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Typical defect analysis of automobile injection parts and its solutions

Update:13-06-2022
Summary:The amount of plastic used in automobiles has become an important indicator to m...
The amount of plastic used in automobiles has become an important indicator to measure the level of automobile design and manufacturing. At present, more than 10% of domestic automobile materials are composed of plastics, from exterior parts such as bumpers, skirts and fenders to dashboards Plastics can be seen everywhere, from interior parts such as door panels and pillars, to engine peripheral parts such as front-end frames, hoods, and intake manifolds. Most automotive plastic parts are injection molded, with short molding cycle, high production efficiency and low manufacturing cost. But injection molding may produce more defects. The editor has collected 5 typical automotive injection molding defects and their solutions.
1. Tiger skin pattern
Tiger skin pattern often appears on bumpers, instrument panels, door panels and pillars and other large-area automotive injection molded parts. It is a surface defect of wavy stripes. The stripes are approximately perpendicular to the direction of melt flow and are formed on the surface of injection molded parts. The engravings with different luster look like patterns on tiger skin, commonly known as tiger skin patterns. Tiger skin pattern is easy to occur on injection molded parts with thin wall thickness and large flow. It has the following characteristics: (1) The surface of the injection molded part appears with alternating light and dark stripes that change periodically; (2) The stripes are roughly perpendicular to the melt (3) The stripes are divided into bright areas and dark areas, the gloss of the bright areas is high, and the gloss of the dark areas is poor; (4) If the appearance surface of the injection molded part is a bright area, the back is a dark area, and the two appear alternately.
reason:
1. The more toughening materials in the material, the more likely the tiger skin phenomenon will appear. The toughened material is stretched and sheared during the injection molding process, resulting in a small amount of deformation, which makes the melt flow unstable, resulting in tiger skin lines. Materials with poor toughness rarely appear tiger skin patterns, such as reinforced materials, non-toughened nylon, polybutylene terephthalate and other materials, which rarely produce tiger skin patterns during the molding process. Polypropylene (PP) used in automobile injection parts is very prone to tiger skin defects due to the high impact resistance required and the addition of toughening components such as elastomers.
2. The thinner the wall thickness of the injection molded part, or the longer the filling flow distance, the greater the flow length ratio, the easier the melt is to produce unstable flow during the filling process, and the easier it is to produce tiger skin patterns. Appropriately increasing the wall thickness of the injection molded parts, or shortening the filling distance of a single gate, can reduce the filling resistance of the melt, ensure the stability of the melt flow, and help eliminate tiger skin lines. However, in today's increasingly demanding automotive lightweight requirements, thinning is the development trend of automotive injection molding parts, so it is not realistic to eliminate tiger skin patterns by increasing the wall thickness.
Solution:
In terms of mold design, increasing the diameter of the runner and expanding the thickness and width of the gate are effective measures to eliminate tiger skin lines. The gate thickness Z is preferably 0.7-0.8 times the wall thickness. The purpose of this move is to reduce the pressure loss of the melt in the runner and gate, and reduce the effect of mold expansion when entering the cavity.
In mold design, straight gates, side gates and fan gates with gradual transition should be used as much as possible, and latent gates and point gates with gradually reduced cross-sectional area should be avoided. Practice has proved that latent gates, point gates or very small side gates are all prone to tiger skin patterns.
In terms of injection molding process, melt temperature, mold temperature and injection speed are several important parameters that affect tiger skin pattern. In general, increasing the melt temperature and mold temperature, and adjusting the injection speed are beneficial to eliminate tiger skin patterns. Using a lower injection speed is beneficial to eliminate tiger stripes because the melt flow is more stable at lower speeds.
2. Poor surface reproduction
Poor surface replication means that the injection molded parts cannot accurately replicate the surface of the mold cavity during the molding process, which is manifested as uneven surface gloss of the injection molded parts.
Cause:
1. Many automotive injection molded parts are designed with dermatoglyphics on the surface. The finer the dermatoglyphic effect, the harder it is to replicate during injection molding. 2. The shape of the dermatoglyphs is very regular, and when they are not connected to each other, it is easy to cause poor surface reproduction due to the trapped air of the dermatoglyphics. 3. Insufficient cavity pressure. In this case, the injection molding process should be set to ensure that the melt has enough pressure in the cavity to replicate the mold surface.
solution:
Generally speaking, high temperature, high speed and high pressure (referred to as the three-high process) are beneficial to reduce the melt pressure loss and make the surface of the injection molded part better replicate the mold surface.
Avoid getting tired. When the exhaust effect of the mold is not good, or the depth of the skin texture is large, or the injection speed is too fast to allow the gas in the cavity to be discharged in time, trapped air will occur, resulting in poor surface reproduction.
3. Sink marks
Sink marks occur on the surface of injection molded parts, which refer to local depressions formed on the surface of injection molded parts. Sink marks usually occur at the location of local thick walls of injection molded parts, or on the back of protrusions such as reinforcing ribs and screw posts. Sink marks are prone to appear on the back of structures such as screw posts, stiffeners or snaps on automotive injection molded parts.
Cause: The sink mark is caused by the lack of effective pressure-holding and feeding in the local position of the injection molded part during the cooling process.
solution:
For the sink marks on the back of the screw column, the commonly used improvement measure is to add a crater structure at the root of the screw column. Reduce the equivalent wall thickness. The smaller the equivalent wall thickness is, the less likely it is to produce sink marks on the surface of the injection molded part. When the equivalent wall thickness at the root of the screw column is close to the wall thickness T of the injection molded part, there is basically no risk of sink marks in the injection molded part.
Automobile injection parts generally use modified PP materials. Compared with engineering plastics, the elastic modulus of modified PP is lower, and the ability to resist deformation is also low. It is necessary to ensure the strength of the injection molded parts from the structural design. A common approach is to add stiffeners to the product design. The selection of the thickness of the reinforcing ribs is related to whether the surface of the injection molded part produces sink marks. For crystalline polymers, since crystallization will cause post-shrinkage, the thickness of the root of the reinforcing rib should be appropriately thinner, and it is recommended not to exceed 1/3 of the wall thickness, otherwise sink marks are likely to be formed on the back; for amorphous polymers, due to post-shrinkage Smaller, the thickness of the root of the reinforcing rib can be appropriately thicker, but it is recommended not to exceed 1/2 of the wall thickness. When the size of the rib is unreasonably designed, it is easy to form sink marks on the back. If possible, design the reinforcing rib behind the transition surface where the two planes meet. Even if there is a slight sink mark, the sink mark can be subtly hidden due to the masking effect of the protective surface.
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Reasonable structural design. Automobile bumpers have strict requirements on the appearance. Even if there are slight sink marks on the injection molded parts, they will be very obvious after painting, which puts forward higher requirements on the design of the structure. Due to the needs of assembly, automobile injection parts are often designed with more buckles behind the appearance surface. The size design of the buckle is very important. If the wall thickness is thinner, the strength of the buckle is insufficient, and it is prone to lack of glue during injection molding; if the wall thickness is thicker, sink marks will occur. The best solution is to reduce the wall thickness at the root of the buckle, which not only solves the strength problem of the buckle, but also avoids sink marks, and at the same time ensures smooth injection molding. The root of the buckle is not thinned, and sink marks are prone to occur during injection molding.
Four. Weld line
When the two streams meet, a weld line forms on the injection molded part. At this time, the tangent of the melt front has an included angle, which is the convergence angle of the weld line, as shown in the figure below. The size of the convergence angle has an important effect on the strength and clarity of the weld line. According to the analysis criteria of Moldflow, when the convergence angle is greater than 135°, the weld line is invisible to the naked eye, and the strength is very high; when 75° < convergence angle < 135°, the weld line is relatively clear, but the strength is still high. , the welding line can be covered by spray paint; when the confluence angle is less than 75°, the welding line cannot be covered by spray paint even after treatment, and the strength is poor.
solution:
For large injection molded parts such as automobile bumpers, instrument panels, door panels, skirts, etc., hot runners with sequence valves are usually used to eliminate weld lines. Generally, the needle valve in the middle is opened first, and when the melt front flows through the next When the needle valve is opened, the needle valve is opened, so as to avoid the formation of more weld lines on the injection molded part during filling. Even if the needle valve type hot runner is used, it is difficult to eliminate the weld line around the hole, but it can also be weakened in the structural design. Usually, structures such as grooves are added around the hole to disturb the generation of weld lines, thereby reducing the effect of weld lines.
When designing the mold, it should be avoided that the gate faces the hole, otherwise the weld line formed will be difficult to adjust. In the injection molding process, the lower injection speed in this section can weaken the weld line, but when the processing temperature or the material melt flow rate fluctuates slightly, the injection molding process needs to be adjusted, which causes certain difficulties in production.
5. Warping deformation
Warpage refers to the large difference between the shape of the injection molded part and the designed shape, usually convex or concave. Cause: PP materials are often used in the design of automotive injection molded parts. Compared with engineering plastics, its elastic modulus is relatively low, so its ability to resist deformation is weak, which requires structural design to ensure the strength of injection molded parts. The front edge of the car dashboard is a part that is easily deformed. Solution: Without increasing the wall thickness, reinforcing ribs can usually increase the strength of the injection molded part and improve the deformation of the injection molded part. However, when the rib design is unreasonable, it will increase the deformation of the injection molded parts.