I have a plastic product with weldline on the surface and we tried different molding parameters , it not works . now we rty to change the thickness . would you have some good idea?

Any part with holes or opening, Which has 2 or more gating and which has high wall thickness ratio, you can't avoid weldlines. Except Improvement to minimize the weldline appearance
Conventional methods:
- higher mold temperatures, higher melt temperature.
- high injection speed and pressure fill
- Air venting at the weldlines location.
- Add ribs directly opposite the weldlines.
- Multiple gating; controlled opening and closing of the valve gates.

New technology;
- Use rapid heating and cooling molding systems. or RTC.It is proven for high gloss surface and weldline improvement. Also help to improve on sinkmark

check your gating position and if its round you need to add cold slug

make blind pocket to accomodate venting

As Mr hh Yong correctly states, you cannot avoid weld lines when you have a part where the melt front divides and then re-connects as the polymer flows around a cored aperture or where two melt fronts meet. Mr Yong is also right when he states that RTC / RHCM technology can completely eliminate visible weld lines on the surface of a part as well as bring many other tangible benefits such as a super high gloss finish and less sink marks. The RTC process involves "rapidly" heating the surface of the steel mould tool to a temperature above the Tg (glass transition temperature) of an amorphous polymer or near to the Tm (melting point) of a semi-crystalline polymer BEFORE injecting the molten plastic. This effectively slows down they formation of the instantaneous frozen skin layer that normally forms as the hot plastic comes into contact with the relatively cool surface of the steel mould and allows the plastic to flow better. Since 2007 Gas Injection Worldwide Limited has been a global market leader in RTC technology and is the only company that can offer 2 RTC processes (Steam Heating and External Induction Heating). If you would like to send me a drawing / CAD file of the part I will ask our Design Engineering team to prepare (Free Of Charge) an RTC feasibility study on your application?

i can't figure out on which surface of your product design encounterd with weld line,but usually it will happen at the side or in the bottom edge of the product,since you have done already,the full adjustment, the parameters of your injection machine,and it will not work out,if you dont mind,why dont you try to eliminate,of all sharp corner of core insert even all possible ribs if any,make it a corner radius,high polish as well,i have encountred already,when i make a mold design for tear tab lid,for bucket pails 18&20 liters,

YOU SHOULD BALLANCE INJECTION POINT [ GATES ] ALSO TRY TO WORK WITH STEAM TECH. ON CAVITY SIDE.