Sinks

[Chr1s]

Does anyone have the real magic formula for designing for zero sinks in ABS. The rato normally talked about is 1/3 wall stock. But I have yet to see a part that uses this that will not show sink. I know other factors like packing out the mold come into it, but has anyone had good results with another ratio.

Thanks,

 

[Soupcat]

Chr1s,
2/3 wall stock surely? I usually advise 0.5T, for example the rib or cross member section being half that of the critical surface section. It really depends on a lot of variables, smaller cavities where the critical sections are close to the gate are easier to pack out (require less pressure in the cavity) than larger cavities where the critical section is quite far from the gate (longer flow path and more pressure required). Surface finish is also a factor as a rough texture can disguise a slight sink witness as can the colour of ABS, i.e. a darker colour will show up sink more than a lighter colour. Can you spark the rib/Boss feature in the smaller section and take it up slowly to a max of 0.5T or 0.6T at the very most? Can you have multiple feed points? A 3-plate tool or multi-nozzle hot runner will help reduce sink too. Hope this helps.

Kieran

http://www.moulddesigner.com

 

[patprimmer]

It also depends on the radii between the ribs and the wall. small radii gives less sink, but is more prone to crack in use. !/2 T, small radii, not to hot in the melt temp, good sized gates, plenty of effective hold up time and pressure, a mould with a good fitting parting line and a machine with enough clamp and a good ring check valve.

If all else fails, use a bit of gas, or blowing agent to pack out the thick spots.

One unavoidable fact is that the mould will get hotter near the junction of the main surface and the ribs, this also leads to sinks and distortion or stress Regards
pat

 

[plasgears]

We have used foaming agent to reduce sinks. The section is dense on the outside and porous in the core, which causes sinks to minimize. This is particularly useful in housings.

Another approach is to put in an unobstrusive undercut opposite the rib. The aim is near equal thickness design.