Thick section Optics questions

[3NGLENN1889]

Hello All! Anyone here experienced in molding thin to thick wall optics pieces in polycarbonate? I'll provide more details depending on reply's.
Thanks G.P.

[Airwinghma]

Greetings;

I've ran thin walled optical type pieces out of Acrylic and san, in molds up to 16 cavities.

Will that get more details?

Scott

[rickbatey]

I've molded PMMA and PC lenses in the past. The PMMA had thick to thin transition but all lenses were for gas pumps. What's your question?
Rick.

[3NGLENN1889]

Sorry for the delay in responding. The part is a rectangle shaped lens gated at the top edge, .100" deep by .250" wide. Material must travel down to a center section that is .850" at its thickest point. The problem is a light weld line smack dab in the middle of this thick section. As this is a light dispersion lens (LED)
engineering is having a cow. Last Friday I jacked the heats up 10* and filled it as fast as I could. Was able to move the the weld line off to the side but still in the thick section. Gave up on it and I'm back at it here again Monday for one last try (little hotter melt, fill a little faster). Mold temp is 225*F on an oiler. Wish me luck.
Any suggestions or tips much appreciated. Simply put again it's a weld line issue in an optics piece. Thanks G.P.

[pjhall]

I've run small and large optics. Is the material PC? If so try 250-275 and fill SLOW.

[rickbatey]

Well I see the root cause is the gate is in the thin section when it should be in the thick section. Higher melt and mold temps will help mask the knit line but I fear you'll never make the part run without too high a scrap rate. I think you need to push the melt as hard as the venting and part cosmetics will allow. Any loss of melt pressure will show as a knit line or poor light transmission.
Why did they gate into the wrong section of the part? Maybe they should attend a DFM class for injection molding?!
Rick.

[3NGLENN1889]

Rick-thanks for the reply. I was heading in the direction you mentioned. was able to get them parts they could test for SAE.
Why did they gate into the thin section? A DFM class? You are sooooo right. These items (AND A ZILLION OTHERS) have been stressed by me for the last 10 years. Any further explanations would involve colorful language not appropriate for this forum. They (engineering) will be getting a copy of this thread. It's great to get info from other sources such as you. Thank you. Have a great day. G.P.