Injection molding through hole into blind hole and solution
In injection molding, through-hole-to-blind-hole refers to a hole structure originally designed to be through that becomes a blind hole with a closed bottom after the plastic part is molded. This defect not only affects the assembly performance and functional realization of the plastic part, but may also cause subsequent processing steps to fail to proceed normally. It is one of the more difficult problems in injection molding. The occurrence of through-hole-to-blind-hole is usually closely related to factors such as poor flow during the melt filling process, poor mold exhaust, unreasonable process parameter settings, or mold structure design defects. For example, when the melt is filling the cavity area corresponding to the through hole, if it encounters excessive resistance or poor exhaust, the melt will not be able to completely fill the bottom of the cavity, thus forming a closed blind hole. In addition, too fast a cooling rate may also cause the melt to solidify before completely filling the through hole, resulting in blockage of the through hole.
One of the main causes of through-holes becoming blind holes is poor mold structural design, typically manifested in improper runner and gate design, poor cavity venting, or core dimensional deviations. Runners and gates are pathways for the melt to enter the mold cavity. If the runner cross-section is too small or the gate is positioned far from the through-hole area, the melt will experience insufficient pressure and velocity upon reaching the cavity, preventing it from filling the cavity. For example, in a plastic part with an elongated through-hole, if the gate is positioned at the end away from the through-hole, the melt will experience excessive pressure loss during flow, unable to advance further by the time it reaches the bottom of the through-hole, thus forming a blind hole. Furthermore, inadequately designed or improperly positioned venting slots in the mold cavity prevent gas from escaping, hindering the melt from fully filling the cavity and resulting in a closed structure. As a key component in forming the through-hole, a core that is too long or misplaced can also fit too tightly against the cavity bottom, preventing the melt from entering and causing the through-hole to become blind.
Improper process parameter settings are also an important factor that causes through-holes to become blind holes, involving multiple aspects such as injection speed, injection pressure, melt temperature, and mold temperature. When the injection speed is too slow, the melt cools faster during the flow process, and the viscosity increases and the fluidity decreases when it reaches the through-hole cavity, making it difficult to fill the entire through-hole area; while an injection speed that is too fast may cause turbulence in the melt when entering the through-hole cavity, entraining air to form bubbles, hindering further filling of the melt. Insufficient injection pressure cannot overcome the resistance in the melt flow process, especially for through-holes with complex structures, which require sufficient pressure to push the melt to the end of the cavity; but if the pressure is too high, it may cause the melt to overflow at the through-hole entrance, which in turn affects subsequent filling. Too low melt temperature and mold temperature will increase the melt viscosity and deteriorate the fluidity, making it easy for the through-hole to be underfilled, eventually forming a blind hole.
To address the issue of through-holes becoming blind holes, solutions can be implemented through mold optimization and process parameter adjustment. Regarding mold optimization, first, the runners and gates should be properly designed to ensure that the melt can reach the through-hole cavity with sufficient pressure and speed. For slender through-holes, multi-point gates or sequential valve gate technology can be used to reduce melt flow resistance. Second, venting grooves should be added at the bottom and around the through-hole. The depth and width of the venting grooves should be determined according to the plastic type, typically with a depth of 0.02-0.05mm and a width of 5-10mm to ensure smooth gas discharge. Furthermore, the core size and installation position should be checked and adjusted to ensure sufficient clearance between the core and the bottom of the cavity to facilitate melt filling. For example, when producing cylindrical plastic parts with axial through-holes, the core can be designed to be stepped, leaving a gap of 0.5-1mm at the bottom to ensure that the melt can completely fill the through-hole.
When adjusting process parameters, the injection speed and pressure should be appropriately set based on the plastic’s characteristics and part structure. For parts prone to through-holes becoming blind holes, a “slow-fast-slow” injection speed strategy can be employed. Specifically, a slow injection speed is employed in the initial stage to prevent melt splashing and air entrainment; the injection speed is increased as the through-hole area is filled to enhance melt fluidity; and the speed is reduced again near completion to prevent overflow. Simultaneously, the injection pressure should be appropriately increased to ensure the melt can overcome flow resistance and reach the bottom of the through-hole. However, care should be taken not to overly increase the pressure to avoid mold damage or flash. Furthermore, the melt and mold temperatures should be increased to reduce melt viscosity and improve fluidity. For example, for high-viscosity plastics like polycarbonate, the melt temperature can be increased by 10-20°C and the mold temperature by 5-10°C to facilitate melt filling of the through-hole cavity. In actual production, the through-hole area can also be fed by increasing the holding pressure and extending the holding time to ensure that the melt completely fills the cavity and avoid blind hole defects. By combining mold optimization with process adjustment, the problem of injection molded through holes becoming blind holes can be effectively solved, ensuring the quality and performance of plastic parts.
