The bonding strength in double-shot injection molding is determined by both „material compatibility“ and „process parameters,“ rather than solely by the types of materials used. Combinations of the same material (e.g., ABS/ABS or PBT/PBT) yield the highest bonding strength, reaching 44–52.7 MPa. Combinations of different materials (e.g., ABS/PBT) show a slight decrease in bonding strength but still achieve a robust 38.5–43.7 MPa. The „mixed material“ approach widely used in the keycap industry—such as PBT+ABS alloys—essentially trades some compatibility for better processability; it sacrifices a degree of bonding strength to reduce the difficulty of injection molding and improve production yields.
I. Same-Material vs. Different-Material Combinations: What the Data Says
Research on double-shot injection molding provides direct comparative data on bonding strength. ABS/ABS combinations show a strength of 44.0–48.4 MPa, while PBT/PBT combinations reach 51.3–52.7 MPa. In contrast, the ABS/PBT combination—involving two different materials—yields a strength of 38.5–43.7 MPa.
This data reveals two key points: First, the self-bonding strength of PBT is higher than that of ABS, which is attributed to PBT’s greater molecular chain rigidity and higher crystallinity. Second, while the bonding strength of dissimilar materials is lower than that of identical materials, it remains quite high; the strength of an ABS/PBT combination is only about 10 MPa lower than that of the single-material counterparts.
II. Material Compatibility: Why ABS and PBT Can „Bond“ Together
Chemically, ABS and PBT are not inherently compatible. ABS is an acrylonitrile-butadiene-styrene copolymer, while PBT is polybutylene terephthalate; the two differ in terms of polarity, crystallization behavior, and coefficients of thermal expansion.
The „mixed material“ solutions frequently employed in the keycap industry typically do not consist of pure ABS paired with pure PBT. Instead, they utilize PBT/ABS alloys or composite materials enhanced with compatibilizers. Patent literature regarding glass fiber-reinforced PBT keycap materials explicitly mentions the addition of a compatibilizer (2–6 parts), which serves to improve interfacial bonding between the PBT and ABS phases. Without a compatibilizer, the interface between dissimilar materials is prone to microscopic delamination, which may lead to complete separation over long-term use.
Even with a compatibilizer, the interface between dissimilar materials remains a „weak link.“ A known risk associated with dual-material (two-shot) keycaps is that differences in the coefficients of thermal expansion between the materials can cause subtle delamination after extended use.
III. The Decisive Influence of Process Parameters on Bond Strength
While material compatibility establishes the „upper limit“ for bond strength, process parameters determine how closely that limit is approached in practice.
Melt temperature is the most critical variable. A 2024 study on the multi-material injection molding of PC, PC-ABS, ABS, and SEBS concluded that the material with the lower melting temperature should be injected during the first shot to achieve superior bond strength. The underlying mechanism is that the high-temperature melt from the second shot must provide sufficient heat upon contact with the surface of the first-shot component to induce „re-melting“ or softening. This allows molecular chains from both phases to diffuse and entangle across the interface. If the material from the first shot lacks sufficient heat resistance, the heat from the second shot may fail to activate the interface, resulting in reduced bond strength.
Mold temperature is equally significant. Research has yielded an empirical equation based on interfacial temperature, with a goodness-of-fit (R²) exceeding 0.96. This indicates that interfacial temperature—governed by both melt temperature and mold temperature—is the core variable for predicting bond strength.
Thermal resistance and positioning accuracy are also critical factors. During the second shot, the incoming melt may induce thermal stress at the interface while simultaneously softening the first-shot component. Furthermore, if the first-shot component is not positioned with sufficient precision within the second-shot mold cavity, the melt may „cross“ the intended boundary, leading to issues such as flash or the encroachment of transparent resin into opaque regions.
IV. Practical Trade-offs in the Keycap Industry
The keycap industry faces a fundamental dilemma: while PBT is more durable than ABS, using pure PBT for injection molding is difficult and results in low production yields.
PBT has a low melt flow index (MFI) and poor fluidity, making it challenging to fill the thin-walled channels required for double-shot molding. Attempting to produce double-shot keycaps using pure PBT often leads to issues such as burrs along the edges of the legends, blurred characters, or even broken pins. Consequently, many products marketed as „PBT double-shot keycaps“ are actually made from a blend of PBT and ABS, or feature an outer layer of PBT combined with an inner layer of ABS.
The trade-off for this hybrid approach is reduced wear resistance. In terms of durability, the general consensus in the keycap industry ranks methods as follows: dye-sublimation > pure PBT double-shot molding > ABS/PBT double-shot molding ≈ ABS double-shot molding. Because the double-shot process often necessitates blending PBT with ABS, such keycaps may exhibit lower resistance to wear (polishing) compared to those made via dye-sublimation or pure PBT double-shot molding.
The bond strength in double-shot molding is a complex issue: the theoretical upper limit is determined by material compatibility, while the actual value is dictated by process parameters. Combining identical materials (ABS/ABS or PBT/PBT) yields the highest interfacial strength. When combining dissimilar materials (ABS/PBT), the use of compatibilizers and process optimization can achieve a bond strength of 38.5–43.7 MPa—sufficient for daily use—though long-term durability is constrained by differences in the materials‘ coefficients of thermal expansion. Ultimately, the industry’s adoption of hybrid material solutions represents a strategic balance between bond strength, manufacturing yield, and wear resistance.
