Against the backdrop of modern manufacturing's shift toward high precision, multi-functionality and green development, multi-material composite molding has become a core approach to boosting product competitiveness. Two-shot injection molding, rotary two-shot molding and overmolding, as key technologies in this field, resolve the drawbacks of traditional assembly processes—such as low efficiency, poor precision and high costs—through integrated molding advantages. Widely applied in consumer electronics, automotive manufacturing and medical devices, these technologies share common technical principles while differing in equipment configuration, production efficiency and application scenarios. A systematic analysis is provided below.
Core Principle
Two injection units work with a dedicated mold to sequentially inject two materials or colors in a single molding cycle. Interface bonding is realized via molecular diffusion or mechanical interlocking to form integrated composite products, eliminating subsequent assembly procedures.
Key Technical Parameters
Mold coaxiality error ≤ 0.02mm; temperature difference between molten materials controlled within 30℃; molding cycle 15%-30% longer than that of single-color injection molding.
Typical Applications
Consumer electronics casings (PC+TPU two-color gradient), automotive interior parts (ABS+soft-touch layer) and medical devices (PEEK+silicone sealing structures).
Equipment Structure and Workflow
The system core comprises a two-station injection molding machine and a servo-driven rotary mold. The workflow is as follows: first injection to form the base material → 180° mold rotation → second injection of composite material → mold opening and part ejection, all completed automatically.
Process Advantages and Application Scope
Positioning accuracy up to ±0.01mm with repeat positioning error ≤ 0.005mm; suitable for mass production (annual capacity ≥ 500,000 units) with a molding cycle of 15-30 seconds per part; applicable to axisymmetric products and complex structural components.
Key Production Control Points
Coordination error between rotation action and injection timing ≤ 0.1s; zoned mold temperature control with cavity surface temperature difference ≤ 5℃; optimized runner design to prevent molten material from scouring the formed base material during the second injection.
Process Characteristics and Technical Differences
Two independent molds are used for sequential molding: first, the rigid base material is injected and cooled, then transferred to the second mold for overmolding with a secondary material. Compared with rotary two-shot molding, it features lower mold costs and higher production flexibility for product switching.
Material Selection and Compatibility Requirements
Priority is given to material combinations with similar polarity; interface bonding strength ≥ 2.5MPa (compliant with ISO 8510 standard); laser micro-nano structuring (Ra3.2μm) on the base material surface can improve bonding strength by 30%.
Solutions to Common Production Issues
For insufficient bonding strength: increase secondary injection temperature by 10-20℃ and extend holding time. For dimensional deviation: adopt precise mold temperature control and ensure cooling time ≥ 1.5 times the base material solidification time. For surface defects: optimize exhaust system and enhance material drying process.
Intelligent Upgrading
Digital twin technology reduces mold testing times by over 40%; AI adaptive process systems adjust parameters in real time, lowering defect rates to below 1.2%.
Green Transition
Application ratio of bio-based materials (PLA+PHA) increases with degradation rate ≥ 90%; recycled material compound modification technology enables circular utilization, cutting costs by 20%.
Application Selection Suggestions
Rotary two-shot molding is preferred for mass-produced precision products; overmolding suits small-batch and multi-variety production; two-shot injection molding combined with in-mold decoration is applicable for high-end functional components.
The advancement of these three technologies fundamentally responds to the manufacturing industry's core demands for efficiency, precision and environmental protection. With the continuous integration of intelligent technologies and green materials, they will further break through limitations in material compatibility and molding complexity, unlocking greater application potential in new energy vehicles, high-end medical equipment and smart wearables. For manufacturers, precise grasp of the technical boundaries and application scenarios of each process, combined with capacity needs and product positioning for selection, is the key to reducing costs, improving efficiency and increasing product added value. In the future, process integration, equipment intelligence and material greening will become mainstream trends, driving multi-material molding technologies toward higher efficiency, greater precision and more sustainable development.
