Common cable ties on the market generally suffer from problems such as insufficient anti-detachment force, insufficient buckle strength, loose buckle installation, wire harness sliding within cable tie, and low precision, affecting product quality and even endangering personal safety of car users.

 

Figure 1 Common wire harness cable ties
(1) Insufficient anti-detachment force. Existing products often use single-stage barbs or simple buckles, with head pull-out force generally below 80N, making it difficult to meet requirements of high-intensity vibration environments. Anti-detachment force is insufficient, especially when cars are driving on roads with poor conditions, wire harness shakes with vehicle body, cable tie is prone to detachment or wire harness becomes loose and worn. Product design needs to increase number of barb steps on cable tie buckle head, barb elasticity, groove depth, and strength of locking elastic sheet.
(2) Insufficient buckle strength. Some wire harness fixing devices are prone to slippage or breakage due to insufficient barb height or stress concentration at the root. First, cable tie structure needs optimization. Product design should increase height and thickness of fixing barbs, relying on increased material to improve cable tie strength. Second, surface precision of injection mold parts needs improvement to reduce stress concentration at interface during injection. Third, mold venting structure needs improvement to increase flow rate of injection material in mold and enhance product density.
(3) Loose buckle installation. Buckle's mounting head and mounting hole do not fit tightly. Insufficient mounting steps and a large gap between barb interface and vehicle body mounting hole result in loose wiring harness fixation. This causes harness to shake during vehicle operation, colliding with vehicle body and producing abnormal noise, affecting driving experience and even endangering safety of passengers. Product design should increase number of mounting head locking steps and reduce gap between mounting head and mounting hole.
(4) Wiring harness sliding within cable tie. An overly smooth interface between wiring harness and cable tie, excessive distance between slot and elastic plate, or a shallow slot can all cause wiring harness to slide within cable tie. When designing product, it is necessary to consider increasing surface roughness of interface between cable tie and wire harness to suppress slippage of wire harness within cable tie loops.
(5) Low product precision. Product has defects such as burrs and shrinkage cavities that do not affect functionality. Some products cannot even precisely inject company logos or important product markings, affecting brand image of car manufacturers. This requires comprehensive consideration of precision issues during mold design and manufacturing.

Based on customer needs and feedback regarding high-strength, tight connections, and non-slippage characteristics of cable ties, structure of traditional cable tie design was improved and perfected. Finished product effect is shown in Figure 2.

 

Figure 2 Oval mushroom-shaped automotive wiring harness cable tie
Newly developed cable tie head adopts an elliptical mushroom-shaped head with multi-step elastic barbs and an elastic sealing cover structure. Specific dimensions are shown in Figure 3. This design employs several key features: first, increasing thickness of cable tie clip head to enhance connection strength and prevent slippage; second, utilizing a multi-step elastic barb and elastic sealing cover structure to reduce gap between cable tie and vehicle body, ensuring a tight connection and eliminating noise caused by wiring harness swaying during vehicle operation; third, increasing depth of slots and thickness of elastic locking plates, while reducing slot spacing, to guarantee quality of cable tie locking and prevent slippage; and fourth, adding anti-slip protrusions to interface between cable tie and wiring harness to increase surface resistance, resolve issue of wiring harness slippage within cable tie.

 

Figure 3. Two-dimensional structure of elliptical mushroom-shaped automotive wiring harness cable tie.

Injection mold design must be based on product's internal and external structural characteristics, combined with product material and its properties (such as flowability and shrinkage rate), focusing on four major structural systems of melt flow, molding, cooling, and ejection. A well-designed mold can improve product yield, shorten molding cycle time, and reduce mold maintenance costs.

Although elliptical mushroom-shaped automotive wiring harness cable tie is injection molded, its wall thickness is only 1.3mm, and its flow length is approximately 177.4mm, resulting in a flow length ratio exceeding 130. Poorly designed venting systems can easily lead to insufficient melt filling or shrinkage deformation, and even trapped air, causing burning defects. Core and insert require venting holes larger than φ0.5mm and venting grooves on parting surface with a depth of not less than 0.03mm. This can reduce trapped air defects by more than 95%.
Product head is relatively thick and has a complex structure, requiring forced demolding. A double ejection mechanism needs to be designed for both upper and lower molds. Due to long runner length, required injection pressure is also high. Traditional mold steels (such as domestic P20) lack sufficient hardness, are prone to wear and deformation, and affect mold lifespan; therefore, requirements for mold steel are also high. Undercut at cable tie buckle has an inclined angle, making demolding difficult, requiring a demolding clearance structure. Mold structure is shown in Figure 4. To improve production efficiency and reduce costs, a 1-mold 4-cavity structure is adopted.

 

Figure 4 Mold structure

Mold venting system adopts a layered layout. A ring-shaped stepped groove, 0.04mm deep and 8mm wide, is created along parting surface, covering end of elliptical mushroom head surface and locking tooth area. Through multiple buffer sections, depth gradually increases to 0.1mm, connecting to external air to improve venting efficiency and reduce risk of overflow. A permeable insert venting groove, 2mm wide and 0.15~0.2mm deep, is embedded in the top of mushroom head cavity. Its multi-venting structure achieves seamless venting, avoiding weld line defects caused by traditional venting grooves. Venting system design is a key aspect of mold design, determining forming quality of cable tie. Mold insert venting is shown in Figure 5.
Mold employs a dynamic collaborative venting mechanism involving ejector gap and slider linkage. A 0.04mm clearance is designed in ejector hole of locking forming area. Combined with chrome-plated surface of ejector, this ensures smooth ejection while allowing natural gas discharge through gap. A spring-loaded vent valve is installed at the end of slider's movement trajectory. It closes during high-pressure injection phase to prevent overflow and automatically opens as slider retracts after pressure holding period, releasing residual gas. Melt flow path is simulated using Moldflow software to identify high-risk areas for gas trapping, such as intersection of mushroom head curved surfaces and base of locking mechanism. Eight micro-vent holes (φ0.5mm × 0.015mm) are added accordingly. During trial molding stage, a pressure sensor monitors gas pressure inside cavity, optimizing vent distribution density to ensure gas discharge rate matches injection filling rate.

 

Figure 5 Mold insert venting

In upper mold ejector assembly, four φ2.5mm flat ejector rods are symmetrically arranged along major axis of mushroom head ellipse. Ejection end face conforms to curved surface of mushroom head, with a contact area ratio ≥80%, reducing local stress concentration. In lower mold ejector assembly, undercut area at locking point uses eight φ1.8mm stepped ejector rods with a step difference of 0.3mm, forming a mechanical limit and ensuring precise demolding during synchronous ejection. Ejector rod spacing tolerance is ±0.01mm to prevent damage caused by misalignment. Structure of upper and lower mold ejector assemblies is shown in Figure 6.

 

Figure 6. Push rod assembly structure
Upper and lower mold ejector assemblies adopt a dual hydraulic cylinder + gear and rack synchronous drive system. Upper and lower ejector plates achieve synchronous displacement through a precision rack. Ejection stroke is 20mm, and speed is adjustable from 0.1 to 0.5m/s. Self-lubricating graphite copper guide sleeves are installed at four corners of ejector plate, in conjunction with φ20mm hardened guide pillars, with a straightness ≤0.005mm/m and a lateral force resistance ≥5kN, ensuring smooth ejection without jamming. A disc spring assembly with a preload of 3000N is added between ejector plate and hydraulic cylinder push rod to absorb ejection impact force of injection molding machine and reduce risk of ejector rod breakage. Main reset is driven by mold closing back pressure, assisted by a nitrogen spring. Pressure is adjustable from 50 to 200 N, providing forced reset with a reset accuracy of ±0.015 mm, preventing interference caused by incomplete reset of ejector pin. Ejector pin base adopts a modular quick-release design, with a single ejector pin replacement time of no more than 3 minutes, extending maintenance cycle to 200,000 mold cycles.

Spring-block type demolding mechanism design is shown in Figure 7. Two symmetrical spring blocks are set in corresponding area of cable tie buckle barbs. Material is SKD61, and inner wall of spring block precisely fits shape of barb, while outer wall is machined with a 15° inclined guide groove. Spring blocks are driven by inclined guide pillars. During mold opening, they move backward with moving mold, and inclined guide pillars force spring blocks to move laterally outward along inclined surface, disengaging them from buckle area. A rectangular spring is installed at the bottom of spring block, pushing it to accurately reset during mold closing.
Lateral parting speed of spring block is synchronized with mold opening speed of main parting surface. Piston rod speed of hydraulic cylinder is adjusted via a proportional valve to ensure spring block completes lateral separation before undercut detaches. Hardened guide strips are installed on both sides of spring block, along with self-lubricating graphite copper guide grooves to prevent jamming. A grid-like mass-reducing groove with a depth of 2mm is machined on the back of spring block to reduce inertial impact. Thickness of spring block is optimized through finite element analysis (FEA), with a base thickness of 6mm and a local thickening to 8mm in barb contact area to ensure a bending strength ≥1200MPa. Spring block employs a double safety mechanism: an additional mechanical limit screw prevents overtravel; when spring fails, mold closing back pressure forces a reset via an inclined plane, avoiding mechanical interference.

Core material is imported S136 mold steel with a hardness of 48~52HRC. Electrical discharge machining (EDM) is used, with precision controlled within ±0.005mm, ensuring no burrs, smooth transition surfaces, and clear markings after cable tie is formed. Core undergoes vacuum heat treatment, carburizing, and nitriding to maintain its toughness, improve surface hardness and wear resistance, and reduce roughness.

Elliptical mushroom-head automotive wiring harness cable tie, through its anti-detachment structure design, reduces risks of short circuits, poor contact, and fires caused by loose wiring harnesses, thus improving reliability of vehicle operation. Improvements to mold's strong demolding and venting system reduced injection defect rate, improved cable tie quality, and extended mold's service life to over 200,000 cycles, facilitating large-scale production. Main innovative achievements are as follows:
(1) Triple optimization of anti-detachment structure. Cable tie adopts a three-tiered gradient barb head and elastic sealing cover structure, making cable tie fit more tightly with vehicle body mounting holes, with a small gap, and elliptical mushroom head structure is easy to install. However, three-tiered gradient barb structure makes extraction more difficult. ANSYS simulation verified that target extraction force is greater than 200N, and cable tie stress is reduced by 18%. Cable tie working surface features 17 uniformly high-damping protrusions, each φ2mm in diameter, 0.4mm in height, and spaced 3mm apart, increasing friction coefficient to 0.9 and enhancing anti-slip properties of cable tie. Buckle tilt angle is increased to 45° and thickness to 3.2mm. ANSYS analysis shows that pull-out force is greater than 300N, an increase of 60%.

 

Figure 7: Mold insert spring block mechanism
(2) Breakthroughs in key mold technologies. Mold venting system uses insert venting grooves and parting surface venting, utilizing push rod gap and slider linkage for dynamic coordinated venting to overcome problem of trapped air. Demolding employs a double push rod ejection mechanism. Upper and lower mold push rod assemblies utilize a dual hydraulic cylinder + gear rack synchronous drive system, allowing upper and lower molds to eject synchronously, preventing cable tie from breaking or tearing during demolding, improving demolding efficiency and quality. For complex parts, buckle barbs use a spring-block demolding mechanism. Spring block's lateral parting stroke is synchronized with main parting surface's mold opening speed, ensuring consistent product demolding. Demolding process is shown in Figure 8.

 

Figure 8. Demolding process.