BASF Corporation
Mechanical Performance of Knit Lines and Welded Butt Joints –
A Key Factor in Designing of Multiple Gated Injection Molded and
Subsequent Welded Thermoplastic Components
Abstract
Previously we reported to Antec’96, GPC’99 and GPC’2000 our studies on the efficiency of various welding
technologies (linear vibration, orbital vibration, hot plate, infra-red/laser and ultrasonic) on the mechanical
performance of welded thermoplastics. Mechanical performance of the welded joints depends from key welding
processing parameters, such as weld-melt temperature, clamp and hold pressure, melt-down (melt collapse),
hold/cooling time, and thickness of inter-phase.
The mechanical properties of the injection-molded thermoplastic components depend on the part and the molding
tool design, and utilized molding processing parameters such as melt and mold temperatures, molding pressure,
injection rate, cooling time, etc. For injection molding of various welded thermoplastic parts, the complex of
multiple gating injection molding tools was used. Both the design of these welded parts and the molding tool
design is very important to get optimum flow patterns and to predict the locations of stress-bearing areas and knit
(molding)1 and weld (joining) lines (planes). In the report to SAE’20012, we discussed importance of the knit
lines in multiple gated rotating plastic part design.
In this study, we are discussing the mechanical properties in local (knit lines and welds) and bulk (molded part)
areas, with the influence of molding and welding conditions. It has been found that for non-reinforced and
reinforced nylon, the mechanical performance in the both in the knit planes and welds areas is approximately
equal to the mechanical performance of used resin (nylon/polyamide).
The results from this study will help designers to accurately interpret the results of structural analysis of welded
parts and complex tensile (as basic) properties, such as strength, deformation and fatigue of nylon based plastics
and to utilize these important material parameters at end-use conditions for a part life assessment.
Introduction
Mechanical performance, of the injection molded and welded thermoplastic components, is very critical
for the various industrial applications (automotive under-the-hood, bumpers, appliances, power-tools, etc.) [1-2,
5-6]. High performance non-reinforced, fiber-glass reinforced and glass/mineral versions of nylon 6 based plastics
were utilized in many applications [6-9]. The automotive and power-tools industries have developed a culture of
reliability and cost effectiveness, in which high risks and adventures are not encouraged [6]. Due to the wide and
ever increasing application of plastics for these industries, mechanical performance of molded and joined/welded
thermoplastic parts depend upon optimized design and processing (including welding) technology and properties
of polymer based materials.
1
Sometimes called the weld line or weld plane [1-2]. In this study we will use the term “knit line” for injection molding, and
the term “weld line” for welding of thermoplastics.
2
“Optimizing Mechanical Performance of Injection Molded Multiple Gated Rotating Thermoplastic Components: Part 1 –
Consideration of Structural Analysis and Knit Line Effects. Part 2 – Knit Line Inter-Phase Integrity” [3-4].
There are many design and technology cases for welded plastic components, which require multiple gated
injection molding systems. Mechanical performance of these welded, injection molded multiple gated plastic
parts such as chassis/bodies (Figure 1), air intake manifolds (Figure 2), etc., depends from uniform distribution
and orientation of flows and welds, and the material property patterns.
Figure 1. Multiple gated injection molded base hollow part of vibration welded thermoplastic body/chassis.
Figure 2. Vibration welded two pieces air intake manifold (both parts are multiple gated)
The process of weld (weld inter-phase) formation during welding is similar to knit/meld line formation in
injection molding. In our reports to
GPC’99 [10] and GPC’2000 [11] we tried to describe the vibration welding process as “micro re-molding”, when
the weld (weld inter-phase) is created where two plastic flow’s front meet and formed weld. For nylon/polyamide
(PA) based plastics, the following welding processing parameters, such as a clamp pressure, melt-down (melt
collapse), melt-weld temperature, thickness of the flows and inter-phase, hold (cooling) time, etc., may affect the
mechanical performance of the weld.
Similarities and Differences in Formation of the Knit, Meld and Weld Lines
Introductory Remarks
The successful design and advanced manufacturing of injection molded and welded thermoplastics components
require a correct combination of knowledge in material, design and processing technology. Both, the welding and
injection molding processes offers a wide degree of freedom of design and optimized mechanical performance of
the components may be obtained only if:
•
The material developers, plastic part, mold and weld tools designers take the numerous specific processing
factors into account
• They will realize these combined effects in finalizing:
- All welding and injection molding processing conditions
- Welding and molding tools design.
In general, the following “key” material and parameters affect the knit (molding) and weld (welding) line integrity
[1-2, 12-16]:
•
•
The extent of physical and mechanical property change depends on the ability of the two melt flows to knit
(joint) together homogeneously.
Type and properties of base resin
•
•
•
Molded part/specimen thickness
Presents of fillers, reinforcements and impact modifiers
Welding and injection molding process conditions such as:
- Temperature and viscosity of the molten polymer/plastic when it meets
- Mold (or welded part) temperature
- Molding (or welding), clamp/hold/back pressure
- Molding (or welding) and cooling/hold time
• Orientation and interaction of the melt flows and stress-strain patterns.
By geometry (shape and sizes) the knit and weld lines are very similar. These sizes depend upon the design of
molded and welded parts (Figures 1-2), including the design of welded joints (Figure 3). By length they are long
and are equal to the length of contour of the welded part(s) or to width/length w of the molded wall (Figure 4).
The thickness t of the knit (or weld) lines is equal to the thickness of a wall (or weld bead). Specific data on
localized material properties in knit and weld areas is very important for the structural analysis and life
assessment of various multiple gated and welded plastic parts such as air intake manifolds, resonators, fluid
reservoirs, chassis, etc.
A
Weld
B
C
D
(w, t)
Figure 3. Welded joint design principles (types and configuration). Legend: A – joint design; B – butt joint
(straight); C – butt joint (T – shape); D – shear / lap joint.
Figure 4. An example of the knit (weld) line formation at central gated cooling fan with a ring (MoldFlow®
data). Legend: Sizes (length x thickness = w x t) of the knit line(s) are equal to width of a ring (w) x thickness of
a ring
Formation of the Knit and Meld Lines
Many thermoplastics of an injection mold will influence the final products and assembled/welded parts
mechanical performance, dimensions and other end-use characteristics. These mold parts include the cavity
shape, gating, parting line, vents, undercuts, ribs, hinges, etc. Additionally, there are many cases such as parts
requiring multiple gates or family mold cavities. The molding tool designer should take all these factors into
account. If the melt is flowing around of any obstacles in the mold tool, such as the inserts, ribs, cores, etc., a
weld or meld line will results (Figure 5-6). The knit and meld lines (planes) are created wherever polymer flow’s
front meet from opposite or parallel directions correspondingly (Figure 5, 7). These lines (planes)3 are significant
for injection molded and subsequent welded thermoplastic part performance because the local mechanical
properties in the knit, meld and weld lines/planes areas differ significantly from those in the rest (total/bulk) areas
of the molded parts.
3
In the mechanical performance and morphology analysis we will use the term “inter-phase” also, as the three dimensional
description of this very complicated by glass-fibers and polymer chains re-orientation areas [10-11].
The number of knit lines (planes) may be determinate (Figure 5-7) by equation (1):
N = G +C D - 1
(1)
Where:
N – number of weld planes (lines)
G – number of gates
C0 – number of the shutoff cores (or pins).
Knit Line 1
Shutoff Core
Core
Knit Line
Meld Line
b
a
Gate 2
Material Flow
→
→
→
Material Flow
Flow Patterns
Material
Flow
Gate 1
Knit Line 2
Figure 5. Knit (weld) and meld lines formation principles. Legend: a – formation of the knit (weld) line; b –
formation of the meld line.
Figure 6. Example of the knit lines formation (two gates system)
Knit lines are weaker than meld lines [1, 5, 14]. The knit lines/planes become likely areas of the crack initiation
and propagation and possible injection molded part failure. These weaknesses can change the mechanical
performance of thermoplastic components. A family of knit lines may be formed from one gate, and the divided
plastic streams joins after flowing through ribs and a ring (Figures 8). Mechanical performance through crosssection of the knit line(s) will depends on melt flow temperature profile. The example of melt flow temperature
distribution at the end of the fill cycle is presented in Figure 9 and Table 1. These results are showing the
possibility for different cooling conditions for the skin and core layers. This will affect crystalinity distribution
through cross-section of molded part/specimen also4.
Figure 9 (a, b, c) shows various plastic flow patterns, orientation patterns related to mechanical property
performance. The melt in the center has a lower viscosity due to its higher melt temperature (see Table 1). As a
result of this distribution, the maximum shear rate occurs not at the contact area with a mold surface, but closer to
center of the melt [1, 13-14].
Orientation of the knit lines in the round components is perpendicular to the tensile stresses, applied to the ring
(Figures 2, 4, 8). The maximum of tensile strength of molded plastic is in the direction parallel to flow (Figure 10
a, b). In order to achieve controlled fill patterns, the mold designer should select the location and number of gates
that will result in the desired areas.
4
“Optimizing The Mechanical Performance In Semi-Crystalline Polymers: Roles of Melt Temperature and Skin-Core
Crystalline Morphology of Nylon”, N. S. Murthy, V. A. Kagan, and R.G. Bray
The methods of neural networks were successfully applied in prediction of the knit lines during injection molding
process. Developed computational simulation packages have had success in predicting filling behavior in very
complicated geometry of various hollow components. Molding parameters are influential on melt flow formation
and finally on the performance of the knit line (inter-phase). A practical thermoplastic part design – melt flow
consideration is to combine concepts of full optimization of mechanical performance of thermoplastic parts where
multiple gating systems were used.
Stress
Knit Line
Knit Lines (a)
Stress
Gates
Gate (c)
Meld Line
Meld Lines (b)
Figure 7. Molding analysis study: Different plastic flow patters create knit and meld lines (MoldFlow® data).
Legend: a – knit line; b – meld line; (c) – molding gate
Figure 8. Molding analysis study – mold filling analysis (time);: An example of the flow patterns and knit lines
formation (MoldFlow® data) in the injection molded housing (nylon 6, 33 wt.% GF).
The number of gates and internal shutoff cores should be considered as an important aspect of:
•
•
Initial thermoplastic part design
Prediction of part performance with the influence of location of stress-bearing areas and weld planes/lines.
Standard mold filling and cooling analysis (MoldFlow® data, Figures 1, 4, 7-9) provides the designer and
technologist with a quantitative data on flow patterns and the knit lines (planes). Critical plastic flow distance,
and the number of gates and internal shutoff cores should be considered an important factor of thermoplastic part
design for required uniform mechanical/physical performance [1, 14]. Unfortunately, it was not possible to find
in the published literature detailed recommendations on nylon based plastic selection and design for multiple
gated injection molded parts containing the knit lines. Published data on mechanical performance of fiber-glass
reinforced nylon in knit line is limited also [2, 13-14]. Previously it was assumed [14] to use for design of nylon
made components the tensile strength of knit line in range from 46 to 93% of base material5 strength (for level of
5
The definition of the strength of base material is not clear enough in used trade literature and published reports. By our
opinion “the strength of base material” is strength of the matrix (base resin).
GF reinforcement from 10 to 30 wt.%6 GF). Above recommended [14] range of the ultimate strength of knit line
is very wide for design purposes and needs to be defined more precisely for the critically loaded components,
when a part weight saving is very important.
a
b
c
Figure 9. Molding analysis study: An example of the melt flow temperature distribution through thickness of
gated from both sides injection molded ISO tensile specimen (MoldFlow® data, nylon 6, 33 wt.% GF). Legend: a
– skin layer/area; b – inter-layer (between the skin and a melt core); c – flow/melt core central layer.
Table 1. Melt Temperature (°C) Distribution Through Thickness of Doubled Gated ISO Tensile Specimen at the
End of Packaging Cycle
Position of the melt layer through thickness
Gate
Knit-Line (Plane)
Skin area (a)
77.4
67.4
Inter-layer (b)
158.5
277.8
Central core (c)
280.2
281.6
Formation of the Weld Lines
In general, process of development of the weld lines, is very similar to knit lines formation, when melt flows are
coming from two opposite located gates. During the welding of thermoplastics two uniform (by geometry) melt
flows will develop and meet at weld plane area and will create the weld inter-phase (Figure 11). As an example,
for weld line formation, we will use the frictional method of welding. Frictional methods (linear vibration,
orbital, spin and ultrasonic) of welding technologies all share the following phases, very important for weld line
formation:
•
•
•
•
Thermoplastic heated in areas where the joints are to be formed
Local melting in joined surfaces areas
Heated surfaces contacting / pressing together for joining
Cooling in the joint interface and other areas.
By geometry (shape and sizes) the weld lines are very similar to knit lines. These sizes of the weld lines depend
upon the design of the welded beads (Figure 3). By length the weld lines are very long (up to 2 m and more) and
are equal to the length of contour l of the welded part(s). The thickness t of the weld line(s) is equal to the
thickness of the narrow part of weld bead.
6
Level of reinforcement of filler by weight (wt.%).
Nylon Based Plastics – Material of Choice for Multiple Gated Injection Molded and Welded Components
Thermoplastics offer to the manufacturers the following key benefits – weight saving, improved performance and
style. Integration of function is a much benefit of plastics molding, turning the assembly of metal air intake
resonators, chassis, fluid reservoirs, etc. There are different types of injection-molded nylons [2, 7-9, 15-16]
providing wide range of end-use performance for many industrial applications. Automotive under-the-hood nylon
use in North America and Europe has grown from 87,500 tons to 165,000 in 1999 and is projected to be 230,000
tons in 2005. Nylons (polyamides) are high performance semi-crystalline thermoplastics with a number of
attractive physical and mechanical properties. Injection molded nylon components including cooling fans and
shrouds are used in many industrial applications, the largest being automotive. There are more than a dozen
classes of polyamide (PA)/nylon resins, including nylon 6, nylon 66, nylon 46, nylon 10, nylon 12, etc.
Shear Thinning Layer
Melt Front
a
Surface Highly Oriented
Melt Front
Plug Flow
Gate
Gate
Core Orientation
Fast Fill
Melt Front
b
Sub-Surface Orientation Melt
Oriented Skin
Orientation
Gate
Figure 10. Melt flow orientation through part/specimen thickness.
Slow Fill
Figure 11. Effects of different flow rates on melt profile. Legend: a – fast fill; b – slow fill.
Three important thermoplastic – molding tool – plastic part interrelations must be considered at the outset by
those specifying polyamide (nylon): nylon 6 is a family of related plastics, not a just a single composition [1, 15].
Reinforced nylon plastics (with 15 – 40 wt.% fiber-glass reinforcements - GF) are commonly used in design of
automotive welded fluid reservoirs [6, 8]. Fiber-glass reinforced and mineral filled nylon plastics (25 – 45 wt.%
GF/MF) are used in design of the welded resonators and various covers also. Typically, the weight of glass-fiber
reinforced molded and welded nylon-based automotive part is 40-55% less than a similar design made from
stamped steel or by casting. The similar weight reduction was achieved for various power tools and lawn &
garden applications.
This family of fiber-glass reinforced or reinforced and filled plastics [2, 6-9, 15-16] can be considered, that all
compositions have the following injection molding advantages for welded components:
•
•
•
•
Good mechanical performance of molded and welded parts after several re-molding/re-grind cycles
(mechanical property losses are minimal, etc.)
Moldability to close tolerances for following welding
Fast overall processing cycles and ejectability (part release from molding tool) is very good
Predictable mold and annealing shrinkage; small tendency for warpage for following welding
•
•
High flow and toughness in thin sections, easy to fill of complicated shapes
Sufficient knit (molding) and weld (welding) lines strength.
Experimental: Knit and Weld Lines Integrity
In this study we used the following grades of nylon 6 based plastics (all heat stabilized - HS series):
•
•
•
•
•
•
Non-reinforced /non-filled nylon
Short fiber-glass reinforced plastics with the level of reinforcement from 6 wt.% to 63 wt.% GF
Mineral filled plastic (40 wt.% MF)
Mineral filled and short fiber-glass reinforced (25 wt.% MF + 15 wt.% GF)
Impact modified plastic (5 wt.% IM)
Impact modified and short fiber-glass reinforced (5 wt.% IM + 33 wt.% GF).
To illustrate the influence of knit lines on mechanical performance of analyzed plastics, the multipurpose test both
sides gated specimens (ISO 3167, with the thickness equal 4 mm) were used. Tensile properties of plastics were
obtained using one side gated ISO specimens. Short fiber-glass reinforcements are (in average): 11-13 µm in
diameter, and 240-320 in length. Mechanical performance of the weld was obtained using welded together
rectangle plaques with the following sizes:
•
•
150 x 55 x 4 mm
150 x 55 x 6 mm.
Sizes (length x width) of the welded plaques are 150 (or 100) mm x 120 mm (approximately). Weld area is equal
to 600 (or 400) mm2 respectively for the plaques 150 mm and 100 mm long. For the tensile test we used welded
specimens machined from welded plaques with cross-section in weld area (width x thickness = 10 x 4 mm). ISO
527 tensile test procedures were used in this investigation. Five tensile specimens were tested at every condition.
All tests were conducted at 23°C for “dry-as-molded” and “dry-as-welded” conditions7. The tensile strength of
material, at knit line and weld is “key data” – the first step in the design of multiple gated and welded
thermoplastic parts, molding and welding process optimization, and comparative analysis of material suitableness
for molding with following welding.
Injection Molding and Welding Procedures
Both, injection molding and welding conditions were optimized by criteria of mechanical performance
(comprehensive tensile properties). Prior to injection molding, all materials were dried by ASTM requirements.
For injection molding conditions we used the recommendation [20], based on the design experiment approach
(Taguchi Method), where the following processing variables were taken unto account:
•
•
•
•
Melt and mold temperatures
Injection and hold pressure
Cooling and holding time
Back pressure.
For welding we used various joining technologies such as linear vibration, orbital vibration, hot plate (contact
method), ultrasonic and infra-red/laser transmission. The influence of the following key welding process
variables were taken into account for linear vibration welding (as an example):
7
ASTM D-4066 specified for nylon/polyamide 6 moisture content wt.% max "as received” equal 0.2%.
•
•
•
•
Weld amplitude
Weld clamp and hold/cooling pressure
Melt-down (collapse)
Welding, holding/cooling time.
For linear vibration welding we used Mini-Welder-II type8 machine. For a bigger sizes of the welding machines
used plaques are too small (and clamp pressure is very high for process optimization).
Knit Line Strength
Overall welded thermoplastic part design should be scrutinized for redundancy and simplified to require the least
less complicated injection mold design. In the design of complicated shapes, knit and meld lines (see Figures 5
and 7) are often unavoidable. Information on the effects of injection molding variables on mechanical
performance of nylons is not abundant, and a part of these results are discrepant. But what does exist for nonnylons gives us the following guidelines [1, 13-14]:
•
•
Increasing both melt and mold temperatures will increase weld line tensile strength
Excessive melt temperatures will degrade the joined polymer causing a general strength reduction of the bulk,
including weakness at the weld plane
Fill rate and packing pressure effects are positive in limited areas only.
•
Table 2. Tensile Strength at Knit Line for Non-Reinforced Plastics
Type of
Resin
Tensile Strength,
MPa
Tensile Strength of Weld,
MPa
Tensile Strength
Retention, %
Ref. [ ]
PSU
65.0
65.0
100
1, 14
SAN
80
60
80
1, 14
PP
30
29.4
86-97.6
1, 14
PP
35
33.6-35
96-100
1, 14
PPS
60
50
83
14
HDPE
25
23
92
14
PA 66
85
-
83-100
14
Higher melt temperature promotes polymer molecular knitting and chains entanglement at the weld inter-phase
and yield less net orientation. Increasing mold temperature promotes slow cooling. In most cases the mold
temperature effects (they are positive) is not so effective as the melt temperature effects. Effects of these complex
changes can also depend on particular grade of thermoplastic, molded part design, and mold and melt temperature
levels. The effects of the inherent knit line integrity were analyzed in [1, 13-14]. In these studies were evaluated
the effects of the knit line on tensile strength (Tables 2 and 3) of several thermoplastics (polysulfone – PSU,
styrene acrylonitrile – SAN, polypropylene – PP, and polyphenylene sulfide – PPS, high-density polyethylene –
8
Mini-Welder-II is the trade name of welding machine from Branson Corporation (Danbury, CT).
HDPE, nylon 66). For non-filled plastics the tensile strength of the knit line is equal approximately or less (up to
17%) to tensile strength of base material (Table 2). It was found for fiber-glass reinforced plastics, that the
percentage of loss in strength at the weld line is greatly increased (up to 80%, Table 3). However, absolute knit
line strength doesn’t increases as more or stronger reinforcement is added.
Table 3. Effects of Reinforcement on Tensile Strength at Knit Line for Fiber-Glass Reinforced Plastics
Type of Resin
Glass- Fiber, wt. %
Plastic Strength,
MPa
Knit Line
Strength, MPa
Tensile Strength
Retention, %
PSU
30
115
70
62
SAN
30
110
45
40
PP
20
65
30
47
PP
30
-
-
34
PPS
10
70
25
38
PPS
40
140
28
20
PA 66
10
-
-
87-93
PA 66
30
-
-
56-64
Table 4. Effects of Reinforcement on Tensile Strength at Knit Line of fiber-glass reinforced nylon 6 based
plastics (Capron®9series).
GF Wt. %
Tensile Strength of Plastic,
MPa
Knit Line Strength, MPa
Tensile Strength Retention,%
0
82
85.8
99.2
14
125
89.1
66.6
33
185
89.2
46.2
50
220
83.3
37.8
63
229
83.8
36.5
The loss of strength at knit line fiber-glass reinforcement is caused by:
• There is a sharp V-type notch at the knit plane surfaces that acts as the stress/strain concentrator
9
Capron® - is a registred trademark for BASF Corporation nylon/polyamide plastic products.
•
•
•
•
Fiber-glass orientation in the knit/weld inter-phase occurs at the right angles to the principal melt flow
patterns
This orientation is contributed to the strength reduction in the knit line (inter-phase area)
Incomplete molecular entanglement or diffusion
Possible presence of the contamination or micro-voids at the knit line (inter-phase).
Absolute value of the melt and mold temperatures affect to microstructure formation at skin and core areas
(Figure 11-12). For injection molded nylon 6 based plastics, rapid quenching produces a skin with lower
cristallinity than at core, which cools more slowly. The melt front is influential to flow rates – slow rate will form
an elliptic flow configuration, that affect to development of V-type notch (Figure 11). Mechanical performance of
various fiber-glass reinforcement nylon 6 plastics at knit-line area is shown in Table 4. When the tensile strength
of the thermoplastic increases from 82.5 to 229 MPa (at the range of glass-reinforcement from 0 to 63 wt,% GF),
tensile strength retention for the knit-line/plane decreases from 99.2 to 36.5% (see Table 4). Analysis of
presented results is showing that the tensile strength at the knit line remains the same for evaluated levels of fiberglass reinforcement (from 0 to 63 GF wt.%) and is equal (approximately) or slightly above of the tensile strength
of non-reinforced plastic (83.3 ÷ 89.2 MPa). Similar results were obtained for nylon 66 based plastic (Table 5)
with the same level of fiber-glass reinforcement.
Tensile strength of mineral filled (MF) plastic is increasing slightly (Table 6); at the same time tensile strength at
knit line decreases by 14.5% (for 40% MF nylon). Impact modifiers (IM) reducing the tensile strength of nylons
by 35% approximately (Table 6). Combined effect of fiber-glass reinforcements (33 wt.% GF) and impact
modifiers (5% IM) is reducing tensile at knit line strength by 60% approximately. However, absolute knit line
strength decreases significantly for impact modified plastics (Table 6).
Table 5. Tensile Strength at Knit Line for nylon 6 and nylon 66 based plastics (0 and 33 wt.% GF)
Type of Resin
GF wt. %
Tensile Strength of Plastic,
MPa
Weld Line Strength,
Mpa
Tensile Strength
Retention, %
PA6
0
86.5
85.8
99.2
PA66
0
xx
Xx
Xx
PA6
33
192.9
89.2
46.2
PA66
33
190.8
88.5
46.3
Weld Line Strength
In our reports to Antec’96, GPC’99 and GPC’2000 [10-11, etc.] we presented results on optimized mechanical
performance of various nylon based plastics. In these studies we evaluated mechanical performance of welded
joint for the following widely used welding technologies:
•
•
•
•
Linear vibration
Orbital vibration
Hot plate (contact)
Ultrasonics
•
Infra-red/laser.
Table 6. Effects of Various Reinforcements (GF), Fillers (MF) and Impact Modifiers (IM) on Tensile Strength at
Knit Line for 6 based plastics (Capron®series).
GF
wt., %
MF wt., %
IM wt.,
%
Tensile Strength
of Plastic MPa
Knit Line Strength,
Mpa
Tensile Strength Retention,
%
0
0
0
86.5
85.8
99.2
0
40
0
90
77
85.5
15
25
0
126
90
71.1
0
0
4
54
51.6
95.5
33
0
0
192.9
89.2
46.2
33
0
5
152
62
40.8
Data on the efficiency of five welding technologies for 33 GF wt.% nylon 6 plastic is presented in Table 7. For
the data comparison we used the following two, welding factor, parameters:
•
•
Welding factor fwm related to the tensile strength of the base material/polymer. Weld factor fwm = tensile
strength of the weld/tensile strength of non-reinforced plastic (base resin)
Welding factor fwpl related to the tensile strength of the reinforced plastic. Weld factor fwpl = tensile strength of
the weld/tensile strength of reinforced plastic.
Table 7: Efficiency Of Welding Technologies For Fiber-Glass Reinforced Nylon 6 Based Plastics (33 Wt. % GF,
At 230C, Dry As Molded, Optimized Processing Conditions)
Weld Factor fwm
Weld Factor fwpl
Linear Vibration
1.08
0.46
Orbital Vibration
1.10
0.47
Hot Plate (Contact)
1.12
0.48
Ultrasonics
0.34
0.15
Laser (Transmission)
0.92
0.41
Type of the Welding Technology
For nylon 6 based plastics, four of the analyzed welding technologies (including infra-red/laser transmission) are
three times more efficient than ultrasonic welding (see Table 7). Slightly higher mechanical performance for
various ultrasonically weld nylon 6 and nylon 66 (Table 8) was published in [19]. The absolute weld line strength
doesn’t increases or decreases for fiber-glass reinforced nylon. Similar results were obtained for nylon 66 based
plastic (Table 8) with the same level of fiber-glass reinforcement.
Linear vibration welding technology is widely used for joining of various hollow multiple gated injection molded
parts. Infra-red/laser transmission welding technology is a new, but will have a wide application also. Weld line
data, obtained for these technologies, is critically important for the plastic parts design and process optimization.
Table 9 is showing efficiency of vibration and infra-red/laser technologies with the influence of fiber-glass
reinforcement. In analyzed in this table range of reinforcement (from 0 to 45 wt.% GF), the tensile strength at
weld line is approximately equal to tensile strength of non-reinforced nylon 6. Some increase of the tensile
strength was found at the range of fiber-glass reinforcement (GF) from 14 to 25 wt.%.
Table 8: Influence Of Fiber-Glass Reinforcement & Fillers On Efficiency Of Ultrasonic Welding Technology For
Nylon 6 and Nylon 66 Similar And De-Similar Joints [19]
Type of Resin and Joint Design
(Similar or De-Similar)
GF wt. %
MF wt. %
Tensile Strength at
Weld Plane, MPa
PA 6 + PA 6
0
0
25 – 40
PA 66 + PA 66
0
0
20 – 30
PA 6 + PA 6
25 / 25
0
30 – 35
PA 6 + PA 6
30/ 30
0
25 – 35
PA 6 + PA 6
0
30
40 – 45
PA 6 + PA 66
30 / 30
30
20 – 24
PA 66 + PA 66
0 / 30
0
25 – 30
Similar Joints
Desimilar Joints
Table 9: Influence Of Fiber-Glass Reinforcement On The Tensile Strength Of Similar Joints (At 230C, Dry As
Molded) Of Linear Vibration And Laser Welded Butt Joints (Optimized Processing Conditions)
Wt.
%,
GF
Tensile Strength of Fiber
Glass Reinforced Plastic
(MPa)
Vibration Welding
Technology Tensile
Strength of Weld (MPa)
Laser Welding Technology
Tensile Strength of Weld
(MPa)
0
82
81
≥ 82
14
125
90.7
N / A*
25
160
90.2
76.9
33
185
83.6
75.4
45
208
80.1
70.4
Information on the effects of welding parameters on mechanical performance of nylons is not abundant, and a part
of these results are discrepant. The vibration welding optimization studies [11-12] gives us to present the
following guidelines [11-12]:
•
•
•
•
Weld amplitude increase and weld pressure decrease will increase weld line tensile strength
Melt-down and thickness of inter-phase increase will increase weld line tensile strength
Increasing weld-melt temperatures (above melting pint) will increase weld line tensile strength
Direction and shape of oscillations do not affect mechanical performance of weld line.
An Analysis of Mechanical Performance of the Knit and Weld Lines
For mechanical performance prediction of welded multiple gated components, we need to use the various physical
and mechanical properties (at total and local areas) of injection molded thermoplastics with the influence of
processing (molding and welding) and end-use environmental and mechanical loading conditions. Tensile
strength of knit and weld lines is critically important for plastic selection for various industrial applications were
welded parts will be stress-strains bearing. Table 10 shows the influence of fiber-glass reinforcement (from 0 to
63 wt.%) on the tensile strength at knit and weld lines for nylon 6 based plastics. Both results are very similar and
they are very close to tensile strength of non-reinforced plastic (base polymer).
Table 10: Influence Of Fiber-Glass Reinforcement On The Tensile Strength (At 230C, Dry As Molded) Of Knit
and Weld Lines (Nylon 6 Based Plastics. Optimized Processing Conditions)
wt. %,
GF
Tensile Strength of Plastic,
(MPa)
Tensile Strength of Knit
Line,(MPa)
Tensile Strength of Weld Line
(MPa)
0
82
85.5
81
6
85
14
125
25
160
33
185
45
208
50
220
83.3
80.5
63
229
83.8
79.2
83.1
89.1
90.7
90.2
89.2
85.6
82.1
Influence of combined effects of fiber-glass reinforcements (GF), fillers (MF) and impact modifiers (IM) is
presented in Table 11. Weld line strength is slightly above of the strength at weld line. AS we discussed
previously, the influence of impact modifiers (IM) is significant. Analysis of presented (Tables 10 and 11) results
is showing that mechanical performance of the knit and weld lines are very similar.
Melt-Flow Orientation and Morphology of the Knit and Weld Lines
Mechanical performance of an injection molded parts and specimens strongly depend on processing technology,
molding tool and part design. To illustrate the influence of injection molding processing technology on fiber-glass
glass orientation and mechanical performance of plastic components, the multipurpose test specimen (ISO 3167)
was analyzed (Figures 8, 14-16) and related to what may happen in injection molded and welded parts.
Tensile test data for both sets of ISO multipurpose specimens is presented in Tables 4-6, 9-11. Fiber-glass
reinforcements display flow orientation during injection molding. The degree of orientation (fiber-glass and
molecular) depends on aspect ratio, plastic part dimensions, injection rate, and gating. A one side/end gated ISO
multipurpose specimen will have fiber-glass orientation mostly oriented along the length axis (Figure 12). Such
plastic flow orientation leads to the clearly defined anisotropy of mechanical properties in fiber-glass reinforced
nylon (see Figure 10 a).
Table 11. Effects of Various Reinforcements (GF), Fillers (MF) and Impact Modifiers (IM) on the Tensile
Strength (at 230C, Dry As Molded) at Knit and Weld Lines (Nylon 6 Based Plastics. Optimized Processing
Conditions)
GF wt.,
%
MFwt.,
%
IMwt.
,%
Tensile Strength of Plastic,
MPa
Knit Line Strength,
MPa
Weld Line Strength,
MPa
0
0
0
82
?
81
0
40
0
90
77
81.5
15
25
0
126
90
84.8
0
0
4
54
51.6
33
0
0
192.9
89.2
33
0
5
152
62
85.6
Double-gated multipurpose specimens develop knit-lines (inter-phase) that are critical for the loading/maximum
stress area that results in the mechanical performance of the specimens (see Figure 10 b). As melt flow fronts
meet, the fibers are turned 90° from the direction of flow. This is 90° from the direction of expected applied
tensile stresses, so fibers are aligned perpendicular to the applied tensile stress and offer little reinforcing effects at
weld plane (Figure 16). Molded part/specimen thickness does not play a major role in the knit line strength
retention.
The observed increase in the tensile strength at the knit-line/plane for 33 wt.% GF nylon 6 from 86.5 MPa (tensile
strength of matrix/resin) to 89.2 MPa (knit-line) can be explained by the following:
•
•
•
As a result of a local reinforcement in the knit-line (inter phase).
Diffusion of the joined layers/flows
As the result of optimized molding conditions.
Conventional techniques to optimize knit-line/plane strength are:
•
•
•
Increase melt and mold temperature
Increase injection pressure
Use fast fill molding
•
Avoid use of external release mold lubricant (mold release).
Flow Patterns
a
Stress
Flow Patterns
Flow Patterns
b
a
Weld Line/Plane (Inter-Phase)
Stress
Knit Line
b
P1
Oscillations
P2
Figure 12. The effect of melt flow orientation on mechanical performance of injection molded parts/specimens.
Legend: a – applied stress is parallel to flow direction (unidirectional fiber-glass orientation); b – applied stress is
perpendicular to the knit (weld) line.
Figure 13. Weld line (inter-phase) formation during welding processing technology. Legend: a – linear vibration
welding; b – inter-phase; P1 – heat affected area at part 1; P2 – heat affected area at part 2.
Flow Direction
Figure 14. Example of orthotropic (unidirectional) fiber glass orientation in molded part/specimen (one side
gated). Data obtained using injection molded nylon 6, 33 wt.% GF multipurpose specimen (ISO 3167) at flow
direction.
Figure 15. Example of random fiber-glass orientation in molded part/specimen (one side gated). Data obtained
from injection molded nylon 6, 33 wt.% GF multipurpose specimen (ISO 3167) close to gate area.
At the gate area, fiber-glass orientations are random for both sets of the injection molded specimens (one and both
sides gated). The tensile strength and modulus reaches a maximum value in the flow (longitudinal) directions
(Figure 15) and up to 50% less in the transverse (perpendicular to flow) direction. For 33 wt.% GF nylon 6 this
decrease might reach up to 50 – 60% of the similar value due to micro-cracks orientation.
Similar fiber-glass re-orientation effects were found for multiple gated injection molded and welded part (Figure
15) and weld line (inter-phase) also (Figure 18). The observed increase [10-11] in the tensile strength of welded
butt joints (up to 85.6 MPa for 33 wt.% GF nylon 6, Tables 9- 10) was explained by the following [10-11]:
•
•
•
As a result of local reinforcement in the weld inter-phase, some fibers (oriented randomly) are crossing the
weld interface (Figure 18)
Optimized thermos-mechanical and weld inter-phase formation (melt-down, thickness of inter-phase) for remelted layers of plastic(s) to be joined
Diffusion of the joined layers.
Knit-Line
Flow 1
Flow
Flow 2
Flow
Weld Inter - Phase
Knit (Weld) Inter-Phase
400 µm
Knit-Line
Figure 16. Example of fiber-glass orientation at knit (weld) inter-phase area in molded part/specimen
(doubled/two sides gated). Data obtained using injection molded nylon 6, 33 wt.% GF multipurpose specimen
(ISO 3167).
Figure 17. Example of fiber-glass orientation at knit-line (weld inter-phase area) in injection molded multi-gated
part. Data obtained using nylon 6, 33 wt.% GF.
Part 1
Weld Inter-Phase
Part 2
Figure 18. Local reinforcement effects at the weld inter-phase (a part of fibers are crossing the weld inter-phase).
Data obtained for linear vibration welded nylon 6, 33 wt.% GF.
Conclusion
•
Knit and weld lines become likely areas of a crack initiation and propagation, and possible molded and
welded part(s) failure (or damage).
•
For the multiple gating system, the melt-flow is largely governed by dimension and shape of the injection
molded components, processing parameters and the location, number and sizes of the gates.
•
Prediction of location and orientation the knit and weld lines is very important in the design of multiple gated
injection molded and welded thermoplastic parts.
•
Prediction of the knit and weld lines should take into account orientation of maximum stress-stresses. It is
recommended that the surface that containing is to be subject to high load10 - bearing should not contain the
knit and weld lines. Orientation of the knit and weld lines should be considered with loading and end-use
conditions.
•
For non-reinforced (or non-filled) nylon the tensile strength at knit and weld lines (inter-phases) is
approximately equal to the tensile strength of used resin (matrix).
•
Fiber-glass reinforced and fiber-glass/mineral nylons have the tensile strength at knit and weld lines, which
are different (significantly less) from the tensile strength of reinforced/filled plastic due to flow patterns and
local fiber-glass re-orientation at knit and weld lines (inter-phases). At the same time for these plastics, the
tensile strength at knit and weld lines, remains the same as for non-reinforced plastics or slightly higher than
the tensile strength of used resin (matrix).
•
The impact modifiers significantly reduce tensile strength at knit and weld lines. For plastics with combined
composition, the allowable working stress at knit and weld lines should be reduced with the influence of
additives (fiber-glass reinforcements, fillers, impact modifiers, etc).
•
For impact modified plastics at the high molding temperature, the additive concentrate, at the part surface and
the weld inter-face, because of a phenomenon called “fountain flow”. For this phenomenon, the melt does not
reach the wall or surface of the frozen wall layer by simple forward advance, but rather tends to flow down to
center of the cavity to the melt front and then flow toward the wall.
•
This can have an important effect on the direction of the flow-induced orientation of the polymer molecules.
If the melt flows around an obstacle of any kind in the cavity, the knit line will result and the V-type notch
may be developed.
References
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Hall, New York, 1145 pages, 1995.
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Paper 01M-121, 16 pages, 2001 (to be published).
10
stresses /strains - bearing
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