1.The Fundamental Purpose of Insulation Design
What exactly is an insulation layer in a refrigerator?At its core, it serves one fundamental purpose: to block heat transfer and convection.
The compressor and refrigeration system continuously “transport heat” via the refrigerant, removing heat from the interior and creating a cold environment inside the cabinet.
The insulation layer exists to prevent the cold inside and the heat outside from reuniting, allowing the cold energy already generated to remain inside the refrigerator for as long as possible.
Let’s imagine an ideal extreme scenario.
If a perfectly efficient insulation layer existed—one that completely isolates cold from heat—the compressor might only need to run briefly, generate a small amount of cooling, and then shut off.
The internal temperature would remain low because virtually no cold energy escapes.
If a 100-liter refrigerator consumed only one kilowatt-hour per month, what would that mean?
2.Why Insulation Matters Even More in Portable Refrigerators
Today, we are discussing a component that may seem insignificant at first glance, yet is absolutely critical to the performance of portable refrigerators—the insulation layer.
This topic will be presented in two parts.
In Part One, we will focus on the overall design objectives, material choices, and process logic, to understand why insulation is particularly challenging in portable refrigerators.
The insulation layer directly determines several key performance metrics, including:
cooling efficiency, energy consumption, overall size efficiency, and ultimately, the user experience.
Compared to household refrigerators, portable refrigerators face stricter requirements.
In addition to basic thermal insulation, they must simultaneously meet two critical goals:
- Thinness: to maximize internal usable volume within limited external dimensions;
- Light weight: weight is extremely sensitive in vehicle applications, affecting portability, installation, and energy efficiency.
A thin and lightweight insulation system directly enhances user experience—
occupying less space in the vehicle, making transportation easier, and improving overall cost-performance and competitiveness.
3.Polyurethane Foam Materials
As a reference, household refrigerators primarily use MDI-based polyurethane systems.
This system offers good environmental performance, controllable cost, and mature processing, making it the industry standard.
Portable refrigerators also rely mainly on MDI systems, but some high-end models adopt high-functionality polyether polyols or modified polyether formulations to enhance thermal resistance and impact durability, meeting automotive-grade standards such as TS16949.
Key industry players include BASF, Dow, Wanhua Chemical, and Hongbaoli.Dow’s vacuum foaming technology, applied in refrigerators for many years, works by improving foam flowability and density uniformity, reducing microcell size, and thus lowering thermal conductivity.This technology can improve energy efficiency by approximately 10% without significantly increasing cost.
In China, Wanhua Chemical reported domestic MDI capacity of approximately 3.1 million tons in 2024, with combined polyether capacity of about 550,000 tons.
Its ultra-low-energy foaming technology can reduce polyurethane foam thermal conductivity by up to 10%, while improving heat distribution.
Recent patents also show that increasing the allowable content of tetramethylsilane while maintaining material stability enables the production of low-density, low-thermal-conductivity rigid polyurethane foams.
In practice, portable refrigerators prefer physical blowing agents such as CO₂ or low-boiling hydrocarbons, balancing safety and weight reduction.
Some high-end products use vacuum insulation panels (VIPs) as auxiliary insulation, with thermal conductivity as low as 0.004 W/(m·K).
However, due to cost, fragility, and volume constraints, VIPs are only used locally and cannot fully replace traditional PU foam.
4.Foaming Process and Manufacturing Technology
The complexity of foaming processes is mainly reflected in equipment requirements, production efficiency, and quality control.
The fundamental goal of foaming is simple:to ensure uniform distribution of insulation throughout the cabinet, forming a continuous and complete foam layer.
Portable refrigerators typically use small customized molds and high-pressure foaming machines, often requiring multi-gun injection to fill complex internal cavities.
Due to size constraints, vacuum foaming used in household refrigerators is not feasible, making formulation flowability and injection precision critical.
Household refrigerator lines can achieve cycle times of about 9 seconds per unit, with annual capacity exceeding 6.7 million units.
Portable refrigerator lines typically require around 30 seconds per unit, or 15–20 seconds for compressor-based models, but small-batch production limits equipment utilization.
Investment for a 500,000-unit portable refrigerator line is approximately RMB 2–3 million, compared to tens of millions for household refrigerator lines.
Advanced multi-gun injection systems, such as 5-gun configurations originally developed for ultra-thin built-in refrigerators, are increasingly applied to portable refrigerators to improve foam uniformity.
Foam cell defects are one of the most critical quality indicators.
Defects indicate insufficient filling, allowing cold leakage.
To mitigate this, vibration suppression and localized infrared inspection are introduced to monitor foam uniformity during production.
5.Thermal Conductivity, Insulation Performance, and Service Life
Measured thermal conductivity of PU foam in portable refrigerators typically ranges from 0.032 to 0.035 W/(m·K), significantly higher than household refrigerators at 0.018–0.022 W/(m·K).
This is mainly due to thickness limitations (30–45 mm) and foam cell coarsening caused by blowing agent systems.
In terms of insulation performance, standard portable refrigerators can maintain temperature for about 8 hours, while high-end models using VIP + PU hybrid insulation can reach up to 3 days.
This difference is driven by both thermal conductivity and insulation thickness.
This highlights the core technical trade-off:
portable refrigerators must be lightweight and thin, yet current materials and processes make it difficult to achieve strong insulation under these constraints.
This is one of the most critical areas for future innovation.
Regarding service life, portable refrigerators are typically designed for 3–8 years.
High temperatures (over 70°C inside vehicles) and continuous vibration accelerate microcrack formation in foam, increasing gas exchange and thermal conductivity.
Tests show that at 70°C, PU foam thermal conductivity may increase by 15–25% within five years.
Under cyclic temperature and vibration conditions, foam aging is significantly faster than in household refrigerators.
For example, in accelerated aging tests alternating between -30°C and 50°C every 24 hours, thermal conductivity rises rapidly in the first 20 days before stabilizing.