A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. Depending on the resin, resin temperature, mold 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.
As noted, the size and shape of the knit line is affected by the molding 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 mold. 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.
Protolabs mold technicians try to minimize 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 mold releases can further exacerbate the problem. Third, it may be possible to improve the situation by working with Protolabs to optimally place the gate(s) so that the knit lines are minimized or moved to a less critical area.
As always, feel free to contact an applications engineer with any questions, at 877-479-3680 or [email protected].