abs surface finish

Hi all. I have recently been molding a 1+1 abs case lid and base made from a pretty standard abs with a black masterbatch. The moldings are fine but the surface finish has some defects. I occasionally get what looks like splay ( material is definitely dry ), plus what looks like a combination of jetting or swirl. I have tried everything from low to high injection speeds, decompression after screw back, screw rotation, various melt temps and back pressures but nothing seems to solve the issue. Could this be an issue of the material degrading in the barrel? I am molding the material at about 225 Celsius (about 430 Fahrenheit) which i know is hot but the tool has a huge gate channel so i need the material hot so i can get it in quickly through the tab gate. I have used this abs hundreds of times with no issue but on this component i just cannot resolve this defect. I have been provided with a sample component so i know that this correct quality can be achieved. I am molding in a 130t Negri Bossi with a 45mm screw. The shot is less than half the barrel capacity. Anyone had similar issues? Any ideas as its driving me nuts?

I suggest there maybe something going at the nozzle, check the nozzle heater band or bands if you have more then one and the placement of the thermocouple. Ideally the shorter the nozzle the better. The placement of the termocouple into the body of the nozzle and not just slipped under the heater band will provide better control as well. I have seen all these defects a number of times with ABS and a nozzle setup that caused splay while the heat cycle was on and blush while the heat cycle was off.

Hello,

I have seen numerous times a nozzle temp reading 450F, but actually 550F, and causing those issues with ABS. I agree with Bryan, and suggest taking a temperature probe, and sticking it inside the nozzle to see actual temp, (also inside hot sprue/manifold if your mold is so equipped.)

Second suggestion, 430F sounds too cold for ABS, I run it at least at 450F, and too cold can also cause splay.

Joel

Hi

What carrier material is used for the masterbatch, sounds as if it could be lamination. ABS can be quite problematic when using carrier materials that aren't suitable, if you look at the moulding under a magnifying glass, can you pick away the surface with a pin? We've supplied masterbatches for ABS many times and seen this happen from time to time, our masterbatch would work on one tool but wouldn't on another even though the ABS was exactly the same material, we'll change the carrier and issue goes away.

Huw

Many thanks for all of your comments. After further investigation i have discovered that the sample component i was provided with was made from a different grade of ABS with a better flow rate which explains why i was having so much trouble getting the material through the tab gate. Thinking about your comment Huw, this might also be an explanation for some of the defects i was experiencing. At the edge of the part their is what looks like a sign of de lamination, which i originally thought was gas trap burning due to a lack of vents. I have been using a universal master batch suitable for ABS but if the material will not blend or is shearing through the small tab gate it would explain some of the defects on the part. A new grade of compounded material is due to arrive tomorrow so i guess its back to square one and start again. Fingers crossed i have better luck. Thanks again everyone for your help and advice.

Running an ABS now (Lustran 348), data sheet specifies 475F - 525F or 246C - 274C
Going below the 475F will result in look like splay or cold flow marks

And if you're using 50% of barrel capacity residence time may be a little short resulting in non-homogeneous melt, or material is not melting all the way through to the center of the pellet

Does the defect look like this? (Parts from earlier today)
Attachment 347Attachment 348

Increasing back pressure completely fixed all the cosmetic issue - which falls in line with achieving a homogeneous melt and melting to center of the pellet as stated above!
Attachment 349

I'm running ~20% Barrrel capacity so if you're 50% capacity you should aim to crank the BP up to get a good mix. See if that helps

Thanks for posting this example Chris. Really helpful for us newbies out here.

Many thanks for all the comments and advice. Problem was completely eradicated as soon as i changed the grade of ABS. Looking through the data sheets it seemed the original ABS which was a grade for extra stiffness (insert funny joke here) was really struggling to get through the tiny tab gate and was causing the material to shear. Very strange considering how often i have used this material in the past but something i will keep in mind for the future. Been doing this years and its still great to hear other peoples advice and ideas. Many thanks again everyone for your contributions. #stilllearningnewthingseveryday