Injection Molding & Advanced
Process Control
Injection Molding &
Advanced Process Control
SPE Automotive TPO Global
Conference 2009
1 - BasM
Polymer Basics
2 - BasM
Material selection is part of the design process and can directly cause failure
A key element in a successful plastic part program is the selection of the
appropriate material for the application.
The process of material selection is complicated by the fact that there are literally
thousands of polymers, blends, alloys, and compounds available.
The selection process can be simplified by comprehending:
 The performance requirements of the part
 How materials are classified
 Testing methods
3 - BasM
POLYMERIZATION PROCESS
Polymer:
 From the Greek Poly meaning many, and Meros (mer), meaning part(s)
Chemical Composition: Example
 Polyethylene: Crack crude oil to ethylene then polymerized (linked together) to form
500-1,000,000 ethylene units
Silo
Silo
Esterifier
1°
Feedstock:

Transfer
System
Esterifier
2°
Crude Oil

Natural Gas

Agriculture
Manifold
Extrusion
High
Polymerizer
Low
Polymerizer
4 - BasM
Pump
Polymer:
 From the Greek Poly meaning many, and Meros (mer), meaning part(s)
Chemical Composition: Example
 Polyethylene: Crack crude oil to ethylene then polymerized (linked
together) to form 500-1,000,000 ethylene units
Key Points:
 Polymers can have the same carbon backbone but drastically different properties
 Polymers with Oxygen (O), Nitrogen (N), Chlorine (CI), Bromine (Br), or Fluorine
(F) probably have to be dried
Time and space do not allow us to review each polymer composition and its
properties. An excellent source for basic polymer descriptions can be found
in the first pages of the yearly Modern Plastics Encyclopedia.
5 - BasM
Chain Lengths
 At room temperature
 C1
= Methane, a gas
 C6
= Hexane, a liquid
 C17
= Kerosene
 C50
= Wax
 C500-1,000,000 = Polyethylene
For strength
 Long molecules are preferred
For processability
 Short molecules are preferred
 Easy flow
 Low viscosity
 Transmits pressure more hydrostatically
 Long molecules
 Difficult to flow
 High viscosity
 Viscosity changes dramatically with changes in flow rate
 Dynamic: flow rate during filling
 Static: flow rate during pack and hold
 Large static pressure losses along the flow path
Resin manufacturing must compromise/balance chain length vs. strength vs. flow.
Most polymers that find commercial use as plastics or rubbers have molecular weights
between 10,000 and 1,000,000.
6 - BasM
Molecular Weight (Mw):
 Molecular Weight = (Molecular Weight of Repeat Unit x Number of Units)
 In practice, when polymers are made, chains or the number of units linked
together varies
 Within the same polymer family molecular weight is a measure of
molecular length:

High Molecular Weight = Long Molecules = Stronger

Low Molecular Weight = Short Molecules = Weaker
7 - BasM
Molecular Length
 Polymers are mixtures of various length molecules
 Not all chains are the same length
 Molecular weight distribution
8 - BasM
Batch to Batch Variations Cause Changes in Viscosity
 Effects of Molecular Weight
 Generally as molecular weight increases, physical properties increase
at the expense of processing ease
 Short chains - easy flow
 Long chains - difficult to flow
9 - BasM
Polymer Chain Degradation
 Polymer chains are strong
 Aramide fibers (flack jackets, tires)
 Chains can be broken by improper processing
 Too much heat
 Too much time at elevated temperatures (drying and melt)
 Too much shear force
 Presence of a chain-breaking chemical (e.g., water)
 Polyester
 Polycarbonate
 Nylon
 Proper drying is essential for these materials
10 - BasM
Degradation
 Improper processing can degrade polymer chains and provide
poor part performance
 Chain degradation will provide a regrind that is easier flowing
(lower viscosity)
 Shorter molecules = poor strength
How can this be detected?
11 - BasM
Polymers
Plastics
Plastics = Polymer + Additives
Additives:







Impact modifiers (rubber)
Fillers
Nucleating agents
Flame retardants
Mold release agents
Antioxidants
Wetting agents







Plasticizers
Fiber reinforcements
Heat stabilizers
Lubricants
Antistatic agents
Coloring agents
Slip aids
12 - BasM
Additive Degradation:
 Additive chain size is small relative to the polymer
 May become volatile during processing which can
result in:
 Accumulation in vents, mold surfaces, etc.
 Change in physical properties (often an
increase)
 Stiffer flow (higher viscosity plastic)
Change in Additive Content is a Prime Difference
Between Virgin and Early Generations of Regrind.
Material is going to vary folks –
that's no excuse!
13 - BasM
Polymer Morphology
Two families of thermoplastics
Amorphous







Random Structure
Broad Melting Point
Often Solvent Sensitive
Poor Fatigue/wear
Low Shrink
No Molecular Cuddling
More Forgiving on dimensional
Control
Semi-crystalline







Ordered Structure
Sharp Melting Point
Solvent Resistant
Excellent Fatigue Wear
High Shrink
High Molecular Cuddling
Problematic on
dimensional Control
14 - BasM
Cooling and Crystallinity
 Extremely important for semi-crystalline materials
 Crystals are non-existent above melting temperature
 Low mold temp, faster cooling, smaller/less crystals, less shrinkage, but may
not be suitable for application
 High mold temp, slower cooling, larger/more crystals, more shrinkage
 Mold and Melt temperatures are critical to maintain dimensional tolerances
15 - BasM
Decoupled Molding Strategies
SM
16 - APT
Train2.jpg
Injection Molding Approaches
Traditional Molding:
 The original method of molding
Train1.JPG
 Fill and pack on first stage
 Use only enough first stage pressure to pack the mold
DECOUPLED MOLDINGSM:
Train2 a fill pack only.jpg
 Use abundant first stage pressure
 Separate the filling or velocity phase of molding from the packing
or pressurization phase
17 - AF
DECOUPLED MOLDINGSM
Definition:
A process control method that addresses how the machine controls are
used to fill and pack plastic into the mold.
Objectives:
 Injection speed reacts only to the machine’s velocity setting(s)
 Constant and repeatable injection speed shot to shot, year to year,
regardless of any effective viscosity changes
 Reduce part variation due to effective viscosity variations
 Gain the ability to fill fast to minimize effective viscosity variations
 Make the process capable of always making good parts
 Reduce cycle time due to increased fill rates
Train2 a fill pack only.jpg
18 - AF
Plastic Temperature
 Screw Configuration
 Barrel & Nozzle Heats
 Screw R.P.M.
 Back Pressure
 Feed Throat Condition
19 - APT
The Screw
Barrel 1a 001
Inside the barrel is a screw.
Most injection molding screws are divided into three zones each with its own task:
 Move plastic - Feed Zone (conveying)
 Melt plastic - Compression Zone (transition)
 Mix plastic - Metering Zone
The difference between the zones is in the channel depth which creates a space
for plastic.
The channel depth on the feed end of the screw is larger than the depth of the
metering end. The ratio of depth difference is called Compression Ratio.
Example - 3:1 Compression ratio screw means the channel depth in the feed
zone is three times larger than the channel depth in the metering zone.
When the screw is rotated, drawing plastic into the barrel, it is conveyed forward
and compressed. This action converts screw work into shear heat.
20 - APT
Compression Ratio
The proper compression ratio screw for the application is a key component for
proper plastic melting and mixing.
Low - 1.5 to 2.5:1 - Shear sensitive materials (PVC)
Medium - 2.5 to 3.0:1 - General purpose
High - 3.0 to 5.0:1 - Crystalline materials (Nylon)
How do you know if you have the correct
compression ratio for your job?
What other features of a screw are important?
21 - APT
Screws come in different lengths and diameters. The flight length in relationship to the outside
diameter of the flights is called the L/D.
 A high L/D indicates a long screw relative to its diameter; therefore, the plastic will have a greater
distance to travel from the feed port to the nozzle
 The time to travel this distance is called the residence time
 Conversely a low L/D is a short screw which will provide faster throughput and a lower residence time
The profile of a screw describes the number of turns in each zone from the feed zone through the metering
zone.
Example: 10-5-5 (20/1 L/D screw) = 10 turns feed, 5 turns transition, 5 turns metering
 The exact screw type and configuration needed depends on the application; however, generally
speaking:
 Amorphous resins need long transition sections
 Crystalline resins need long feed sections
What is a good guide for residence time?
How can we know what it is?
22 - APT
Mixers
Mixing Screw
Mixing Screw.jpg






Breaks up tornado-like flow of melted plastic in the screw flights
Is placed in the metering zone to mix melted plastic
Must not cause high localized shear
Provides more uniform melt temperature
Provides more uniform color dispersion
Faster purging / quicker color change
23 - APT
SCREW RPM: Should be as slow as possible without
increasing cycle time.

Reduces maximum and overall shear rate

Shears material more uniformly

Reduces fiber breakage in filled material

Provides more melt homogeneity

Think circumferential speed when moving to another machine
Screw RPM determines the maximum shear rate of the plastic
24 - APT
25 - APT
Back Pressure




Injection cylinder works as a pump pushing oil back to tank
Changes the amount of heat added by shear
Changes temperature of the melt quickly (temporary)
The change will not be totally sustained unless zone temperatures are
also changed
 Changes the amount of shear mixing or homogenization
 May increase fiber breakage in fiber-filled materials
26 - APT
 Feed throat temperature effects melting and moisture splay
 Must be cold enough to prevent clumping and hot enough
for dew not to form (105 °F or 35 °C minimum)
27 - APT
1.
Set all zones at the desired melt temperature
2.
Run the machine on cycle
3.
Take the melt temperature
4.
Adjust the center and rear zones to achieve desired melt temperature
5.
This may not be possible on machines with an incorrect screw for the
material being molded
If screw recovery problems exist, do step 6
6.
Set the rear zone to make the plastic stick to the barrel and to obtain
faster screw recovery (temperature at which minimum screw recovery
time is achieved with a constant RPM)
28 - APT
Plastic Flow
Injection Speed and Distance
plasticbarrel1 - 2004.jpg
29 - AF
During filling two things must be available:
 Pressure in abundance (Ri)
 Flow sufficient to achieve the desired flow rate (pumps or accumulators)
plastic flow rate key - 2004.jpg
On electric machines available plastic pressure and injection rate are
analogous to Ri and flow rate
30 - AF
Force can be intensified in a hydraulic system


The determining factor for force intensification is the square inch area on which the
hydraulic pressure is applied
If the piston in a cylinder has more area than the area where the force is being applied then
the output force will be greater than the input force
Pressure increases inversely proportional to the area ratios.
A1
=?
A2
psip?

Force=Pressure a 001.jpg
For example, if two pistons of different size are connected by a rod, the pressure existing
on the smaller area will always be greater
 This principle also applies to the cap side and the rod side of a normal double
acting piston
32 - AF
33 - AF
NonNewtonian.JPG
34 - AF
35 - AF
THE RUNNER
The main purpose of the runner is to distribute the plastic to all the cavities in such a way as
to fill all the cavities at the same shear rate and direction, at the same temperature, and
at the same pressure gradient.
Good
Best
Better
Better
Two are almost
as good
36 - AF
Hot Runner Manifold System - 2 Drops
The runner can be kept liquefied through the use of a hot runner manifold.
The mold a1 - 001.jpg
37 - AF
If the mold does not treat the plastic material the same in each
cavity. This fact may render the process uncontrollable.
This is why no mold can be more controllable than a single cavity
mold.
38 - AF
How can we complete the fill of these two glasses
at the same rate (time)? What must be adjusted?
41 - AF
Heated systems require the use of a valve gate to seal the gate.
Of critical importance is the number of gates and their location.
What is the primary reason for using a valve gate?
42 - AF
Complex 8-Cavity Independent
Always remember that the weld lines are the weakest area of a plastic part
 Varies with material
 Difference in strength is greater in fiber-filled material
 Always determines the strength of screw bosses and depressions where
knit lines are present
 Improper venting enhances the effect
 Weld line strength varies with how the part is packed
Most part failures occur at the weld lines
48 - AF
Dynamic Pressure Loss
16,490 psip
9,628 psip
8,345 psip
Parts psi a 002
5,245 psip
49 - AF
DECOUPLED MOLDINGSM
Decouple injection speed from first stage hydraulic pressure.
 Make sure abundant hydraulic pressure is available to the injection cylinder - at least 10% more than the peak
hydraulic pressure observed at the injection cylinder gauge
 Use the speed control to exclusively control injection speed

Any adjustment of fill speed should change the fill time

The peak hydraulic pressure must not reach the maximum setting
 Separate filling the cavity from packing and/or holding
 Fill to 95-99% full (not packed)
Fill at a constant “CONTROLLED” rate.
 Fill time should remain constant
 Peak hydraulic pressure should vary as viscosity varies from all causes
Fill as fast as far as possible.
 May need to be relatively slow

Lens molding

Shear sensitive material like PVC

Poor mold venting (not acceptable in the long run)
The best fill set up fills the mold quickly with as few speeds as possible to make good parts.
 Sometimes a slow start is necessary

Jetting in hot runner molds

Reducing some types of splay or blush at the gate
Cut off or end filling by:
 Stroke
 Cavity pressure
Don’t flash!
 Speed doesn’t flash a properly built, properly clamped mold
 The sudden, complete filling of the cavity flashes the mold
50 - AF
How many programmed injection steps are required?

As few as possible
 Normally no more than three (3)
 Ten (10) is overkill
How should these programming steps be
determined?


Equal divisions over the shot size
Each step size determined by the set-up person
 Preferred
 Fewer steps
 Simpler setup
 Should be able to cancel unused steps (KIS)
 Should be able to change overall fill speed with
one command
Injection Steps.jpg
51 - AF
Fill
Purpose:

To fill the cavity and produce part(s) that are 95-99% filled
Goal:

Fill as fast as possible under control

Separate speed from pressure

Have adequate pressure available
During Fill:

The injection unit is brought forward so the nozzle is in contact with the mold sprue
bushing

The moving platen is brought forward, the mold halves meet and clamping force is
built

When the proper force is built, oil is directed to the injection cylinder, pushing the
piston forward

Since the screw is attached to the piston in the cylinder, the screw is also pushed
forward

As the screw moves forward, without rotation, the ring on the non-return valve is
pushed back into the closed position and the screw acts as a plunger, pushing
plastic out of the barrel
52 - AF
The material is forced through the nozzle of the machine and into the
part of the mold called the sprue bushing.


The stream of melt can be divided and distributed into different areas of
the mold via the runner system
The plastic then flows through the gates and across the cavity(s)

At the end of fill, the part is 95 to 99% filled, or “short”
Speed of fill can be “constant” or
“programmed”. First stage pressure
setting must never be reached.
53 - AF
Plastic Pressure
54 - APG
55 - APG
56 - APG
Plastic Pressure Gradient
(Static Not Dynamic)
Pack and Hold
 Packing Pressure Over Time
 Packing Velocity to a Cavity Pressure
 Holding Pressure & Time
57 - APG
Decoupled ІІ Molding
 Objective: fill the mold fast, transfer by screw position when the
cavities are 95-98% full. The ram inertia is used up just before
the cavities fill out, and 2nd stage hold pressure is used to
complete the filling and pack out the parts
58 - APG
59 - APG
Example of Decoupled II Molding
(2-Stage Control)
Better
But Not
The Best
What effect will a +/- 10% viscosity change have inside the mold?
Can parts be contained if a viscosity change happens which
requires re-centering the process?
How?
61 - APG
62 - APG
Decoupled II Process
What is the post gate peak cavity pressure variation? __________
23.76%
What is the end of cavity peak cavity pressure variation? __________
34.10%
Formula: ((High - Low) divided by Original) X 100 =% change
63 - MM II ALM
Plastic is compressible 1/2 to 3/4
percent per 1,000 psip.
This means overall dimensions will change on average 1/2 to 3/4 percent per
1,000 psi of plastic variation.
Other factors:
 Direction of flow vs. transverse to flow direction
 Long fillers such as glass
 Crystallinity
Controlling pressures
in the mold cavity is the
key to minimizing part variation.
REMEMBER:
A short shot is zero pressure in the cavity
at the end-of-cavity.
64 - APG
Group Practice
Post Gate psip 12,000
Pressure Loss
10,000
.500
± .002
Material
EOC 2,000 psip
Highly Nucleated Polypropylene
Morphology
Crystalline
+10% viscosity change
What is the Pressure Loss?
11000
What are the New Dimensions? .495
Crystalline
+20% viscosity change
12000
Short
ABS
Amorphous
-10% viscosity change
11000
.4975
65 - APG
Pressure
Cause

Short Shots

Flash

Sinks

Dimensional Variations

Warp

Gloss Gradient

Strength

66 - APG
Decoupled III: Three stage molding
 Objective: fill the mold fast, profile by screw position. When the cavities
are 85-95% full, transfer to a slow, controlled velocity pack stage.
Packing is complete when the cavity pressure or screw position transfer
completes packing the part(s)
67 - APG
Decoupled III Molding
68 - APG
Process Pivot Point 2a.jpg
69 - APG
Example of Decoupled III Molding
The
BEST!
What effect will a +/- 10% viscosity change have on the
end of cavity pressure of this mold? How much better is
this than Decoupled II in the previous example?
70 - APG
Decoupled III Process
2.82%
What is the post gate peak cavity pressure variation? __________
What is the end of cavity peak cavity pressure variation? __________
11.81%
71 - APG
Mold Cooling
How do we know when cooling
in a mold is adequate?
72 - AC
Cool The Plastic
 Design for mold temperature uniformity
 The hottest spot in the cavity determines the cycle time
 Locate coolant lines to adequately cool critical area
 Inside sharp corners
 Around the gate, runner and sprue
 Thick part sections
 Cooling lines should be in the mold inserts
 Sliding cores, as well as lifters, must be cooled
 Coolant channels and connectors should be standardized
 All coolant circuits should have provisions to detect flow problems for
process control
COOLING IS THE LONGEST PORTION OF THE MOLDING CYCLE
73 - AC

How do we know when the cooling or heating system in our
home is adequate?
When every area is at the temperature we want it to be

We are filling
something with
heat, then
removing it.

In order to optimize cooling we must understand what limits it

Heat and heat content

Heat conductivity and flow
74 - AC
Heat Content of Each Shot
This total heat content multiplied by the shot weight is transferred
into the mold each shot.
The mold is
a leaky heat
bucket.
75 - AC
If the cycle is interrupted heat continues to flow out of this leaky heat bucket
called a mold until the mold reaches room temperature.
During startup heat flow starts when the plastic contacts the surface of the mold.
The temperature difference between the plastic and the mold is the thermal
pressure causing heat flow from the plastic into the mold.
To understand how fast heat can flow, we must understand thermal conductivity
and thermal impedance or resistance.
76 - AC
Heat Flow

Heat flows through matter like water seeps through soil

Temperature difference is a measure of thermal pressure

In the winter the temperature difference between the inside of a wall and
the outside is the thermal pressure making heat flow out through the wall

The ability of a material to conduct heat is called it’s thermal conductivity
77 - AC
To compare polypropylene to tool steel:
21 .070 (from Table 5) = 300 times more conductivity
12” tool steel size 300 of conductivity = .040” thickness of polypropylene
12” x 1 = Equivalent Thickness
300
BTU’s have a hard
time passing
through plastic or
air.
To compare tool steel to air:
21 .014 (from Table 5) = 1500
12” 1500 = .008” of dead air space has the same thermal impedance as 12” of tool steel
78 - AC

A dirty mold surface or reduced cavity pressure can reduce cooling rates by
as much as 20%

A mold with scale in cooling lines can reduce cooling rate by as much as 50%
Scale
79 - AC
Plastic Cooling
General piping rule:

Use the fewest flow circuits that will provide turbulent flow
in all channels with a maximum coolant T between the
inlet and the outlet of 4°F (2°F on critical jobs)
Monitor or control the flow in all parallel flow channels

Without turbulent flow in the channels, the outer layers of
coolant near the walls insulate the center of flow and
efficiently reduce the volume of fluid available to carry the
heat away
80 - AC
The ability to transfer heat is increased with turbulent flow as shown below:
Heat transfer can be done without turbulent
flow. More turbulent is better than less.
81 - AC
The following tables show the flow necessary in commonly used mold cooling lines to
obtain turbulent flow.
FLOW THROUGH MOLD COOLING LINES FOR TURBULENT FLOW:
Flow in GPM for water to obtain turbulent flow:
RN =
3160 x GPM
VD
5000
Where V = Kinematic Viscosity
(Centistokes)
D = Diameter in inches
Note: Twice the flow rate is needed for 50/50 ethylene glycol/water mix
for turbulent flow.
82 - AC
The tables below show the flow necessary in commonly
used mold cooling lines to obtain turbulent flow:
Temperature / °F
32° F
50° F
100° F
140° F
180° F
200° F
Kinematic
Viscosity
Water
1.79
1.31
0.69
0.47
0.35
0.31
50/50 Water
Ethylene
Glycol
3.6
GPM NEEDED FOR TURBULENT FLOW:
Note: Twice the flow rate is needed for
50/50 ethylene glycol mix for turbulent flow.
Pipe Size
Temperature
I / D Inches
140° F
100° F
50° F
32° F
1/8
5/16
0.25
0.50
0.75
1.25
1/4
7/16
0.40
0.70
1.25
1.50
3/8
9/16
0.50
0.75
1.50
2.00
1/2
11/16
0.60
1.00
2.00
2.50
83- MMI
Note that ethylene glycol increases the viscosity and doubles the flow
necessary to achieve turbulent flow.

Ethylene glycol should not normally be used in cooling circuits but it is OK
if abundant capacity exists. (Reduces efficiency)
Thermal conductivity of coolant changes with temperature.
Viscosity of coolant changes with temperature.
Calculating the potential heat transfer coefficient is complex and time
consuming and makes understanding the finer points of heat transfer at the
coolant interface difficult.
Computers Programs Make It Easy!
84 - AC
When possible, use higher flow rates and coolant no colder than 50°F to obtain
required cooling.

System is more efficient

Eliminates need for use of ethylene glycol
Remember the potential cooling capacity depends on the total cooling impedance
of the system which includes:

The plastic

The plastic mold interface

The mold material

Any air gaps

The scale or lack of scale in the cooling channels
All must be optimized and maintained for continued constant cooling rates and
times.
85 - AC
How to pipe a mold:
Strategy

Treat each cavity the same

Separate cooling circuits for each cavity or:

Use enough flow so that the temperature difference between in and out
will create essentially uniform temperature throughout the mold

Pipe the mold the same each time




Same size lines
Same length lines
Same size disconnect fittings
Same manifold position to each mold circuit
86 - AC
Critical dimensions for cooling channels are:
 Diameter (should they be round?)
 Depth (the average distance from the center of a cooling channel
to the surface of the part)
 Pitch (the average distance between the cooling channels)
As in a building, the criteria for judging if the
environmental system is adequate
is if all areas are at the desired temperature for
the intended use or result.
87 - AC
Temperature Map of the Molded Part
300°
280°
+
+
190°
+
70°
160°
210°
+
+
+
88 - AC
How should cooling channels be connected?
Series
 Uses least coolant
 Has largest pressure loss
 Largest temperature differential
 All channels get the same
coolant flow
Parallel
 Provides maximum cooling if
coolant is available
 Uses the most coolant
 Lowest pressure loss
 May waste resources
 Flow channels with highest
restrictions get least cooling
89 - AC
Combination of series and parallel
 Best: A balance of the two
90 - AC
This allows easy detection of
cooling problems.
 Blocked line
 Insufficient available coolant
 Coolant interruption
 Coolant line breaks (auto shut-off)
Coolant can be prioritized to
areas where it is needed.
 Hard to cool areas
 Smaller lines
 Areas far away from coolant
surface
Q = KA P
If a line is plugged, piped wrong, or
the flow to the manifold from the
central system changes, then the P
or pressure difference will change.
For a given flow rate Q there is a
specific P if all channels are open. If
not, A changes and P will change.
K = A flow constant
A = Area of all flow channels
91 - AC
Trouble killing
92 - Troublekilling
Short Shots
Plastic’s Point of View
Shorts shots mean zero pressures at the end-of-fill. There was no
packing phase in the cycle. If no machine changes of pressures or flow
rates were evident, this generally means a viscosity change has occurred.
The viscosity is increased to a condition where the plastic will no longer
flow.
93 - Troublekilling
Short Shots, cont.
Causes for Short Shots
Possible Corrective Action
Plastic temperature is too low.
Measure melt temperature using the 30/30
approach.
Injection rate is too slow
Monitor fill time and compare it to correct fill
time.
During fill, insufficient injection
pressure was available to maintain
fill speed set.
Raise first stage pressure.
During fill, pressure is abundant
and fill time is too slow.
Check for nozzle obstruction.
Purge into the air and observe the injection
pressure.
Material is too high viscosity.
Change material.
Increase pack and hold pressures to
compensate for material increase in
viscosity.
94 - Troublekilling
Splay
Plastic’s Point of View
Possible Corrective Action
Splay is plastic road kill!
Dry material.
Splay is a gas or liquid disbursed over the
surface of a plastic part and manifests
itself as tracks. Splay can be caused by
several liquids or gases which can be
moisture from undried material, trapped
air, degraded polymer molecules, or
degraded additives. Splay can be
generated by moisture condensing on the
surface of the mold and smearing across
the mold surface by the plastic flow.
Increase back pressure to remove trapped air.
Change the screw type to remove trapped air.
Check material temperature and residence
time to eliminate molecular degradation and
additive degradation.
Keep moisture from condensing on the mold
surface.
Remove sprue break.
Reduce or eliminate decompression.
Check L/D of screw; if lower than 16:1, try
higher L/D or different screw design.
95 - Troublekilling
Flash and Short Shots at the Same Time
Plastic’s Point of View
Possible Corrective Action
This generally indicates
a change in the
pressure distribution
during fill due to a
dramatic viscosity
change. It can also be
caused by insufficient
clamp tonnage.
Check clamp tonnage availability.
Check viscosity (Injection Fill
Integral).
Check fill time.
Check melt temperature using
30/30 approach.
Check mold temperature.
Determine that the material is
correct.
96 - Troublekilling
SINKS AND VOIDS
Plastic’s Point of View
As material cools and shrinks in a mold, insufficient packing will become evident
as either sinks on the outer portion of the part or voids in the center. Sinks and
voids are evident in thick sections which are the last to cool or in areas
extremely remote from the gate or very near the gate. Classic sinks in thick
areas and away from the gate indicate insufficient packing and possibly
increased viscosity of material. Sinks near the gate generally indicate lack of
gate seal and possibly decreased viscosity, which is many times due to
increased plastic temperature causing lack of gate seal. Decreased packing
generally causes sinks away from the gate while increased packing can cause
sinks at the gate end of the part if the increased pressure in the part causes the
gate discharge after injection.
97 - Troublekilling
Causes of Sinks and Voids
Possible Corrective Action
Increase in viscosity of plastic
Improper melt temperature check using 30/30 approach.
Packing and holding pressures
are too low (end of fill sinks
and thick sections only).
Raise pack and/or hold
pressure.
Injection time is too short to
allow gate seal.
Increase injection
forward/hold time.
Mold temperature is too high to
affect gate seal.
Lower mold temperature.
Voids are sometimes mistaken
for bubbles. Bubbles are
trapped gas while voids are
vacuum voids due to a lack of
material during cooling. SEE
BUBBLES for further
information.
98 - Troublekilling
Dimensional Variations
Dimensional variations in plastics are caused by changes in the pressure
distribution throughout the cavity and, for semi-crystalline materials, changes
in cooling rate. Dimensional variations in both types of material can also be
caused by changes in the post mold, cooling, and stabilization environment. In
order to best analyze dimensional variations, the problem should be
specifically categorized as on the next page:
Good Part Template
99 - Troublekilling
Dimensional Variations, cont.
Good Part Template
Part is too small all over:
Plastic’s Point of View
Possible Corrective Action
Plastic pressure throughout the
cavity is too low.
Increase packing pressure to obtain
required in-mold pressures.
100 - Troublekilling
Dimensional Variations, cont.
Part is too large all over:
Plastic’s Point of View
Possible Corrective Action
The pressure distribution
throughout the cavity is too high.
Reduce packing pressure to
obtain cavity pressure required.
101 - Troublekilling
Dimensional Variations, cont.
Part is too small near the gate:
Plastic’s Point of View
Possible Corrective Action
The effective plastic
pressure near the gate is
too low while the pressure
in the rest of the part is
okay. This generally is
caused by the gate being
unsealed.
Increase injection forward/holding
time or determine the root cause,
such as possible increase of plastic
temperature.
102 - Troublekilling
Dimensional Variations, cont.
Part is too large at the gate:
Plastic’s Point of View
The pressure distribution in
the mold at the gate end is
too large; the rest of the
pressure is okay.
Possible Corrective Action
Reduce packing pressure; possibly
change the pack rate if possible.
Check melt temperatures; may be too high.
Check material viscosity (Injection Fill
Integral).
Allow discharge by reducing injection
forward/hold time.
103 - Troublekilling
Dimensional Variations, cont.
Part is too small at the end of cavity:
Plastic’s Point of View
Pressure distribution is okay
at the gate but is reduced
from normal at the end of fill.
This generally indicates a
change in plastic viscosity.
Plastic’s Point of View
Check fill time.
Check melt temperature using 30/30
approach and adjust as appropriate.
Check viscosity (Injection Fill Integral).
If viscosity is too high, increase fill
speed until the viscosity is correct.
104 - Troublekilling
Dimensional Variations, cont.
Part is too large at the end of cavity:
Plastic’s Point of View
This means that the pressure
distribution in the cavity is okay
at the gate and too high at the
end-of -fill. This indicates a
lowering of viscosity.
Possible Corrective Action
Check melt temperature by using
30/30 approach.
Check fill time.
Check viscosity (Injection Fill
Integral). If viscosity is too low,
reduce injection speed until viscosity
105 - Troublekilling
number is correct.
Dimensional Inconsistency
Plastic’s Point of View
Possible Corrective Action
Inconsistent dimensions mean inconsistent
pressure gradients throughout the part. This
could indicate that on some shots, the gate is
sealing or not sealing. Otherwise the pressure
distribution in the cavity is varying on a shot-toshot basis. If this is the case, check to see if
there are large variations in parts shot-to-shot
or if there are trends over time. Trends over
time indicated temperature variations or
material lot variations, whereas shot-to-shot
inconsistencies indicating packing fluctuations
over short periods are either caused by gate
seal changes or pressure changes due to
check ring leaks, etc.
Run a series of short shots to check the
check ring consistency.
Monitor hydraulic pressure consistency.
Do a gate seal analysis and possibly
increase injection forward time.
Monitor viscosity fluctuations (Injection Fill
Integral).
Monitor in-mold pressure to determine
variability.
106 - Troublekilling
Warp
Plastic’s Point of View
Warp is an inconsistent deformation of the part making it nonconforming to the shape of
the cavity. This is generally caused by some kind of stress induced in the part during
filling, packing, or cooling. Warp is a complex phenomena and is often caused by a
summation of many forces, a few of which are dominant.
In amorphous material, the effect of crystallinity does not exist. Therefore there is an
extra dimension in the crystalline or semi-crystalline materials. Parts filled with long thin
fibers, such as glass, have another factor; the orientation of which is not present in
unfilled materials.
107 - Troublekilling
Warp, cont.
Crystalline Materials
Possible Corrective Action
In crystalline materials, a large portion of warp is
crystallinity induced, caused by non-uniform cooling.
In amorphous materials compressive stress
gradients caused by non-uniform packing pressures
throughout the part are generally predominant. In
addition, the orientation stresses caused by flow and
subsequent relaxation of the stresses during cooling
can cause non-uniform stresses in parts. In
analyzing warp problems, it is important to
categorize them as crystalline or amorphous, fiberfilled or unfilled, and then proceed. In the semi
crystalline material, comparing the first shot out of a
cold or uniformly-temperatured mold to one run later
as the mold warms up will give an understanding of
whether or not this is the problem. If the first part out
of the mold is not warped, then the issue of nonuniform cooling is probably the predominant
situation.
Redesign the cooling to minimize hot spots
and cold spots in the mold.
Change the material to an amorphous resin.
Redesign the part to a more rigid structure
using complex shapes or ribs (ribs are least
desirable).
Another quick check is to run the part using an
amorphous material. Many times an easy flow ABS
can be substitutes for polypropylene. If the ABS part
does not warp and the polypropylene part does,
crystallinity issues are probably the cause.
108 - Troublekilling
Warp, cont.
Plastic’s Point of View
Crystalline Materials con’t.
Analyzing warp in parts made of high aspect (long)
fiber-filled materials involves comparing parts
molded with the fiber to that of the unfilled resin to
determine the degree of change. Flow during fill
orients fibers in the direction of flow, but they do not
de-orient as many molecules do during cooling.
Most often, warp caused by orientation of fibers can
only be corrected by changing the flow directions in
the part or changing the design of the part.
109 - Troublekilling
Warp, cont.
Amorphous Unfilled Materials
Possible Corrective Action
Generally, amorphous warp is caused by a combination
of orientation stresses and a packing stress gradient. A
packing stress gradient can be reduced by reducing
viscosity, generally by increasing fill rates and/or
temperatures.
Increase fill speed.
Decrease fill speed.
Increase melt temperature.
Increase mold temperature.
Also, when Decoupled III is being used, optimizing
pack speed can minimize the packing stress gradient.
Orientation stress can be reduced by raising the plastic
temperature, slowing the fill, and/or reducing the
cooling rate. If the packing stress gradient is the
predominant factor, increasing speed will generally
reduce warp. If orientation stress is the primary effect,
increasing speed will cause the warp to worsen.
Another important factor is whether or not the gate is
sealed. Many times the packing stress gradient can be
reduced by intentionally not sealing the gate which will
reduce the compressive stress gradient by allowing
back flow, thus making the part flat. This is especially
true in center-gated parts, both semi-crystalline and
amorphous.
Reduce injection time to allow gate to
back flow or discharge.
110 - Troublekilling
Knitline Weakness or Cosmetic Problems
111 - Troublekilling
Knitline Weakness or Cosmetic Problems, cont.
Plastic’s Point of View
Possible Corrective Action
A knitline is truly a glue joint in that
the two plastic flow fronts join and do
not generally re-entangle. The
exceptions are some crystalline
materials which are welded together
above their melting point. In a classic
knitline formation, the same problems
apply to a knitline as that of making a
good glue joint. The material must be
at a low enough viscosity, the flow
front must be clean, there must be
sufficient pressure to cause bonding,
and the pressure must be held for a
sufficient time to allow the material to
solidify. Also, there is often air
entrapment. Knitlines must be well
vented. If all of the elements are
available, knitline integrity should
exist.
Check viscosity (Injection Fill Integral) and
correct if necessary.
Check melt temperature.
Check fill time.
Check mold temperature.
Be sure the part is properly packed.
Raise packing pressure. Be sure that the gate is
sealed.
Check gate seal time.
Maintain proper cycle.
Be sure that the cavity is adequately vented at
the knitline area.
112 - Troublekilling