1、 Formation mechanism of weld lines
Weld marks are one of the common defects in the hollow forming process in Dalian. They refer to visible marks formed when two streams of molten plastic meet and recombine in the mold due to insufficient bonding. This phenomenon not only affects the appearance of the product, but may also reduce the mechanical strength and sealing performance of the product.
The formation of weld lines is mainly related to the following factors:
Insufficient melt temperature: When the melt temperature is too low, the flowability of plastic molecules is poor, and the two melts cannot fully fuse when they meet.
Insufficient injection pressure: Insufficient pressure can cause the melt to not fully fill the mold, especially at the junction of the melt.
Uneven mold temperature: If the mold temperature is too low or uneven, it will accelerate the cooling of the melt and affect the welding quality.
Poor exhaust: The gas inside the mold cannot be discharged in a timely manner, which can form air pockets or burn at the fusion joint.
Material characteristics: Some plastics such as PP, PE, and other crystalline materials are more prone to forming obvious weld lines.
2、 Optimize process parameters to avoid weld lines
1. Increase the melt temperature
Properly increasing the melt temperature can improve the problem of weld lines. After the temperature rises, the fluidity of the plastic melt increases, and the molecular chains are more likely to diffuse and entangle with each other. However, it should be noted that excessive temperature may lead to material degradation. Suggestion:
Set the temperature range according to the material supplier's recommendation
Adopting segmented heating to ensure uniform melting of the melt
Regularly calibrate temperature sensors
2. Adjust injection pressure and speed
The injection pressure and speed have a direct impact on the welding quality:
Increase injection pressure: increase the bonding force of the melt at the confluence
Optimize injection speed: too fast can cause spraying phenomenon, too slow may cause premature cooling of the melt
Adopting multi-stage injection control and appropriately increasing pressure at the melt convergence point
3. Control the pressure holding stage
The pressure holding stage is equally important for eliminating weld lines:
Set appropriate holding time and pressure
The holding pressure is generally 60-80% of the injection pressure
The holding time should be sufficient to allow the melt to fully fuse at the confluence
3、 Mold design and improvement
1. Optimize gate design
The position and quantity of gates directly affect the formation of weld lines:
Reasonably arrange the sprue to make the flow path of the melt symmetrical
Increase the number of gates and reduce the distance of melt flow
Using fan-shaped gates or thin film gates to improve melt distribution
2. Improve the exhaust system
Good exhaust can effectively reduce weld lines:
Add exhaust slots at locations where weld lines may form
The depth of the exhaust groove is generally 0.02-0.05mm
Special exhaust materials such as porous metals can be used
3. Mold temperature control
Accurate mold temperature control is crucial:
Using a mold temperature machine to maintain uniform mold temperature
Raise the temperature locally in the fusion zone
Consider using a hot runner system to reduce temperature loss
4、 Material selection and processing
1. Material Selection
The tendency of different materials to form weld lines varies:
Choose materials with good melt flowability
Consider adding anti weld trace additives
Avoid using high viscosity materials to make complex products
2. Material pretreatment
Appropriate pretreatment can improve the quality of welding:
Fully dry the material to avoid moisture interference
Especially important for moisture absorbing materials such as PET, PC, etc
Control the proportion of recycled materials, too high will lead to a decrease in performance
5、 Other auxiliary measures
1. Use gas assisted technology
Gas assisted hollow forming can improve weld lines:
Gas pressure helps to fully bond the melt
Reduce differences in product wall thickness and improve flow balance
Especially suitable for large thick walled products
2. Follow up processing
For formed weld lines, the following measures can be taken:
Local heating repair
Surface treatment such as spraying, electroplating, etc. to cover up traces
Mechanical processing to remove obvious defects
To avoid the problem of weld marks during the hollow forming process, it is necessary to comprehensively consider multiple aspects such as process parameters, mold design, and material selection. By optimizing the melt temperature, injection parameters, improving the mold structure, selecting suitable materials, and taking appropriate auxiliary measures, it is possible to reduce or even eliminate weld lines, improve product quality, and enhance market competitiveness. In actual production, analysis and adjustment should be carried out according to specific situations, and if necessary, mold flow analysis software can be used for prediction and optimization.
