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In injection molding, weld lines (also known as weld lines) represent a type of molding defect where, during filling, the flow front of the resin diverges around ribs or bosses within the mold structure and subsequently converges, resulting in visible line-like marks. It is generally impossible to completely eliminate weld lines in typical injection molding. There are two main reasons why weld lines are considered defects:
Weld lines on appearance parts, especially after secondary processes such as plating or painting, can become more pronounced. If weld lines occur where functional strength is critical, they can lead to insufficient product strength and potential cracking. Additionally, poor gas escape around the weld lines can cause short shots or gas burns.
In injection molding, weld lines invariably form at the points where the flow fronts converge during filling. If they appear at positions anticipated during mold design, they are usually not problematic. However, unexpected locations or excessively pronounced weld lines are issues. During filling, the molten resin flows more readily towards easier paths, and conversely, not towards more difficult paths. This can result in pronounced weld lines or weld lines deviating from desired locations. Below are the main factors contributing to weld line formation:
Poor filling balance can lead to the formation of weld lines. To ensure even filling balance, the following elements should be checked:
A low mold temperature can cause the filling speed to be insufficient, allowing the flow front to cool and solidify before converging, thus strengthening the weld line.
As production progresses, gas contamination accumulates in the mold, worsening gas escape. Since weld lines occur at flow front convergence points, failing to adequately vent gas through gas vents can lead to pronounced weld lines or even gas burns and short shots.
Long injection times allow the flow front to cool and solidify by the time it converges, strengthening the weld line and also leading to sink marks.
To prevent weld lines proactively, the following points should be checked before production:
Particularly in multi-gate molds, poor filling balance can strengthen weld lines and cause their location to shift. If the valve gate's heater in an HR mold is partially broken or the valve slides poorly, the filling volume from that gate decreases, shifting the weld line's position. Even if the product does not short, a shift in the weld line position can fail to meet the mechanical function. It is crucial to compare with a good quality sample to check the position and intensity of the weld lines.
The flowability (MFR) of the material can vary by type and material lot, as can the effects of additives or colorants. Changes in flow can alter the location of weld lines. Materials prone to strong weld lines can be improved by adjusting the injection pressure and speed at the weld line location, altering the gas escape.
Complex mold structures can cause the flow front to cool and solidify before converging, leading to strong weld lines and sink marks. Particularly, thin sections of the product tend to exhibit stronger weld lines because the molten resin flows with difficulty. If various measures do not resolve the weld lines, and they fail to meet aesthetic or functional standards, consulting with the customer to consider changing the mold structure may be an option. Adjusting the thickness and position of ribs and bosses can improve weld lines.
Measures to counteract weld lines due to the mold structure include:
Cleaning of the parting line (PL) gas vents alone is not sufficient to remove all gas contamination. Accumulated gas contamination directly affects the product. Regular overhauls every tens of thousands to hundreds of thousands of shots are effective.
So far, we have discussed the main causes of weld line formation and countermeasures for each. However, despite these efforts, completely eliminating weld lines is challenging. To accurately detect the occurrence of weld lines, pay special attention to the following timings:
At startup, it is crucial to perform a thorough initial inspection to ensure that the quality meets the standards. Judging the appearance location and intensity of weld lines can be difficult. If judgment is challenging, it may be prudent to defer the decision to the quality control department and wait for their approval before proceeding.
Weld lines generally worsen in correlation with the cumulative shots of the mold. This worsening is due to the clogging of gas vents. Taking time samples and performing regular quality checks is crucial.
Changes in weld line occurrence are particularly noticeable when material lots change. The gas produced during melting can directly affect the weld lines. Special attention is needed during changes in material lots.
We have summarized points of concern for addressing defects and improving productivity in injection molding factories, particularly regarding weld lines. Weld lines are defects where striated lines appear at the convergence points of the flow. By not overlooking weld lines, aim to establish higher precision in molding techniques.
