This article will take an in-depth look at thermal shocks.
The article will bring more understanding on topics such as:
This chapter will explore thermal shock chambers, including their design, construction, and operation.
Thermal shock chambers are specialized climatic chambers used for testing materials under extreme temperature changes. They subject materials to rapid transitions between high and low temperatures to identify defects or potential failures, particularly in the electronics industry where early-life failures are a concern.
The material is placed in a mobile basket that transports it rapidly between a hot compartment and a cold compartment, often cycling between the two. This process occurs quickly, typically within 10 seconds, across multiple test chambers containing two or more compartments.
The temperature of the test compartment and the testing methods are dictated by the specific requirements of the standard, which defines the exact cycle to be performed. These tests aim to evaluate how the product reacts to thermal shock. During these tests, the material may experience varying rates of heating or cooling in different areas, depending on its properties. Significant volume contraction or expansion can result in substantial mechanical stress on the sample, potentially leading to failure.
It is crucial to consider the reliability of electronic boards used in aeronautics, where reliability is a key requirement. Electronic boards can experience faults during severe thermal shocks, especially if the shock exceeds the material's maximum resistance. Issues such as pin breakage or weld failure may occur if materials with differing coefficients of thermal expansion are used.
Using a thermal shock chamber is crucial for identifying potential defects in products, production processes, or components before manufacturing begins. It helps ensure that all necessary adjustments are made to enhance product performance and functionality.
Thermal shock chambers come from environmental chambers. The aim of an environmental testing chamber is to check the results of a range of physical, climactic, and other exceptional conditions on an object. They are constructed to make environments which an object may face during its usage. Researchers depend on them to offer controlled states which can be altered, such as temperature variances, humidity, and low or high pressure, to examine and test the characteristics of a product.
Environmental testing ensures the quality of products by subjecting sample materials to various tests. These evaluations determine the product's performance and reliability, revealing its potential for rust, corrosion, or emissions.
The design of thermal shock chambers includes:
The cabinet is made of galvanized steel, featuring a control cabinet on the right and a refrigeration cabinet at the rear. It has two doors at the front: one for the low-temperature test zone and one for the high-temperature test zone. The insulation is provided by ultra-fine polyurethane foam and glass wool, offering excellent thermal performance. The base is constructed with a steel channel structure for strong load-bearing capability. For ease of handling and movement, the cabinet is equipped with support feet and casters.
The high-temperature tank is located in the upper section of the test chamber. Due to the potential for high temperatures to affect the physical properties or measurements of the materials used, these materials must be suitable for extreme temperature conditions. Constructed from 1.0mm 304-grade stainless steel, which is resistant to both low and high temperatures, the tank is highly durable. The front of the high-temperature tank serves as a high-temperature test zone, designed for use in thermal shock testing or other high-temperature evaluations.
A circulating air duct, equipped with a rotating fan, heating element, and impeller, is positioned behind the tank. Air is heated within the duct, and the airflow is directed to the front test area for cyclic testing. When a test product is placed in the low-temperature area, the tank remains in a heat storage state. Once the sample is removed from the low-temperature zone, the high-temperature test mode can be quickly reactivated.
The low temperature tank is located at the bottom of the test chamber. Given that low temperatures can adversely impact most matrix materials, the materials used in the low temperature tank must meet specifications for low-temperature environments. It is constructed from 1.0mm thick 304-grade stainless steel, which is resistant to both low and high temperatures and ensures durability.
The front of the low-temperature tank serves as a test area for low-temperature and thermal shock testing. At the back of the tank is a circulation system equipped with an impeller, a circulating fan, a heating element, a regenerator, and an evaporator. Air flows through the evaporator, where it absorbs heat and reaches the required low temperature for testing. The air then circulates through the air duct and the test area, completing the cycle for the test.
When the test material is placed in the high-temperature zone, the tank operates in cold storage mode. Once the sample is moved from the high-temperature area, the tank can quickly revert to low-temperature test mode.
Inside the basket, there are 2 sample racks. The location of the sample holder is shiftable up and down. The basket is constructed from a 304 grade stainless steel tubular square. The bottom and top are sealed plates that function to seal the low temperature area and the high temperature area. The sealing part is done by a silicone belt. Furthermore, it does not warp under low and high thermal shock situations, and the sealant performance is good.
The basket's movement between the low and high-temperature areas is powered by a cylinder. This system uses the combined action of the cylinder, roller, and wire rope to move the basket for temperature impact testing. This setup avoids issues such as casing deformation and the limitations of conventional motors.
As the basket moves up to the high-temperature test area, the hauling force of the cylinder compresses the tension on the lower part of the lift, as well as between the low and high-temperature zones, effectively sealing the area.
When the basket moves to the low-temperature test zone, the weight of the hanging basket compresses the separator between the uppermost part and the low and high-temperature zones, providing a seal. The quality of this seal directly impacts the performance of the test.
The materials used in constructing thermal shock chambers include:
When molten zinc comes into contact with steel, a chemical reaction occurs, causing the zinc to bond to the steel's surface. This forms a protective zinc layer that helps prevent corrosion. The most common method for applying a zinc coating to steel is hot-dip galvanizing.
Longevity is a big factor in the quality feature of galvanized steel. It is able to live up to 20 years in severe exposure to water and up to 100 years in normal circumstances. Water is the main taxing problem for steel, causing corrosion and rust. Durability and reliability are a key aspect of galvanized steel. The tough coat, made of zinc is corroded first aiding to further shield the steel. The zinc coat is outstandingly resistant to rust. The way hot dipped galvanized steel functions protects all components of the thermal shock chamber cabinet, including those usually inaccessible zones.
Type 304 stainless steel contains at least 8% nickel and 18% chromium, with a maximum of 0.08% carbon. It is classified as a nickel-chromium austenitic alloy. Its notable features include excellent welding and forming properties, resistance to oxidation and corrosion due to the chromium content, and high durability, including at cryogenic temperatures. These attributes make it an ideal material for constructing low-temperature tanks in thermal shock chambers.
The operation of a thermal shock absorber involves:
To effectively control temperature, the chamber must be able to both cool and heat. Additionally, it should ensure that the temperature is distributed uniformly throughout the test compartment.
Addressing specific technical concerns related to air distribution in the test compartment allows for precise standardization of temperature values over time and throughout the chamber. This ensures that all surfaces and parts of the product are exposed to the same temperature.
The test compartment is uniformly cooled by compressing and then expanding a refrigerant gas. Climatic chambers are typically categorized into two main types based on temperature ranges: single-stage chambers, which can achieve minimum temperatures of -40°C, and double-stage (cascaded) chambers, which can reach minimum temperatures of around -70°C.
Hot air is circulated through the inner part of the test compartment via ventilation. The heating and cooling processes are managed by PLC programming, with the operator configuring the cycle parameters to ensure the desired performance.
The chamber must be able to perform both dehumidification and humidification, and it should distribute humidity evenly throughout the test compartment. Direct humidification is achieved using an electric humidifier, which introduces steam through an opening in the airflow just after the air circulation fan. This method ensures that the humidification process is free from aerosol. A specialized algorithm manages the humidifier to enhance its reliability.
The chamber is dehumidified by a mechanical system based on the cold finger principle, which also serves the cooling function. According to this principle, moisture in the air condenses on the surface of a product at a lower temperature when exposed to a higher ambient temperature. The evaporator, being the coldest part of the thermal shock chamber, plays a crucial role in reducing humidity levels in the test compartment as needed.
The specifications are:
Things to consider are:
Specification tables for thermal shock chambers detail the achievable maximum and minimum temperatures. Most manufacturers set the maximum temperature between +150°C and +180°C, with an optional range extending up to +200°C if required. While the maximum temperature is generally consistent across models, the minimum temperature varies depending on the mechanical cooling system used, leading to two broad types of chambers:
In some cases, a thermal shock chamber with dual refrigeration does not need to achieve extremely low temperatures but should provide a rapid cooling rate at lower temperatures.
The temperature exchange rate, measured in °C or Kelvins per minute, indicates how quickly the temperature in the testing chamber changes. This rate can vary significantly between models, ranging from 6°C/min to 10°C/min.
The temperature exchange rate depends on the cooling capacity of the compressor and the heating power of the heating elements in the chamber. Generally, a more powerful compressor leads to a faster cooling rate, while having more heating elements increases the heating rate.
When testing a product, factors such as material, size, weight, and shape should be taken into account. The product size helps determine the appropriate volume for the test chamber, ensuring it is large enough to accommodate the sample comfortably. It is generally recommended that the size of the test sample should not exceed one-third of the chamber's volume, though the shape of the product also requires special consideration. In all cases, it is crucial that air can circulate freely around the sample to ensure uniform temperature distribution and minimize variations across the entire surface, within the specified test tolerances.
The weight of the test piece is a crucial parameter, as large volumes can negatively impact test performance. The performance of a climatic chamber, including temperature gradients, is specified and calculated with the chamber empty, meaning without any objects inside. Consequently, if the required temperature exchange rate for a test sample is close to the value stated for an empty chamber in the data sheet, it is important to perform a verification check.
Weight must be considered for another reason as well: test chamber racks are built to support samples only up to a certain maximum weight. It's crucial to consult the rating plate to identify the maximum allowable weight for the test sample. If the sample exceeds this limit, the chamber will need reinforced brackets to accommodate the extra load.
Performance considerations for testing a sample include:
When the test sample is connected to a power source, it may generate heat. In some cases, this may be negligible, but in other situations, it must be considered, as it can impact the chamber's performance. This is particularly relevant because performance metrics are often specified with an empty chamber, which does not account for heat dissipation or the volume of the sample.
As a result, when the test sample is in operation, the chamber will handle the heat generated by the sample without affecting its performance, allowing the test values to be accurately achieved.
There are special situations where the test sample may emit explosive, flammable, corrosive, or toxic substances, potentially releasing harmful gases depending on the temperature range. These conditions must be managed carefully.
The various types of thermal shock chambers include:
Firstly, there is the three-box thermal shock chamber. This type features a low-temperature zone, a high-temperature zone, a test area, a control cabinet, and a cold cabinet. The three-box design requires substantial heating and cooling capacity. Test products are placed in one of two product carriers, which are moved between the zones to create extreme thermal stress. The cold zone is continuously occupied by at least one sample product carrier.
This design enhances the efficiency of the cabinet’s cooling system, allowing for greater product testing capacity compared to conventional thermal shock chambers. Heaters are installed in the cold area for defrosting, enabling the chamber to function as a temperature cycling cabinet when not in use for thermal shock testing. An advantage of this design is that the test piece remains stationary, eliminating the need for a basket transfer device.
Secondly, there is the vertical lift thermal shock chamber. This design includes a low-temperature area, a high-temperature area, a gondola, a control cabinet, and a cold cabinet. A single sample carrier moves between these areas, exposing the sample to significant temperature changes. The vertical lift mechanism minimizes external environmental impacts. However, because the chamber's low and high-temperature zones are arranged vertically, a larger test area results in a longer overall chamber length.
As a result, this design can lead to operational inconvenience, making it more suitable for smaller test chambers. The vertical lift chamber is advantageous in that it requires minimal floor space, making it ideal for compact laboratories. It offers quick conversion times and requires lower cooling and heating capacities.
Thirdly, there are horizontal mobile thermal shock chambers. This design features a low-temperature area, a high-temperature area, a mobile basket, a control cabinet, and a cold cabinet, all arranged horizontally. This configuration is suitable for larger test boxes.
Among these, the horizontal moving type and the vertical lifting type are both two-box thermal shock chambers. Their advantages include reduced heating and cooling requirements and easier temperature control.
Table 1: Differences Among Three Types of Thermal Shock Chambers
| Three-Box | Vertical Lift | Horizontal Mobile | |
|---|---|---|---|
| Characteristics | Test zone is fixed Low temperature and high temperature converting in one box | Down and up two boxes Low-temperature and high-temperature converting through the basket mobilizing up and down | Right and left box Low temperature and high temperature converting by mobilizing the basket right and left |
| Application | The three-box type is best for not so demanding tests | The vertical lift type is best for small parts and components | The horizontal mobile type is best for large and medium equipment |
This chapter will explore the applications and advantages of thermal shock chambers.
Thermal shock chambers can be applied to do product testing for the following industry divisions: building materials, automotive, chemicals, timber, electronics, cosmetics, aerospace, plastics, metal, tobacco, pharmaceuticals, textile, packaging industry, bio-tissue engineering, biotechnology, ceramics, veterinary and human medicine, beverage and food, surface technology, microbiology, and insect and plant growth.
In the food, cosmetics, and pharmaceutical industries, environmental and stability chamber monitors are essential for meeting international regulatory standards. Thermal shock chambers are used to control and measure temperature and humidity (such as mean kinetic temperature). Advanced aging studies conducted in these chambers help establish safe shelf-life levels and determine expiration dates.
In microbiology and biology, thermal shock chambers can be used to observe the effects of humidity, temperature, and other conditions on the growth of algae, plants, insects, viruses, and small animals, such as fruit flies (Drosophila). They also facilitate the cultivation of organs, cells, and tissues, as well as insect rearing and plant growth.
The aerospace industry relies on thermal shock chambers to simulate outer-space conditions through thermal vacuum, thermal experiments, and vacuum environments. This allows for testing of space system devices under extreme temperature and climate conditions. Thermal shock chambers are also used to test portable life support systems for astronauts, as well as cryogenic equipment, high-pressure oxygen systems, and other instruments to assess their performance under reduced altitude and pressure conditions.
In the automotive industry, thermal shock chambers are used to simulate extreme conditions such as intense sunlight and hot road surfaces. Vehicle manufacturers routinely perform these tests, often using drive-in chambers located within their own testing facilities. These chambers recreate real-world scenarios, including typical humidity levels, air and road temperatures, as well as extreme conditions, to evaluate vehicle performance. Drive-in car testing chambers are designed to be sealed and resistant to expansion and contraction.
In addition to complex research protocols employed by quarantine bureaus, universities, major manufacturers, and research organizations, thermal shock testing also plays a crucial role in quality control for everyday items such as electrical devices, plastics, batteries, paper products, and metals.
Many consumer products are purchased at face value, often without consideration of the rigorous testing they underwent to become commercially available. This highlights the critical role thermal shock chambers play in manufacturing and refining the performance and features of these products. They have been instrumental in advancing technology to its current state while ensuring the reliability and safety of everyday items.
Inspecting products for reliability before they enter manufacturing is crucial to ensure they can withstand various environmental conditions. This approach helps reduce costs associated with warranties and recalls. Additionally, component testing can enhance competitiveness by aiding in the design and construction of more durable products, ensuring that components are ready for consumer use.
Reliability testing also aids customers in meeting supplier fundamentals. The most common forms of environmental testing include temperature and humidity.
The product design is validated to ensure it performs reliably under normal conditions. This validation, typically completed during the R&D phase, ensures that the product can handle the conditions it will encounter throughout its lifespan. During the production phase, the goal is to pass all tests and meet the specifications without failure, resulting in a robust and dependable product.
Product validation ensures that the product meets the regulatory requirements and specifications for which it is intended. This process involves simulations similar to design validation to identify potential gaps or faults in the design or manufacturing phases that could lead to failures.
Environmental stress screening applies various stresses, such as thermal cycling, to uncover latent defects in products during customer use. For newly manufactured or repaired components (particularly digital ones), the risk of failure can have significant consequences. Therefore, living products or components must demonstrate greater reliability compared to similar products or components that have not undergone such screening.
Producers of these four types of evaluations simulate real-world conditions, including temperature and humidity. Many products undergo standard testing procedures commonly recognized by organizations such as the IEC, UL, and the military.
Several key industries benefit from using thermal shock chambers for product reliability testing. These markets, including consumer electronics, aerospace, and automotive, conduct tests to ensure the protection and performance of their products. Testing is essential for electronics to ensure they function reliably under various environmental conditions and climates.
The limitations of the various types of thermal shock chambers are:
When temperatures are lowered and raised, significant requirements for preheating and pre-cooling arise. This necessitates substantial energy and power storage, which increases costs accordingly.
For large and medium-sized test chambers, using basket components can be challenging, and the maximum height of the test box can be cumbersome, making operation difficult.
The basket drive components require higher resistance to both low and high temperatures, while the mobile rails must meet stricter flatness standards.
The condenser should be cleaned every three months. For air-cooled systems, regular overhauls and cleaning are necessary to maintain optimal heat transfer and ventilation performance. For water-cooled systems, it is essential to keep pressure and temperature within the specified range and ensure adequate flow rate. The condenser should also be regularly cleaned and descaled to maintain consistent heat transfer performance.
To maintain low temperatures for extended periods in a thermal shock chamber, the system should be set to 110°C. The chamber door must be opened for 2 hours to allow for defrosting. After each test, the temperature should be adjusted to near ambient levels. Additionally, the chamber should be allowed to cool for about 30 minutes before cutting off the power supply and cleaning the inner walls.
The thermal shock chamber requires regular cleaning to maintain optimal performance. Each sample may vary in cleanliness, and the forced air circulation causes the evaporator to collect numerous small particles, such as dust, which must be cleaned regularly.
The circulating air blade and condenser fan of the low-temperature test chamber must be regularly cleaned and balanced. Similar to the evaporator, these components collect dirt and small particles due to the varying operating environment. Therefore, routine cleaning of the test chamber is essential.
If the thermal shock chamber needs to be relocated, it is essential for the company's technicians to oversee the process to avoid damage. If a customer handles the relocation independently, a qualified electrician must inspect the circuitry to ensure it is safe before powering up the chamber. Failure to do so could result in damage to the test chamber components.
If the thermal shock test chamber has been in use for an extended period, it should be energized for at least 60 minutes every two weeks. Additionally, the operation of related components should be regularly tested to ensure proper functioning.
Thermal shock chambers are used to put the products to serious shocks through the repeated and sudden passage to low temperature areas from high temperatures, to ascertain defective parts or those contingent upon infantile. The purpose of these tests is to inspect the reaction of the product when it is put under a thermal shock. In these instances, the material may heat up or cool with differing speeds in differing parts reliant on the material. If this relates to huge contraction in volume or increase, the sample may suffer huge mechanical stress, thus leading to failure. There are three types of thermal shock chambers which are horizontal mobile, vertical lift, and three-box.
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