In the refrigerator manufacturing process, polyurethane foam material is the key insulation layer. Its hardness, structural stability and closed-cell rate directly affect the insulation performance and energy efficiency level of the refrigerator. However, in actual production, many companies will face problems such as “softening”, “loose structure” or “poor load-bearing performance” after foaming. This article will start from the perspective of polyurethane foam raw materials , systematically analyze the common reasons for foam softening, and put forward targeted formulation and process optimization suggestions.
Imbalance in raw material ratio
The core raw materials of the foam system are polyether polyol (Polyol) and isocyanate (MDI). The ratio of the two determines the hardness, elasticity and structure of the final foam.
1. Abnormal polyol ratio
- Insufficient use of polyols : will cause the foam structure to be hard and brittle, thus affecting its elastic recovery properties.
- Excessive use of polyols : The foam may expand fully at the beginning, but shrink and collapse during the curing process, forming a softened feeling or irregular holes.
2. Improper isocyanate index
The isocyanate index is defined as the NCO/OH molar ratio and is generally controlled between 100-120.
- The index is too high (>130) : It will cause excessive cross-linking of the foam, resulting in the phenomenon of “brittle shell + soft collapse inside”, the closed cell rate will increase but the rebound will decrease.
- The index is too low (<90) : The reaction is incomplete, and low-strength foam with loose structure and irregular pores is easily formed.
Recommended control range : Isocyanate index 105-115, flexibly adjusted according to different refrigerator or box sizes.
Data reference: “Study on the Relationship between Polyurethane Foam Structure and Performance” by East China University of Science and Technology
Foaming agent selection and addition amount
The foaming agent determines the rate and quality of cell formation in the foam system. Improper use will directly lead to the softening of the foam.
1. Chemical foaming agent – water
Water reacts with isocyanate to produce CO₂ gas, which is the driving force for foam expansion.
- Increased dosage (>2.5 pbw) : The foam density decreases significantly, but the foam wall support capacity is weakened, resulting in structural softening.
- Insufficient dosage (<1.0 pbw) : Insufficient foaming leads to closure of cells and severe shrinkage.
2. Physical foaming agent – dichloromethane, HFO, etc.
This type of low boiling point solvent vaporizes quickly during the reaction and assists in the formation of a microporous structure. Using too high a ratio , such as more than 10 pbw of dichloromethane, will destroy the stability of the pore wall and cause the foam to collapse.
Insufficient volatilization : Residual liquid solvent can cause the foam to be soft and leave odor.
Recommended dosage (double foaming system) :
- Water: 1.2–1.8 pbw
- Dichloromethane or HFO-1233zd: 8–10 pbw
Data reference: China Plastics Association “PU Rigid Foam Insulation Materials Handbook (2023)”
Effect of process parameters on foam hardness
Even if the raw material ratio is reasonable, if the process control is not precise, it may cause foam softening problems.
1. Foaming temperature control
- The temperature is too high (>35℃) : The reaction is violent, the center of the bubble is easy to burn, and the edge area is insufficiently cured, resulting in local softening.
- The temperature is too low (<18℃) : the fluidity is poor, the foam cannot fully expand, and soft areas with uneven density are produced.
Stabilizer Adjustment
- Excessive catalyst : The reaction is too fast, the bubbles are formed before they are stable, and the pore structure is incomplete.
- Insufficient foam stabilizer : It will cause foam aggregation and foam collapse, which is especially obvious in large boxes or complex structures.
Recommended mold temperature control : 25-30℃, and the temperature of the box insulation zone after foaming is controlled at 38-42℃.
Influence of ambient humidity and storage conditions
The humidity and temperature in the ambient air will also have an indirect impact on foam performance.
1. The air humidity is too high
When the humidity is high (>80% RH), the moisture in the air may react with the isocyanate prematurely, reducing the content of the effective component, resulting in loose foam and reduced strength.
2. Raw materials absorb moisture and deteriorate
If MDI and some polyethers are not tightly sealed during storage, they will easily decompose after absorbing moisture, thus affecting the efficiency of the foaming reaction.
It is recommended to control humidity : <65% RH; all raw materials should be stored in sealed containers and used within 48 hours after opening.
Conclusion and Suggestions
The softening of refrigerator foam after foaming is not caused by a single reason, but the result of multiple factors such as raw material ratio, foaming agent usage, process parameters and environmental humidity. To address this problem, companies should combine their own formula system and equipment characteristics to improve from the following aspects:
- Establish a raw material batch management system to ensure that key indicators such as hydroxyl value and moisture content are within the qualified range;
- Accurately control the isocyanate index and blowing agent ratio, and develop a density-strength correlation curve;
- Invest in automated equipment for high-precision measurement and real-time temperature control;
- Increase the frequency of foam physical property testing to prevent quality fluctuations caused by system drift.
Through systematic investigation and data monitoring, the refrigerator foaming raw material system can achieve the triple goals of “high strength + high closed-cell rate + low energy consumption”, bringing more stable product performance and lower after-sales costs to the company.