Case Study of Injection Moulding Defects - Automotive Molding
Time：2019-03-25 15:21:23 / Popularity： / Source：
- Car door board ejector impression - auto body side molding
Main process parameters of materials
| Car door | Injection parameters | ||||
| Name | Car door panel | Injection pressure | 40MPA | ||
| Material | PP | Feeding method | Side gate | Holding pressure | 18MPA |
| Color | gray | Weight | 1400G |
Picture below shows defect of top plate of car door panel. Top white position corresponds to a rounded apex on back side, where ejector impression affects aesthetics of car door panel and is an unacceptable apparent defect for auto body side molding.
Possible cause: Main reason for ejector impression is that ejector exerts too much force, causing an apparent defect in position of ejector pin.
Improvement measures: Through optimization of mold structure and moulding process, force applied by ejector pin of auto body side molding is effectively reduced, and possibility of ejector mark is reduced. Solutions adopted in this case are considered from following two aspects:
1. Mold structure: When draft angle of auto body side molding is small or polishing effect of rib position is not good, stripping resistance of part will be increased. Force exerted by ram on the part is increased, resulting in whitening or crowning. Improvement is to improve polishing of ribs or to increase draft angle of part.
2. Moulding process: When holding pressure is too large or holding time is too long, parts will be overfilled. At this time, coating force of core on auto body side molding is large, so that ejection force of ejector is also very large, and finally phenomenon of whitening or topping occurs. Improvement is to reduce holding pressure or reduce holding time.
2) Improved rendering
Some injection molded parts cannot have a ejector mark because of application requirements. For transparent parts, if there is a ejector, it will affect its appearance. Or because there is no position setting ejector for thin wall thickness, it can be solved by using a hidden ejector.
- Car bumper flower from car bumper molding
Main process parameters of materials
| Name | Car bumper |
| Material | PP |
| Colour: | Black |
| Mold temperature | 55℃ |
| Gate method | side gate |
Possible reason:
1. Product mold: Car bumper molding used in this case contains hot runner and cold runner respectively, but there is no cold well on the cold runner, which is directly connected to workpiece.
2. Moulding process: For injection moulding process, it is observed that some of needle valve gate sealing is not strict, there is residual cold material on the gate of car bumper molding, and pressure of closed needle valve is very low, which does not function as a sealing glue.
Improvement measures: In this case, air pressure of sealing needle is increased by using a pneumatic booster pump, and cold material disappears after hot gate no longer leaks.
Cold runner structure in the car bumper molding
Hot runner needle valve drive and pressure
- Comparison of before and after improvement
Following figure shows structure of a typical hot runner nozzle. It can be seen that front end of nozzle is in contact with stencil. Although contact area is limited, heat at the front end of nozzle is transmitted to cold stencil. In addition, there is sometimes a cooling circuit around nozzle due to need for cooling, which further increases possibility that nozzle tip temperature is too low. Temperature at front end of nozzle is low, resulting in a small amount of cold material at front end of car bumper molding. Especially when sealing needle is not tight, melt overflows and cold material is more likely to form. Therefore, when hot runner nozzle is turned to cold runner, it is recommended to provide a cold well on cold runner.
- Car bumper peeling - car bumper molding
Main process parameters of materials
| Product parameters | Injection parameters | ||||
| Name | Car Bumper | Injection Moulding Machine | 3200T | ||
| Material | PP+EPDM+TD20 | Weight | 3200g | Mold temperature | 18℃ |
| Color | black | Gate method | Side gate |
Possible reason:
Moulding process: For this case, it may be caused by cold material.
Inspection revealed that material temperature and hot runner temperature were normal on car bumper molding, and open hot runner used in car bumper molding was also normal. Due to manual pick-up during production, when pick-up time is long, each hot nozzle has different degrees of flow. After shortening pickup time, it was found that hot nozzle flow phenomenon was effectively improved.
Therefore, it is possible that cold material generated by rogue is flushed into cavity to cause delamination, and site also finds that it is length of worker's pickup time that determines whether next mold product produces delamination.
Improvements:
In terms of moulding process: injection distance of injection moulding machine was changed from 16mm to 20mm, hot nozzle no longer produced rogue, and there was no layering peeling on workpiece.
Figure Improved rendering
2) Key tipsCold material rushing into cavity may cause delamination. Source of cold material may be from that material temperature is too low, nozzle temperature is too low, flow channel is not provided with a cold well or length of cold well is insufficient, hot runner temperature is too low, and mold temperature is too low.
- Car rear bumper bulging - car bumper molding
Main process parameters of materials
| Name | Car rear bumper |
| Material | PP+EPDM+TD15 |
| Colour | Black |
| Weight | 3800G |
| Gate method | Side gate |
1) Possible cause analysis and improvement measures
Possible reason:
1, Product structure: Observe that bulge is located at corresponding arc position on both sides of trunk lid. Arcs on both sides of trunk are bulging, and chances are different; bulge sometimes bulges obviously, sometimes only slightly uneven and it is difficult to find. But it is very obvious after painting; if it is baked at 600℃ with a hair dryer, and slight bulge quickly bulges. Take a product with a slight uneven surface, and see a hole in cut section.
Baked at 600℃ with a hair dryer, and slight drum kit quickly bulges.
Cut through section and found a hole
2, Product mold: Injection molded parts corresponding to position of bulge does not have a venting groove, and gas is difficult to discharge during injection moulding process.
Car bumper molding does not have a venting slot
Adding a venting slot
Improvements:Product mold: Add a venting groove on car bumper molding.
In terms of moulding process: find screw position corresponding to bulge part, which is injection molded at low speed. Finally solve bulging problem.
3, Comparison of before and after improvement
Use of hair dryer baking is also a way to identify cause of bulge. In this example, a hair dryer was used to bake at 600℃, and a slight bulge quickly bulged, indicating that it was caused by air bubbles.
- Apparent defects of fender - rubber fender trim molding
Main process parameters of materials
| Name | Fender |
| Material | PP |
| Color | black |
| Gate method | Large Gate |
Possible reason:
In terms of product structure: design of rubber fender trim molding is unreasonable. Cooling time of parts is related to maximum wall thickness of parts. When parts are thicker, cooling time of parts should be increased to ensure that no sink marks are observed. However, too long cooling time will increase molding cycle and a reasonable design should be made.
Improvement measures: Average wall thickness of workpiece in this case is 2.5mm, but wall thickness of 5mm appears in local area. In order to ensure appearance of surface, cooling time is extended from 20s to 30s, but molding cycle is extended. In practice, insert can be used to accelerate cooling, or thick-walled part can be hollowed out to achieve both strength requirement and cooling time.
Part thickness design
Improved rendering
2) Key tipsWall thickness of part affects cooling time. When designing rubber fender trim molding, attention should be paid to uniform wall thickness.
- Car dashboard tiger skin - car dashboard molding
| Name | Dashboard |
| Material | PP |
| Color | Black |
| Mold temperature | 45℃ |
| Gate method | Needle valve gate + Side gate |
Possible reason:
In terms of moulding process: injection moulding speed of original moulding process of this product is faster. By raising material temperature to 255℃, actual mold temperature is about 49℃, and appearance of product is not improved. Valve gate method is used for car dashboard molding. It is noted that interval between the first valve gate and other valve gates is as short as 3 s. When slow moulding process is used in actual car dashboard molding, the first valve gate may have insufficient pressure due to opening of other gates, resulting in instability of melt filling front pressure and instability of flow, thereby forming a tiger skin pattern.
Improvement measures: In order to provide sufficient pressure to maintain stable flow of the first gate melt front, by closing all other gates and filling with a slow speed 15, main appearance of workpiece disappears.
Improved rendering
2) Key tipsStudies have shown that production of tiger skin is not directly related to viscosity of melt. More research has shown that instability of flow process is main cause of tiger skin grain.
Seven, car bumper pressure line - car bumper moulding
Bumper belongs to outer surface of car, and apparent quality requirements are strict, and generation of pressure line will have a serious impact on its apparent quality.
Main process parameters of materials
| Name | Bumper |
| Material | PP |
| Color | black |
| Mold temperature | 35℃ |
| Gate method | Needle valve gate |
Possible reason:
Car bumper moulding: In this case, gate G5 is arranged near hole around fog lamp. When gate is opened, pressure on both sides of hole reaches equilibrium pressure line again due to influence of hole.
Improvements:
2. Process: By delaying opening time of G5 gate, previously solidified melt solidified layer is thickened and pressure line is weakened.
Improved effect
Pressure line described in the case is a submerged streamline, which often appears in the area where weld line is located. Mechanism of pressure line appears as shown in figure below. Solution is to reduce pressure difference around weld line as much as possible, or pressure difference is not sufficient to drive melt being solidified.
Mechanism diagram of pressure line
- Size of luggage rack is deformed
Some brand of car luggage rack bracket
Main process parameters of materials
| Name | Baggage bracket |
| Material | PC/ABS |
| Injection moulding machine | ECOPOW 550 |
| Gate method | 5 submerged sheets into glue |
possible reason:
Product structure: In this case, product structure is relatively simple, and it is a strip with a slight strip of curvature. There is no problem in molding, main problem is inconsistent size after molding.
Improvement measures: By optimizing injection moulding process to form large parts, hot runner sequence valve opening time is extended from 8s to 10s. Glue injection time is extended from 9s to 15s, and cooling time is extended to 60s. End point of glue is 32mm which is reduced by 40mm. After mold is released, dimensions of parts are 1436~1437mm. Minor modifications were made to mold and this size issue was resolved.
2) Improved effect
Problem in this case is essentially due to improper mold opening of mold. Test size is 1439mm for a moment, and mold size is about 1445.2mm if mold is opened at a shrinkage of 0.5%. Therefore, this problem is caused by unreasonable mold opening. In order to ensure smooth production in later stage, size calculation must be done at molding design stage.
- Car sub-dashboard tiger skin - car dashboard molding
Obvious tiger skin patterns on auxiliary instrument panel
Main process parameters of materials
| Name | Sub-dashboard guard |
| Material | PP |
| Color | Black |
| Mold temperature | 45℃ |
| Gate method | Needle valve gate + side gate |
possible reason:
Analysis found that design of runner and gate in car dashboard molding of this case is unreasonable, gate is small and rectangular, there is no gradual transition, pressure loss is large, and pressure cannot be provided enough to maintain stable flow of gate melt front.
Surface problems caused by unreasonable design of runners and gates
Improvements:Increase size of runner 2mm on site, increase width and thickness of gate, and use a gradient gate, as shown below:
Improvements
Improved rendering
Due to small size of flow path and gate of car dashboard molding, pressure loss in flow path is large. As a result, filling pressure is insufficient, melt flow front cannot maintain a stable flow, and tiger skin defect is likely to occur.
- Car grille size deformation
Grille from some car factory
| Name | Car Grill |
| Material | car PP |
| Colour | Black |
| Gate method | 4 hot runner into glue |
Possible reason:
Product mold: Molding design size is 898.3mm, and moulding process uses four different processes for injection moulding. Results of injection molded parts are shown in following table. Sizes of four kinds of process injection molded parts are larger than design size, but there is still a large gap during assembly.
Different process injection molded parts results
| No. | Size | Weight g (with material) |
| 1 | 902.16 | 945.0 |
| 2 | 901.64 | 937.8 |
| 3 | 901.28 | 925.0 |
| 4 | 899.79 | 819.0 |
2) Improved effect
There are assembly problems with dimensional problems, and there are generally assembly problems. Therefore, it is necessary to pay attention to mutual cooperation between parts during assembly, so that size problem can be satisfactorily solved.
- Car central control panel weld line - car dashboard molding
Picture below shows central control panel of car. Due to multiple weld marks in trapped position, it is trapped in multiple streams. And subsequent processing of parts has a spraying process, and trapped area cannot be covered. Part mold adopts three-point hot runner open gate feeding, which cannot be controlled by timing.
| Name | Car Central Control Panel |
| Material | PC |
| Color | Black |
| Gate method | Hot runner open direct glue |
Possible reason:
In terms of moulding process: since trapped gas is merged by three strands of molten material at the same place, wind is formed, and convergence angle of flow makes trapped position in the middle of workpiece, and gas cannot be discharged. There was some improvement after trying to reduce rate of injection during moulding process, but it did not completely eliminate trapped air. In addition, lowering of rate of injection tends to cause shortage of other parts, so this method should not be used.
Improvement measures: Increasing flowability of melt by increasing bottom hot runner by 20℃, so that three melts meet at different places to form two weld marks, and trapped gas defects are eliminated.
Figure Improvement measures
Improved rendering
2) Key tipsWhen hot runner mold cannot control opening time, melt filling flow rate can be controlled by adjusting temperature of each hot runner nozzle. Although it is unstable, method is very effective to car dashboard molding. In this way, it is possible to adjust filling behavior of melt of each hot runner nozzle and change position of weld mark position.
- Car bumper gas trap defect - Car bumper moulding
| Name | Car bumper |
| Material | PP |
| Color | Black |
| Gate method | Fan gate |
Possible reason:
Moulding process: When car bumper moulding itself is poorly exhausted, process conditions are very important. There is a fixed flower in the place where weld line appears, most likely due to trapped gas at weld line. Therefore, it is necessary to pay attention to whether injection speed is reasonable. If speed of melt passing through position of weld line is too fast, charring of material will be decomposed and a flower defect will occur.
Improvement measures: In this case, defect will not occur when injection speed is reduced from 60 to 40.
FIG Improved rendering
2) Key tipsIn process adjustment process, following auxiliary measures are needed to ensure elimination of trapped air:
1. It must be ensured that temperature of melt cannot be lowered too much, so that process of slowly dissolving gas can be continuously performed.
2. Just try to reduce amount of trapped air. Improve exhaust of each position of car bumper moulding, especially near trapped position.
- Battery cover floating fiber- automotive molding
| Product parameters | Injection parameters | ||||||
| Name | Battery lower cover | Injection molding machine | SUPER JACK 500t | ||||
| Material | PA6+30% GF | Mold temperature | 95℃ | Injection pressure | 115bar | Injection time | 4s |
| Color | black | clamping force | 500t | ||||
| Weight | 20000g | Gate method | Point gate | Pressure holding pressure | 75bar |
Possible reason:
A. Moulding process:
(1) Melt temperature is low. When melt temperature is low, viscosity of material increases, and fluidity of melt decreases. As a result, pressure loss of melt during filling process increases, and cooling rate when melt contacts surface of cavity increases.
(2) Holding pressure and holding time are too small.
Improvements:
(1) This case improves fluidity of melt to some extent by raising melt temperature.
(2) In this case, by optimizing pressure-holding process curve, thermal expansion and contraction effect of part will be reduced to a certain extent, and floating fiber will be masked.
B. Product mold:
When mold temperature is low, cooling rate when melt comes into contact with surface of cavity increases.
Improvement measures: In order to prevent formation of surface hardened skin at initial stage of filling, in this case, it is easy to obtain a more remarkable effect by raising temperature of mold.
c. Product structure:
Defect of this case is specifically analyzed from following aspects in terms of product structure:
(1) Is gate size appropriate? When a small gate is used to form a part, gate tends to cool faster, resulting in melt not being pressed into cavity during subsequent holding phase. With thermal expansion and contraction effect of part, glass fiber will pierce surface of part to form a floating fiber.
Improvement measures: Increasing size of gate can make pressure retention more sufficient, thus reducing thermal expansion and contraction effect of part.
(2) How is exhaust effect? When automotive molding is poorly ventilated, gas in cavity cannot be discharged in time during filling process, forming a gas flow layer, so that workpiece can not be well covered on surface of automotive molding, thereby causing surface of glass fiber to form a surface.
Improvement measures: In molding design, it is necessary to strengthen exhaust in a position where air trapping is likely to occur.
(3) Is there a spray phenomenon? When gate size is too small or wall thickness of workpiece varies greatly, it is easy to form a spray at a gate position or a thick wall thickness, and spray causes a part of melt to be cooled relatively quickly to form a surface hardened layer.
Improvement measures: Pay attention to uniform variation of wall thickness during design and avoid spraying.
Improved rendering
2) Key tipsWhen forming a floating fiber defect, we often use a three-high moulding process, namely high mold temperature, high material temperature, high injection pressure and injection speed. Reason why three high process is used is to improve fluidity of melt and make parts more full. Thereby reducing effect of surface hardening layer, thermal expansion and contraction effect on floating fiber.
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