Combination of plastic product and aluminum alloy (3) - Aerospace Products
Thermal shock fracture
Generally speaking, the temperature resistance of plastics can be known from the physical property table, such as the minimum temperature that can be reached when the original basic physical properties can be maintained at low temperature, and the same is true at high temperature. Materials have their range of temperature, which is not the same. Some perform very well at low temperatures, while others withstand high temperatures at high temperatures. However, in the range of temperature resistance, they generally refer to slow To reach this temperature does not reach this temperature instantly. If it changes from one temperature to another temperature with a considerable gap in a very short time, most materials cannot meet such conditions, but here, it is necessary to discuss What is described is not the impact caused by extremely short-term temperature changes, but the impact caused by repeated changes between two temperatures. Most products used in aerospace will encounter such temperature shocks. The environment changes back and forth between low temperature and high temperature. Generally, the low temperature and high temperature are within the range that the material can withstand. However, in the face of continuous and repeated temperature shocks, this shock effect is very likely to cause thermal shock fracture of the product.
What is thermal shock fracture
Thermal shock fracture refers to a phenomenon in which a product breaks in the rapid "low temperature and high temperature repetition" of the surrounding temperature. At first glance, it seems that the product does not bear much external force and internal stress. How could it break? But the biggest problem is here. As shown in the table below, thermal shock cracks are prone to occur in the following products.
- Inserts with metal
- With screw lock
- Pair inserts of different materials
- Various material crimping parts
The common point is that this kind of situation is likely to occur in "products made of two parts with different physical properties". However, a single product is also prone to thermal shock fracture when the wall thickness is significantly uneven, although the proportion of this case is relatively low.
Thermal shock fracture is also called thermal shock fracture or hot and cold cycle fracture, but they all refer to the same thing.
Mechanism of thermal shock fracture - thermal stress
As mentioned above, thermal shock fracture is a phenomenon that the product breaks during the rapid "low temperature ⇔ high temperature repetition" of the surrounding temperature. Let's discuss what happened during the temperature change process.
First, take the metal insert shown in Figure 2 as an example to illustrate. If the metal insert is an injection molded product in which the metal is held in the mold and surrounded by resin, stress occurs in the product during cooling during the injection molding process. In other words, plastics "want" to shrink by their molding shrinkage, while metals "don't let" them shrink. Although it is a rough estimate, if the shrinkage rate of the plastic after molding is X (%), the tensile strain of X (%) will also appear in the circumferential direction of the plastic, and as the temperature changes, the plastic will When linear expansion occurs (high temperature side) or contraction (low temperature side), the strain in the circumferential direction also changes accordingly.
For example,
• T low :Low temperature
• Thigh :High temperature
• CTE low: Coefficient of linear expansion (low temperature ← room temperature)
• CTE high: Coefficient of linear expansion (room temperature → high temperature)
Then:
ε low temperature = X + (23-T low) × CTE low × 100
ε high temperature = X + (23-T high) × CTE high × 100
Such strains appear on the low-temperature side and the high-temperature side, respectively. Since the value of (23-T) is positive on the low temperature side and negative on the high temperature side, the plastic will further adhere to the metal insert on the low temperature side, thereby increasing the circumferential strain in the plastic and increasing the internal stress. If it is faced with a rapid temperature change, the internal stress in the plastic product will also rise sharply. On the high temperature side, because the linear expansion of the resin is much larger than that of the metal, the strain will decrease and the internal stress will decrease and ease. sign. That is to say, with the rapid "low temperature and high temperature repetition" of the surrounding temperature, the strength and weakness of the strain will of course be repeated in the plastic. Changes in the strength of strain will naturally cause repeated impacts of stress (thermal stress), and this kind of material fatigue damage decay (accompanied by the pulsating impact of this thermal stress) and long-term creep damage (due to short-term maintenance In the environment of high temperature and low temperature), the fracture caused by two different degrees of damage can be called thermal shock fracture.
However, the thermal stress change due to the pulsating impact is not always fixed, but with each "repeated temperature change between low temperature and high temperature", the initial strong molecular bonds will be strongly stretched and strained, and the molecular bonds will gradually weaken each other. Arrangement and combination will gradually decrease (weakness) under the action of stress relaxation; fractures mostly occur in uneven areas or areas with large variation points in injection molding (such as the melting glue meeting area of products, areas with large thickness differences, The structure near the gate area, etc.) and as a mechanical singularity, such as sharp corners (sharp corners), mechanical force bending, etc., so it is difficult to predict how long the service life before thermal shock fracture, That is, it will break after repeated repetitions of low temperature and high temperature.
3. Extend the thermal shock fracture life
In order to prolong the thermal shock fracture life, it is very important to reduce the above-mentioned strain (generated stress and residual stress) in response to design changes in terms of stress concentration and uneven wall thickness during design. Secondly, the development details of the mold must be optimized, from the selection of materials, the design of mold cooling, the design of runners, the design of gate position and shape, the reasonable design of flow length ratio, whether to use co-molding or not, and whether to use related auxiliary molding And the optimization of molding conditions, as well as the details and preparations before and after mass production, can minimize the stress impact caused by the product in the face of repeated temperature changes between low and high temperatures, so that the life cycle of plastic products can be greatly extended. At the same time, in the case of the same wall thickness, generally speaking, the service life of thermal shock fracture of small-sized products will be longer than that of large-sized products.
