In injection production process, products often have various quality defects, such as cracking, warping deformation, poor size, shrink marks, weld lines, color difference, flash, insufficient strength, gas traps and black spots. For a long time, designers have lacked scientific system analysis for causes of these problems, resulting in long blind adjustments and large waste of raw materials, but they cannot solve these problems, affecting quality and delivery time of products. In severe cases, it will cause large-volume returns and reduce competitiveness of enterprise.
To solve defects of product molding, it is not enough to rely on past experience, root cause of defect needs to be found. Now injection molding principle and process are briefly explained first, then causes of weld line of injection products are analyzed. Defects are solved by adjusting injection moulding process parameters and mold design optimization, and actual case is combined with simulation analysis.

 

Figure 1 Complete injection molding cycle

Dried granular resin is sent to barrel equipped with heating device through hopper of injection molding machine to be heated and melted. Under push of screw, melt is injected into injection mold cavity by high speed and high pressure, melt is gradually solidified and shaped after a set time of pressure-holding cooling; mold is opened and molded products is extruded to complete a molding cycle. A complete injection molding cycle is shown in Figure 1.
Analysis of causes of weld lines

Weld line is commonly referred to as a bond line, which means that when two or more streams of molten metal meet when molten plastic is filled into injection mold cavity, front portion of flowing melt has been cooled, so that it does not fuse well at the confluence to cause defects, as shown in picture 2.

 

Figure 2 Common weld lines

Weld line on the part is like a scratch or a groove, especially at deep or transparent locations. Weld mark affects appearance quality and structural strength of product, and is liable to cause product to break.
When injecting a cavity, main conditions for molten plastic to merge and form a weld line are:
01 Determined by functional structure of product itself, such as various holes, grooves, inserts and uneven wall thickness.
02 Determined by mold design, such as  number of gates, location, cooling system, and exhaust.

Difference in product structure and different molding conditions in production process, mold making, form of weld lines are also different.
01 According to fusion depth of cross-section of weld line, it is divided into: surface-type weld line, that is, weld line only appears on the surface layer of product, which affects appearance quality of product and has little effect on structural strength of product; inner-layer weld line, weld line not only appear on surface layer, but also extends into inner layer of product, which affects appearance quality and structural strength of product; all through-type weld lines, that is, weld line runs through entire surface of product, seriously affecting structural strength of product and easily causing product to break.
02

 

Figure 3 Stream front angle

According to process of forming weld line, it is divided into: encounter type weld line, molten plastic fills cavity from different gate positions, weld front face meets weld line formed at the front, and convergence angle of front edge of encounter type weld line is generally less than 135°, as shown in Fig. 3. Adjacent weld line is caused by structure of product. When filling, stream merges and forms a weld line. Convergence angle of two streams at leading edge of adjacent weld line is greater than 135°. Messy weld line has a fountain flow when melt is filled faster, and weld line has an irregular curve due to unstable flow.

Causes of weld lines are as follows:
01 Material temperature is too low and melt fluidity is poor.
02 Injection pressure is too small.
03 Cavity exhaust is poor.
04 Number and location of gates are not properly designed.
05 Other causes, such as fluidity and contamination status of plastic melt itself, reasonable structure of product.
From analysis of causes of formation of weld lines, it is known that to reduce weld line, it is necessary to reduce occurrence of confluence after flow splitting during injection or extrusion process.

In the selection of product materials, in the case of structural strength of product, it is preferred to use materials with low viscosity, small molecular weight, no filler or non-reinforced fluidity.

Structure is further optimized when product is designed. In the case of structural function of product, try to avoid designing structure of holes, slots, etc., and optimize it to be blind holes or circles to avoid flow branch. Wall thickness difference of product should be designed to be less than 30%, and wall thickness should be designed as evenly as possible.

Reasons and countermeasures related to process parameters:
01 Adjust molding conditions to improve fluidity of melt flow front. Mold temperature is low, set temperature of barrel (especially the nozzle) is low, injection speed is slow, and fluidity of material is low. Solution strategy is: gradually increase mold temperature and barrel temperature, increase injection speed and quickly form a fusion site. The lower holding pressure, the more obvious weld mark. In addition, if gate size becomes smaller and gate position is unreasonable, appearance and strength of fusion will be affected. Solution: Increase holding pressure, increase material temperature and mold temperature, speed up injection speed, increase gate cross-sectional area, adjust product wall thickness, and select materials with good fluidity.
02 High-speed injection is filled in the order of filling volume, slow injection is carried out in the order of wall thickness, and weld mark can be appropriately displaced. In most cases, it is impossible to adjust process parameters to completely avoid weld line. All you can do is to fade weld mark and reduce its brightness, or move weld mark to a non-critical appearance level that is not obvious or completely invisible. .

Reasons and countermeasures related to mold structure design:

If mold structure only affects appearance of product, position and length of weld line are adjusted by changing position of gate and number of gates, or portion produced by weld line is treated as a dark glossy surface.

Increasing cross-sectional area of gate, increases melt flow and makes it easier to apply pressure.

Exhaust port is weak, and melt fusion site may also be flow end. If exhaust gas is poor, appearance and strength of fusion site of product may be affected. Solution strategy is to check depth and size of exhaust port, ensure that exhaust is unobstructed, and avoid blockage of mold scale; or add a venting groove to provide a push rod at the place where product is welded to facilitate exhaust.

If cooling water channel is too close to melt, melt at fusion zone will be too fast due to temperature drop, viscosity will increase and it will not be fully fused, weld line will be easily formed. Therefore, design of cooling system should be reasonable; Cold material gathering area is cut and removed after molding.

Mold cavity and core surface are coated with an excessive amount of release agent, which will cause a layer of oil film to be sandwiched at melt junction, which will not fuse well and reduce welding strength.

 

Figure 4: Welding line length adjustment

Original gate position of product is on the side of existing gate position, weld mark is long, appearance is obvious, appearance of product is affected, and customer judges that it is an unqualified product, as shown in FIG. 4 . By changing gate position and number of gates, length of weld line is adjusted, length on design surface is controlled within an acceptable range. Weld line on the surface of product after mold trial becomes short and appearance is not obvious, customer judges that it is a qualified product. By using simulation software simulation and verification, reasonable flow channel size and gate size can be designed to reduce number of mold modification. At the same time, position of weld line of product can be predicted, reasonable gate position setting can be made to improve molding quality of product.

 

Figure 5 Process optimization of weld line on appearance surface

Original weld line of product is obvious in appearance, which affects appearance of product and it is a substandard product. Product shown in Figure 5 is designed on the side of cavity plate near weld line to design cooling water channel, mold temperature is adjusted by hot water or hot oil to improve fluidity of melt front; Appropriately increase surface roughness at the core to reduce fluidity of core side melt, position of weld line changes toward core direction, thereby reducing influence on the front appearance of product.
A large amount of trapped air is generated at the joint between core side and cavity. Partially removing inserts and processing them into a piece of form facilitates discharge of trapped air, promotes better fusion of two melts, improves appearance quality of product. After above measures are improved, weld line on the surface of product is optimized, and no obvious traces are visible on the surface of product.

 

Figure 6 Thin wall of product

At thin wall of product shown in Fig. 6, wall thickness of thin wall of product is thickened by drainage, so that melt flow rate in this region is accelerated, and melt flow front edge is better fused. By partially removing insert and processing it into a piece, gas trapped in this area is smoothly discharged to prevent fusion portion from being trapped, thereby avoiding occurrence of weld lines on the surface of product, affecting appearance and strength of fusion portion of product.

 

Figure 7 Product structure change

In button type product shown in Fig. 7, original welded joint position is on appearance A surface, which is a defective product. Improvement measures: structure of product is changed, wall thickness at inner side of A side is increased, intermediate melt flow rate is increased, melt flow front edge is moved forward, weld line position is moved to side wall C, and side wall of C is non-first appearance surface. Or thinning wall thickness of non-appearing lip B, so that melt velocity of peripheral lip is slowed, fusion front of intermediate melt is moved forward, and position of weld is moved to side wall C. Because of structural changes involved in product itself, contact customer, and finally choose to modify wall thickness of lip B.