Case study of injection molding defects

Case Study of Injection Moulding Defects - Automotive Molding
As an automotive engineer, we can understand a lot of things, but we still have a headache when
we encounter some actual injection defects in auto parts. Don't worry, this article is a
professional analysis of 13 cases of automobile injection moulding defects with solutions.
First, car door board ejector impression - auto body side molding
Automobiles generally include four door panels for left, right, front and rear assemblies. PP is
generally used as material, and apparent quality of auto body side molding is very high.
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.
1) Possible cause analysis and improvement measures
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

3) Key tips
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.
Second, car bumper flower from car bumper molding
Car bumper is a safety device that absorbs external impact and protects front and rear of body.
Generally, a PP having a good mechanical strength is selected.
Name Car bumper
Material PP
Colour: Black
Mold temperature 55℃
Gate method side gate
Flower-like cold material near car bumper not only affects apparent quality of part, but also
affects its mechanical properties. How to deal with it?
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
2) Comparison of before and after improvement
3) Key tips
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.

Third, car bumper peeling - car bumper molding
Surface of automobile bumper has a large surface and defect defect rate is low, so it is difficult to
find skin defect, but this defect seriously affects appearance quality of workpiece.
Product
parameters
Injection
parameters
Name Car Bumper Injection Moulding Machine 3200T
Material PP+EPDM+TD
20
Weight 3200g Mold
temperature
18
℃
Color black Gate method Side gate
Figure below shows that bumper surface of a project has a layered peeling, and defect rate is
3-5%, which appears randomly on the surface of workpiece. Due to large surface of part and low
defect defect rate, it is difficult to find, but it seriously affects appearance quality of part.
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 tips
Cold 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.
Fourth, car rear bumper bulging - car bumper molding
Rear bumper of car plays a very important role in personal safety.
Name Car rear bumper
Material PP+EPDM+TD15
Colour Black
Weight 3800G
Gate method Side gate
Following figure shows bumper of a car in a project. Surface has a bulge. Position is roughly fixed.
Located at corresponding turning position of trunk lid, chance of producing a bulge is high, and
generation of bulging may adversely affect its apparent quality.
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
3) Key tips
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.
Fifth, apparent defects of fender - rubber fender trim molding
Apparent defects in automobile fenders seriously affect appearance. How can we make a
reasonable rubber fender trim molding to avoid such defects?
Name Fender
Material PP
Color black
Gate method Large Gate
Picture below shows fender. In moulding process, due to long molding cycle, molecules have
sufficient molecular chain relaxation or crystallization time in mold, which causes molecular chain
or its aggregation state to be “free”. Rough, affecting apparent quality of product. Excessive
molding cycle will seriously affect normal production efficiency and increase production costs.
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 tips
Wall thickness of part affects cooling time. When designing rubber fender trim molding,
attention should be paid to uniform wall thickness.
Six, car dashboard tiger skin - car dashboard molding
Tiger skin pattern is characterized by a line of lines perpendicular to direction of flow, which looks
like pattern on tiger skin. As shown in figure below, obvious tiger skin defect on dashboard of car
seriously affects appearance of workpiece.

Name Dashboard
Material PP
Color Black
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 tips
Studies 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.
There is a clear pressure line around bumper fog light, which affects appearance and surface
quality of product.
Name Bumper
Material PP
Color black
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:

1. As shown in the figure above, by adding a sufficiently deep choke groove near G5 gate,
pressure difference between left and right holes is reduced, and pressure line defects become
light.
2. Process: By delaying opening time of G5 gate, previously solidified melt solidified layer is
thickened and pressure line is weakened.
Figure 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.
Figure Mechanism diagram of pressure line
Eight, size of luggage rack is deformed
A brand of car luggage rack bracket, material used is PC / ABS material. After inspection on

product, size was found to be shorter. Size required by OEM was 1438±1 mm, but after molding
was only 1435mm. Inappropriate size does not meet assembly requirements. Dimensional
change of plastic parts is essentially caused by uneven plastic shrinkage.
Some brand of car luggage rack bracket
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.
Nine, car sub-dashboard tiger skin - car dashboard molding
There are obvious tiger skin patterns on auxiliary instrument panel, which will affect appearance
of parts and have little effect on performance strength.

Obvious tiger skin patterns on auxiliary instrument panel.
Name Sub-dashboard guard
Material PP
Color Black
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.
Ten, car grille size deformation
Grille of a certain automobile factory, due to size problem, found that there was a large gap after
assembly, which failed to meet production requirements. Dimensional change of plastic parts is
essentially caused by different degree of shrinkage of various parts of plastic. For injection
molded parts with assembly requirements, dimensional deformation is an unacceptable molding
defect.
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.

Table 4 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
Improvement measures: Since dimensions of parts meet design requirements, it is necessary to
further check whether dimensions of other assembly parts are appropriate.
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.
Eleven, car central control panel weld line - car dashboard molding
Part of welded part of workpiece is trapped in gas, and it is easy to burn when it is serious, which
has a great influence on appearance.
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 tips
When 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.
Twelve, car bumper gas trap defect - Car bumper moulding
Trapped air will form defects such as flow marks and gas marks on the surface of product. As
shown in figure below, trapping phenomenon on the surface of car bumper will lead to loose
structure and strength, which will affect appearance quality and strength of product.
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 tips
In 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.
Thirteen, battery cover floating fiber- automotive molding
Floating fiber, also known as exposed fiber, refers to phenomenon that glass fiber contains a glass
fiber piercing surface of workpiece during injection moulding to produce a white surface. In
figure below, there is a relatively obvious floating fiber in central part of lower cover of battery.
Floating fiber will seriously affect appearance of part, especially for parts with high appearance
and high gloss.
Product
parameters
Injection
parameters
Name Battery Injection molding SUPER

lower cover machine JACK 500t
Material PA6+30% GF Mold
temperature
95℃ Injection
pressure
115
bar
Injection
time
4s
Color black clamping force 500t
Weight 20000g Gate method Point gate Pressure
holding
pressure
75b
ar
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 tips
When 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.

Case study of injection molding defects

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