In the refrigerator manufacturing process, polyurethane (PU) rigid foam is the core material of the insulation layer, and its molding quality is directly related to the energy-saving performance and structural stability of the whole machine. However, in actual production, foam shrinkage is a common but difficult problem, which not only affects the appearance of the product, but also reduces the insulation effect, and may even cause serious consequences such as deformation and cracking of the inner tank.
According to production experience and case analysis, the reasons for refrigerator PU foam shrinkage can be summarized into the following four categories:
1. Foaming process problem is the core root
Free foaming expansion is out of control.
During the free foaming stage, the foam continues to expand after gelling, forming an elongated cell structure. When the foaming is finished and the temperature drops, the internal gas condenses and the volume shrinks, resulting in significant shrinkage, with a shrinkage rate of usually 2%-10%.
Multiple pouring or re-foaming will make the cell structure unstable and aggravate the shrinkage problem.
Excessive filling is limited.
Ideally, the foam can fill the mold space before gelling, achieving isobaric expansion and uniform closed cells. However, since the refrigerator liner structure cannot withstand high-pressure filling, once the foaming is insufficient or the filling pressure is improperly controlled, the ideal spherical cells cannot be formed, and the shrinkage will naturally increase.
2. Changes in ambient temperature affect foam stability
Low temperature can easily lead to poor molding
. If the ambient temperature is lower than 15°C, the foaming reaction slows down, the aging time is prolonged, the cell wall is prone to collapse during molding, the overall foam density decreases, and the shrinkage rate increases. Actual measurements show that under the same raw material formula, the foaming volume at 15°C is about 15% less than that at 25°C.
High temperature is not a benefit either.
If the ambient temperature or mold temperature is too high and the reaction rate is too fast, the pores may close prematurely, making it impossible to release the internal pressure of the bubble, forming abnormal structures such as “empty bubbles” and “cracked bubbles”, and reducing stability.
3. Imbalance in raw material ratio is a common cause
Improper ratio of black and white materials
In polyurethane foaming raw materials, the precise ratio of black material (isocyanate) and white material (polyether mixture) is required:
- There is too much black material, the reaction is violent, and cavitation is easily formed due to rupture of the bubbles.
- There is too much white material, the reaction is not sufficient, the foam is too soft and the supporting strength is insufficient.
Foaming agent (such as cyclopentane) ratio problem
Adding too much foaming agent will cause excessive expansion, thin cell walls, and easy shrinkage or collapse;
If insufficient addition is made, complete cells cannot be formed, resulting in cavities and depressions.
Excessive amounts of materials in Group A affect the structure.
For example, if the catalyst, foaming agent, and emulsifier in Group A exceed the standard, the strength of the adhesive will decrease and the bond will be weak; if the reaction is too fast, closed cells will be formed and the internal stress will not be fully released.
4. Equipment and operation factors cannot be ignored
volume is insufficient
, the foam cannot fill the mold cavity, and the overall density is low, causing local shrinkage or depression.
Poor spray gun mixing or abnormal temperature,
aging or inadequate cleaning of the spray gun mixing head, and low material temperature (especially in winter) will all lead to incomplete mixing reaction, uncontrolled foam structure, and further shrinkage.
5. Solution Suggestions
Process Optimization
- Use one-time sufficient pouring (about 90%-100% filling) to avoid re-issuance in the middle;
- Control the gel time to avoid the bubbles from continuing to expand disorderly;
- Use post-curing stabilization solutions, such as mold insulation.
Temperature control
- The production environment temperature must be maintained at ≥18°C ;
- Preheat the mold to above 25℃. It is recommended to preheat for more than 30 minutes in winter;
- The preheating of raw materials should be controlled within the recommended range (22-24℃ for white material and 25-28℃ for black material).
Recipe Adjustment
- The effects of different ratios on the cell structure were tested by small tests;
- Increase the drawing time of the combined polyether, recommended to be ≥60 seconds, to enhance the cell stability;
- Adjust the proportion of foaming agent to balance the expansion force and closed-cell structure.
Improvement of mold structure
- The top of the liner mold should be designed with an inclined surface at a certain angle;
- Add reasonable exhaust holes to ensure that foam gas can escape effectively and reduce stagnant air areas.
summary
The shrinkage problem of refrigerator PU foam is not caused by a single factor, but the result of multiple factors such as process, temperature, raw materials, equipment, etc. Mastering its essential laws, accurately adjusting process parameters, and combining mold and equipment upgrades are the key to ensuring stable foaming quality and improving product qualification rate.
It is worth mentioning that the PU foaming process of refrigerator and solar water heater inner tanks has a high degree of commonality in structure and principle, and can serve as a technical basis for mutual reference.
If you are interested in PU foaming production lines, process optimization or intelligent injection systems, please feel free to contact us to visit our factory showroom or request a technical brochure.