Weld line

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Weld lines are weak points of a com­po­nent. In plas­tics tech­nol­o­gy, we know sta­t­ic and dy­nam­ic weld lines. Sta­t­ic weld lines are formed, for ex­am­ple, dur­ing the weld­ing of ther­mo­plas­tic mold­ed parts. Dy­nam­ic weld lines, on the oth­er hand, are formed dur­ing in­jec­tion mold­ing by the con­flu­ence of mass flows (be­hind cav­i­ties, due to chang­ing wall thick­ness­es or gates of the mold). Here, flow fronts col­lide and weld. The low­er the tem­per­a­ture and pres­sure, the low­er the strength of the weld line. If com­plete weld­ing of the ma­te­r­i­al fronts is not pos­si­ble be­cause the melt has al­ready cooled down con­sid­er­ably, the weld line will show a groove or indentation.

What problems does a weld line cause?

If ten­sile stress­es are present at the po­si­tion­ing of the weld line dur­ing sub­se­quent ap­pli­ca­tion, this will re­sult in ex­ces­sive stress and pos­si­bly break­age. In ad­di­tion, weld lines can pro­duce un­de­sir­able col­or de­fects and dull ha­los. The mere pres­ence of weld lines is al­so prob­lem­at­ic when it comes to vis­i­ble com­po­nents, such as the high-class cock­pit in a pas­sen­ger car, the con­trol but­ton on a wash­ing ma­chine or that of a dish­wash­er. For rea­sons of strength, which can min­i­mize the sta­bil­i­ty of the com­po­nent by up to 50% de­pend­ing on the ma­te­r­i­al, and sur­face qual­i­ty, the de­sign of plas­tic com­po­nents should be such that weld lines are avoided.

Early detection of weld lines

Spe­cial sim­u­la­tion soft­ware for in­jec­tion-mold­ed com­po­nents can be used to iden­ti­fy and pre­vent weak points, such as fill­ing prob­lems, free-steel for­ma­tion or weld lines, in the planned mold as ear­ly as the de­vel­op­ment and de­sign phase. 

Solution for weld lines

The weld lines are an in­te­gral part of plas­tic in­jec­tion mold­ing and can sel­dom be pre­vent­ed, even by specif­i­cal­ly plan­ning the mold with in­serts for holes or the like. With un­change­able and fixed re­quire­ments for the fi­nal prod­uct, their avoid­ance of­ten proves difficult.
It is pos­si­ble to pre­vent the weld line from flow­ing to­geth­er by in­creas­ing the melt and/or mold tem­per­a­ture. How­ev­er, this has the neg­a­tive ef­fect of length­en­ing the in­di­vid­ual in­jec­tion mold­ing cy­cle. It al­so in­creas­es en­er­gy consumption.
High­ly dy­nam­ic high-speed tem­per­a­ture con­trol sys­tems, which are in­te­grat­ed in­to the in­jec­tion mold and al­low very rapid sup­ply and re­moval of ther­mal en­er­gy on­ly at the prob­lem area, pre­vent the for­ma­tion of weld lines. Neg­a­tive ef­fects, such as cy­cle ex­ten­sion or in­creased en­er­gy con­sump­tion, do not oc­cur with such sys­tems. They can be di­rect­ly in­cor­po­rat­ed in­to the de­sign of the mold or retro­fit­ted to ex­ist­ing molds.
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