» Does preheating/drying material have big effect on flow pattern during mold filling?
Does preheating/drying material have big effect on flow pattern during mold filling?
Hi! Holiday greetings to all. Just a quick question, what happen if we increase the drying temperature higher than the spec given by material suppliers? For instance, 90 degree C instead of 70 degree C? Does that affect the flow pattern during mold filling? If let say, product released from mold does have a weld line at edge when using standard temperature, will increasing the temperature help reduce/ relocate the weld line? Thank you for sharing.
Increasing the drying temp. probably will not have an effect on final part outcome. Especially, PP, which is not very hygroscopic to begin with. Increasing mold and stepping up barrel temps. will help reduce cycle times, but changing viscosity of the polymer will only worsen flash, parting, weld lines. If you run too hot and too much dwell you will only degrade the problem and worsen the flash. But, if you run too cold, you may not have decent packing and the material will have stressors built-in possibly leading to microfracturing.
The best way to solve your problem is minimize packing pressure and dwell time.
I have seen nylon lose plasticizers when over dried (temp and time issue), causing issue with knit lines, weld strength (bottle was vibration welded), and impact strength. For PP, I have seen improvements to the part finish when molded (PP+talc), due to drying.
Now if you want to move a knit line; fill rate, mold and melt temps are the only methods, providing you don't have a valve gated tool. There is one other trick, a melt flow helper. This is just a localized thickening of the wall stock, done to the non-visible side of a part, to assist the melt front in getting into an area and pushing the knit line into a more acceptable place. These do not always work, due to part cosmetics or tool design. But they can help you keep a customer.
Hope this helps, Rick.
Thanks to Donn and Rick for your response. Donn, we tried minimize packing pressure before. The result is, short shot (insufficient filling) Right now, we are planning to modify the tooling, i.e adding some extra features at hidden area to redirect the flow. Just as what more or same said by Rick. But, the customer did inquire stepping up the drying temperature. That's what made me wonder..how effective it is. Another mold trial means another delay to project schedule. Increasing the cost too..
I would suggest that in the future, you include two to three tunning loops in your start up programs. There is ALWAYS those small things that either no one saw coming, or unexpected results (like knit line placement or dimensioanl/cosmetic issues). You can increase the drying temp, but a quick test of the moisture content can settle the point. Or you can test the theory....Do you have the material suppliers spec sheet? What do they specify? I know that one brand of ABS/PC calls for a much higher drying temp than several others. I know that the increased drying temp helps with cosmetic issues (lowers gloss and gate blush), but nothing as far as knit lines or dimensions. Rick.
First, I wish a very happy new year to you & your families. All the polymers absorbs moisture from atmosphere. But %age of moisture absorption varies from one polymer to another. %age of moisture is widely effect flow fattern of molten material. Predrying of material is depend upon temperature and time. If we increase the predrying temperature higher than manufacturer recommendation, It may result in variation of colour / bonding strength between monomers will be weak. It would be better to increase the time. Mould temperature controller can be used to maintain uniform temperature of mould in order to have uniform flow of molten material inside mould.
Air vent can be provided near joint line to release gases/air.
I agreed 100% with Kumar.
Pre drying PP will not improve welding lines or increase the melt flow index, especially PP.
The injection gate is very important, do not forget to make a lot of radius on the sprue and like the colleague said, good venting gases are crucial. If you want a better flow, you will need to choose another grade of PP with a better melt index!
I have a question: If I overdry a resin (for ex. PA6) will the melt flow index decrease?
If I have problem with flashes, after for example 4 hours of production, can it be a reason of "wet" polyamide?