20 MOST COMMON QUALITY ISSUES IN
YOUR PLASTIC PRODUCTION
CHAIRMAN’S DISCLAIMER
The manufacturing related issues and their respective solutions listed in this
guide is solely intended for educational purposes. Although we as NexPCB
have supported the manufacturing of hundreds of startups in the last 15
years, and have full confidence in our expertise in doing so, there can be
virtually unlimited number of culprits behind a manufacturing problem, and
throughout research to find out the exact issue may be necessary.
We hope you use this guide to make more comprehensive inquiries with
your manufacturer. But please take our recommendations with a pinch of
salt and always do your own research. Before making costly changes in your
manufacturing, always ask an expert how things should be handled.
As you scale manufacturing higher, there will be issues with your product at
some point. These issues can be catastrophic mistakes or healthy management
exercises depending on how you handle them. If you find value in this guide
and need more in-depth assistance, you can contact us and let us know how
we can assist.
Thunder ZHANG
Co-founder, NexPCB
TABLE OF CONTENTS
04
State of Manufacturing Plastics
05
Who is This Guide for?
06
Nomenclature Discussed
07
Process Related Manufacturing Problems
08
Short Shots
10
Sink Marks
14
Flashing
17
Bubbles
19
Blisters
21
Warpage
24
Jetting
26
Shrinkage Marks
28
Cracks
31
Burn Marks
34
Ejector Pin Marks
37
Mold Related Manufacturing Problems
38
Drag Marks
41
Vacuum Voids
44
Gloss Differences
47
Gate Blush
50
Material Related Manufacturing Problems
51
Weld Lines
54
Black Specs
57
Silver Marks
60
Splay Marks
63
Color Streaks
66
Avoiding Future Plastic Defects
4 | 20 Most Common Quality Issues in Your Plastic Production
STATE OF MANUFACTURING PLASTICS
2021 AND BEYOND
Plastic business isn’t an easy one to break in. Apart from costly machinery, well-trained staff and long manufacturing
lines, you also need process expertise to maintain a high level of production yields.
Especially with the advent of Covid-19 induced supply shortages across the raw materials industry, we are seeing even
some of the reliable manufacturers abandoning their quality standards to cut costs. Adulterating pellets with chalk and
excessive recycled materials, using damaged molds and cutting injection hold-times to cope with the surge in demand is
becoming more rampant across Asia.
Therefore it is more important than ever to work with
reliable, vetted factories in order to avoid the product
defects we will discuss in this guide. With manufacturers
largely ignoring small batch orders to deal with the
demand surge coming form the west, working with a
partner like NexPCB can help you manufacture plastic
parts safely without having to skip Pilot Runs. And as
a rule of thumb, we advise you to never skip Pilot Run
manufacturing if you are dealing with a part that was
never manufactured before. Contact NexPCB for a free
design review of your parts before investing your time and
money on costly tooling and molding.
5 | 20 Most Common Quality Issues in Your Plastic Production
WHO IS THIS GUIDE FOR?
01 PRODUCT ENGINEERS
02 QA ENGINEERS
This guide can help you design better products and better
molds to go with them. Even if you’re not handling mold
design, figuring out their shortcomings early on can avoid
structural and aesthetics issues involved down the road.
03 PROCUREMENT MANAGERS
QA Engineers will especially find this guide useful since it
can allow them to modify the manufacturing processes
according to the solutions listed.
04 HARDWARE STARTUP FOUNDERS
If you’re in the process of testing out different suppliers,
this guide can help you identify possible product risks
associated with each vendor.
As a hardware startup, you have very little wiggle room
to make wrong decisions when it comes to plastic parts
and casings, as many issues can set you back weeks if not
months, and cost a lot. Use this guide in your NPI Process.
DISCOVER
IDENTIFY
ELIMINATE
the errors in your manufacturing as
early as possible. Familiarize yourself
with the vocabulary so that you can
communicate them more effectively.
the culprit behind the errors.
Figure out whether its caused by
the mold, the material, the process
management or a combination of any
of these.
through making the right
adjustments and changes in your
process.
WRITTEN AND EDITED BY
CHARLIE CHEN
VP of R&D & Engineering
CHRIS HOWES
VP of Revenue and Strategy
ORKUN Z. OZTURK
Marketing Development Lead
6 | 20 Most Common Quality Issues in Your Plastic Production
NOMENCLATURE DISCUSSED
Taken from xcentricmold.com
Taken from ewmfg.com
7 | 20 Most Common Quality Issues in your Plastic Production
PROCESS RELATED
MANUFACTURING PROBLEMS
Process related problems are the most common
setbacks you can face in manufacturing plastics.
That’s due to the fact that there are many variables
that can affect the final result. As you can imagine,
factories can get chaotic at times, with orders for
many different products piling in.
On a personal account of this hypothesis, we once
found out that a window that was left open in the
winter caused parts to cool down too rapidly on the
factory floor. A less steeper temperature curve was
achieved after making necessary changes, which in
return solved the problem with blisters.
In order to avoid molding process related issues,
make sure the machinery is in good condition,
serviced regularly and proper pilot runs are
completed before manufacturing parts in series.
Give proper handling instructions to the staff
and set the temperature, injection speed and
injection pressure correctly, along with optimizing
hold times. Common product defects with
manufacturing process issues as the main culprit
are listed below:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Short Shots
Sink Marks
Flashing
Bubbles
Blisters
Warpage
Jetting
Shrinkage Marks
Cracks
Burn Marks
Ejector Pin Marks
8 | 20 Most Common Quality Issues in Your Plastic Production
1. SHORT SHOTS
Other Names
Non-fills, short molds
Problem Type
Aesthetic, Structural
Prevalence
Common early on
1
We call this phenomenon a „short shot” due the
fact that it is caused by the molding shot falling
short of filling the mold. In other words, the melted
plastic can’t manage to occupy all the space
designed by the mold and creates an unfinished
and deficient product.
9 | 20 Most Common Quality Issues in Your Plastic Production
SHORT SHOTS
TROUBLESHOOTING
Problem
Solution
The equipment is not suitable
Change the equipment. It is highly recommended to choose an injection equipment that can
allow for at least 15% more injection rate than the
injection volume needed (for both plastic and
condensed material)
The runner is blocked by a cold material
Disassemble the mold and injectors to clean
the nozzles of any cold slug formations. A larger
sized runner may also be needed
The melt temperature is too low
Increase hold time to improve this defect. Make
sure to consult the manufacturer’s data-sheet on
the minimum melt point of the plastic.
Mold temperature is too low
Pre-heat the mold to the temperature of process requirements before the injection machine
starts; if the temperature of the mold can’t reach
the target, check if cooling system is causing it.
Improper mold vent placement
Reposition the mold vent to a suitable place and
potentially increase its size. It is generally acceptable to place the vent at the end of the fill (depth
0.02-0.04mm, width 5-10mm)
The material exhibits bad viscosity
Act on the material or on the injector. You can
improve the placement of the runner, enlarge
the gate or the size of the nozzle as a few examples. You can also put some additional ingredients in the raw material to alter its viscosity
Excessive use of lubricants
Reduce the amount of lubricants or make some
adjustment spaces between the cylinder and
the screw
Process
Mold
Material
10 | 20 Most Common Quality Issues in Your Plastic Production
2. SINK MARKS
Other Names
Teeth marks, shrinkage
Problem Type
Aesthetic, structural
Prevalence
Very common
Taken from: Polymer Testing
2
During the injection process, there are times when
parts of the mold don’t get enough pressure to
set the plastic. When this happens, it can cause
additional stress on that area where the rate at
which the plastic shrinks is slower than other areas.
The sink marks appear when the surface of the
plastic is pulled in by this stress because the surface
isn’t solid and the melt isn’t complete.
The sink marks will appear at places where the melt
is slower or the wall section is thicker. An example
of this could be the edge between the rib, brace
and surface of the product.
11 | 20 Most Common Quality Issues in Your Plastic Production
SINK MARKS
TROUBLESHOOTING
Problem
Solution
Improper material temperature (too high or
too low
Adjust material temperature (see material data-sheets)
Insufficient cooling time
Extend cooling time before ejecting from the
mold
The nozzle is too narrow or too wide
Adjust nozzle width according to the design of
the mold and the kind of material used
Improper mold temperature (too high or too
low)
Adjust mold temperature
Bad mold vent at sink mark
Modify the mold vent design or placement
Wall thickness is too thick at rib or brace
Adjust the mold to make the wall thickness as
uniform as possible within the design
Insufficient charge into the mold
Increase the charge amount
Process
Mold
Material
12 | 20 Most Common Quality Issues in Your Plastic Production
SINK MARKS
TROUBLESHOOTING
Problem
Solution
Improper gate placement or the slow injection Increase gate number or place gate at a location
process
where the wall is thicker
Process
Mold
Material
13 | 20 Most Common Quality Issues in Your Plastic Production
SINK MARKS
SHRINKAGE RATES OF DIFFERENT PLASTICS
Material
Shrinkage
Rate
Material
Shrinkage
Rate
Shrinkage
Rate
Material
GPPS
0.5
PA66
0.8~1.5
PC
0.5~0.7
HIPS
0.5
PA6
1.0
PMMA
0.5~0.8
SAN
0.4
PPO
0.6~0.8
PVC(Hard)
0.1~0.5
ABS
0.6
POM
1.5~2.0
PVC(Soft)
1~5
LDPE
1.5~4.5
CAB
0.5~0.7
PU
0.1~3.0
HDPE
2~5
PET
2~2.5
EVA
1.0
PP
1~4.7
PBT
1.5~2.0
PSF
0.6~0.8
14 | 20 Most Common Quality Issues in Your Plastic Production
3. FLASHING
Other Names
Burrs, Flash
Problem Type
Aesthetic
Prevalence
Very common
3
During the injection process, sometimes a little
bit of plastic can seep out of the die joint and
make a small rib. This is called flashing. Flashing
is not a good phenomenon in injection molding
production, and it can cause die joint damage if the
flashing can’t be immediately cleaned from the die
joint and the clamp. This type of damage will cause
more evident flashing in future products. For this
reason, it’s very important to look out for flashing
marks in injection molding production and remove
them before they turn into a persistent problem.
There are many reasons for flashing to occur in
injection molding. Some examples include very
high injection pressures or terminal injection
speeds, and inadequate clamping force.
15 | 20 Most Common Quality Issues in Your Plastic Production
FLASHING
TROUBLESHOOTING
Problem
Solution
The injection pressure is too high or the injection speed is too fast
Adjust the speed of the injection at the nozzle so
as to inject the plastic at the correct rate for the
chosen material
Clamping force is too high
Adjust the pressure of the clamps to loosen the
grip
The joint face isn’t fit or the accuracy is poor
Repair and check the mold or make the joint
face more precise
Mold clamping force isn’t enough (especially
true if the part has flashing around edges)
Increase the clamping force
The clamping area of mold is too large
Change the machine to one with larger mold
clamping force or the mold design to accommodate a smaller clamping area
Partial flashing due to gate blush
Re-balance the gate either naturally (preferred)
or artificially.
Mold deformation
Use a harder material for the mold
Process
Mold
Material
16 | 20 Most Common Quality Issues in Your Plastic Production
FLASHING
TROUBLESHOOTING
Problem
Solution
Pressure drop before the material can set
Conserve the clamping pressure according to
cooling temperature
The material of mold is bad or easy to wear
and tear.
Choose better mold materials with proper heat
treatment
The viscosity of the plastic is too low (like PA or
PP).
Change the plastic material with a high viscosity
one or adjust with a suitable filler
The material temperature or mold temperature is too high
Make adjustments to the respective temperatures of the mold and the material
Process
Mold
Material
17 | 20 Most Common Quality Issues in Your Plastic Production
4. BUBBLES
Other Names
N/A
Problem Type
Aesthetic, Structural
Prevalence
Less common
4
When the mold cavity fills up, it is possible that the
flow of the plastic is too quick and air can’t escape
quickly enough from the mold. When this happens,
bubbles can form in the materials. These may be
very large and very small. The presence of air sets
this issue apart from vacuum voids, which are
caused by a similar yet not identical set of causes.
The way to confirm whether the defect is a bubble
or a vacuum void is by heating up the issue area
with a heat gun or a lighter to see whether the
plastic gets sucked in or pops. A bubble with air
trapped in it would pop, while a vacuum void would
sink, causing a depression due to environment
pressure.
18 | 20 Most Common Quality Issues in Your Plastic Production
BUBBLES
TROUBLESHOOTING
Problem
Solution
Back pressure is low or the melt temperature
is too high
Increase back pressure or lower the melt temperature
The screw rotation speed or the injection
speed is too fast
Decrease the speed of the screw or the injection
pressure
The gas can’t be expelled quickly enough
Make the mold vent larger to allow air to escape
Incorrect gate size
Decrease the size of gate or use a different shape
Raw materials are not fully dried
Increase the intensity of the drying process
Process
Mold
Material
19 | 20 Most Common Quality Issues in Your Plastic Production
5. BLISTERS
Other Names
Delamination, blistering
Problem Type
Aesthetic
Prevalence
Uncommon
5
There are times that, when a part has been
manufactured and released from the mold,
the surface has an area where the volume has
expanded more than its surrounding area. This
phenomenon is called blistering. Blisters elevate the
surface of the product with a thin, film-like layer.
While the main culprit behind blisters is excessively
high injection speeds, it is in essence caused by the
air within the mold being unable to vent out. Before
redesigning your vents, check whether slowing
down injection fixes your issue.
20 | 20 Most Common Quality Issues in Your Plastic Production
BLISTERS
TROUBLESHOOTING
Problem
Solution
Non-uniform drying of different areas
Ensure effective cooling by adjusting the cooler,
making the temperature of mold cooler, increasing the hold-time, and/or decreasing the melt
temperature.
Injection is too fast
Make the mold injection speed slower, reduce
the injection pressure.
Cycle time is too quick
Increase the cycle time
Wall thickness is inconsistent
Optimize the structure of plastic and avoid having areas where the plastic part is too thick or
too thin.
Process
Mold
Material
21 | 20 Most Common Quality Issues in Your Plastic Production
6. WARPAGE
Other Names
Deformation, Warp
Problem Type
Aesthetic, Structural
Prevalence
Uncommon
Taken from: Proceedings of the 4th International Conference on
Green Design and Manufacture 2018
6
Warpage means that the shape of the injection
parts are inconsistent with the requirements of the
design drawings and presents visible deformations.
These deformations are usually caused by uneven
shrinking of the plastic parts, or due to stress that
during demolding. The most common underlying
cause of warpage is the use of glass-reinforced
plastic molding, large surface areas or very thin/
structurally weak products.
22 | 20 Most Common Quality Issues in Your Plastic Production
WARPAGE
TROUBLESHOOTING
Problem
Solution
The finished product has not been cooled and
set when it is taken out of the mold
(1) Decrease mold temperature
(2) Extend cooling time
(3) Decrease material temperature
The packing is too dense, forming internal
stress
Reduce pressure and hold-time
The injection pressure or holding pressure is
too high
Reduce injection pressure or holding pressure
Insufficient injection volume leads to shrinkage and deformation
Increase the charge, increase back pressure
The shape and thickness of the finished product is asymmetric
(1) Use clamping device to fix after demolding
(2) Change product design
Gate feeds are not balanced
Balance the gates for a balanced feed
The ejection system is unbalanced
Improve ejection system or change ejector
mode
Process
Mold
Material
23 | 20 Most Common Quality Issues in Your Plastic Production
WARPAGE
TROUBLESHOOTING
Problem
Solution
The mold temperature is not even across the
cavity
Improve the mold temperature so that the
point-to-point temperature is even
The plastic parts stick to the mold
Repair mold surface and/or add release agent
The front and back mold temperature is not
adaptable, the temperature difference is large
or unreasonable
Adjust the temperature according to the difference between the front and back dies
Plastic shrinkage anisotropy is too high (tendency to react to pressures from different
directions in a different rates)
Use plastic with a low shrinkage anisotropy
Process
Mold
Material
24 | 20 Most Common Quality Issues in Your Plastic Production
7. JETTING
Other Names
N/A
Problem Type
Aesthetic
Prevalence
Common
7
If the melt is injected too fast at the gate, the
surface of the parts (in front of the edge gate) will
produce a serpent-shaped line. This is commonly
called jetting.
Jetting mostly occurs with edge gates. When the
plastic melt flows through the nozzle, runner, gate,
and other narrow areas at high speed, it suddenly
enters the open and relatively wide areas of the
mold where the melt material will bend forward
along the flow direction like a snake. The molten
plastic will then cool down quickly after contact
with the mold surface. Because this first-to-enter
part of the material can’t have good fusion with
the rest of the resin subsequently entering into the
cavity, this phenomenon results with a visual defect.
25 | 20 Most Common Quality Issues in Your Plastic Production
JETTING
TROUBLESHOOTING
Problem
Solution
Injection speed is too fast (in-gate)
Reduce the injection speed
Improper placement of gate (Direct injection
into the cavity)
Change gate position (Move to the corner)
The gate is too small or unfit for application
(edge-gate)
Employ a wider edge gate. Even better, use overlap gate
Plastic has very high viscosity
Change the plastic raw material with a lower
viscosity one. (Higher the viscosity, more the
jetting). Add lubricants or flow promoters in the
mix.
The material temperature or mold temperature is too high
Reduce the material temperature and/or mold
temperature
Process
Mold
Material
26 | 20 Most Common Quality Issues in Your Plastic Production
8. SHRINKAGE MARKS
Other Names
N/A
Problem Type
Aesthetic, Structural
Prevalence
Common
8
In places where the wall thickness jumps by a large
margin, bite-like marks can form due to uneven
shrinkage. Since it’s due to uneven contractions on
a surface , they are called a shrinkage marks.
27 | 20 Most Common Quality Issues in Your Plastic Production
SHRINKAGE MARKS
TROUBLESHOOTING
Problem
Solution
Injection pressure and speed are too low
Increase injection pressure accordingly.
Hold time too short
Increase hold time and cooling time
Not accounting for plastic contraction in the
mold design process
If the raw material has high contraction rate, it
will result with shrinkage marks. Use low shrinkage plastic and suitable mold release agents.
Poor melt flow of the mold
Moldflow simulation can help target the right
gate size and wall thickness at the inlet
Process
Mold
Material
28 | 20 Most Common Quality Issues in Your Plastic Production
9. CRACKS
Other Names
Fissures, splits, chips
Problem Type
Aesthetic, Structural
Prevalence
Uncommon
9
After injection molding, the surface of the plastic
parts can get cracked in a number of different
ways. Although most of these will be partial cracks,
in some extreme cases complete fracture of the
material can take place.
The issues mentioned here only accounts for the
ones arising during the manufacturing process. The
solutions are intended to solve those in particular.
If you’re facing product issues related to usage
wear and tear, you might want to go back to
your design and make necessary changes before
manufacturing future batches.
29 | 20 Most Common Quality Issues in Your Plastic Production
CRACKS
TROUBLESHOOTING
Problem
Solution
Injection pressure too high or terminal injection speed too fast
Reduce injection pressure or terminal injection
speed
The pressure is too high or the hold time is too Reduce injection pressure or shorten time hold
long
time
Part ejection is too volatile
Slow down the ejection speed
The gate is too small and the shape and position are not suitable
Increase gate size, change gate shape and position
The draft Angle is not enough, the mold is not
smooth or inverted
Increase draft Angle, polish mold, lower back
pressure
The temperature of the metal insert is low
Preheat the metal insert
Too much stress on the mold
Control or improve internal stresses, try annealing treatment
Process
Mold
Material
30 | 20 Most Common Quality Issues in Your Plastic Production
CRACKS
TROUBLESHOOTING
Problem
Solution
Poor mold exhaust (trapped air)
Improve the effect of vents
Excessive humidity in the feed material
Dehydrate the resin, lower or eliminate recycled
plastic raw material usage.
Melt temperature or mold temperature is too
low/uneven
Raise melt temperature or mold temperature
(can be formed with less injection pressure) and
make mold temperature uniform
Process
Mold
Material
31 | 20 Most Common Quality Issues in Your Plastic Production
10. AIR BURNS
Other Names
Burn Marks, Gas Burns
Problem Type
Aesthetic, Structural
Prevalence
Uncommon
10
Air burns present themselves as black and brown
marks caused by overheating of the part due to
air or other gases trapped within. Proper venting
is required to release these gases and stabilize the
temperature of the melt in the process.
Venting will depend on proper number and width
of channels, and a slower, steady injection. A more
subtle, but more frequent issue is the change of
color in white pellets into a slight shade of yellow.
32 | 20 Most Common Quality Issues in Your Plastic Production
AIR BURNS
TROUBLESHOOTING
Problem
Solution
Injection speed too fast
Slow down the injection
Injection pressure too high
Reduce injection pressure
Poor mold venting
Increase size or count of vents (vacuum injection
molding)
The gate is too small or in the wrong place
Change gate or its position
Excessive clamping force (the vent becomes
smaller)
Reduce the clamping force or clamp type
The vent or is blocked
Clean the blockage in the vent
Excessive melting temperature (viscosity reduction)
Reduce melt temperature
Process
Mold
Material
33 | 20 Most Common Quality Issues in Your Plastic Production
AIR BURNS
TROUBLESHOOTING
Problem
Solution
Poor thermal stability of plastics (easy to decompose)
Process
Mold
Material
Switch to plastic with better thermal stability
34 | 20 Most Common Quality Issues in Your Plastic Production
11. EJECTOR PIN
MARKS
Other Names
Visible Ejector Mark, Pin Push
Problem Type
Aesthetic
Prevalence
Common
11
When the plastic part is ejected from the mold, the
ejector pin will often leave deep or shallow traces
on the plastic part. These traces can be shallow
enough and away from the product face. In that
case they can safely be ignored. But when these
marks are causing non-uniform, deep, asymmetric
marks on the part surface, they can cause
problems. Although one could think that ejector pin
marks are solely caused by the part sticking to the
mold and therefore it is a mold issue, in reality it’s
more closely related to the manufacturing process.
If these traces are too large, it is called visible ejector
marks. In the most extreme cases, the ejector site
can be elevated above the part surface.
35 | 20 Most Common Quality Issues in Your Plastic Production
EJECTOR PIN MARKS
TROUBLESHOOTING
Problem
Solution
Ejection speed too fast
Make ejection speed lower
Excessive injection pressure or holding time
Reduce the pressure appropriately
Terminal injection speed too fast
Slow down injection speed towards the end.
The mold temperature is too low or too high
Adjust the mold temperature appropriately
Draft angle too small
At least 0.5 degrees draft on all vertical angles
Unbalanced runner system
Artificially or naturally balance the mold.
The ejector pin is not in sufficient quantity or
position
Increase ejector pin number or change ejector
pin position
Process
Mold
Material
36 | 20 Most Common Quality Issues in Your Plastic Production
EJECTOR PIN MARKS
TROUBLESHOOTING
Problem
Solution
Air trapped in mold
Ensure venting is adequate
Thin walls where the ejector pin marks appear
Increase wall thickness where the marks appear.
The ejector pin size is too small or the ejection
speed is too fast
Increase ejector pin size or slow down ejection
speed
Process
Mold
Material
37 | 20 Most Common Quality Issues in your Plastic Production
MOLD RELATED
MANUFACTURING PROBLEMS
Mold related problems may arise either due to
bad mold design or wear and tear after many
cycles. Always make sure to run the necessary
computer simulations on the mold design before
going further with it. And ask your manufacturer
about the lifetime of your mold. While changes
in the mold design can be made later on either
through improving surface quality, adding vents,
or artificially balancing load, they are duct tapelike solutions compared to starting with the right
design from the beginning.
12.
13.
14.
15.
Drag Marks
Vacuum Voids
Gloss Differences
Gate Blush
38 | 20 Most Common Quality Issues in Your Plastic Production
12. DRAG MARKS
Other Names
Surface Abrasion, Scratches
Problem Type
Aesthetic
Prevalence
Common
12
When a part is being ejected from the mold,
sometimes its surface can suffer from abrasion if
the ejector system is not aligned or the draft angles
are not sufficient enough to remove the part from
the mold within allowed resistance. If the problem
isn’t a misaligned ejector system, in most cases the
mold needs to have some tool work done on it and
in some cases it needs to be changed completely
with a better mold design.
39 | 20 Most Common Quality Issues in Your Plastic Production
DRAG MARKS
TROUBLESHOOTING
Problem
Solution
Flashing on the side of the mold cavity
Polishing the surface of the mold cavity and removing flashes
Draft angle is not enough
Increase the draft Angle of the die cavity
The inner side of the cavity is too rough
Change rough grain to fine grain or smooth step
structure
Excessive clamping force (mold cavity deformation)
Reduce clamping force and prevent cavity deformation
Front mold temperature is too high or cooling
time is not enough
To Reduce cavity temperature or extend cooling
time
The mold opens too quickly
Slow down the mold opening starting speed
Mold locking end too fast (mold cavity impact
collapse)
Slow down the terminal locking speed to prevent the cavity from collapsing
Mold
Process
Material
40 | 20 Most Common Quality Issues in Your Plastic Production
DRAG MARKS
TROUBLESHOOTING
Problem
Solution
Excessive injection pressure or hold-in duration
Mold
Process
Material
Reduce injection pressure or hold-in duration
41 | 20 Most Common Quality Issues in Your Plastic Production
13. VACUUM VOIDS
Other Names
Internal Voids
Problem Type
Aesthetic (if transparent), Structural
Prevalence
Uncommon
Taken from 3 Space
13
Shrinkage in the interior of a plastic part is often
called a vacuum void. It is generally found in an area
where the plastic is very thick or where multiple
channels come together to form a rib or a wall.
These areas are especially susceptible to uneven
cooling or shrinking, which is the underlying cause
of voids.
When the part is cooling down after the injection is
completed, areas of thicker plastic will tend to dry
at a different rate than thin ones. If the hardness of
the plastic isn’t high enough or there isn’t enough
fiber mixed into the raw material, voids can form
within the plastic as it shrinks unevenly. What
separates bubbles from vacuum voids is the lack of
air. A simple heating test can reveal the exact issue.
A vacuum void will create a debris while a bubble
would burst outwards.
42 | 20 Most Common Quality Issues in Your Plastic Production
VACUUM VOIDS
TROUBLESHOOTING
Problem
Solution
The mold temperature is too low
Increase the mold temperature
The wall thickness is too large (i.e. the boundary of the rib or the brace of the wall)
Optimize the product structure design, try to
keep the wall as thin as possible
The gate is too small or the location of gate is
suboptimal.
Make the gate larger or change the gate location
The runner is too long or too thin.
Shorten and/or widen the runner
Injection pressure is too low or injection speed
is too slow
Increase the injection pressure or speed
Pressure drop or hold-in time insufficient
Sustain the pressure and increase hold-in time
The melt temperature is too low or short shot
Increase the temperature or increase amount of
charge
Mold
Process
Material
43 | 20 Most Common Quality Issues in Your Plastic Production
VACUUM VOIDS
TROUBLESHOOTING
Problem
Solution
Cooling time in mold is too long
Decrease the cooling time in mold or use water
bath cooling
Back pressure is too low
Increase overall back pressure on the mold
Mold
Process
Material
44 | 20 Most Common Quality Issues in Your Plastic Production
14. GLOSS
DIFFERENCES
Other Names
N/A
Problem Type
Aesthetic
Prevalence
Common
14
When the plastic part is formed, the finished
surface can lose its original luster and form an
opalescent film, or is in a fuzzy state.
The surface gloss differences between plastic parts
is mostly due to the surface state of the mold. If the
surface of the mold is not polished well or has mold
scale, the surface of the finished product will not
get a good gloss. Excessive use of release agents
or oil release agents is also a cause of poor surface
gloss. If the material is hygroscopic or contains
volatile or foreign matter mixed in (pollution), this
can also cause gloss differences.
45 | 20 Most Common Quality Issues in Your Plastic Production
GLOSS DIFFERENCES
TROUBLESHOOTING
Problem
Solution
The surface of the mold is permeated with
water or oil
Wipe clean water or oil to see if it solves the gloss
difference. Can also try washing with non-abrasive substances
The inner surface of the mold is not smooth
Polish the mold or remove remnants of the previous charge.
Mold cavity is scaly
Remove mold scale through treating it chemically or by polishing its cavity.
The density of the melt is not enough or low
back pressure
Increase back pressure/hold-in time
The charge decomposes due to overheating or
Reduce melt temperature or back pressure.
stays in the cylinder for too long
Material cools down prematurely due to small
runner and gate size
Increase the size of runner and gate
Injection speed is too slow or the mold temperature is uneven
Improve injection speed or cooling system
Mold
Process
Material
46 | 20 Most Common Quality Issues in Your Plastic Production
GLOSS DIFFERENCES
TROUBLESHOOTING
Problem
Solution
The cylinder is not cleaned
Clean the cylinder thoroughly
Excessive mold release agent in the mold
Control the amount of mold release agent and
wipe it clean
The raw material isn’t dry enough
Fully dry ingredients
Mold temperature or material temperature is
too low
Raise mold temperature or material temperature
Mold
Process
Material
47 | 20 Most Common Quality Issues in Your Plastic Production
15. GATE BLUSH
Other Names
Cloudiness, Haze, Fog
Problem Type
Aesthetic
Prevalence
Common
15
If the gate is too small and the injection speed is
too fast, the melt flows violently and pulls air into
the melt. This will leave cloudy marks in the gate
location, turning points, and step positions of the
plastic parts. It’s easiest to see gate blush at the
gate with materials like ABS, PC, and PPO.
48 | 20 Most Common Quality Issues in Your Plastic Production
GATE BLUSH
TROUBLESHOOTING
Problem
Solution
The gate is too small or improperly positioned
Increase gate size or change gate position
Excessively long or thin runners (melt is easy to
Shorten or widen runners
cool)
Air trapped within mold due to bad vent
Adjust the mold vent
Increase cooling fluid or enlarge the cooling
The cooling channel is too small or insufficient. channel, or place channel near the largest part
of the mold
Can’t figure out maximum injection temperature required with the current mold setup
Moldflow simulation can help target the right
gate size and wall thickness at the inlet
The injection speed is too fast
Multistage injection of material to slow down the
injection speed of corresponding parts
Raw materials are not fully dried or are decom- Fully dry the raw materials and don’t use therposed by overheating
moset materials
Mold
Process
Material
49 | 20 Most Common Quality Issues in Your Plastic Production
GATE BLUSH
TROUBLESHOOTING
Problem
Solution
The viscosity of plastic is high and the fluidity is
Switch to higher viscosity plastic.
poor
The melt temperature is too high or the mold
temperature is too low
Process
Mold
Material
Reduce the material temperature (to prevent decomposition) or increase the mold temperature
50 | 20 Most Common Quality Issues in your Plastic Production
MATERIAL RELATED
MANUFACTURING PROBLEMS
Low quality or adulterated plastic materials
will set a ceiling on the quality of the finished
product. Apart from causing plastic viscosity,
color quality and melting temperatures to be
unpredictable, choosing the wrong plastic
material for the job will result with part failure
in the long run. Although most problems with
material as the major culprit cause aestheticonly problems, they shouldn’t be ignored.
16.
17.
18.
19.
20.
Weld Lines
Black Specs
Silver Marks
Splay Marks
Color Streaks
51 | 20 Most Common Quality Issues in Your Plastic Production
16. WELD LINES
Other Names
Meld Line(slightly different)
Problem Type
Aesthetic, Structural
Prevalence
Uncommon
16
When two or more plastic streams meet at different
temperatures as the plastic is filling the mold cavity,
the two streams may not correctly mix together.
When this happens, a weld line is formed where
they interface.
52 | 20 Most Common Quality Issues in Your Plastic Production
WELD LINES
TROUBLESHOOTING
Problem
Solution
The plastic material isn’t fully melted or is under-drying
(1) increase the temperature of feed cylinder;
(2) Make the back-pressure higher;
(3) Increase the screw rotation speed;
(4) Make sure the material is fully dried
The material isn’t pure or contaminated with
impurities
Check the material quality to make sure it hasn’t
been exposed to external pollutants or change
to a different material
There is too much mold release
Decrease the mold release agent or try not to
use it if not needed
The mold temperature is too low
Increase the heat applied to the mold
The gate is too small or the location of gate
isn’t suitable
Increase the size of gate or change the gate location
There is excess air inside the mold
(1) make the mold vent larger at location with
weld line;
(2) Check if the mold vent is blocked or not or
use vacuum forming
The main runner and sub-runner are too thin
or too long
Make the size of runner channel wider and/or
shorter
Material
Mold
Process
53 | 20 Most Common Quality Issues in Your Plastic Production
WELD LINES
TROUBLESHOOTING
Problem
Solution
Injection speed is too low
Increase the injection speed
The injection pressure is too low
Increase pressure at the nozzle.
The cold-slug well is too small
Increase the cold-slug well size or set flash gate
at location with the weld line
Material
Mold
Process
54 | 20 Most Common Quality Issues in Your Plastic Production
17. BLACK SPECS
Other Names
Dark spots
Problem Type
Aesthetic
Prevalence
Common
17
On transparent plastic parts, white plastic parts,
or light colored plastic parts, black specs often
appear during the injection molding production.
The black specs on the surface of plastic parts can
affect the appearance quality of products, resulting
in a high rejection rate, waste, and high cost in the
production process.
The problem of black specs is difficult to avoid in
injection molding and needs to be controlled from
the raw material, crushed material, ingredients,
feeding, environment, and production processes
to reduce the spec rate. The main reason for the
black impurities to appear is that the foreign
material within the plastic melt will degrade at high
temperatures.
55 | 20 Most Common Quality Issues in Your Plastic Production
BLACK SPECS
TROUBLESHOOTING
Problem
Solution
The raw material is mixed with foreign matter
or the drying bucket is not cleaned
1. Check the raw material for dark spots;
2. The drying bucket should be thoroughly
cleaned
Color pigment diffusion is poor, resulting in
condensation point
Add diffuser or change the quality of the toner
Impure or contaminated water outlet
Good control of water outlet (best use dust free
workshop)
The raw material of the superheated melt attaches to the inner wall of the cylinder
1. Thoroughly shoot remaining melt;
2. Clean the molding cylinder thoroughly;
3. Lower melt temperature;
The mold gate is too small, injection speed is
too fast
1. Increase gate size;
2. Make the injection speed lower;
Debris in the barrel causes the material superheated decomposition
Check the nozzle, ring and feed tube for wear/
corrosion or replace the machine;
Dust falls into the air during mold opening
Adjust the wind and wind direction of the cooling fan (it is best to turn off the fan), cover the
injection molding machine with film
Material
Mold
Process
56 | 20 Most Common Quality Issues in Your Plastic Production
BLACK SPECS
TROUBLESHOOTING
Problem
Solution
The nozzle is blocked or the nozzle tip is too
small
Remove residue material or enlarge the hole
diameter
Crusher/mixer not cleaned up
Thoroughly clean up the crusher/mixer
Material
Mold
Process
57 | 20 Most Common Quality Issues in Your Plastic Production
18. SILVER MARKS
Other Names
Silver Streaks
Problem Type
Aesthetic, Structural
Prevalence
Common
Most common silver mark pattern will resemble brushed aluminum
18
Spatter lines formed on the surface of the plastic
along the direction of melt flow are called silver
marks.
The emergence of silver marks is generally due
to the rapid start of the injection which causes
the air in the melt and mold cavity unable to be
discharged quickly enough. This results in the
appearance of silver filaments on the plastic
surface. Silver marks not only affect the appearance
of the plastic parts but also reduces the mechanical
strength of them as well. The formation of silver
marks is mainly due to the gases in the melt surface
of the plastic part. By finding the source of these
gases, we can find out the methods to solve the
defects.
58 | 20 Most Common Quality Issues in Your Plastic Production
SILVER MARKS
TROUBLESHOOTING
Problem
Solution
Raw material contains water
Dry the raw material thoroughly (within the allowed moisture content)
Excessive material temperature (melting decomposition)
Lower melt temperature
Material containing other fillers (e.g. lubricant)
Reduce its usage or replace with other suitable
additives
Pigment decomposition (Pigment has poor
temperature resistance)
Choose a pigment with high-temperature resistance
Mixed raw materials or poor thermal stability
Replace raw materials or use plastics with good
thermal stability
Excessive ash content
Reduce ash content
When the molten material flows from the thin
wall to the thick wall, it expands. The volatiles
vaporize and contact with the surface of the
mold and become silver
Improved die structure design
Adjust the relationship between injection speed
and position
Material
Mold
Process
59 | 20 Most Common Quality Issues in Your Plastic Production
SILVER MARKS
TROUBLESHOOTING
Problem
Solution
The gate is too large/too small or in an improper position
Improve gate size or adjust gate location
Poor mold exhaust or low mold temperature
Improve mold exhaust or mold temperature
Injection speed too high (shear decomposition or air entrainment)
Make the injection speed lower
Air is trapped inside the barrel
De-gas the mold
Low back pressure (poor degassing)
Increase back pressure appropriately
Material
Mold
Process
60 | 20 Most Common Quality Issues in Your Plastic Production
19. SPLAY MARKS
Other Names
Splash Mark
Problem Type
Aesthetic
Prevalence
Common
19
Splay mark is a water wave pattern which can not
be removed after the part is formed. It is mostly
seen on the plastic parts with a smooth finish.
Splay marks are caused by the first melt into the
cavity cooling too fast while the hot melted plastic
is forced into the front to push the initial melt
forward, which causes the formation of water wave
lines. They can be eliminated by increasing melt
temperature and mold temperature, accelerating
injection speed and increasing hold-in times. If the
cold material remaining at the front end of the
nozzle enters the mold cavity directly, it will also
cause water ripple formations. Therefore, a cold
material well should be opened at the end of the
main channel to effectively prevent the occurrence
of the splay mark.
61 | 20 Most Common Quality Issues in Your Plastic Production
SPLAY MARKS
TROUBLESHOOTING
Problem
Solution
Poor melting and plasticization of raw materials
1. Raise the barrel temperature
2. Increase the back pressure
3. Increase screw rotation speed
The poor fluidity of melting material
Change material with good fluidity
Mold temperature or material temperature is
too low
Raise the mold temperature or material temperature
The gate is too small or at an inappropriate
position
Increase the gate or change the gate position
The flow channel is too long or too fine (melting prematurely cools down)
Shorten or thicken the runner
Slow injection speed at the splay mark
Increase the injection speed at the splay mark
appropriately
The cold material well is too small or insufficient
Increase or increase the cooling well
Material
Mold
Process
62 | 20 Most Common Quality Issues in Your Plastic Production
SPLAY MARKS
TROUBLESHOOTING
Problem
Solution
Increase pressure and pressure maintaining
time
Pressure too low or time too short
Material
Mold
Process
63 | 20 Most Common Quality Issues in Your Plastic Production
20. COLOR STREAKS
Other Names
Color differences
Problem Type
Aesthetic
Prevalence
Common
20
The surface of the plastic part or the part where the
flow direction changes will produce a local color
deviation phenomenon, called color streaks.
There are many reasons for color streaks including
uneven pigment diffusion during the injection
molding process, incomplete cleaning of the barrel,
mixture with other colors in the raw materials, the
unstable proportion of recycled material, and poor
melt plasticization.
64 | 20 Most Common Quality Issues in Your Plastic Production
COLOR STREAKS
TROUBLESHOOTING
Problem
Solution
Improper melt plasticization
Improve plasticity with additives or plastic quality
Pigment clumps or poor diffusion
Grind or replace the pigment
Too little dispersion aid
Properly increase the dosage of the dispersant or
replace it
The compatibility of plastics and pigment is
poor
Replace the plastic or pigment in masterbatch
Low material temperature or low back pressure
Increase material temperature, back pressure,
and screw rotation speed
Unclean cylinder (containing other residues)
Thoroughly clean the cylinder
The inner wall of screw rod and feed tube of
injection was damaged
Repair or replace damaged screw/barrel or machine
Material
Mold
Process
65 | 20 Most Common Quality Issues in Your Plastic Production
COLOR STREAKS
TROUBLESHOOTING
Problem
Solution
Residual melt is stuck on the head of the nozzle
Material
Mold
Process
Clean the residual melt on the outside of the
nozzle
66 | 20 Most Common Quality Issues in Your Plastic Production
AVOIDING FUTURE PLASTIC DEFECTS
We have seen many products failing to
scale due to improper handling of their
manufacturing.
That’s one of the biggest reasons why we prefer not
to jump stages in our NPI process. Because every step
starting form prototyping a product can offer valuable
information about the production of its parts.
If you’re looking to scale, we advise you to pay attention
to the issues discussed in this guide. If you are still at an
early stage, and you don’t have a dedicated manufacturing
team, you should consider working with a contract
manufacturer to avoid costly mistakes.
TAKEAWAYS:
•
Do your research on Failure Mode Identification
It is not realistic to expect 0 plastic defects hundred
percent of the time. Identify Failure Modes based
on how critical they are and focus on fixing critical
problems first.
•
Conduct Design Validation Testing(DVT)
Manufacture at least 40 DVT samples and test them
for design-related product defects along with housing
fit.
•
Run Moldflow Analysis
Most of the time you won’t be designing molds inhouse. Generally the factory’s takes care of that. But
that doesn’t mean you shouldn’t be involved in its
design process. Ask your factory to run Moldflow
Analysis on the proposed mold design before you take
them to manufacturing.
NexPCB has a network of
900+
verified suppliers at your service
MANUFACTURE PLASTICS
AND MORE WITH NEXPCB
NexPCB’s distributed supply chain makes it accessible for startups and growing businesses to
manufacture in world-class facilities. We eradicate most problems that you could face before they
happen thanks to a professional project management team dedicated for every order.
CONTACT US
Bldg.1, Floor 3 Linghang Innovation Park Zone B, 428 Dongqing Road
Ningbo, China
www.nexpcb.com