Dear colleagues,
I would like to ask you if you have experienced problems with dimension which is out of specification and try to “improve it” using process settings.
Due to fact ABS is styren family so it is amorphous there is not enough space for shrinkage adjustments.
I have thought of mold and melt temperature and holding pressure could influence the dimension.
Thanks for any tips

Increase size = increase Holding pressure and/or holding time, gate/runner size, reduce mould temp
Decrease = opposite to above!

Also inc cooling = inc size

It used to be very common to use parameters to change dimensions. Modern practice is to optimise the the parameters for a particular mould. In the past on a 10 inch dia PP part we have changed the dimensions by1/8 of an inch by changing parameters as Darren mentions above and the part has still performed well.

HI. Jiri:
For molding parameter just has very small change for the dimension. Like high temperature and a little less packing pressure etc. shorten cooling time. But normally 0.55% is for small parts. If for big parts then it is better 0,45%. Some specially case. even 0.3% also possible shrinkage. middle size from 100 to 400mm. 0.5%; Bigger then less shrinkage. Thinner also need bigger shrinkage. more gates to reduce the injection pressure also helpful.

Thanks for comments, we have a problem with already finished mold and was not present at the beginning of the project, unfortunately.
To be more precise, I am talking about cylindrical part (ring intersection) where inner diameter is smaller then it should be. 45,75+-0.1 is by measurement sometimes smaller than 45,5. There is also problem with the ovality of a part. I do not want to change mold yet.
So by your recommendations I should recommend supplier to:
-decrease holding pressure
-decrease cooling time
-increase mold temperature


What about melt temperature?

If you want to make small your part ,you should increase temperature so when you take part on the tool that part must be hotter than . because, hot product, can more shrink outside of the mold.
For small product you should;
İncrease melt temp,
İncrease hot runner temp(mold temp)
Decrease cooling time,
Decrease holding pressure,
Decrease feeding back pressure ( When screw going back )
You can increase chiller set point.


For Big product you should;
Decrease melt temp,
Decrease hot runner temp ( mold temp)
İncrease coolng time
increase holding pressure
increase feeding back pressure
decrease chiller set point.

If the part is oval and the wall thickness is even all round then the gate is usually the problem. One side gate causes the part to be oval.
Get your vendor to weigh the parts with minimum hold on time. Increase the hold time and hold pressure until you find the maximum weight that can be obtained. If the hold on time does not increase the weight after 2 secs or so the gate is too small this depends on wall thickness if a thicker wall then the 2 sec rule should be increased. Once you have reached the maximum weight with a minimum tool temp normally around 40C for ABS and the parts are still too small then you need to make a new core. Also use a longish cooling time bearing in mind the the hold on time is also cooling time. Also ensure that the core is central with no deflection so that the wall thickness is even all round. One side gate could be causing deflection.

Thanks to conclude this topic. But there is a one think I could not clearly understand. SHRINKAGE INCREASES WITH INCREASING MOLD TEMPERATURE.
I have always thought that higher mold temperature supports crystallization, decreases residual stress,ect. Could somebody try to explain this phenomenon or send me a link where is a clear explanation?
Ken,
thanks for 2 gate advice. I will look over this possibility.

Increasing mould temp can sometimes help to pack the part better so reduces shrinkage but it is also related to cooling times as a long cooling time means you are using the cores as a cooling fixture. But of course on some materials with a memory after a few days ???

Re gates a diaphragm gate is the optimum but might not be practical 2 gates might stop core deflection but for ovality on a 45mm dia really four gates is the least.