Application experience of rear mold slide mechanism
The rear mold slide mechanism is a key device for achieving undercut demolding in injection molds for complex plastic parts. The rationality of its design directly determines product quality and production stability. In the production of automotive interior parts, a dashboard side cover had three internal undercuts. A rear mold inclined guide post slide mechanism was used for demolding. Initially, the clearance between the slide and the guide groove was too large (0.05mm), causing the slide to wobble during movement and straining at the undercuts, resulting in a high defect rate of 12%. By adjusting the clearance to 0.02-0.03mm and adding wear blocks to the bottom of the slide using SKD11 material with a hardness of HRC55, the slide movement precision was improved and the straining phenomenon was eliminated. The installation of the wear blocks also extended the mold maintenance cycle from 80,000 to 150,000 cycles, significantly reducing production costs.
The power transmission method for the rear mold slide should be determined based on the undercut size and production cycle. For large-scale plastic parts with long slide strokes, hydraulic drive offers advantages over mechanical drive. The rear mold of a refrigerator door liner had an undercut as long as 150mm. The slide was originally driven by an inclined guide pin. However, due to the excessive stroke, the guide pin length reached 200mm. This resulted in bending deformation due to melt pressure during injection molding, and the slide’s positioning accuracy varied by as much as 0.1mm, causing dimensional deviations in the part. Switching to a hydraulic cylinder drive, with precise cylinder stroke control via a servo valve, resulted in repeatability of ±0.01mm, significantly improving part dimensional stability and raising the CPK value from 1.2 to 1.8. Furthermore, the hydraulic drive allows for adjustable slide speed, allowing for a low speed (5mm/s) during the initial core pulling phase to prevent part deformation and a high speed (15mm/s) later in the process to shorten cycle time. This reduced production time per mold by 4 seconds, meeting the demands of high-volume production.
The locking mechanism design of the rear mold slide is crucial for preventing slide back during injection molding. Improper locking methods can lead to flash or dimensional deviation in the molded part. The slide in the rear mold of a mobile phone charger housing used a wedge-type locking mechanism. However, because the wedge-type angle was 1° less than the slide’s pitch, the locking force was insufficient. During high-pressure injection molding, the slide experienced a slight slide back (0.03mm), resulting in a 0.05mm flash on the edge of the molded part. By adjusting the wedge-type angle to 1° greater than the slide’s pitch and adding a spring-assisted preload, the locking force was increased by 30%, the slide back was controlled to within 0.005mm, and the flash defect was completely eliminated. Furthermore, an oil groove on the contact surface between the wedge-type slide and the slide, regularly filled with high-temperature grease, reduced wear between the two and extended the service life of the locking mechanism to over 200,000 mold cycles.
The design of the cooling system for the rear mold slide is often overlooked, but in fact, sufficient cooling of the slide can effectively improve production efficiency and product quality. The slide of the rear mold of a laptop computer bottom case is responsible for forming the inverted USB interface. Because there is no cooling water channel inside the slide, the cooling time of this area is 8 seconds longer than other parts, which becomes a bottleneck restricting the production cycle. By drilling a φ4mm spiral cooling water channel inside the alloy steel slide, the cooling water can be close to the inverted forming surface, and the cooling time is shortened to the same as other parts. The production cycle of each mold is reduced by 8 seconds, and the average daily production capacity is increased by 15%. At the same time, the stabilization of the slide temperature reduces its thermal deformation, the dimensional accuracy of the USB interface is improved from ±0.05mm to ±0.02mm, the consistency of the plug-in and pull-out force is significantly improved, and the customer complaint rate has dropped by 90%.
The use of rear mold slides in specialized environments requires targeted protective measures. For example, in the injection molding of high-temperature materials (such as PA66+GF30), the slide mechanism is susceptible to high temperatures and can become stuck. In an engine intake manifold made of PA66+GF30, the molding temperature reaches as high as 280°C. During continuous production, the rear mold slide expanded due to heat, eliminating the clearance between the slide and the guide groove, causing the slide to become stuck and halting production. The technical team implemented a thermal expansion compensation design: the slide material was replaced with H13 mold steel, which has a lower thermal expansion coefficient, and the clearance was increased to 0.08mm. Graphite wear-resistant sheets were also embedded in the guide groove to enhance lubrication. These improvements ensured the slide remained flexible even at high temperatures, allowing continuous production of 50,000 molds without any sticking. The equipment utilization rate increased from 85% to 98%, creating significant economic benefits for the company.
