A few days ago, I talked with a colleague about the welding line.
Their upstream customers have very high requirements on the welding line.
I asked whether this defect could be eradicated? You may have had similar experiences with such a thorny problem.
Today, I’ll talk to you about the weldng line, the generation mechanism of the welding line, the history of the welding line, and the solutions to the welding line.
1. Mechanism of welding line
Welding line, also known as fusion line, bonding line, etc. the welding line is produced because the cooled molten resin from the leading edge in different directions fails to fully fuse at the joint.
It often occurs at the confluence of two streams, or at the place where one stream converges again after bypassing some geometric shapes (such as holes) on the workpiece.
The material flow of gate 1 and gate 2 finally forms a welding line on the product surface, as shown in Fig. 1.
Fig. 1 welding line
Let’s take another look at the melt flow pattern before the formation of the welding line.
Because the mold temperature is very low relative to the melt temperature, a layer of frozen layer will be formed on the surface layer, and a gap will appear at the end after the confluence.
The plastic grease cannot completely replicate the mold surface.
A higher melt temperature will produce a better feeding effect, but the orientation of the surface plastic molecular chains will not overlap, so a welding line is generated, as shown in Fig. 2.
Fig. 2 copy of welding line
The two resin streams at the welding point will not mix with each other (because they advance while semi curing in the jet stream).
Therefore, if the temperature is low, the surface layer will become thick, the texture will be obvious, and the strength will be reduced, because the adhesive force between the two becomes weak.
2. Hazards of welding line
The welding line mainly brings two problems.
The first is the structural problem.
The parts are more likely to be broken or deformed at the welding line, especially when the quality of the welding line is very low.
This weakness is a more serious problem in areas where the parts are stressed.
The second is visible defects on the appearance.
The welding line may cause lines, grooves or color changes on the surface of the parts.
This may not be a problem if the welding line is located on the surface of a non critical part (e.g., the bottom).
3. Influencing factors of welding line
when two resin streams meet, fusion will occur.
At this time, the lower the temperature of the two, the more obvious the fusion.
On the contrary, if the temperature of the two resin streams is high, the adhesive force will be enhanced and the appearance will become less obvious.
At the fusion point, the two melted resins are pressed, and the adhesion condition here depends on the pressure applied thereto.
The lower the holding pressure, the more obvious the fusion and the lower the strength.
As curing progresses, pressure transfer becomes more difficult.
In addition, if the gate runner size becomes small and the gate position becomes poor, the appearance and strength of the fusion will deteriorate.
When the front of two streams meet, the smaller the angle is, the more obvious the fusion line is; the larger the angle is, the less obvious the fusion line is.
In Moldflow, the fusion line is formed when the melt polymerization angle is less than 135 ° and the fusion line is formed when the melt polymerization angle is greater than 135 °.
The fusion line is invisible to the naked eye and acceptable.
Fusion is the confluence point of the resin and may also be the flow end.
At this time, if an exhaust port is not well provided at this position to exhaust the gas, the appearance and strength of the fusion will deteriorate.
(5) Filling speed
When the filling rate is too slow, the melt flow front cannot reach the filling end, and the farther the melt is from the gate, the easier it is to freeze, thus making the quality of the welding line low.
Or the flow length is too long. When the welding line is formed at the end, the melt front temperature decreases too much, resulting in low quality of the welding line.
4. Solution of welding line
First, raise the temperature
When the melt temperature is low, increase the melt temperature to better fuse the front of the two streams, but pay attention not to exceed the degradation temperature of the material;
The mold temperature is low, and the mold temperature is increased to better fuse the material flow front.
The root of the formation of the frozen layer in the melt is that the temperature is lower than the material conversion temperature.
If the mold temperature is higher than the conversion temperature by increasing the temperature, the melting line will be fundamentally solved.
Technically, the above described effects can be achieved by the processes of hot and cold and electromagnetic heating.
Of course, the cost is higher than that of ordinary molds.
Fig. 3 shows the welding line effect of high gloss molds.
The left side is ordinary molds, and the right side is ordinary molds.
The mold temperature is raised to more than 150 ℃ by hot and cold, and the material conversion temperature is about 145 ℃.
Fig. 3 Comparison of welding lines
Second, for insufficient filling end pressure
If the filling end pressure is low, increasing the holding pressure can make the two melt fronts fuse better;
In this way, the material flow front with the highest temperature will meet at the cavity wall, and these areas will also obtain the maximum holding pressure to ensure that the plastic shear thinning property plays a role.
Third, the angle is too small
Adjust the wall thickness of the product or the position of the gate to increase the confluence angle of the melt.
Fourth, for trapped gas
Adding exhaust gas at the filling end can make the front ends of the two melts combine better.
It shall be ensured that the exhaust position is correct, and the exhaust with improper position will not play an exhaust role.
Fifth, the filling rate is too slow
Ensure that the shear thinning property of plastic plays a role. Increasing the filling rate will increase the temperature of the material flow front, increase the shear thinning effect, and thus improve the strength of the bonding line.
When the filling distance is too long, it should be noted that the filling distance should not be too long (L / T = length to wall thickness, which should be less than 250:1).
If L / T is greater than 250:1, it is usually considered as thin-walled molding.
The flow length ratio can be reduced or a new gate can be added.
Of course, this may cause a new weld line gate to be improperly positioned.
5. Evaluation standard of welding line
In Moldflow, the welding line is formed when the melt polymerization angle is less than 135 ° and the welding line is formed when the melt polymerization angle is greater than 135 °.
The experience summarized by a company is shared below.
The welding line is visible when the confluence angle of the welding line is less than 75 °, but not when the angle is greater than 75 °, as shown in Fig. 4.
At the same time, all enterprises can summarize the industry standards suitable for their own enterprises according to the appearance requirements, structural characteristics and material types of their own products.
Fig. 4 welding line angle and depth
The forming conditions help to determine the quality of the welding line or the fusion line, and the strength of the welding line is affected by the formation temperature of the welding line and the pressure applied to the welding before the parts freeze;
The pressure is 0 (zero) when the welding line is formed.
Generally, the melt temperature when the welding line is formed is not lower than 20 °C under the injection temperature, and “high-quality” welding can be formed.
6. Strength analysis of welding line
If you want to see the impact of the welding line strength on the product structure and predict the welding surface of the strength reduction coefficient, you can perform the welding line strength analysis.
Before the analysis, you need to test the welding line strength property of the material.
With the property of welding line strength, carry out Moldflow flow flow pressure maintaining analysis, and then import the welding line results into ANSYS or ABAQUS for analysis, so as to predict the impact of welding line on product structure strength.
In combination with the line strength analysis, it will be shared in future articles, so there will not be too much introduction here.
The generation of the welding line is closely related to the structure, mold design, process and material of the product.
In the case of holes or multiple gates, it is difficult to avoid the welding line, and the welding line can be diluted through the process.
However, the high light mode is required to completely eliminate the welding line.
At the same time, the impact of the welding line on the structural strength of the product needs to be known, and the strength of the welding line can be analyzed and predicted.