A knit line in a plastic injection-moulded part is created when two separate plastic flows meet within the mould and "resolidify" along their interface. Depending on the resin, resin temperature, mould temperature, and filling speed, knit lines can vary from virtually invisible to something that looks like cracks in the plastic. And in some cases (e.g., long thin features with resins like LCP) the knit lines can have reduced mechanical properties and be a cause of part breakage. So for reasons ranging from cosmetics to functionality, it is important to know as much as possible about why they happen and how to reduce their impact on your design.
Minimising Knit Lines
As noted, the size and shape of the knit line is affected by the moulding parameters, but its location will be primarily governed by the geometry of the part. The primary cause of knit lines is the way the plastic flow rejoins after it goes around a metal core in the mould. So for this reason there is a knit line (visible or not) downstream from every hole that goes through your part. And for similar reasons there is a knit line between every two gates on the part.
Proto Labs mould technicians try to minimise the appearance of any knit lines, but they must balance this with other challenges like avoiding sink or blush, achieving the desired surface texture, etc. So anything you can do to help avoid knit lines when designing the part would be a benefit.
Here are a few things to consider. First, thicker walls will slow down the cooling rate of the resin and thereby help to improve the appearance and strength of any knit lines. Second, the resin you select may make a difference. For example, unfilled materials will tend to have stronger knit lines than filled materials. In fact, knit line strength will decrease with higher filler content as well as with longer fibers. In addition, thermoplastic materials such as Santoprene, or other materials that contain additives like flame retardants, lubricants, and mould releases can further exacerbate the problem. Third, it may be possible to improve the situation by working with Proto Labs to optimally place the gate(s) so that the knit lines are minimized or moved to a less critical area.
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