【Abstract】Weld line is an important factor affecting quality of plastic parts. Study found that: under same process conditions, strength of weld line area is only 10% to 92% of original material, which seriously affects normal use of plastic parts. This article introduces formation process of weld marks, influencing factors and solutions to eliminate weld marks.

Weld marks (called water traps) are transfer marks formed when molten plastic flowing out of gate at different positions of mold converges. Seams formed when melts merge are divided into melt lines and weld lines. Performance of melt lines is significantly better than weld lines. Generally speaking, a melt line is formed when confluence angle is greater than 135℃, and a weld line is formed when confluence angle is less than 135℃, as shown in Figure 1. Performance of melt line is significantly better than that of weld line. Melt angle has an important influence on performance of welt seam, because it affects sufficient degree of molecular chain fusion, entanglement and diffusion after fusion. The larger melt angle, the better performance of weld seam.

 

Figure 1 Difference between melt line and weld line
"Weld mark" in a plastic part refers to a three-dimensional area where morphological structure and mechanical properties formed when two strands of molten material are in contact are completely different from those of other parts. There are two basic types of the most common weld marks in plastic 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 mark formed at meeting of melt fronts entering cavity from different gates is called cold welding; the other is when mold cavity is equipped with cores and inserts, melt is divided into two strands when passing through obstacle. After bypassing obstacle, weld line formed by two strands of melt rejoining is called thermal weld line. It can be seen that form and number of gates in injection molding process are unreasonable, plastic parts are more prone to defects of weld marks. Generally, number of weld marks produced is not greater than number of gates. For use of gates, when length of plastic part exceeds 150mm, an additional gate should be added to prevent plastic part from being insufficiently filled due to difficulty of remolding; when distance between gates is too close, it is easy to produce wrinkles due to mutual impact of material flow when melt fills cavity. Plastic part shown in Figure 2 is earmuff ring, material is ABS, and its shape features, that is, wall thickness at transition from arc to flat edge gradually becomes thicker, and the whole is annular and thin-walled.

 

Figure 2 Earmuff ring
Note: Height from A to B is increased from 4mm to 10mm
Height from B to C is reduced from 10mm to 4mm and the rest is 4mm

As shown in Figure 3 and Figure 4a, earmuff ring has obvious defects such as insufficient filling and weld marks; weld marks can be determined according to gate positions at I, II and III, analysis of melt flow in mold cavity. Mold has 3 gates. When melt is injected into these 3 places, there must be 3 confluences in flow of melt under ideal conditions. Their approximate positions are where K 1, K 2 and K 3 meet as shown in Figure 4a, that is, these 3 locations are prone to defects such as insufficient filling or weld marks during injection molding process. According to Fig. 3, it can be seen that positions of insufficient filling and weld line defects are consistent with above.
In Figure 4a, three gates I, II, and III are all side gates, and they are all directly facing large core. Under action of pressure, front of high-temperature melt directly impacts large core, then splits and fills. At this time, temperature of front of melt will rapidly decrease to form a condensation film. Since cavity is only 1mm thick, flow of melt to junction K 1 is longer, temperature of melt front will drop further during flow, so that it has solidified before it flows to junction K 1, which causes defect of insufficient filling. Due to short process at Huijia K 2 and Huijia K 3, although defect of insufficient filling does not occur, front of melt will inevitably cause a large drop in temperature after impacting large core, two melts are poorly compatible, which leads to weld marks at junction.

 

Figure 3 Defects of earmuff ring

In view of fact that earmuff ring has insufficient filling and a large number of weld line defects, analysis shows that root cause of these defects is unreasonable structure of designed gating system (including gate position and number, etc.), so solution is mainly to start with gating system .
(1) Option 1: An economical solution based on original injection mold structure. This solution only slightly modifies gating system of injection mold to achieve immediate results, and cost of modification is minimal. Specific measures are as follows.
a. Since there are 3 lateral gates, there are 3 melt streams converging, which can produce 3 weld marks. Therefore, to reduce number of weld marks, number of lateral gates must be reduced. Designed three gates are relatively concentrated. For this reason, middle gate is sealed first, only gates on both sides are retained, so number of weld marks is reduced by one, as shown in Figure 4b.
b. When melt in rectangular lateral gate fills mold cavity, it will directly impact larger core and cause its own temperature to drop significantly. Therefore, rectangular lateral gate should be changed to a fan gate, specifically, 3*2*3mm side gate should be changed to a 2*3*1mm*90° fan gate.
c. Set up a cold material hole at place where weld mark is generated to store cooled and oxidized melt front, thereby improving welding condition at weld mark and increasing strength of weld.
d. Reasonably trim corner R to reduce direct impact of most of melt on large core and cause a faster cooling, so as not to reduce fluidity of melt.
Treatment results: number of lateral gates has been reduced by one; defect of insufficient filling has been solved; weld line is no longer obvious, strength of weld line has been improved.
(2) Scheme 2: This scheme only has a fan-shaped gate, because width of cavity at left end 1 of Figure 4a is 2mm, width of cavity at right end K 1 is 1mm. According to principle that gate or sheet gate should be set in wide cavity, fan gate is designed at position of weld line K 1 at left end as shown in Figure 4a. Size of fan gate is 3*1mm*120°, which can make melt fill cavity along fan gate, avoiding melt directly impacting large core and causing a significant cooling; set a cold spool at weld line, that is, front of melt enters cold slug hole during process of filling mold cavity with melt, so as to achieve purpose of improving welding condition at weld line.
Treatment result: defect of insufficient filling has been completely solved; number of weld marks is only one, marks are no longer obvious, strength of weld marks has been greatly improved.

 

Figure 4 Defect analysis of earmuff ring and injection mold rectification plan
a — —Analysis diagram of earmuff ring defects and melt filling mold b — —Earmuff ring injection mold rectification plan diagram
1. Core 2. Gate 3. Split runner 4. Main runner 5. Cold slug hole

From perspective of improving quality of plastic parts, second option is an ideal one, plastic parts will have a better molding effect than the first option. However, second option means scrapping existing molds, which will cause direct economic losses. Generally, second option can only be used when designing new molds or duplicating existing molds, so this scheme cannot be easily adopted as a last resort.
Through introduction of above cases, it is proved that reasonable design of structure of mold gating system, such as reasonable adjustments to form and quantity of gates, can achieve purpose of solving defects such as insufficient filling and welding marks.