Quality control in the production process of automobile wiring harness

Quality control in the production process of automobile wiring harness
This article combines the actual production process of automobile wiring harness based on the critical functions of tangent-spray-crimping, ultrasonic welding, heat-shrinkable tubing, stranded wire, assembly covering and accessories, electrical performance testing, appearance and size inspection. The quality control of the wire harness has refined the critical monitoring points. The list of the key monitoring points of each process is implemented at each wire harness supplier. It is committed to pursuing the stability of the supplier's process and manufacturing zero defects.
 
With the continuous improvement of automotive safety, economy, comfort, environmental protection and functional requirements, automotive wiring harnesses have become complex. Due to the increase of functions, the growth of branches, and the rise in connectors, automotive wiring harnesses have become a frequent issue to automobile failures. Therefore, the quality requirements for automotive wiring harnesses in the production process are getting higher and higher. Establishing and sorting out the critical monitoring points of each method has set a guaranteed screen for the quality control of the wiring harness and cable assembly, not matter the wiring looms are for on road vehicles or off-road UTV ATV, or harnesses for leisurely golf-carts.
 
1. The structure and function of the wire harness and cable assemblies
 
Automotive low-voltage wiring harness is an assembly composed of wires, connectors, terminals, staples, sheaths, tapes, corrugated pipes, PVC pipes, brackets, heat-shrinkable tubing, and fuse boxes. The wiring harnesses are connected and transmit electrical energy and electrical signals to the electronic appliances in the car, which has the image title of "automobile nerve". The functions of each wiring harness in the vehicle are different. They thus are divided into the main wiring harness, cockpit wiring harness, instrument panel wiring harness, door wiring harness, ceiling wiring harness, engine wiring harness, etc.
 
2. Common wiring harness defects
 
Common wiring harness defects include terminal exit, relay/fuse assembly error, wrong buckle position, poor relay operation, short circuit, insulation damage, open circuit, poor contact, etc. In addition to the natural factors that cause failures (excessive service life, oxidation/deformation of the wire harness ends), electrical equipment failures that cause damage to the wire harnesses, and subjective human factors when assembling or repairing auto parts, there are many defects in the wire harness or wiring looms. It can be prevented or eliminated during the production process.
 
3. Quality control in the production process
 
The general manufacturing process of automotive wiring harnesses includes incoming inspection, raw material storage, wire cutting, crimping, ultrasonic welding, shrinkable heat tubing, stranded wire, terminal insertion, assembly covers and accessories (felt, tape, PVC pipe, corrugated pipe, Staples, cable ties and brackets, etc.), assembly, wiring harness inspection (appearance, size, electrical performance and air tightness, etc.), packaging and storage.
 
The schematic diagram of the process is shown in Figure 1.
 
Based on personnel training, equipment maintenance, and routine inspections, the quality of the wiring harness will still fluctuate due to many reasons. Therefore, through professional advantages and experience feedback, we refined the key monitoring points for each process and implemented them at the supplier to help them improve the first-time pass rate and 0 km quality performance.
 
3.1 Tangent-coding-crimping
 
1) Equipment
①The application parameters of the equipment are set by the process or production supervisor and cannot be manually interfered with by the operator;
②The interface for setting parameters on the machine is closed for the operator;
③ Whether it is automatic crimping equipment or semi-automatic crimping equipment, pressure monitoring devices are required;
④Once an abnormality occurs, the equipment will automatically destroy the non-compliant crimping or automatic locking system.
 
2) Process
①Confirm that the wire is not bent, knotted, stressed, or dropped to the ground;
②Manual re-cutting, peeling and crimping operations are not allowed, and there are no prohibited tools (scissors, pliers, etc.) in the workshop;
③Before crimping, CFM (Crimp Force Monitoring) teaching should be carried out;
④ The crimped terminal is protected by a wire cup, where the airport has no contact with the bottom of the wire cup, and the wire cup cannot be tied to the airport but tied to the insulator;
⑤In the case of double crimping, if it is stacked crimping, the wire with a smaller cross-sectional area should be placed underneath;
⑥ In the case of side-by-side crimping, the cross-sectional area of the two wires should be the same.
 
3) Compliance
① The insulation of the wire after the wire is cut is not damaged, and the cut is flush;
②The copper wire has no scratches, no blemishes, no oxidation;
③The ink jet font is suitable for the cross-section of the wire, and the logo is consistent;
④ The crimping height, section analysis, and pulling force strength of the terminal must meet the technical requirements.
The visual standard of terminal crimping is shown in Figure 2.
 
3.2 Ultrasonic welding
1) The equipment checks the equipment parameters (welding energy, amplitude, pressure, width, etc.) before the start of each batch and uses standard samples for error-proof verification. It can automatically prevent errors such as multi-line and few-line errors.
 
2) Process
①Ultrasonic welding and heat shrinkable tubing is carried out by the same operator at the same station;
②Rework is not allowed for any bad welding, and splicing with other tools is not permitted;
③The operator needs to wear gloves to prevent the ultrasonic welding from being contaminated.
 
3) Compliance
①Check the welding drawing force and water tightness of each batch;
②The dimensional characteristics of aligned welding at both ends and one-sided ear welding must meet the technical requirements;
③The ultrasonic welding part is rectangular, no copper wire forming sharp corners, no continuous cracks, no over-burning/melting insulation layer, no welding bead;
④ The wire core is not overlapped on the wire insulation layer, the wire is not bent, and it is drawn straight from the nugget, and there is no scattered wire core. The welding length is equal to or greater than 5 mm.
The ultrasonic welding position is shown in Figure 3.
Image
 
3.3 Heat shrinkable tubing
1) Equipment
①The heating time, heating temperature and cooling time have been defined; in the cooling phase, the product is locked on the equipment.
 
2) Process
① After the wire harness is inserted into the heat-shrinkable tube, the insulating skin can be peeled off;
②The heat shrinkable tube is inserted into the side with more wire harnesses and moved to the side with fewer wire harnesses (to reduce the risk of wire strand rotation) and can be centred by standard operations or tooling.
 
3) Compliance
①The surface of the heat-shrinkable tube shrinks neatly without over-burning;
②Waterproof heat shrinkable tubing needs to flow evenly after heat shrinking, and the distance is greater than 1 mm;
③The waterproof performance after heat shrinkage is tested with water-tightness testing equipment, and good and bad samples are used for verification.
Waterproof heat shrinkable tubing is shown in Figure 4.
 
3.4 Stranded wire
1) Equipment
①The process parameters such as the length before twisting, the length after twisting, the non-twisting length, the speed, the twisting distance, etc. have been defined;
②At the beginning of each batch, check the non-invasiveness of the twisted claws;
③Check that the tracks of the stranding equipment are aligned and the carrier is running smoothly;
④Confirm the positioning tolerance of the carrier.
 
2) Compliance
①The distance between twisted wires and the length of non-twisted cables must meet the technical requirements;
②The stranded wire has no loops.
The stranded wire area is shown in Figure 5.
 
3.5 Covers and accessories
1) Process
①For the connector, the stop size of the cover is shown in Figure 6a, L=L1+10 mm. Among them, L1 is the distance between the outermost wires at the entrance of the connector, and L is the distance between the cover and the connector.
② Avoid stress on the branch circuit, as it may cause the terminal to damage the terminal locking hook of the connector.
③For ring terminals, the stop size of the cover is shown in Figure 6b, and the distance from the cover to the inner side of the crimping wing (the side of the ring end) is 0-20 mm.
④Check that there is no covering on the ring terminal (functional area).
 
There are generally two ways to cover felt and foam materials. Figure 7a is a flag type, the length of the mating part is 25% of the harness circumference, with a tolerance of ±5%; Figure 7b is an overlapping type, the overlapping part covers 25% of the harness circumference, with a tolerance of ±5%.
 
Adhesive tape mainly plays the role of bundling, insulation, flame retardant, noise reduction, wear-resistance and marking in the harness. The main types currently in use are PVC tape, cotton webbing, polyester tape, and easy-to-tear tape (paper tape). The winding methods on the harness branches are divided into pitch winding (the interval between adjacent windings = tape width), tight winding (the overlap between adjacent windings = 1/2 the tape width), and point winding (if there is a For metal fixing buckles, the metal parts should be wound with easy-tear tape to avoid cutting the wires during the logistics process).
 
The function of the fixed buckle is to constrain the direction of the wiring harness on the car body to avoid interference with environmental parts or sensitive parts of the car body. The fixed buckle is similar to a standard feature, and its fixed interface is standardized. There are four main types of selected interfaces on the car body: round holes with a diameter of 6.5 mm, elliptical holes with a diameter of 6.2 mm × 12.2 mm, studs with a diameter of 5.0 mm, and steel or plastic plates with a thickness of 1 to 3 mm. Most of the fixed buckles are tied to the wire harness by the cable tie (the fixed part and the tie part are integrated or separated). There are also fixed buckles that are set on the wire harness by wrapping tape; there is also a selected clip. The pin is specified on the corrugated pipe by matching with the diameter of the corrugated pipe. The door wiring harness will use a sealed fixed buckle; this type of fixed buckle adds a sponge foam or rubber pad to the improved surface for waterproofing. According to the design rules, the wiring harness must be effectively fixed every 300 mm on the vehicle body.
 
2) Tooling
The assembly board must ensure the direction of the circuit branch and provide the assembly direction of the sheath, bracket, connector, and positioning clip.
 
pay attention:
① The diameter of the covering such as the bellows is consistent with the diameter of the wire harness;
② During the process of assembling the bellows, there is a risk of scratching the wires, causing the breakdown of the insulation layer during later use. Tools should be used to install the bellows;
③ The tape needs to close and lock the terminal of the corrugated pipe, and the video shall be wound at least two times;
④ Scientifically manage the cable tie guns and regularly check the clamping force of the cable tie guns. If there are multiple cable tie guns in the same station, the management method should be used to avoid using the wrong cable tie gun.

3.6 Wiring harness inspection
3.6.1 Electrical performance test
1) Equipment
①Using electrical measuring equipment to ensure the continuity of the wiring harness;
②The layout of the electrical measuring station should avoid mechanical stress on the wiring harness (such as pressure, tension, impact on the branch circuit, etc.);
③There is no missing circuit in the electrical test program, which is entirely consistent with the drawing;
④It is not allowed to pass the electrical test after the same connector is crossed;
⑤The branch with shrapnel/airbag branch/branch with relay needs to pass the secondary electrical test (the branch is turned on after the shrapnel is disconnected, and the branch is turned on after the shrapnel is closed);
⑥ The size and direction of the air pressure need to be paid attention to during the airtightness test. The air pressure direction is divided into two ways: blowing and inhaling;
⑦Close the secondary lock of the connector according to the recommendation of the connector supplier, and be able to detect whether the secondary lock is locked (for example, Molex 120 hole type connector, you need to use Molex special tools or electric test bench pneumatic automatic lock to close the secondary Secondary lock);
⑧The probe in the detection module is retractable, only contacting the surface of the terminal in the connector, and will not affect the plating of the terminal.
 
 
2) Process
Each time the test is interrupted or a defect is detected on the electrical test bench, the test sequence needs to be reset, and a complete electrical test is redone, and the electrical test program can automatically restore to the original test point; the same electrical test bench tests multiple items, When there are various parts, you need to pay attention to the switching between programs to avoid batch errors; use a vision system or electrical measurement system to ensure the functionality and existence of the fuses and relays; do not manually shake the connectors or components during the electrical measurement Actions.
 
3) Compliance
 ① Only after passing the electrical test can the electrical test label be printed to define the position, size and content of the title;
②The content shall include the name of the supplier, supplier number, model, part number, part name, production date/hour, shift, operator number and other required information;
③ The whole process needs to pay attention to the protection of terminals and pins, especially gold-plated terminals and small deformable terminals and pins (such terminals are very easy to deform, and they are not easy to detect after deformation, and there may be a risk of poor contact after installation).
 
3.6.2 Appearance and size inspection
 
After passing the electrical test, visual inspection and dimensional inspection are required.
The content of appearance inspection includes:
① Whether the connector is damaged or deformed;
② Whether the handle position of the connector is correct;
③Whether the sealing plug and sealing plug in the connector are present, undamaged and installed in place;
④Whether the buckle is used correctly and installed in home;
⑤ Whether the direction of the pin is correct and whether the fixing method meets the requirements;
⑥Whether the white fixed-point tape on the branch is missing;
⑦Whether the corrugated pipe is used correctly, has no deformation and completely covers the wire;
⑧ Whether the label is clear and correct, etc. 100% appearance inspection items and customer requirements items.
 
The frequency of dimensional inspection is in accordance with customer requirements. The length of each branch, branch position, cover length, buckle position, sheath position, etc., are measured and recorded.
 
Fourth, the improvement results of the wiring harness supplier.
 
After sorting out the list of crucial monitoring points for each process of automotive wiring harness, we implemented it at the wiring harness supplier. Take a supplier as an example. We conducted this audit in the first quarter of 2016 and reviewed it in the third quarter. Figure 8 shows the first time quality (FTQ) data (PPM) of the supplier from January to August 2016. It can be seen from Figure 8 that the FTQ data in April dropped by 62.45% from January and remained at a relatively stable level from April to August.
 
V. Conclusion
 
Under the requirements of the rapid development of automobiles in the direction of intelligence and electronics, onboard electronic and electrical equipment is constantly increasing, and the wiring harness, as the central part of the automobile control circuit, plays a vital role in the performance and quality of the entire vehicle. Therefore, how to strictly ensure the quality of the wire harness during the wire harness production process has become a vital issue. This article analyzes and sorts out the key monitoring points of each method by studying the process and quality control of the wire harness production process. This was formed into a standard list, which was implemented and audited by the harness supplier, which effectively helped the supplier to improve the first-time pass rate and 0 km quality performance.