Application experience of slide mechanism of injection molding inner mold
The inner mold slide mechanism (also known as the inner core pulling mechanism) is a key device for preventing internal undercuts (such as internal threads and grooves) in plastic parts. Its design requires a balanced core pulling force, stroke, and mold space. A cosmetic bottle cap mold with internal threads uses an inner mold slide mechanism with a 30mm core pulling stroke, eliminating the mold release issue with traditional ejectors and increasing production efficiency fivefold compared to manual thread stripping. The inner mold slide drive method is selected based on the core pulling force. Cylinder drive is suitable for high core pulling forces (>50kN). For example, the inner groove of an automotive water pipe connector has a core pulling force of 60kN. A 50mm diameter cylinder, coupled with a guide slide, ensures smooth and non-binding core pulling. For smaller core pulling forces (<10kN), an inclined guide post can be used. For example, the inner groove of an electronic connector has a core pulling force of 8kN. The inclined guide post has a 15° angle, resulting in a compact design and 40% lower cost than cylinder drive. The clearance between the slide and the cavity needs to be controlled at 0.01-0.03mm. A mold had flash on the plastic part due to the gap being too large (0.05mm). After adjustment, the flash was eliminated and the appearance qualification rate increased from 90% to 99%.
The guidance and positioning accuracy of the inner mold slide directly affects the size of the plastic part. High-precision slides and positioning devices are required to ensure that there is no offset during the core pulling process. The slide selection must match the slide weight and speed. A 5kg inner mold slide uses a linear slide (accuracy level C3) with a parallelism error of 0.01mm/m, which is 60% higher than the positioning accuracy of ordinary sliders. When the slide speed exceeds 50mm/s, a buffer device must be added. A mold has reduced the impact noise from 85dB to 70dB by installing a polyurethane buffer block at the end of the stroke, while also reducing component wear. The positioning device usually uses a combination of a locating pin and a locating block. After the core is pulled into place, the locating pin is inserted into the hole of the locating block, with a repeatable positioning accuracy of ±0.005mm. In this way, the inner hole slide mechanism of a precision gear controls the inner hole size deviation to ±0.01mm, meeting the matching requirements. The return accuracy of the slider is equally important. A mold had a cavity misalignment due to inadequate return, and the wall thickness deviation of the plastic part was 0.2mm. After adding a travel switch detection, the misalignment fault was eliminated.
The cooling design of the inner mold slide is often overlooked. In practice, it’s crucial to ensure consistent temperatures between the slide and the cavity to prevent stress in the plastic part caused by temperature differences. Cooling water channels, typically 6-8mm in diameter , are required within the slide and connected in series with the cavity water channels. In one case, the slide temperature was 10 °C higher than the cavity, resulting in sink marks on the inside of the part. Replacing the water channels eliminated these sink marks. For small slides ( <50mm ), beryllium copper inserts can be used to aid cooling. For a micro connector, the inner mold slide utilizes beryllium copper (thermal conductivity 200W/(m・K) ), which achieves five times the cooling efficiency of steel slides ( 40W/(m・K) ) , maintaining a stable slide temperature of 60±2 °C. The sealing of the cooling water channel must be reliable. The aging of the sliding water channel sealing ring of a certain mold caused water leakage, affecting the sliding movement. After replacing the high-temperature resistant sealing ring (temperature resistant to 150°C), the sealing life was extended from 10,000 mold times to 50,000 mold times.
Failure prevention for the inner mold slide mechanism requires attention to wear and lubrication, and regular maintenance can significantly extend its service life. The contact surface between the slide and the guide rail requires a coating of high-temperature grease (resistant to 180°C), replenished every 1,000 molds produced. Insufficient lubrication caused the guide rail to wear 0.1mm, costing 2,000 yuan to replace. After standardized lubrication, the wear rate was reduced by 80%. Wearing parts (such as inclined guide pins, sliders, and springs) require regular inspection. The diameter of the inclined guide pin of one mold decreased by 0.5mm due to wear, reducing the core pulling force. Premature replacement avoided production interruptions. The slide limit device must be secure. The slide limit block of one mold became loose, causing core pulling to overtravel and collision damage to the cavity. After reinforcing the limit block and adding a lock nut, the same type of failure has not occurred again. For materials containing glass fiber, the slide must use a wear-resistant coating (such as TiN). For a PA66+GF30 plastic part mold, after the slide was coated, the wear was reduced from 0.02mm/10,000 mold times to 0.005mm/10,000 mold times.
Inner mold slides used in special operating conditions require targeted measures to address the challenges of high temperature, high pressure, or corrosive environments. For inner mold slides used with high-temperature materials (such as PEEK, processing temperatures of 380°C), high-temperature-resistant steel (such as H13) must be used, and cooling strength must be increased. For a PEEK mold, the slide cooling channel diameter was increased from 8mm to 12mm, increasing flow by 50%. The slide temperature was kept below 180°C, preventing material adhesion. For slides used in high-pressure molding (injection pressures > 200MPa), increased clamping force is required. For one mold, increasing the contact area of the locking block (from 50mm² to 100mm²) ensures the slide retains its shape under high pressure, improving part dimensional stability by 30%. For inner mold slides used with corrosive materials (such as PVC), anti-corrosion treatment (such as chrome plating) is required. Chrome plating (0.05mm thickness) on the slides of a PVC pipe fitting mold reduced corrosion by 90%, extending its service life from 30,000 to 100,000 cycles. In addition, automated inspection of the inner mold slide is essential. A certain production line installs proximity switches to detect the slide position. When the slide is not in place, the machine is shut down immediately to avoid batch scrapping, and the defective rate is reduced from 2% to 0.1%.
