1 Introduction
In many couplings of automotive parts, tightening is a more common way of fixing and coupling than welding, bonding and snap-on coupling. Among them, the self-tapping screw tightening assembly method has been widely used in automobile plastic parts assembly because of its series of advantages.
However, in the actual production process, due to design, manufacturing, operation and environmental reasons, the plastic parts may cause failures such as cracking and slipping (not reaching the specified tightening torque) during the self-tapping tightening process, resulting in loose parts. If it is light, it will produce abnormal noise, and if it is heavy, it will cause the parts to fall off, and may even affect the vehicle function and driving safety.
To this end, the author has classified and summarized the problem of self-tapping screw tightening failure of plastic parts analyzed or understood.
2. Introduction to self-tapping screw connection
Self-tapping is the process in which the thread is not pre-drilled by the coupling member, and the thread is directly drilled and tapped by the self-tapping screw during the coupling.
For couplings made of thermoplastics, the internal threads in the mounting holes are forcibly extruded by rotating self-tapping screws; while the parts made of higher hardness thermosetting plastics have internal threads in the mounting holes. It can be cut by a rotating self-tapping screw.
Self-tapping screws are basically made of metal materials, and their important dimensions include the large diameter, small diameter, pitch and tooth angle of the thread.
The main advantages of using a self-tapping screw to tighten the joint are as follows.
a. Easy to automate;
b. high joint strength;
c. low probability of stress cracking;
d. Low cost.
The common types of self-tapping screws are shown in Figure 1. After tightening, the connection status is shown in Figure 2.
Figure 1 Common self-tapping screws
Figure 2 Connection status after the tapping screw is screwed into the mounting hole of the plastic part
3. Common self-tapping screws tightening failure problem and analysis
3.1 Plastic parts mounting hole cracking
During the self-tapping tightening process, the area where the plastic component mounting hole is located is subjected to complex effects such as pressing, friction and torsion, and tensile stress is generated in the circumferential tangential direction of the hole.
Cracking of the mounting holes during self-tapping tightening is a common current production quality problem that can lead to production line downtime in severe cases. The main causes of this problem are as follows.
3.1.1 The weld hole has low weld line strength
For the mounting holes (usually cylindrical) on the injection molded part, there must be a core mechanism that gives the shape of the mounting hole inside the injection mold. During the injection molding process, when the molten plastic stream reaches the mounting core, the stream is split into two strands that pass around the core and merge again to form a weld line that runs substantially parallel to the axis of the mounting hole. The strength at the weld line is typically between 20% and 80% of the strength of the normal material.
If the mold structure is unreasonable or there are problems with the injection molding parameters (such as the gate is too small, the melt temperature is too low, etc.), the welding strength will be too low. When the self-tapping screw is screwed in, the mounting hole wall is welded along the force. Trace cracking, the proportion of such cracking cases in the failure of plastic parts tightening is very high.
For example, when a hexagonal plastic nut (Fig. 3) molded with POM is fixed with a tightening torque of 2.5 N·m, a through crack occurs on one side of the nut, as shown in Fig. 3. The position of the gate and the weld mark is centered on the mounting hole. 180° to each other. After analysis, the material and size of the nut are no problem, mainly because the mold temperature is low during injection molding, resulting in too low weld line strength and cracking when tightened.
In another case, the drawings required that the plastic parts be made of PP-GF20, and there was no tightening problem before. Later, a batch of parts cracked about 10% when the self-tapping screws were tightened.
The analysis found that the supplier incorrectly used PPGF30 material on the failed batch parts, and the injection molding parameters did not change. Under the same injection molding conditions, due to the poor fluidity of the PP-GF30 material, the fusion strength at the weld line is lowered, and cracking occurs at the weld line when tightening.
Figure 3 Plastic hex nut cracked at the weld line
3.1.2 The mounting hole is too high
Under normal circumstances, the mounting hole size of the plastic parts that need to be self-tapping and the size of the self-tapping screws to be selected must be properly designed to ensure a reliable connection after installation, and the force applied to the mounting holes is appropriate. Within the scope.
However, due to improper selection of design parameters, or failure to achieve the designed size due to manufacturing problems, the problem of tightening failure may occur.
For example, if the self-tapping screws selected are relatively thick or the mounting holes are relatively thin, the tensile stress generated inside the wall of the mounting hole during the self-tapping tightening process may be too high, and the wall of the mounting hole may be cracked.
For example, after a plastic parts supplier switches from A supplier to B supplier, 25% of the plastic part mounting hole wall cracks when the self-tapping screws are tightened (Figure 4). According to the analysis, the parts of the two suppliers use PA6-GF30 with a water content of 1.3%, and the processing also meets the requirements of the drawings. Therefore, it is suspected that the size of the mounting holes of the self-tapping screws or the B supplier plastic parts is problematic.
Figure 4 Plastic parts mounting hole wall cracking
The dimensions of the self-tapping screws specified in the drawings are: thread outer diameter D = 7.00 mm, screw core diameter d = 5.00 mm, see Figure 5. After testing, the screw size meets the requirements of the drawings, so the size of the mounting hole of the plastic part becomes the detection focus. The shape of the mounting hole of the plastic part is shown in Fig. 6. The dimensional results are shown in Table 1.
Figure 5 Self-tapping screw size
Figure 6 plastic mounting hole
As can be seen from Table 1, the D1 diameter of the parts produced by the B supplier is too small, even smaller than the diameter d of the self-tapping screw core.
Therefore, when the self-tapping screw is screwed into the mounting hole from top to bottom and gradually approaches the bottom hole, the pressing force of the screw on the wall of the mounting hole becomes larger and larger, and the strength of the wall of the mounting hole cannot resist the pulling caused by the pressing. When the stress is applied, the mounting hole is finally cracked.
3.1.3 The mounting hole wall is too thin
Similar to the case described in 3.1.2, if the wall thickness of the mounting hole of the plastic part is too thin, it may not be able to withstand the tensile stress in the circumferential direction of the mounting hole caused by the self-tapping screw being screwed in, causing cracking.
Another problem worth noting is that the wall of the mounting hole of some plastic parts is partially thin due to deformation of the mold core and the like.
For example, a plastic component mounting hole is cracked after tapping the self-tapping screw (Fig. 7). Figure 8 is a CT scanning sectional view of the mounting hole. It can be seen that the thinnest part of the mounting hole wall is only 1.03 mm (position 5); During the screwing process, the mounting holes are cracked on the side;
Figure 7 Plastic parts mounting hole cracking
Figure 8 Plastic parts mounting hole CT scanning and dimensioning
The thickest part on the other side reached 1.59 mm (position 6) and cracking never occurred. According to the analysis, the uneven wall thickness of the mounting hole of the plastic part is caused by the displacement of the position of the metal mandrel forming the mounting hole in the injection mold.
3.1.4 Local stress is too high in the mounting hole
In the case of injection molding of plastic parts, if the injection parameters or mold structure are unreasonable (such as too fast cooling rate, etc.), high internal stress will be generated in the parts after injection molding. The plastic nut shown in Fig. 9 is cracked at the gate position indicated by the arrow during the screwing of the self-tapping screw.
According to the results of microsection analysis, the flow near the gate of the plastic nut is extremely disordered, resulting in excessive stress (Fig. 10), so that cracking is likely to occur near the gate position when the self-tapping screw enters.
3.1.5 Degradation of plastic materials
If the injection temperature is too high, the material stays in the barrel for too long, or the material is not sufficiently dried, etc., the plastic material will be degraded during processing, and the mechanical strength of the plastic part will be reduced.
For example, if a part molded by PC is not sufficiently dried due to raw materials, the PC will be seriously degraded during injection molding, causing the material to be brittle, and the part mounting hole will be broken when the self-tapping screw is screwed in instead of ordinary cracking.
Figure 9 Plastic nut cracking at the gate
Figure 10 Plastic nut microsection
3.2 Self-tapping and tightening connection can not reach the specified torque (slip)
Self-tapping screws for plastic parts are usually designed with a reasonable tightening torque.
However, sometimes, when the self-tapping screw is screwed into the plastic mounting hole and the design torque has not been reached, the screw slips in the mounting hole and causes the thread on the inner wall of the mounting hole to buckle.
This is another common type of tightening failure. The main reasons for this problem are the following.
3.2.1 The mounting hole is too large or the tapping screw is too thin
When the inner diameter of the mounting hole of the plastic part is too large or the self-tapping screw is too thin, the depth of the thread invading the inner wall of the mounting hole is too shallow during the screwing of the self-tapping screw, so that the pressing effect between the thread surface and the inner wall of the mounting hole is The friction is too small to achieve sufficient tightening torque.
3.2.2 There is a loose area inside the installation hole wall
When plastic parts are injection molded, due to the low clamping force or low holding pressure, there is a loose area inside the wall of the mounting hole, which reduces the rigidity of the mounting hole and causes large deformation under the action of screw pressing, thus causing tightening torque. Too low, the specified tightening torque cannot be achieved, and the looseness inside the mounting hole wall is sometimes difficult to find even with CT scanning.
For example, a plastic part is made of ABS material. The production line found that the tightening torque of a batch of parts is low, only 1.8 N·m, and the process requirement is 3.0 N·m.
It was found through analysis that the mounting hole of the piece is at the end of the flow of the entire part, the material density of the part is only 1.09 ~ 1.10 g / cm3, and the material density of other places away from the mounting hole can reach 1.15 g / cm3, thus inferring There is a loose area in the wall of the mounting hole of the piece, which causes the tightening torque to be too low.
3.2.3 Plastic parts material hardness is low
If the hardness of the material selected for the plastic part is low, the pressing force and friction between the thread surface of the self-tapping screw and the inner wall of the mounting hole are too small, so that a sufficient tightening torque cannot be achieved.
For example, a POM base is fixed by self-tapping screws. At the production site, it is found that the parts of the A supplier are tightened without problems, and the parts of the B supplier are 100% slipped when tightened, as shown in Figure 11.
The analysis showed that the B supplier's parts contained rubber toughening components, which increased the toughness of the POM, the ball indentation hardness was only 56 N/mm2, and the parts slipped when tightened to 1.25 N·m; The non-toughened POM material used by the A supplier has a ball indentation hardness of 89 N/mm2 and can fully meet the 2 N·m torque required by the process when the part is tightened.
A supplier's parts B supplier's parts
Figure 11 Comparison of the plastic parts of the two suppliers after tightening
In addition, for glass fiber reinforced plastic materials, if the glass fiber content of the selected material is insufficient, the hardness of the material may be lowered and the specified torque may not be reached when the self-tapping screw is tightened.
4. Other important factors affecting the tightening of self-tapping screws
4.1 There are injection holes inside the mounting hole wall
Holes in the wall of the mounting hole of the plastic part may occur due to excessive thickness of the mounting hole wall or the mold temperature is too low. In particular, parts made of semi-crystalline plastic materials (such as POM, PA66 or PA6) are relatively more prone to holes. The effect of the hole on the tightening of the self-tapping screws is complicated and depends on the specific situation.
4.1.1 has no obvious effect on tightening
If the number of holes is small, the size is not large, or is far from the inner wall of the plastic component mounting hole, the self-tapping screw can still smoothly reach the specified torque, and the mounting hole does not crack. In this case, if the plastic part is not cut or CT scanned, there is no hole in the plastic part.
4.1.2 Causes low tightening torque
For more flexible plastic materials, such as unreinforced PA66 or POM, if there are many holes, the pore volume is large, or the hole is especially close to the inner wall of the mounting hole, when the self-tapping screw is screwed in, the inner wall of the part mounting hole is squeezed by the screw. A large deformation occurs underneath, resulting in a too low tightening torque and the inability to achieve the specified tightening torque.
After the CT scan of the PA6 part shown in Figure 12, a large number of holes are visible in one side of the mounting hole, and the specified torque cannot be achieved when tightened.
Figure 12 Holes in the wall of the plastic mounting hole (CT scan)
4.1.3 causing cracking
For plastic materials with high rigidity and low toughness such as PPS-GF30 and PBT-GF30, if the hole formed inside the wall of the component mounting hole is large, especially if the shape of the hole is extremely irregular (such as a structure with sharp corners), it will result in The edge of the hole produces a high concentration of stress. When the self-tapping screw is screwed into the inner wall of the extrusion, cracking occurs at the starting point of the stress concentration point, which causes cracking of the plastic part.
It can be seen from the above discussion that it is not necessary to have a hole in the wall of the plastic component mounting hole which may affect the tightening of the self-tapping screw.
In the analysis of the specific tightening failure problem, even if it is confirmed that there are holes in the plastic parts, further analysis and verification are required. Of course, adjustments and optimizations should be made from a design and process perspective to eliminate holes as much as possible.
4.2 Ambient temperature
The performance of plastics is greatly affected by the ambient temperature. Therefore, the failure of plastic parts in the assembly process tends to be regular with seasonal temperature changes.
For example, when the northern part enters the winter, the failure of the plastic parts to break during assembly (including the plastic parts tightened with self-tapping screws) will increase.
This is because after the temperature is lowered, the mobility of the plastic molecular segments is weakened, the rigidity of the material is increased, the toughness is lowered, and cracking may occur during the tightening process. Excessive ambient temperatures can cause the tightening torque to become low and even slip.
In addition, the temperature increase caused by the frictional heat generation during the tightening process is also a factor to be considered, especially when the rotation speed of the tightening tool is too high, the temperature rise of the friction can even cause the inner wall of the mounting hole of the plastic part to be melted, thereby greatly reducing Tighten the torque. As a rule of thumb, the tightening tool speed is preferably set below 600 r/min, but the specific values ​​should be determined after testing.
4.3 Water content of plastic parts
Plastic materials containing polar groups usually absorb moisture in the air and have an impact on material properties. In particular, nylon materials commonly used in automotive parts (such as PA6 and PA66) have a higher proportion of amide groups in the molecular structure. Significant hygroscopicity.
When the water molecules enter the nylon material, they form a hydrogen bond with the amide group and act as a plasticizer, so that the rigidity of the nylon material is greatly reduced, which may result in the tightening torque not reaching the standard.
Especially in the hot and rainy southern regions, if the nylon injection molded parts are stored for too long, the torque of the plastic parts may not meet the requirements after a large amount of moisture is absorbed in the air. If the water content of nylon is too low, it will lead to excessive rigidity and too low toughness, and cracking may occur when tightening (especially when the ambient temperature is low).
Therefore, the water content of nylon materials should be controlled, and the water content should be within a reasonable range by humidification or water immersion. It is recommended that the water content can be controlled for common PA6 and PA66 and its glass fiber reinforced materials. Within the range of 1.0% to 2.5%, of course, the specific data also needs to be verified by the corresponding tests.
4.4 Environmental Stress Cracking (ESC)
After the self-tapping screw is screwed into the mounting hole of the plastic part, there is an extrusion and expansion between the screw and the inner wall of the mounting hole of the plastic part due to the interference fit between the two, and the tensile stress exists inside the wall of the mounting hole of the plastic part. .
If the tensile stress is too high, and the outer surface of the mounting hole wall is in contact with some solvent which has a strong effect on the plastic used, it may cause cracking or even breakage of the outer wall of the mounting hole of the plastic part under the action of stress and chemical reagents. Environmental stress cracking (ESC).
Therefore, do not use any known or unknown chemical reagents (such as alcohol, isopropyl alcohol, acetone, and other cleaning agents) to wipe or clean the plastic parts that have been screwed into the self-tapping screws.
5 Conclusion
The cause of the problem that causes the self-tapping screws of the plastic parts to be tightened is complicated. However, as long as the influence of various factors is mastered and clarified, the various analysis methods can be used to find the cause of the specific problem at the fastest speed and the most economical cost. And give reasonable advice to solve the problem as soon as possible.
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