processing | Weld lines Overfl ows overcome weld lines


High performance plastics supplier Solvay Specialty Polymers has completed a study showing that overfl ow tabs – small reservoirs that connect to a mould cavity via a small channel to allow


overspill of injected plastic – can improve weld line strength of injection moulded fi bre reinforced structural compo- nents.


The study examined


structural parts moulded in reinforced grades of the company’s KetaSpire poly- etheretherketone (PEEK) and AvaSpire polyaryletherketone (PAEK) resins. It showed a minimum increase of 50% in fl exural strength could be obtained using two-gate part designs with an overfl ow tab, as compared to two-gate samples with no tab, due to the impact on melt fl ow and fi bre orientation. “The study confi rms that


weld line strength can be optimsed for our reinforced grades of KetaSpire PEEK and AvaSpire PAEK by using overfl ow tabs, thus providing greater overall performance in structural applications,” says Guy Van Meulebeke, CAE senior specialist for Solvay Specialty Polymers. As a rule of thumb, the


strength of a reinforced polymer with a weld line is typically reduced by about 50% compared to no weld line, according to Solvay, due to poor orientation of the fi llers at the weld line with respect to the load direction. By modify- ing the fl ow of the resin in the cavity, an overfl ow tab (also


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Avaspire resins, the use of overfl ow tabs is not limited to these two high performance polymers, according to Solvay’s processing experts.


“The overfl ow technology can indeed be used also for other fi bre reinforced polymers - actually for all reinforced polymers,” says Van Meule- beke. “We made tests on a few Ixef and Amodel grades, which all showed a strong improve- ment of the weld line quality.” While the test work was


performed on simple two gate samples, Van Meulebeke says the overfl ow approach can be applied to any project where a weld line is formed. So it could be used in multiple-gated


Solvay’s moulding study shows the use of overfl ow tabs can improve weld line strength in fi bre reinforced polymers. Top: sample part showing off-centre location of the overfl ow tab. Centre: short shot sample showing the tab is the fi nal point to fi ll. Bottom: short shot sample with no overfl ow tab showing the weld line location


known as spillover, relief tab, or overfl ow well) can favour- ably change the fi bre orienta- tion distribution, the research- ers say. To quantify weld line quality, Solvay’s engineering team modifi ed a test specimen mould to create an overfl ow tab. The tab was positioned slightly off centre from the weld line. The off-centre placement of


the overfl ow tab is critical as it disturbs the fi bre orientation by creating an underfl ow in the


INJECTION WORLD | March 2014


molten layer in the region. Three different 4mm thick


test samples were produced using reinforced grades of KetaSpire and AvaSpire polymers. Flexural strength was found to increase by more than 50% for all resin grades. Solvay says the fl exural strength results obtained for the two-gate samples with tabs was only 15% to 20% less than one-gate samples with no weld line. While the reported results are based on Ketaspire and


components or in sequentially- fi lled parts provided the overfl ow tab is positioned appropriately to disturb the orientation. He says the technique has already been successfully applied in a number of real components, including washers and some more complex parts.


Van Meulebeke says it is possible to use mould simulation software to optimise the position and


dimensions of the overfl ow tab. However, he says it is not


currently possible to model the benefi t.


“We used Moldfl ow to


optimise the position and the dimensions of the overfl ow tab. But there is no way to predict the mechanical performances of the improved weld line quality, even by performing coupled fl ow-mechanical calculations,” he says.


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