10.2417/spepro.005957
New solutions for metal/plastic
hybrid design via nano-molding
technology
Yuanqing He, Xiaoyu Sun, and Harold Ho
Novel glass-fiber-reinforced compound solutions enable high adhesion
between metal and plastic, improved mechanical properties, superior
chemical resistance, and desirable aesthetics for consumer electronics.
In the consumer electronics industry, the popularity of metal/plastic hybrid design is growing rapidly. Designers prefer to use metal as the
main housing and chassis material for portable electronic devices (such
as cell phones, tablets, and laptops) due to the functional benefits it
provides in terms of both aesthetics and useful properties (e.g., electromagnetic shielding). However, metal falls short in key areas (e.g., radio
transparency, colorability, cost, and the secondary processing required
to achieve freedom over geometric design) in which thermoplastics can
deliver. Because of these issues, hybrid design solutions—able to combine the benefits of metal and plastic according to the exact functional
requirements—are often preferred.
Various techniques can be employed for metal/plastic hybrid design:
see Figure 1(a). In recent decades, nano-molding technology (NMT)1–6
has been widely adopted to replace traditional metal-insert moldings in
the consumer electronics industry: see Figure 1(b). To create nano- to
microsized holes, the metal surfaces are pretreated using an etching
process. The plastic part is then insert-molded directly onto the metal,
creating a strong bond at the interface. Due to the ease with which
intricate hybrid designs can be formed using this single-step injectionmolding process, the system cost of manufacturing metal parts is reduced compared to traditional metal-insert-molding routes.
Plastic resins used in designs such as these must be affinitive to metal
and compatible with the NMT process. Many thermoplastic resins are
suitable for this purpose.7 However, because the metal must be colored, a secondary processing step called anodization is often used after
the hybrid part is molded. In anodization, the material undergoes multiple steps of exposure to corrosive acids and solutions to achieve the
desired color. Chemical resistance, especially acid resistance, therefore
becomes a requirement for material selection, limiting the use of certain
polymers such as polyamides. Additionally, the resins can be reinforced
Figure 1. (a) Typical methods for metal/plastic hybrid design. In traditional metal-insert molding, a mechanical-lock design physically connects the plastic and metal parts. In-mold adhesive works by adding
an adhesive agent to improve the bond between the metal and plastics.
For metallization, a thin layer of metal is applied as a coating on the
plastic (e.g., aluminum), creating a metallic appearance. NMT: Nanomolding technology. (b) NMT-treated metal has small cavities on the
metal surface that are filled with plastic during the injection-molding
process, enabling the fabrication of hybrid metal/plastic devices.
with glass fibers to reduce shrinkage and enhance mechanical performance. The linear-expansion coefficients of the plastic compounds also
need to match those of the metals. Otherwise, excessive inner stress
could potentially lead to gaps and movement at the interface, leading
to reduced bonding.
Because of these factors—and the requirements for colorability, low
cost, and an inherent affinity to metal—crystalline polymer polyphenylene sulfide (PPS) and polybutylene terephthalate (PBT) resins have
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10.2417/spepro.005957 Page 2/3
Table 1. Technical data sheet of our THERMOTUF polybutylene terephthalate (PBT) compound portfolio compared with popular PBT/glass fiber
(PBT/GF) and polyphenylene sulfide/GF (PPS/GF) compounds.
Unit
Specific gravity
Tensile modulus
Tensile strength
Tensile elongation
Notched izod impact
energy
Mold shrinkage, flow,
3.2mm
Mold shrinkage, xflow,
3.2mm
Bonding force*
*SABIC internal
method
SABIC
WF008N
SABIC
WF006N
Competitive
PBT
(30wt% GF)
1.5
7900
95
2.5
147
Competitive
PPS
(30wt% GF)
1.5
8000
110
2.5
130
Competitive
PBT
(40wt% GF)
1.7
–
130
2.9
150
MPa
MPa
%
J/m
1.7
10300
127
3
151
%
0.2–0.4
0.1–0.2
0.2–0.4
0.4–0.6
0.4–0.6
%
0.6–0.8
0.2–0.4
0.6–0.8
0.6–0.8
0.6–0.8
MPa
30
35
30
26
36
Figure 2. Diagram of the injection-molded parts used for our bonding
strength test. (a) Lap joint. (b) Butt joint.
–
100
2.3
145
become popular selections, due to their inherently good chemical
resistance and ability to be compounded with fillers. PPS, a highly
crystalline resin, can form very good bonds during the NMT process.
However, its negative attributes include poor processability and aesthetics issues (including its limited color space, rough surface, difficulty
to paint/lacquer, and very poor weatherability). Semicrystalline resin
PBT, on the other hand, has good processability, is easily colorable,
has higher weatherability, and is halogen-free. It also has high stiffness, tensile strength, wear resistance, and low friction. Additionally,
its relatively low impact resistance and shrinkage can be improved via
formulation and proper processing.
Our variety of THERMOTUF metal/plastic compounds aim to
achieve high bonding strength between metal and plastics for NMT
applications.8 WF008N and WF006N are blends of proprietary polycarbonate PBT with glass fiber (GF, 40 and 30wt%, respectively).
We designed these GF-reinforced PBT compounds to enable excellent
bonding strength with metals, particularly aluminum, and to improve
the impact resistance and dimensional stability compared with other
GF-reinforced PBT compounds. In addition, these compounds are capable of superior aesthetics, with a wide color space and exceptional
UV resistance. These features make our PBT compounds an excellent
candidate for NMT, especially in consumer electronics applications.
As shown in Table 1, WF008N and WF006N exhibit well-rounded
mechanical performance compared to competitive PBT and PPS
compounds. Both products provide similar or improved bonding
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10.2417/spepro.005957 Page 3/3
The authors would like to thank the following team members for their
great efforts on this project: Zhenke Wei, Bing Guan, Norio Ozawa,
Takamune Sugawara, and Mark Park.
Author Information
Yuanqing He
SABIC
Mt Vernon, IN
Figure 3. Bonding strength of THERMOTUF WF008N and WF006N
GF blends compared to the PBT-based materials currently used for
these applications.
strength and much better shrinkage performance, and are also white
colorable. The L* (degree of lightness, determined using the CIE 1976
color space) can reach 91C and, after the anodizing process, the E (color difference) shift is less than three, indicating superior color
stability.
Figure 2 shows a diagram of the bar parts we fabricated for testing. ISO 19095, the standard test of adhesion-interface performance in
plastic-metal assemblies,9 is widely accepted by the industry for this
purpose.4 To test WF008N and WF006N, we developed a bondingstrength test method based on this standard. First, the metal parts are
pretreated to create nano- and microsized holes on the metal surface by
a chemical etching process. Within an effective treatment timeframe,
plastic is then injection-molded onto the pretreated aluminum insets.
Finally, bonding strength is measured on a standard tensile test machine by recording the force required for the parts to reach breaking
point as a result of pulling strength.
As shown in Figure 3, the bonding strength of the THERMOTUF
WF008N compound is similar at both joints. The WF006N compound
showed significantly improved bonding strength vs. the incumbent
PBT-based materials that are currently used for this application.
In summary, we have developed two proprietary PBT/GF compounds that are suitable for use in NMT. Our tests have shown that
when these compounds are employed in metal/plastic hybrid devices,
the bonds that they achieve are stronger than those provided using thermoplastic materials currently on the market. In the next stage of our
research, we intend to increase our fundamental understanding of bonding behaviors for NMT (e.g., the correlation between metal cavity size
and bonding strength, and the critical building blocks of blends) to provide new technologies and solutions for better metal/plastic hybrid design. Advancements in this area will enable the development of lighter
portable electronic devices with greater resilience.
Yuanqing He earned her PhD in chemical engineering from the State
University at Buffalo, NY, and is currently a lead scientist of Technology & Innovation at SABIC.
Xiaoyu Sun
SABIC
Exton, PA
Xiaoyu Sun has a PhD and is also a member of SPE, Sigma Xi, IEEE,
and ACS. She is currently a staff scientist of Technology & Innovation
at SABIC.
Harold Ho
SABIC
Taipei, Taiwan
Harold Ho is a senior business development manager and works in the
consumer electronics and innovative plastics industries at SABIC.
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c 2015 Society of Plastics Engineers (SPE)