Know about knit lines? If knot - we hope this Design Tip will be useful for you.

A "knit line" in a plastic injection molded part (see figure) 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 above, 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.

Protomold 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, materials that that tend to outgas a lot (e.g. Santoprene) or 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 Protomold to optimally place the gate(s) so that the knit lines are minimized or moved to a less critical area.

You can visit the Protomold Design Guide for other helpful Rapid Injection Molding design information.

At Protomold we work hard to deliver all three, but it might be interesting to hear from our customers how you'd prioritize them. Let us know what you think, and we'll include the results in next month's Rapid Injection Molding Design Tips. Please suggest any ideas you may have for future survey questions. Just e-mail them to us at marketing@protomold.com.

Which priority most closely matches what you need from your prototype parts supplier? A : 1st Quality, 2nd Price, 3rd Speed B : 1st Quality, 2nd Speed, 3rd Price C : 1st Price, 2nd Quality, 3rd Speed D : 1st Price, 2nd Speed, 3rd Quality E : 1st Speed, 2nd Price, 3rd Quality F : 1st Speed, 2nd Quality, 3rd Price

We asked "What did you think of the first Rapid Injection Molding Journal?"

Of the approximately 200 people that responded, when given the choices of Excellent ("Can't wait for the Fall issue."), Good ("A decent start. Could be better."), Fair ("Saw it. Read it. Tossed it."), Poor ("Why did you send this to me again?") and Not applicable ("What journal? I never saw it.")? it looks like the people that received it didn't think it was too bad. We're working to make the next issue more informative, so if you would like to get your own copy in the mail but didn't get it the first time around, please register on our web site or just send an email with your mailing address to marketing@protomold.com. If you'd first like to check out the Summer issue, you can find a PDF version on our web site.