When convergence angle of welding marks is less than 135 °, a suture is generated, and when it is larger than 135 °, a fusion line is generated. It can be noted that when sink angle is 120 ° ~ 150 °, surface traces of suture will disappear.
It is generally believed that quality of sutures is inferior to fusion lines, because after sutures are formed, fewer molecules fuse with each other across sutures. Increasing temperature and pressure in suture and fusion line areas can improve their strength. When considering strength and appearance of plastic, it is generally not allowed to produce sutures, especially reinforced plastics with fibers, because fibers usually fit parallel to sutures; they cannot cross sutures.

 

(1) Increase temperature of barrel of injection moulding machine
If material temperature is within a reasonable range, try not to raise material temperature first.
(2) Increase temperature in runner
Temperature of runner is mainly affected by factors such as mold temperature, form of runner, layout of waterway, and rate of fire. It is result of other factors.
(3) Reduce temperature generated by shear force.
Shear heat will increase front temperature of melt, which is not inconsistent with desalination of welding marks. Shear heat is usually closely related to factors such as rate of injection, gate form, runner cross-section, material, and mold surface processing, which is difficult to accurately control.

(1) Increase original temperature of mold. This is one of main ways to improve weld markings. However, it is necessary to pay attention to whether there is an optional oil temperature machine. It is necessary to consider whether the gap between guide post and oblique pin is burned or irregular due to thermal expansion of steel. It is also necessary to consider whether mold surface is heat treatment, mold life, molding cycle, cooling efficiency and other factors.
(2) Maintain temperature at operating point.

Increases injection speed of injection moulding machine. Increasing injection speed is beneficial to improve welding marks. It can even be improved by high-speed motors. However, it also need to consider appearance requirements such as spray marks, whether to cause cracking of shearing superheated material, whether product will produce flashes due to increased shot speed, whether there are stress marks, and injection pressure is too high.

1. Reducing cross-sectional area of runner.
If originally reasonable runner is reduced in cross-sectional size, it may lead to shrinkage and small size, increased pressure, deformation of molding support, and burrs.
2. Reduce length of runner (excluding hot runner)
Under condition that product quality is satisfied, use a shorter runner length and a smaller cross-sectional size to obtain the best gating system under condition This is a problem that every CAE engineer must consider. If your runner has been designed properlyly, then shorten runner length, usually there is not much space. And this factor does not seem to be too obvious in improving weld marks.

(1) Improve finish of cavity.
(2) Increase or decrease wall thickness of product to change material flow rule. One of factors to improve welding mark and change thickness is consent of customer or relevant unit.

Injection pressure is small, and injection speed cannot be increased, which is inconsistent with increasing injection rate.

If welding marks are at the end of filling, this method has a more obvious effect, which can enhance strength of joint.

1. Make full use of exhaust capacity of injection moulding machine (if injection moulding machine has this function)
2. Increase exhaust slot of mold.
This is one of improvement factors. If you add an exhaust insert or exhaust block in joint line area, effect will be more direct. Joint line area can be known from Moldflow analysis. This result is very accurate.
3. Locally increase overflow pocket.
Good way, sometimes very effective. Cold material at bonding line can be led to overflow cavity and trimmed off later. It should be noted that bonding line in many places, especially surface, is difficult to add overflow pockets. And consider whether trimming is convenient, trimming cost, quality after trimming, etc.

 

Gate factor is the most direct factor. In the early stage, moldflow optimization must be introduced to guide bonding line to non-appearance surface or improve quality of joint. It is very difficult to improve it after trial, and mold may become worse and worse, there may even be a problem after solving another problem. And it will affect delivery time, increase number of trials, increase cost of trials (cost of each trial is not low), and so on.

A good way, using moldflow to optimize gate position and position of bonding line, and selecting a reasonable delay time can improve bonding line. This is common on products with a demanding appearance of over 8000, such as TV face frames. What needs to be considered is use of hot runner valve gates for sequential control. Number of hot runner valves is more than conventional. Cost of hot runner is high. If output is not large, cost is main consideration. And even with this method, there will still be one or two bonding lines. It is necessary to consider location of weld marks and method of desalination. It should also be noted that timing control molding machine is different from traditional molding, and it is not usual slow, fast, and slow process.

There are some ways to add:
 1. Use moldflow to optimize design of feeding position in the early stage, try to avoid bonding line at obvious appearance or weak structure, and make angle of front edge of melt larger.
 2. Make binding line in the early stage of filling process, such as injection time is 2 seconds, and melt front binding time is 1 second.
 3. Without affecting filling quality, try to reduce number of gates.
 4. Male mold surface (core) is treated with biting (coarser grains change orientation direction of polymer chain when melt merges, making it messily merge).
 5. If front of melt bond is mostly cold material, try to lengthen cold material cavity of runner.
 6. If bonding line is just at weak point of structure after test, consider method of flow leader or cutting thickness to change flow velocity of melt front to change bonding position.
 7. Adding an electric heating rod, principle is also to increase temperature of junction to increase degree of disorder of molecular chain.
 8. Variable mold temperature technology, such as rapid cooling and heating, high temperature nitrogen, infrared, instantaneous heating of electromagnetic surfaces, etc. Moldflow6.2 has a rapid cooling and thermal analysis module. This method also raises temperature of joint, which works well. But cost is not low, equipment, technology, steel material requirements are high, and some patent issues are involved, which makes it difficult to apply.
 9.Imd technology (including IMR, IMF) directly covers surface, and is widely used in electronic products such as Notebook. Its cost and technical requirements are high. Range of applications is limited.
 10. For products that have a certain combination line and are really difficult to improve, it is recommended that customers or related departments use paint spraying or change color of plastic. This is a helpless move and next best thing (but sometimes it does).

 

Weld markings, shrinkage, deformation, etc. are the most common quality anomalies in molding. There are many factors that affect results, including product structure design, mold structure, molding equipment, molding process, polymer material performance, etc. Each factor has a different degree of influence and affects each other, so that problem solving method of each product is different, that is, law is uncertain. We need to analyze specific problems, find causes of problems, think about main influencing factors, and target them to find the most direct solution. So as to solve problem in a shorter time. What's more important is to prevent problems before they occur. In product design stage, Moldflow optimized design was introduced to integrate software with actual situation. Problems that may arise are dealt with early in design, and subsequent problems are much less.