Performance requirements and problem solving strategies for PU foam in refrigerator production

Key indicators of rigid polyurethane foam, such as density, thermal conductivity, and compression strength, directly affect the thermal insulation performance and structural stability of the refrigerator. They need to be precisely controlled within the density range of 23±2kg/m3 to 32±2kg/m3 to ensure that the thermal conductivity is ≤0.020kcal/mh℃ and the compression strength exceeds 1.2kg/cm2. At the same time, the closed cell rate must reach more than 93% to avoid shrinkage and deformation.

Performance parameters and requirements of PU foam

▲ Correlation between density and thermal conductivity

In the refrigerator production process, the appropriate density and thermal conductivity of rigid polyurethane foam are crucial to the thermal insulation performance and need to be strictly controlled within the specified range. The density is calculated as density = weight/volume, and the unit is usually kg/m³. In production, the suitable density of the rigid polyurethane plastic free foam we use is 23±2kg/m³, and the average density of the foam after infusion is controlled at 32±2kg/m³. Thermal conductivity is another key indicator, which indicates the ability of the material to conduct heat, and the ideal value should be ≤0.020kcal/mh℃. It is worth noting that when the density is in the range of 30~40 kg/m³, the thermal conductivity of the material reaches its minimum value.

▲ Compression strength

Rigid polyurethane foam is known for its excellent compression strength, which helps improve the stability and rigidity of refrigerators. To ensure sufficient strength at low temperatures , the foam strength should be maintained at 1.0kg/cm² (981pa) when the gas in the closed-cell bubbles condenses below 0°C. For rigid polyurethane foam used in refrigerators, if the compression strength exceeds 1.2kg/cm² (1177pa) under normal density conditions, its performance can be considered quite ideal.

▲ Adhesion characteristics

The adhesion of rigid polyurethane foam to steel and thermoplastics depends on many factors , including the mechanical bond between steel and plastic, the mechanical properties of the foam, the degree of gelation, and the properties of the formulation and stock solution. These factors together determine how well the foam adheres when it comes into contact with these substrates. Ensuring good adhesion properties is critical to the stability of the entire refrigerator structure.

▲ Dimensional stability

The rigid polyurethane foam used for refrigerator insulation has strict requirements on its dimensional stability in low temperature environments . Generally, the volume shrinkage of the foam should be controlled within 1% to ensure its dimensional stability. If the dimensional stability of the foam is insufficient, the strength may decrease under low temperature conditions, resulting in the shrinkage of the foam, and even causing serious consequences such as deformation of the inner and outer shells and rupture of the inner shell.

▲ Closed-cell rate

The closed-cell rate has a direct impact on thermal conductivity. A high closed-cell rate helps improve thermal insulation performance , and is usually required to be at least 93%. In the foaming process of rigid polyurethane foam, a refrigerator insulation material, if the formula is not accurately controlled, the amount of foam stabilizer added is insufficient, or the water content in the formula is too high, the closed-cell rate may decrease.

Common Problems and Solutions in the Production Process

▲ Infusion foaming problems and solutions

During the production process, the occasional voids in the foam are mainly caused by the poor fluidity of the reaction liquid. Adjusting the mold temperature and the raw material ratio is the key to solving this problem. There are many factors that affect the fluidity of the reaction liquid, which require detailed analysis and adjustment:

Mold temperature : If the mold temperature is too low, the foam fluidity will deteriorate. It is recommended to increase the mold temperature to 35~45℃. If production starts when the mold temperature does not meet the standard, it is recommended to increase the amount of material on the basis of the original injection volume, and then remove the excess material after the temperature reaches the standard.

Raw material temperature : When the raw material and mold temperature are too high, part of the foaming agent will escape, resulting in insufficient filling and voids. The raw material temperature should be adjusted to 20~23℃.

Catalyst dosage : Excessive catalyst will accelerate the gelation reaction, so its dosage should be strictly controlled.

Exhaust hole : If the exhaust hole is blocked or improperly positioned, air will not be able to be discharged, resulting in an “air trap” phenomenon. Ensure that the exhaust hole is opened correctly.

Surface contamination : If the inner wall of the box shell is contaminated by grease or water, it may cause bubble collapse. Make sure the surface is clean and dry.

▲ Shrinkage after placement

Sometimes, foam will shrink after it has been placed. This is due to possible quality issues with the raw materials or due to improper formulation. Ensuring that the chemical reaction is complete is key to preventing foam shrinkage:

Quality of raw materials : It is recommended that raw materials be tested to ensure that no ingredient has lost its effectiveness due to prolonged storage.

Formula adjustment : If the foam density is too low, resulting in shrinkage, the amount of foaming agent should be appropriately reduced.

Ratio of polyol components to isocyanate : After calibration and confirmation that the ratio is inaccurate, it is necessary to check the status of the foaming machine filter and carefully calibrate the flow rate to ensure the optimal ratio.

In addition, the problem of coarse and uneven cells will also lead to increased heat loss and increased thermal conductivity, which may be caused by the degradation of the foam stabilizer silicone oil. Therefore, it is necessary to ensure that the raw materials are stored within the appropriate temperature range and to make appropriate supplements and adjustments.

Through these measures, problems that may be encountered in the production process can be effectively solved, thereby ensuring that the performance of polyurethane foam in refrigerator production meets the standards.