Weld lines are an important factor affecting quality of plastic parts. Study found that under same process conditions, strength of weld line area is only 10-92% of original material, which seriously affects normal use of injection molded products. For example, in automobile industry, unqualified plastic parts directly lead to decline of automobile quality, even endanger safety of human life. Therefore, it is of great practical significance to study formation process of weld lines, influencing factors and to find ways to eliminate weld lines.

 

Formation mechanism of weld lines
 "Weld line" of a plastic product refers to a three-dimensional area in which morphological structure and mechanical properties formed when two strands of molten material are in contact are completely different from other parts.
There are two basic types of weld lines that are most common in injection molded parts:
One is due to structural characteristics or large size of plastic parts, in order to reduce melt flow and filling time, when two or more gates are used, weld line formed where melt fronts entering cavity from different gates meet, is called cold weld line;
The other is that when cavity is equipped with a core and an insert, melt is divided into two strands when it passes through obstacle, and the two strands of melt reunite after bypassing obstacle to form a weld line, which is called thermal welding mark.
In addition, when wall thickness of workpiece is too large, resistance of fluid flowing through cavity is different. Resistance is small at wall thickness and flow rate is fast; while wall thickness is large and flow rate is slow. Due to this difference in flow velocity, melts from different wall thicknesses converge at different flow rates, and finally a weld line is formed at confluence.

 

Influencing factors of weld lines and measures to improve and eliminate them
Since weld lines have an important influence on quality of plastic parts, people have done a lot of research on their formation mechanism and performance evaluation, and put forward a variety of solutions.

Increasing temperature can accelerate relaxation process of polymer and reduce time for molecular chain entanglement, which is more conducive to full fusion, diffusion and entanglement of front-end molecules of material, thereby improving strength of weld line area. Experiments show that increasing melt temperature is beneficial to reduce depth of V-shaped groove on the surface of plastic part. When melt temperature increases from 220℃ to 250℃, depth of V-shaped groove decreases from 7 μm to 3 μm.
Effect of temperature on tensile strength of weld line of PA66 injection molded products containing 33% glass fiber reinforced, study found that tensile strength of sample with or without weld line will increase with increase of melt temperature; Influence of tensile strength of weld line is not linear. When temperature is relatively low (such as 70℃), tensile strength of weld line changes significantly with increase of temperature; but when temperature rises to a certain level, change is relatively gradual.
With PA66 (35% glass fiber reinforced), experiment with ABS also obtained similar conclusions. Using simulation, it was found that mold temperature and melt temperature have different effects on strength of weld lines formed by different materials. It is found by combination of experiment and simulation that among parameters of injection molding, melt temperature has the greatest influence on weld line strength of ABS plastic parts.

Injection pressure is an important factor for plastic melt filling and molding. Its function is to overcome resistance of plastic melt when it flows in the barrel, nozzle, gating system and cavity, to give plastic melt a sufficient filling speed, to compact melt to ensure quality of injection molded products.
Increasing injection pressure helps to overcome resistance of runner and transmit pressure to melt front, so that melt is fused at high pressure at weld line, increasing density at weld line, thereby improving strength of weld line. Increasing holding pressure can not only provide more kinetic energy to movement of melt molecular chain, but also promote mutual bonding of two melts, thereby increasing density of weld line area and strength of weld line.

Increasing injection speed and shortening injection time will reduce flow time before fusion of melt fronts, reduce heat loss, and strengthen shear heat generation, which will reduce melt viscosity and increase fluidity, thereby improving strength of weld line. Strength of weld line is very sensitive to injection time and will increase as injection time decreases.
However, if injection speed is too large, it is easy to produce turbulent flow (melt fracture), which seriously affects performance of plastic parts. Usually, low pressure and slow injection should be used in injection molding, and then injection speed should be adjusted according to shape of plastic part. In actual production, in order to shorten production cycle and avoid turbulent flow, more moderate and high injection speeds are used.
Injection speed affects flow behavior of melt in cavity, as well as pressure and temperature in cavity and performance of product. When injection speed is large, flow rate of melt through the mold gating system and cavity is also large. Strength of weld line of product is also improved.

After selecting material and setting process parameters, design of mold becomes the most critical step. A reasonable mold structure can reduce generation of weld lines, improve strength of weld line area or reduce impact of weld lines on the overall performance of plastic part.

According to formation principle of weld lines, filling multiple gates will inevitably form multiple branch flows. If number of gates is N, number of weld lines is at least N-1. It can be seen that the more number of gates, the more weld lines are generated. If melt from front of each gate cannot be well fused, weld lines will be aggravated, which will seriously affect quality of plastic parts.

Residual gas generated due to poor exhaust is compressed in mold cavity during injection process, which will not only burn product but also lead to appearance of weld lines. When melt is filled, gas is easily extruded to confluence of melt, resulting in an interlayer between the two streams, which ultimately hinders fusion of streams, which not only promotes formation of weld lines but also reduces strength of weld lines.
When PS plastic parts are fully exhausted, strength of weld line is 36.5MPa, but only 17.5MPa when there is no exhaust. Fully exhausting mold or using vacuum bleed air will help reduce and eliminate weld lines.
In addition, opening a cold slug hole at location where weld line appears can make melt front meet at cold slug hole (commonly known as a garbage pin). Increasing temperature of melt when ends of plastic parts meet can not only reduce generation of weld lines, but also enhance strength of weld lines.

The lower mold temperature, the less conducive to full fusion of melt. When designing mold, if cooling water channel is too close to fusion of melt, melt at seam will not be able to fuse due to decrease in temperature and increase in viscosity, resulting in obvious weld lines.
Improper design of cooling system will also cause temperature distribution of mold to be too different, so that when plastic melt is filled, filling speed of different parts of cavity is different due to temperature difference, resulting in weld lines. Therefore, reasonable design of temperature control system will make temperature fluctuation of mold evenly distributed and reduce generation of weld lines.
Now rapid cooling and rapid heating injection mold temperature control and adjustment system is realized, so that surface temperature of mold cavity can be quickly changed according to needs of various stages in injection molding process. Common advantage of this rapid cooling and rapid heating mold temperature adjustment system is that molding surface can be rapidly and uniformly cooled, can be rapidly heated to a temperature close to or equal to melting temperature of raw material, so quality of molded product is good, it is not easy to produce molding defects such as weld lines and flow marks.
Another method is to use magnetic effect of battery coil to rapidly heat surface of mold, then cool mold with cooling water to control temperature of mold, which can improve quality of plastic parts and avoid generation of weld lines.

 

Surface roughness of cavity and core also affects filling flow rate of plastic melt. If surface roughness value is too large, melt flow rate will slow down, condensation layer of mold wall will be thickened, material flow section will be reduced, flow resistance will increase, melt temperature difference will expand, and welding strength of branch flow will decrease. When machining mold, if surface roughness of cavity is inconsistent, weld line will also be generated due to different filling speed of melt.

Utility model relates to a mold structure that can eliminate appearance of welding marks of injection molding products with holes. Method can be used for plastic products with various types of holes on the surface, and can completely eliminate weld lines at holes, thereby improving product quality.
There is also a special injection molding mechanism. In actual production, injection molding is performed when movable and fixed mold plates are not locked (gap 2mm), and secondary mold clamping is performed when injection volume is 70%-80% of required injection volume. Since there is no shunting effect of core when mold is not completely closed, no weld lines will be formed, so generation of weld lines can be completely eliminated.

In highly automated mass production process of products, almost all large injection molded parts use hot runner systems. For such plastic parts, multi-gate glue feeding can ensure complete filling of cavity and improve filling efficiency, but it will inevitably produce branched flow, which will lead to appearance of weld lines. Method of opening gate valve needle in sequence can make molten material flow fuse to both ends of cavity in turn, thereby solving problem of weld lines.
Sequential valve pin gate technology is a new injection molding technology developed in recent years. Its theoretical model was first proposed by Moldflow Company of the United States, and GE Company of United States first applied it commercially in production of thin-walled parts.
Sequential valve pin casting technology is to add a valve device that can control opening and closing of original hot runner gate. Opening and closing of injection gate determines its working mode according to specific requirements. After injection mold gating system adopts valve needle gating control, flow direction and flow of melt can be controlled through gate to achieve purpose of eliminating or reducing weld lines.
Opening or closing of gate is driven by a hydraulic cylinder or a cylinder, connection between gate and ejector pin in hot runner is a cylindrical ejector pin. When using sequential valve gate technology to form products, it is generally divided into four steps. During the whole process, a gate can be opened and closed twice at most:
(1) At beginning of injection molding, open middle gate of mold, inject melt into cavity, and then divide flow to both sides;
(2) When melt just flows through gates on both sides, close middle gate and open gates on both sides at the same time;
(3) When melt fills the entire cavity and starts to feed and maintain pressure, open all gates;
(4) Before end of pressure holding, close all gates, cool the product, and open mold to take out parts.
Sequential gate technology has following characteristics: eliminate welding lines of large plastic parts; reduce molding pressure and related residual stress; shorten molding cycle; effects of injection pressure and holding pressure are independent of each other, pressure of cavity can be adjusted by closing and opening nozzle.
When molding large plastic parts, traditional simultaneous multi-point injection can make melt fill the entire cavity, but due to existence of weld lines, it is difficult to make product quality meet ideal requirements. It has been proved by production practice that sequential valve gate technology effectively solves weld line problem in injection molding.

To eliminate or reduce influence of weld lines and improve strength of weld lines, you can also start with selection of matrix resins and additives. Since there is often a stress concentration at weld line, heat treatment can be used to eliminate or reduce internal stress formed during molding process for injection molded products and improve mechanical properties of product.
For amorphous plastics, treatment method is to place it in an oven with an appropriate liquid or hot air at a temperature of 10-15℃ higher than glass transition temperature and an inert atmosphere for a certain period of time; for semi-crystalline plastics, it should be placed at a temperature below the melting point. Place for a certain period of time under similar conditions of 50-80℃, usually 40-50min/mm, and then cool plastic to room temperature. In addition, researchers also tried double push die method and vibration-assisted injection molding method to reduce generation of weld lines.

Double-push mold filling method is a molding method proposed by researchers in study of material self-reinforcing technology. System consists of a left injection system, a right injection system and a mold with two gates. Initially, two gates are filled at the same time, and when cavity is filled, a flat weld line area is formed in the middle. At this time, control system drives uncured material to move left and right. Due to large shear stress and early solidification near mold wall, it is difficult for material to move, so weld line forms a V-shaped structure.
Then, material moves left and right under action of left and right control systems. Repeated this several times, surface where weld line is located will form a zigzag state, thereby improving bonding strength of the two streams and improving mechanical properties of weld line. Similar to above method is double plunger pushing method invented by Bevis. During operation, reciprocating motion of piston drives reciprocating motion of melt filling cavity, achieving expected effect of double pushing method.

 

Vibration-assisted injection molding is a new type of polymer molding method that uses external force fields (including mechanical vibration, sonic vibration and ultrasonic vibration, etc.) to plasticize and process raw materials, thereby greatly improving the overall performance of plastic parts.
Its mechanism of action is to pulsate polymer through local vibration. This method mainly uses electromagnetic vibration to make material more completely plasticized in barrel, has a lower mold filling viscosity, and is easy to remove gas, which is beneficial to improvement of strength of weld line.
Existence of weld lines will not only cause quality problems on the surface of plastic parts, but also affect strength of plastic parts. Welding is unavoidable for most plastic parts, but by improving structure of plastic parts and molds, implementing active control, influence of weld lines can be eliminated or greatly reduced, so that appearance and mechanical properties of plastic parts can meet design requirements and application requirements.
In addition, crystallinity, relaxation time and other properties of raw materials are also important reasons for occurrence of welding, and selection of appropriate resins is also important for eliminating or reducing weld lines.