Technical Difficulties of Micro Injection Molds for Smart Wearable Device Housings
Smart wearable housings include watch middle frames, bracelet bottom shells and sensor brackets, which are micro thin-walled plastic parts. The products are small in overall size, with locally thin walls, and a large number of tiny buckles, positioning posts, sealing grooves and optical windows. They have strict standards for appearance, dimensional accuracy, assembly clearance and weather resistance. Mold development needs to take micro structure forming, deformation control and mass production stability into consideration. Many technical difficulties are concentrated in processing, molding, demolding and process control links.
Difficulties in Machining of Micro Structure Cavities
Tiny ribs, micro buckles, thin-walled areas of 0.15mm to 0.3mm and micro sealing ridges on products are difficult forming points. Such structures cannot be directly processed by conventional CNC, and need to be completed by wire EDM, mirror spark erosion and optical grinding. Electrode manufacturing itself is difficult. Micro electrodes are easy to deform and wear quickly. Slight deviation of discharge gap will cause over-tolerance of rib wall thickness and poor forming of undercuts. The position degree and coaxiality of cavity features are high. A large number of micro features are concentrated in a small area. Once the processing reference is offset, the tolerance chain of multiple features will be superimposed, directly leading to assembly interference in the later stage. The mold generally adopts split insert design. Micro inserts are small in size and weak in rigidity, easy to shift and deform during processing and assembly. The positioning and locking structure of inserts needs special optimization to prevent displacement after long-term production.
Difficulties in Gating and Filling Balance
The proportion of runners in micro plastic parts is relatively high. Cold material and shear heat have an amplified impact on products. Many wearable housings adopt PC, PC/PMMA, glass fiber reinforced PEEK and modified PPS. Some materials have poor fluidity, and the filling resistance in thin-walled areas is large. The gate size cannot be too large to avoid gate residue affecting appearance and assembly. Too small gate will increase shear, produce high-temperature carbon deposits and silver streaks, and also cause insufficient filling. The flow balance of multi-cavity molds is difficult to control, and the weight and dimensional consistency of products in each cavity are easy to fluctuate. Although valve hot runners can improve filling, the flow channels of micro nozzles are thin and easy to block and carbonize, with a narrow temperature control range. Slight temperature fluctuation will cause material degradation. It is difficult to control the position of weld lines. If weld lines fall on sealing surfaces and stressed buckles, the structural strength will be reduced, affecting waterproof performance and durability.
Difficulties in Controlling Exhaust and Burning Defects
The ribs inside the micro cavity are dense and the filling path is long and narrow. The air inside the cavity is difficult to discharge quickly when the melt advances. The exhaust groove cannot be wide. Once the clearance exceeds the standard, tiny flash will be produced. Flash stuck in buckles and sealing grooves is difficult to clean and directly destroys the waterproof structure. If the exhaust clearance is too small, burning, short shot and bubbles will appear at the end. Burning and air marks on the optical window will cause product scrapping. Conventional exhaust layout is difficult to cover all the ends of tiny ribs. With the assistance of mold flow analysis, distributed exhaust is set at the melt convergence positions and thin-walled ends, and the exhaust depth and length are controlled at the same time, taking both exhaust effect and flash prevention requirements into account.
Difficulties in Preventing Damage of Demolding Mechanism and Micro Structures
Many undercuts of wearable housings are tiny and thin-walled. Scratch, whitening and deformation during demolding are high-frequency problems. If slider core pulling is adopted for tiny undercuts, the slider stroke is small and the matching clearance is extremely difficult to control. Large clearance produces flash, while small clearance easily scratches the product surface. When inclined ejector structures are adopted, the inclined ejector body is slender, insufficient in rigidity and easy to jam and break during movement. Ejector pins and sleeves are tiny in size. The ejection force is concentrated on a small area. Slightly fast ejection speed will cause whitening and cracking of thin walls. There is limited space for ejector layout. Many areas are appearance surfaces and optical surfaces, where ejector mechanisms cannot be arranged. Only the position with good rigidity can be selected for ejection according to the distribution of product wrapping force, and the ejection synchronization is controlled to avoid twisting of the product caused by uneven stress.
Difficulties in Deformation and Dimensional Stability
Most smart wearable housings need to be precisely matched with glass, sealing rings and main boards, with strict requirements on flatness and profile tolerance. Thin-walled structures cool fast, and shrinkage differences in different areas will bring warpage and distortion. Glass fiber reinforced materials have obvious anisotropic shrinkage, and the deformation direction is difficult to predict. The impact of mold temperature fluctuation on micro plastic parts is much greater than that on conventional plastic parts. Uneven mold temperature will amplify shrinkage differences and increase the dimensional dispersion of products in the same batch. It is difficult to set the cavity shrinkage compensation accurately at one time for micro sizes, with low error tolerance. Repeated verification is required in the trial mold stage, and a stable mold temperature control system is matched to reduce molding differences between each mold.
Difficulties in Mass Production Wear Resistance and Maintenance
Many wearable products adopt wear-resistant and chemical-resistant modified engineering plastics, some of which are added with glass fiber. Continuous production will wear the micro cavity, rib cutting edges and sealing ridge positions. Slight wear of the cavity will change the wall thickness, and the assembly size will change accordingly. Micro inserts, inclined ejectors and tiny ejector pins are wearing parts. The spare parts have long processing cycle. The assembly requires high operation skills for workers, and it is easy to scratch the surrounding mirror cavity during disassembly and assembly. It is difficult to clean the mold. Carbon deposits in narrow runners and exhaust grooves are inconvenient to clean, and the size of cleaning tools is limited. Improper operation easily scratches optical surfaces and sealing surfaces, requiring special maintenance processes and tools.
Difficulties in Trial Mold Verification and Inspection
The product features are tiny, and conventional calipers cannot measure accurately. Video measuring instruments and CMM are required to detect tiny rib thickness, undercut size and sealing ridge height. There are many inspection points and it takes a long time. It is difficult to identify defects during trial mold. Tiny air marks, scratches and micro deformation are not easy to find with naked eyes, and problems are exposed only in the assembly stage. In addition to dimensional inspection, waterproof assembly, drop test and button linkage assembly verification need to be carried out synchronously. Many products are qualified when checked separately, but uneven clearance and jamming appear after assembly, increasing the workload of trial mold iteration.
