Design and Machining of Injection Molds for Mobile Phone Middle Frames
2026-05-26 13:44:11
Injection Molds
The mobile phone middle frame is a core structural component of smart phones, which undertakes multiple functions including appearance decoration, load bearing, and integration of buttons and interfaces. It features thin wall thickness, complex shape and strict dimensional tolerance, putting forward high requirements on structural design, material selection and machining technology of injection molds. Combining with product characteristics of mobile phone middle frames, this paper elaborates technical points from mold overall design, core structure planning, material selection, full-process machining technology and trial run optimization.
Product Characteristics and Preconditions for Mold DesignThe mobile phone middle frame is a long frame structure with numerous special-shaped holes, grooves for buttons, charging ports, camera holes and card trays around it. Its wall thickness is generally controlled within 0.8 to 1.2 mm, belonging to typical thin-walled precision plastic parts. The product surface requires high gloss without weld lines, sink marks, bubbles and warpage. Uniform assembly dimensions must be ensured to meet the assembly precision of the whole device.
Carry out product process analysis before formal mold design. Determine demolding direction and draft angle. Adopt small draft angle for thin-walled areas to prevent surface scratches. Sort out holes, undercuts and concave structures, and plan layout of demolding mechanisms such as lifters and slides. Determine cavity quantity according to production volume. Single-cavity molds are applied for small and medium batch production, while symmetrical double-cavity molds are adopted for mass production. Ensure consistent runners, cooling systems and ventilation conditions of each cavity to avoid quality differences among products.
Overall Structural Design of Molds
Parting Surface, Cavity and Core DesignTwo-plate molds are preferred for mobile phone middle frames. Set parting surfaces along the maximum outline of the frame to ensure products stay on the moving half after mold opening. Adopt split structure for cavities and cores. Split complex holes and local grooves into independent inserts, which facilitates machining, polishing and later maintenance, and reduces deformation by releasing machining stress. Reserve sufficient polishing allowance on appearance forming surfaces to achieve mirror finish finally.
Gating System DesignFocus on fast filling and low flow resistance for thin-walled parts. Apply standard main runners and circular cross-section sub-runners to improve melt fluidity. Side gates or fan gates are arranged on non-appearance sides of the frame to avoid gate marks on product surfaces. Adopt multi-point gating for ultra-long frames to balance melt flow, shorten filling time, reduce internal stress and relieve product warpage. Adjust cross-section size of runners and gates according to material fluidity to balance filling speed and sprue removal.
Cooling System DesignUneven cooling easily causes deformation and dimensional deviation of thin-walled products, so cooling systems are the key of mold design. Adopt conformal surrounding cooling channels close to forming surfaces of cavities and cores with uniform spacing. Add denser channels at corners, ports and heat concentration areas. Match channel diameter reasonably to ensure smooth cooling water circulation and distinguish inlet and outlet directions strictly for uniform mold temperature control. Design straight or baffle internal cooling structures inside slender cores to avoid local overheating and inner wall deformation of products.
Demolding and Core-pulling Mechanism DesignA large number of through holes, blind holes and undercuts on sides and ends of middle frames require matching slide and lifter core-pulling mechanisms. Use lifters for small and shallow undercuts due to compact structure and stable movement, and adopt slides for deep and long undercuts. Control fitting clearance of slides strictly to ensure flexible sliding without flash. Combine thin ejector pins and ejector sleeves for ejection systems. Arrange ejection points evenly away from appearance surfaces and stressed areas to prevent penetration and whitening of thin-walled products. Install wear plates and limit structures on all moving parts to maintain long-term operation accuracy.
Ventilation System Design
High-speed melt filling of thin-walled cavities easily causes air trapping, resulting in burns, flow marks and incomplete filling. Set continuous exhaust grooves on parting surfaces, melt flow ends, weld lines and around holes with depth of 0.01 to 0.015 mm. Use clearance of inserts, lifters and slides for auxiliary ventilation to exhaust internal air comprehensively and guarantee complete filling and good appearance quality.
Mold Material Selection
Molds for mobile phone middle frames require high hardness, wear resistance, polishing performance and corrosion resistance. Select high mirror finish mold steel for core forming parts including cavities, cores and inserts. This type of steel has dense structure, small deformation after heat treatment and can reach mirror finish after polishing, which is suitable for production of high-gloss appearance parts. Choose wear-resistant mold steel for moving parts such as slides, lifters and ejector pins to extend service life. Select high-quality structural steel for mold plates and support plates to ensure overall mold rigidity and prevent plate deformation in long-term production. Conduct flaw detection and hardness test for incoming raw materials, and perform aging treatment on blanks to release internal stress in advance.
Key Points of Mold Machining TechnologyFollow the standard procedure: rough machining, semi-finishing, heat treatment, finishing, polishing and assembly. Remove most material allowance in rough machining with uniform reserved allowance and control cutting parameters to reduce cutting stress. Control heating, holding and cooling procedures strictly in heat treatment, and apply multi-stage tempering to eliminate internal stress and minimize part deformation.
Carry out finishing after heat treatment. Process profiles and datums by CNC milling, machine narrow grooves, special-shaped holes and insert fitting surfaces by wire EDM, and process complex dead corners by EDM. Unify machining datums in the whole process and control dimensional tolerance strictly. Use precision grinders for flat surfaces and fitting surfaces to guarantee flatness, parallelism and fitting accuracy.
Polish step by step from coarse oil stone to fine abrasive paper and diamond paste along demolding direction. Avoid dimensional changes caused by excessive polishing and achieve mirror finish on appearance surfaces. Remove all burrs after machining and match all fitting parts one by one to ensure reasonable clearance and flexible movement.
Trial Run and Post OptimizationCarry out trial run after mold assembly. Adopt low speed and low pressure at the initial stage to check the operation of mold opening, closing, core pulling and ejection. Adjust injection temperature, pressure, speed and mold temperature gradually, and observe filling status, appearance defects and deformation of products. Optimize ventilation systems, cooling channels and gate layout aiming at flow marks, sink marks and warpage. Conduct local trimming for out-of-tolerance dimensions. Solidify process parameters after qualified trial run and complete mold acceptance.
Perform daily maintenance in mass production, including regular cleaning of exhaust grooves, inspection of wear of moving parts and maintenance of cooling channels. Troubleshoot minor faults timely to sustain mold accuracy and production stability.
SummaryInjection molds for mobile phone middle frames belong to high-precision thin-walled molds. Multiple factors including product structure, melt filling, cooling, demolding and ventilation need to be considered comprehensively in design stage, and layout of various mechanisms should be planned reasonably. Control accuracy and deformation of each procedure strictly in machining, and select matched mold steel and elaborate polishing technology. Coordinate design, material selection, machining and trial run to manufacture qualified molds meeting requirements of appearance, dimension and mass production, and support stable batch production of mobile phone middle frames.
