Post Shrinkage Nylon 6- 30% GF

[mahesh_a_patil]

Hi,

Need some directional help

Material: Nylon 6- 30% GF

In a physical trial, part warpage is within the torelance limit (+-0.2mm) after immediate measurement, but after 24hrs if I measure the same distance then it is out of limit (+-0.8mm). I know post shrinkage is the inherent property of Nylon material. 

May I know how we can resolve this post shrinkage (warpage) issue? 

MaheshP

[madhukeshwart · Accepted solution]

Hi @mahesh_a_patil,

The post-molding shrinkage is an effect of secondary crystallization; the increase in the degree of polymer crystallinity leads to an increase in stiffness and decrease in impact strength of the polymer material.

By changing temperatures, pressures, and packing and cooling times, it is possible to mitigate shrinkage. By applying pressure to a liquid plastic, the molecules can be compressed into a smaller volume which enables more material to be injected into the mold to compensate for shrinkage.

1. Most such issue must be addressed soft mold/Proto mold to get right shrinkage to be applied to mold, which includes post shrinkage. 

2. By increasing  temperatures& pressures, and packing time - it is possible to mitigate shrinkage. By applying pressure to a liquid plastic, the molecules can be compressed into a smaller volume which enables more material to be injected into the mold to compensate for shrinkage.

3.By cooling rapidly, we can ensure semi-crystalized part  & May be by quenching the part in cold water immediately after ejection of part out of mold.  it arrests post molding shrinkage.

4. Creating cooling fixture, and allowing to cool in fixture. so that locating parts & critical dimensions are ensured. 

5. Need to do trails and fix the procedure to operate for molding.

[h-fujitaDXJU5]

Hi MaheshP

I think that most important thing to control shrinkage of GF filled PA6 is gate location.

In my experience, no packing condition affects warpage (shrinkage).

Fiber orientation dominants anisotropic shrinkage. 

My advice for avoiding post shrinkage is using hotter mold and higher injection velocity.

Post shrinkage is result of post crystallization. You must crystallize in closed mold.

So I recommend high mold temperature at least above 50 degree C.

And you also consider water absorption of Nylon after molding and real using environment.

(While 24 hours water absorption is little, when a week after molded part absorb many humidity.) 

Regards, Hirohisa.

[ryan.fuhr]

Just want to reinforce the post molding water absorption can be a big deal in PA resins. Most of my experience is with 6/6 Nylon (most with 20-40% GF), but on tight tolerance parts, we have had customers add "part must be measured within 48 hours of molding" to help resolve this.

I work in a northern climate where the dew point drops to well below 0 deg F each winter and summer vs. winter makes a difference on part dimensions a week after molding.

If you want to test if this is the root cause, build a desiccant chamber and high moisture chamber and place a population (say, 5 each) of equivalent freshly molded parts in each (e.g. 10 shots in a row, even shots in one chamber, odd in the other). Measure them 3-4 days later. Any difference in the populations would be due to the extremes of moisture content.