This article will take an in-depth look at thermal shocks.
The article will bring more understanding on topics such as:
This chapter will discuss what thermal shock chambers are, their construction, and how they function.
Thermal shock chambers are climatic chambers for thermal shock testing that are utilized to put the material to serious shocks. This is accomplished through the repeated and sudden passage to low temperature areas from high temperatures to ascertain defective parts or those contingents upon infantile mortality (primarily in the electronics industry).
The material is put inside a mobile basket that carries it to a hot compartment from a cold compartment and also vice versa for multiple cycles. These chambers contain two test compartments or more and the movement of the material from one to the other is done very quickly (typically within 10 seconds).
The test compartment’s temperature and the testing means depend on the standard’s needs that determine the exact cycle to be carried out. 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 a huge contraction in volume or increase, the sample may suffer huge mechanical stress, thus leading to failure.
One needs easily to consider electronic boards utilized for aeronautics for which being reliable is an unequivocally crucial prerequisite. Faults may happen in the electronic boards during a serious thermal shock. That is if the shock is bigger than the maximum material resistance. Possible issues that may rise in defective materials are the breaking of pins or the welds failure if the material utilized have differing coefficients of thermal expansion.
The usage of a thermal shock chamber is important to realize possible defects in products. Also their production process or components and to function, before the production begins, all the required adjustments to raise product performance.
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 guarantees companies of their products’ quality. Sample materials are put to many tests to ascertain their performance and reliability. Results can conclude if a product has the likeliness of rusting and corroding or if it releases some kind of emissions.
The construction of thermal shock chambers involves:
The cabinet is constructed of galvanized steel. On the right there is a control cabinet and the refrigeration cabinet is at the back. There are two lower and upper doors in front. These are the low temperature test zone and the high temperature test zone. The insulating substance is polyurethane foam and glass wool, ultra-fine, which has excellent thermal insulation attributes. The lowermost is a steel channel structure with proper load enduring performance. In order to accommodate the handling and movement of the machine, the base is furnished with support feet and casters.
The high temperature tank is positioned in the upper portion of the test chamber. Since high temperatures are able to alter the physical attributes or measurements of the materials utilized in the high temperature tank, these materials must meet the needs for usage in extreme temperature environments. It is constructed of 1.0mm 304 grade low and high temperature resistant stainless steel, which is durable. The high temperature tank front is a high temperature test zone for usage in high temperature testing or thermal shock testing.
A circulating air duct is behind the tank, with a rotating fan, heating, and device impeller. Air is heated inside and airflow in the front test area is distributed for cyclic testing. When a test product is placed in the low temperature area, the tank is in a heat storage state. And when the sample is moved from the low temperature area, high temperature test mode can be reinstated in a short while.
The low temperature tank is positioned on the lower part of the test chamber. Since low temperatures negatively affect almost every matrix material, the materials utilized in the low temperature tank should adhere to the requirements for usage in a low temperature environment. It is also constructed from 1.0mm thick 304 grade low & high temperature resistant stainless steels, which is durable.
The face of the low temperature tank is a test zone for low temperature for usage in low thermal shock test or low temperature test. A circulation duct is at the back low temperature tank, furnished with an impeller, a circulating fan, a heating device, a regenerator and an evaporator. Through the evaporator, air absorbs heat, it attains the low temperature needed by the test, and the flow of air in the air duct and the test zone are joined into one, and the cycle test is done.
When the test material is put in a high temperature area, the tank is in a cold storage mode. And when the sample is transferred from the high temperature area, low temperature test mode may be restored in a brief time.
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 motion of the basket in the low and high temperature areas is driven by a cylinder. The collaboration of the cylinder, the roller and the wire rope is utilized for pulling the basket to do the temperature impact test. This prevents the drawbacks of the deformation of the casing and movements of the conventional motor.
When the basket ascends to a high temperature test area, the tension on the lower portion of the lift and the low and high temperature area is compressed by the hauling force of the cylinder to act as a sealing method.
When the basket shifts to a low temperature test zone, the separator between the uppermost part and the low and high temperature zone is compressed by the weight of the slacking basket to act as a sealing method. The hanging basket seal quality affects test performance directly.
The materials used in making thermal shock chambers include:
When steel makes contact with zinc in a molten state, a chemical reaction happens, and zinc bonds onto the steel’s surface. Thus, a zinc layer on steel acts as a corrosion protective layer. The most general way of putting a zinc coat onto steel is hot dipping 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 has a minimum of 8% nickel and 18% chromium, mixed with 0.08% carbon maximum. It is defined as a Nickel-Chromium austenitic alloy. Some of its characteristics are welding and forming properties; oxidation/corrosion resistance alluding to the chromium material. Also deep drawing quality and brilliant durability, even down to cryogenic temperature, defined as extremely low temperature. These characteristics make it perfect for the construction of low tank temperature tanks in the thermal shock chambers.
The functioning of a thermal shock absorber involves:
For temperature control, the chamber should be capable of performing two tasks: cooling and heating. It also should also be capable of dispensing the temperature uniformly inside the test compartment.
Specific technical concerns about the dispensing of air in the test compartment makes it possible to attain huge standardization of the values of temperature over time and all over the chamber space, guaranteeing that all surfaces and parts of the product are put to the same temperature.
By compressing and subsequent expanding of a refrigerant gas, the procedure will uniformly cool the test compartment. There are two base temperature values which climatic chambers are generally classified. First is single stage, a minimum temperature of -40°C. Second is double-stage (cascaded system), a minimum temperature of around -70°C.
The hot air is moved by ventilation through the inner side of the test compartment. The action of the two heating and cooling operations is controlled by PLC programming. The operator sets the cycle parameters to guarantee the needed performance.
The chamber should be capable of performing two tasks: dehumidification and humidification. It also should be capable of distributing humidity evenly in the test compartment. Direct humidification is attained by use of an electric humidifier. This inputs steam via a hole in the airflow just after the air circulation fan. This guarantees humidification is free of aerosol. A dedicated algorithm controls the humidifier for better reliability.
A mechanical system centered on the cold finger principle dehumidifies the chamber, making use of the same mechanical system utilized for cooling. Based on this principle, when a product with a low temperature is exposed to a high ambient temperature, the air moisture condenses on the colder product’s surface. The evaporator is the part with the lowest temperature in the thermal shock chamber. A dedicated sector of this chamber is thus utilized to reduce the levels of humidity in the test compartment when needed.
The specifications include:
Things to consider include:
Specification tables of the thermal shock chambers outline the maximum and minimum temperatures achievable. Almost all thermal shock chamber constructors set maximum temperature from + 150 °C to + 180 °C, though if needed this can be made to reach + 200 °C as an option. Though the maximum temperature is almost at all times the same, the minimum temperature of the chambers utilizing a mechanical cooling system enables them to be separated into two broad types:
At times a thermal shock chamber with dual refrigeration does not have to reach extremely low temperatures below the needed level of cooling, but has a quick cooling rate at low temperatures.
The rate of decrease or increase of temperature in the testing chamber is referred to as temperature exchange rate (in °C or Kelvins per minute) and may differ immensely in each model, from 6 °C/min to 10 °C/min.
The temperature exchange rate apparently relies on compressor cooling capacity and the heat of the heating rod in the chamber. In a way, a stronger compressor results in a rate of cooling. Hence if more heat bars are in the chamber, the heating rate is faster.
Product material, product size, product weight and shape of product being tested should be considered. The product size allows the determination of the volume of the test chamber, which is big enough to facilitate comfortably. It is commonly best practice that the size of the test sample mustn’t exceed a third of the test chamber volume, though special consideration must be offered to the product under the test’s shape. In all instances air must be able to flow freely to guarantee that uniformity and variation of temperatures are almost similar (within test specified tolerance) over the complete test sample surface.
Test piece weight is an essential parameter, since big volumes can harmfully affect the performance of a test. Climatic chamber performance, like temperature gradient, specified and calculated with an empty chamber, that is without any object in the test chamber. Therefore, when the needed value of the needed temperature exchange rate for a test sample is in proximity to the temperature exchange rate value stated in the data sheet (empty chamber), it is important to do a home check.
Weight should be taken into consideration for another purpose: the test chamber racks are manufactured to support samples up to some certain weight. Therefore, it is important to see the rating plate for the test sample maximum weight. If the test sample exceeds the permissible maximum weight, the chamber accessories will have a reinforced bracket.
The performance considerations for a sample to be tested include:
When the test sample is on a power source it may dissipate heat. In some instances this may be inconsequential, but in other instances it must be accounted for, since it could affect the performance of the chamber, as mentioned above is at times shown with an empty chamber that has no heat dissipation and no volume.
Consequently, when the test sample is in process, the chamber will dissipate the heat coming from the test sample without changing its performance, test values are achieved in this case.
There are also special situations where the test sample may output explosive, flammable, corrosive, or toxic substances that may create possibly harmful gasses, depending on the temperature range. They can be achieved.
The different types of thermal shock chambers include:
Firstly, the thermal shock chamber has a three-box type. The three-box type comprises a low temperature zone, a high temperature zone, a test area, a control cabinet, and a cold cabinet. The three-box structure needs a huge amount of heating and cooling. Products being tested are put in a product carrier, of which there are two and moved between the areas creating extreme thermal stress. The cold zone is occupied by at least one sample product carrier at all times.
This design creates efficient usage of the cabinet cooling system, offering more product testing quantity over general thermal shock designs. Heaters are put in the cold area for defrosting, allowing the area to function as a temperature cycling cabinet when it’s not being utilized for thermal shock testing. Its benefit is that the test piece does not move. There is no need for a basket converting device.
Secondly, the thermal shock chamber has a vertical lift type. The vertical lift type comprises a low temperature area, a high temperature area, a gondola, a control cabinet, and a cold cabinet. One sample carrier mobilizes between each area, subjecting the sample to vivid changes in temperature. Because it is via the basket lift conversion, it prevents the impact of the external environment. Nevertheless, owing to the vertical installation in the low and high temperature ranges, if the test area is huge, the total length of the testing box is comparatively long.
Consequently it will result in an inconvenient operation. Thus, this type is commonly appropriate for small test chambers. A benefit of the vertical lifting chamber is it utilizes little floor space, making it perfect for smaller labs. There is quick conversion time, low cooling capacity, and heating needed.
Thirdly, thermal shock chambers have a horizontal mobile type. This type also comprises a low temperature area, a high temperature area, a mobile basket, a control cabinet, and a cold cabinet, set up horizontally in a low temperature area and a high temperature area and appropriate for a huge test box.
Amongst them, the horizontal moving type and the vertical lifting type are two boxes of thermal shock chambers. Its benefit is less heating and cooling is needed and easy temperature control.
Table 1: Differences Between Three of the Thermal Shock Chamber Types
| 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 discuss the applications and benefits 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 instance of food, cosmetics, and pharmaceutics, environmental and stability chamber monitors are necessary to adhere to the rules put up by international regulatory agencies. Thermal shock chambers can control and measure temperature, and humidity (for instance, mean kinetic temperature). Advanced aging studies in thermal shock chambers aid in determining the safe shelf-life level and expiry dates.
In microbiology and biology, thermal shock chambers can be utilized to watch the effects of humidity, temperature and other conditions on the growth of algae, plants, insects, viruses, and tiny animals (like the Drosophila, also called fruit flies). They allow the culture of organs, cells and tissues, also insect rearing and plant growth.
The aerospace industry depends on thermal shock chambers to give thermal vacuum, thermal experiments, and vacuum which recreate outer-space situations so that space system devices may endure extreme situations of temperature and climate. Portable life support systems for cosmonauts are tested using thermal shock chambers. Cryogenics equipment, high pressure oxygen system, and more instrumentation are used to test the effects of lowered altitude and pressure conditions.
In the automotive industrial sector, thermal shock chambers are utilized to replicate conditions like severe exposure to sunlight and hot roads. All vehicle manufacturers conduct these tests, many of which utilize drive-in chambers found within the company testing buildings. They replicate real-world conditions, like normal humidity levels, air and road temperatures, as well as severe conditions to inspect the vehicle’s performance. Drive-in car testing chambers are sealed and resist expansion and contraction.
Aside from complicated research protocols utilized by quarantine bureaus, universities, big manufacturers and research organizations, thermal shock testing finally discourses the quality control check of every-day items like electrical gadgets, plastics, batteries, paper products, and metals.
Many of the products presently attainable to consumers are attained at face value, with little regard for the rigorous testing that they went through so that they become available commercially to the end user. This is a testimony to the part that thermal shock chambers play in manufacturing and developing the performance and features of marketable products. They have played a huge role in progressing technology to present levels while ensuring the reliability and safety of the products utilized every day.
It’s important to inspect products for dependability before discharging parts into manufacturing, guaranteeing that products may dependably endure different environmental conditions. It’ll lower costs related to warranties and recalls. Components testing may also be competitive because it helps in designing and constructing a more robust product. So the component is prepared for consumer usage.
Reliability testing also aids customers in meeting supplier fundamentals. The most common forms of environmental testing include temperature and humidity.
The design of the product is validated to guarantee the basic operation stays smooth in a normal environment. It will face during its life span in a specification driven and may be finished in the R&D stage. During the production stage, the aim is to succeed in the test and achieve the specifications without failure and the outcome is a robust product.
The reason for product validation is to guarantee that the product achieves the requirements regulations and specifications for which it is meant. The product validation process utilizes simulations like those of design validation to predict gaps or faults in the design or manufacturing phases which may cause failures.
When customers use the product, environmental stress screening uses a variety of unique stresses such as thermal cycling to put latent defects. In newly repaired or manufactured components or products (digital usually), the probability of failure means devastating consequences. The living products or components must have more outstanding dependability compared to similar unscreened products or components.
Producers for these four types of evaluations simulate actual field conditions such as temperature and humidity. Many products are put through conventional testing procedures that are often accepted by the IEC, UL and army.
A few important markets use thermal shock chambers and profit from product reliability testing. The markets do the tests for their products’ protection such as consumer electronics, aerospace, automotive, etc. Any electronics will need testing to ensure it will appropriately work in virtually any experience or climate.
The drawbacks of the different thermal shock chambers are:
When the temperature is lowered and raised, the requirements for preheating and pre-cooling are high. The energy and power storage components are laid in a big amount, and the cost is respectively increased.
For the large and medium test chambers, the basket parts are not easy to apply, and the maximum height of a test box is big, which is not suitable for operation.
The basket drive part has more requirements for low and high temperature resistance, and the mobile rail has more requirements for flatness.
The condenser must be cleaned regularly every three months. For air cooling, the condenser must be overhauled regularly, and the condenser must be purified to make it have good heat transfer performance and ventilation. For water-cooled cooling, it must be ensured its pressure and temperature is in the specified range. It should also be ensured that there is an equivalent flow rate, and the condenser must be regularly cleaned and scaled to achieve its continual heat transfer performance.
For a thermal shock chamber to do low temperature for a long time, as a cycle, it must be set to 110 °C. Box door must be opened for 2 hours for defrosting treatment. And after each testing, the temperature must be set close to ambient temperature. Work must be close to 30 minutes, the power supply must be cut off, and the inner wall of the chamber cleaned.
Thermal shock chamber must be regularly cleaned. The cleanliness level of each sample is different and in the action of enforced circulation of wind, the evaporator condenses numerous small particles like dust, which must be regularly cleaned.
Circulating air blade and condenser fan of the low temperature test chamber must be cleaned and balanced. Just like the cleaning evaporator, since the operating environment of the chamber differs, the condenser fan and circulating air blade will condense dirt and some small object particles. Therefore the test chamber must be cleaned regularly.
In the case there is a need to shift the thermal shock chamber, the company's technicians must guide the operation to prevent damage to the test chamber. If a customer shifts the chamber alone, an electrician should authorize that the circuitry is safe before starting. Otherwise, the test chamber related parts will be burnt out.
If the thermal shock test chamber has been utilized for a long duration, and it must be frequently energized for at least 60 minutes every half a month, and the working of related components of the thermal shock chamber must be tested.
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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