured according to the specific products to be produced.
How to better meet the technical requirements of the coating and lamination process, and efficiently produce high‑quality products, places high demands on the design, engineering, and manufacturing of coating machines.
Therefore, the design of a coating machine requires a thorough understanding of customer needs, combined with the equipment manufacturer's practical experience, to complete the entire process from communication, validation, design, procurement, manufacturing, installation and commissioning, to trial run and final handover.
According to the production process requirements for coating or coating lamination, a coating machine or coating production line typically includes the following basic units: unwinding, pulling/traction, coating, drying, and winding.
Depending on the specific technical process requirements, it may also include devices such as web guiding, accumulators, lamination units, substrate surface cleaning, corona treatment, defect detection, and coating thickness measurement.
The constituent units of a coating machine include: unwinding unit, splicing unit, accumulator unit, pulling/traction unit, coating unit, drying unit, pulling/traction unit, winding unit, tension control system, and drive/control system.
2. Basic Criteria for Selecting a Coating Machine from a Process Perspective
1. Required coating thickness and allowable tolerance
Understand the required dry coating amount and dry thickness, the actual wet coating amount and wet thickness, as well as the acceptable coating thickness tolerance.
Use experimental data to determine the appropriate coating method and drying requirements.
2. Target coating speed
Understand the coating speed to determine the suitable coating method and drying conditions.
3. Coating surface morphology
The surface morphology of the coating is mainly influenced by the coating method and the substrate. Different coating methods use different equipment and have varying effects on coating uniformity; the greater the substrate irregularity, the more difficult levelling becomes, and rough substrate surfaces make it difficult to form a smooth, flat coating.
If a flat coating surface is required, a metering/doctoring coating method such as doctor blade, doctor plate, or doctor rod should be chosen.
If a profiled/contoured coating surface is required, a pre‑metered coating method such as gravure, slot‑die, or slide curtain should be chosen.
Given the compatibility of coating methods in practice, it is necessary to obtain the relevant parameters.
4. Coating liquid viscosity and rheological properties
When selecting a coating method, the viscosity and rheological properties of the coating liquid are critical information. Based on the dependence of viscosity on shear rate, fluids can be classified as Newtonian, dilatant (shear‑thickening), or pseudoplastic (shear‑thinning).
According to changes in viscosity with time, fluids can be further divided into Newtonian, thixotropic, and rheopectic fluids. Different fluids have different effects on coating flow and levelling.
For example, pseudoplastic and thixotropic fluids are effective in preventing sedimentation, floating, and cratering, but they have adverse effects on levelling and gloss.
In general, the greater the deviation of the coating liquid's rheological behaviour from Newtonian, the more likely various quality problems will occur during the coating process.
This is especially true when the coating liquid exhibits dilatant behaviour, because its viscosity increases rapidly with increasing coating speed, requiring greater mechanical force to spread the liquid. As a result, the higher the coating speed, the more difficult it is to obtain a uniform coating; moreover, excessive force may tear the substrate or even damage equipment such as doctor blades. When the coating liquid exhibits thixotropic behaviour, the levelling of the adhesive layer during coating is very poor, making it difficult to achieve a smooth and uniform adhesive layer. These factors have a significant impact on the choice of coating method.
