Overview
Hot melt adhesive lab small coaters are key equipment used to apply hot melt adhesive materials to various substrate surfaces in a controlled manner in a laboratory environment. Its core function is to simulate actual production process parameters, providing reproducible coating samples for adhesive formulation development, performance evaluation, and process optimization. Operating such equipment requires a deep understanding of its working principles and a mastery of standardized operating methods to ensure the accuracy and reproducibility of experimental data.
How it works:
The equipment usually consists of a preheating melt system, a precision coating head (such as a comma scraper, wire rod or slit extrusion type), a substrate conveying platform, a temperature control system and a pressure regulation system. The workflow is to heat a solid hot melt adhesive in the melt chamber to a set temperature to melt it, and then form a layer of specific thickness and width on the substrate in uniform motion through the coating head. Once cured, test samples are available. The core control parameters mainly include melt temperature (Tm), coating temperature (Tc), substrate temperature (Ts), coating speed (v), and coating gap (G) or pressure (P). Together, these parameters determine the quantification of the coating (usually expressed in gram g/m² or thickness μm) and the apparent quality.
Operation process
Standardized operation procedures are the basis for obtaining effective experimental data. Before operation, be sure to consult the equipment manual and refer to the relevant material testing standards (such as sample preparation methods involved in some ASTM and GB/T standards).
1. Pre-operation preparation:Confirm that all parts of the equipment are clean and there is no residual colloid from the previous experiment. The preheating temperature of the melt cavity, transmission pipeline and coating head is set according to the recommended processing temperature range of the hot melt adhesive. Temperature stabilization usually takes 15-30 minutes. At the same time, prepare and cut the substrate to the appropriate size.
2. Parameter Setting and Calibration:Set the coating speed, coating gap or pressure according to the experimental design. Calibration of the coating gap is critical and can be verified using standard feeler gauges. The coating width is adjustable via the bezel on the device. The initial parameter is recommended to start with the intermediate value recommended by the material supplier.
3. Coating Process Operation:Add an appropriate amount of hot melt block to the melt cavity. Once the colloid is completely melted and the temperature is stable, the coating procedure is initiated. The initial stage is manually guided to ensure that the colloid is evenly applied to the starting end of the substrate before starting the automatic conveying. Closely observe the uniformity and continuity of the coating line.
4. Post-Treatment and Cleaning:After coating, immediately remove the substrate from the heat source platform to allow it to cool and cure naturally. Equipment cleaning should be carried out before the temperature drops to a safe range but the colloid is not completely hardened, and special cleaning tools and appropriate detergents should be used to remove residual adhesive to prevent blockage of the flow channel.
Parameter optimization
Each process parameter does not act independently, and its interaction determines the coating effect. The following relationships can be used to understand the fundamentals of quantitative control:
For scraper coating, the wet film thickness (h) is approximately satisfied: h ≈ k × g, where k is the coefficient related to the rheological properties of the compound and G is the coating gap. The actual dry film quantification (W) can be expressed as: W = h × ρ × (1 - φ), where ρ is the colloidal density and φ is the volume shrinkage rate during the curing process.
Temperature is the core variable affecting the viscosity (η) of the compound, and its relationship usually conforms to the Arrhenius formula: η = A × exp(E).a/RT), where EaIt is the activation energy of the viscous flow, R is the gas constant, and T is the thermodynamic temperature. Viscosity directly affects the leveling and thickness uniformity of the coating.
The following table summarizes the impact trends and adjustment strategies of the main parameters:
| Control parameters | The main effect on the coating effect |
| Melt temperature (Tm) | Insufficient temperature leads to high viscosity and uneven coating; Too high may cause thermal degradation. |
| Coating head temperature (Tc) | Maintains the fluidity of the compound at the coating point and prevents premature cooling and condensation. |
| Coating speed (v) | too low speed may make the compound spread too thick; Too high may lead to wire drawing, discontinuous coating. |
| Coating Gap/Pressure (G/P) | The core mechanical parameters that directly control the thickness of the wet film. |
| Substrate temperature (Ts) | It affects the wettability of the compound to the substrate and the initial curing speed. |
When optimizing, it is recommended to use the univariate method, that is, to fix other parameters, and systematically adjust one parameter to observe the changes in the appearance, quantitation and subsequent bonding properties of the coating sample, so as to determine the optimal process window.
Maintenance points
Problems such as uneven coating, streaks, bubbles, or broken glue wires may be encountered during operation. These problems often stem from improper parameter settings, poor equipment condition, or changes in material properties. For example, longitudinal streaks may be related to the cleanliness or damage of the edge edge of the coating head; Lateral fluctuations may be related to conveying speed stability or fluctuations in compound viscosity. Regular maintenance includes cleaning all heating components and runners, checking for wear on the coating head, calibrating temperature sensors and motion control units, and lubricating drive components.
Notes:
The operating temperature of hot melt adhesive is usually above 120°C, and burns must be prevented. Heat-resistant gloves and protective glasses should be worn during operation. Make sure the equipment is well grounded and the heating area is insulated. The working environment should be kept ventilated to avoid inhaling possible thermal decomposition products for a long time. The power supply should be cut off when the equipment is abnormal or when it is deactivated for a long time.
References
1. Adhesive Coating Process Test Method Guide, China Adhesive Industry Association.
2. ASTM D4497 - Standard Test Method for Testing the Apparent Viscosity of Hot Melt Adhesives.
3. Application of Material Rheology in Coating Process, Chemical Industry Press.
4. General technical conditions for laboratory coating equipment and relevant machinery industry standards.