Design Method for Improving Bonding Force of Overmolding Injection Mold

Overmolding products often have problems such as delamination, peeling and forced separation of hard and soft rubber layers. In addition to materials and injection molding processes, mold structure design is the core key affecting bonding strength. Reasonable mold structure, parting layout, gate feeding mode, sealing and positioning design can strengthen the bonding effect of hard and soft rubber from structural interlocking, melt fusion and close fitting, fundamentally solve the common problem of insufficient overmolding bonding force, and provide basic guarantee for stable mass production of bonding quality.

1. Mold Forming Design of Product Embedding Structure(1) Reserve Mechanical Undercut and Clamping Groove Structure

Small undercuts, grooves, bosses and through-hole structures are directly designed in the mold cavity at the hard rubber molding area, allowing the post-injected soft rubber to embed and engage to form physical interlocking. Relying not only on pure chemical bonding, but also forming buckle-type interlocking structure directly through the mold, greatly improving the peeling resistance, suitable for overmolding products with large stress working conditions.

(2) Make Rough Texture on Bonding Surface Mold Surface

Fine texture etching and grain blasting are carried out on the mold surface corresponding to the hard and soft rubber bonding area to increase the micro roughness of the hard rubber surface. After filling, the soft rubber can be embedded in the texture gap, increasing the contact engagement area, avoiding delamination caused by easy peeling of smooth plane, which is an effective way to improve bonding force with low mold cost.

(3) Widen Bonding Lap WidthAppropriately increase the fitting width of the hard and soft rubber overlapping edge during mold design, reduce the unilateral narrow-edge overmolding structure. Sufficient lap width can improve the melt fusion range, reduce local stress concentration, and prevent edge warping and peeling off.

2. Optimization of Gate and Runner Layout(1) Arrange Gate Close to Bonding SurfaceThe soft rubber gate should be arranged as close as possible to the hard and soft rubber bonding area, allowing the high-temperature melt to directly wash the fitting interface, and use the residual heat of the melt to slightly melt the surface layer of hard rubber to realize molecular fusion and bonding. Feeding far away from the bonding surface will cause excessive cooling of the melt, poor interface fusion and obvious decline of bonding force.

(2) Select Reasonable Gate TypePriority is given to side gate and fan gate for smooth and gentle feeding, avoiding excessive flow velocity and shear overheating caused by small point gate and fine submarine gate. Stable flow velocity can make the soft rubber evenly cover the bonding surface, reduce cavities and gap isolation layers, and ensure the overall bonding uniformity.

(3) Thickened Runner with Smooth TransitionThe mold runner is designed with enlarged cross-section and reduced right-angle corners to lower melt flow resistance, ensure stable soft rubber material temperature and sufficient fluidity, fully fill the bonding gap, and avoid poor bonding interface caused by insufficient filling and air holes due to poor flow.

3. Mold Positioning and Sealing Structure Design(1) Precise Positioning of Hard Rubber to Prevent DeviationThe mold is equipped with precision positioning columns and limit steps to ensure no deviation and shaking of hard rubber parts during secondary overmolding. Once the hard rubber is displaced, it will cause uneven local gaps and excessive fitting gaps, making the soft rubber unable to fit closely and directly causing local delamination.

(2) Fitting Surface Sealing to Prevent Flash and VoidThe parting surface and overmolding periphery are well sealed precisely to prevent soft rubber flash and material leakage resulting in insufficient filling; meanwhile, avoid over-tight sealing pressing hard rubber to form hidden gaps. A closed and uniform fitting environment can make the soft rubber fully wrapped and seamlessly fitted.

(3) Avoid Poor Exhaust on Bonding SurfaceExhaust grooves are reasonably arranged at the end of soft rubber filling and dead corners to discharge internal air in time. Air trapping will form bubbles and isolation layers on the bonding surface, blocking the contact and bonding of hard and soft rubber, which is an important hidden cause of invisible delamination. Mold exhaust must match the flow direction of melt.

4. Mold Design of Temperature and Cooling Water Channel(1) Strengthen Water Channel Layout Around Bonding AreaEncrypt the cooling water channel around the hard and soft rubber bonding surface in the mold to control uniform and stable mold temperature. Maintaining a reasonable mold temperature appropriately can slow down the cooling of soft rubber, extend the interface fusion time and improve bonding strength; too low mold temperature will cause rapid solidification of melt and false surface bonding.

(2) Zonal Temperature Control to Reduce Shrinkage Difference

Carry out zonal water channel design for the hard rubber insert and soft rubber covering area, reduce the cooling shrinkage difference between the two, avoid internal stress caused by inconsistent shrinkage, pulling the bonding interface and causing natural delamination and warpage peeling later.

5. Adaptive Design of Demolding and Slider Structure

(1) Avoid Damaging Bonding Surface During EjectionEjector pins and ejector blocks of the mold should avoid key areas of hard and soft rubber bonding, adopt large-area ejection to prevent internal injury and hidden cracks on the bonding surface caused by local pressing, and avoid subsequent peeling and cracking from the bonding interface under stress.

(2) Close Fitting of Slider and Moving PartsFor overmolding molds with sliders and moving parts, the slider fitting surface shall be well researched and matched to prevent gap material leakage and insufficient glue filling. Meanwhile, the slider moves stably without colliding with hard rubber inserts to ensure complete covering contour and regular bonding boundary.

ConclusionTo improve the bonding force of overmolding injection molding, mold design should be comprehensively controlled from five dimensions: mechanical embedding molding, gate and runner layout, positioning and sealing, exhaust and water channel temperature control, and demolding slider adaptation. By making physical interlocking structure through the mold, optimizing the melt feeding and fusion conditions, ensuring close fitting and uniform cooling, it can not only make up for the shortcomings of material bonding, but also stabilize the overmolding quality for a long time, effectively solving common defects such as delamination, peeling and opening.