Weld line strength

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Pre­cise state­ments about the strength of a weld line are dif­fi­cult or im­pos­si­ble to make. New sim­u­la­tion mod­els should make it pos­si­ble to pre­dict the weld line strength.

Ba­si­cal­ly, weld lines are con­sid­ered to be ar­eas of in­suf­fi­cient strength on the mold­ed part. They oc­cur where the melt flow in an in­jec­tion mold splits in­to sev­er­al streams at an ob­sta­cle and then flows to­geth­er again. The weld line (weld line, flow line), which is vis­i­ble as a line, forms at the flow fronts that meet. As soon as an in­jec­tion mold has in­serts for holes or sim­i­lar, the weld line risk in­creas­es. If the melt cools too quick­ly at the mold wall of the mold, the merg­ing mass flow will no longer bond op­ti­mal­ly. Small notch­es are formed, which rep­re­sent a weak point in terms of strength, and this can be as much as half the ba­sic strength! Me­chan­i­cal stress­es at these points can cause pre­ma­ture fail­ure. A frac­ture usu­al­ly runs along the weld line. In or­der to pre­vent fail­ure of the mold­ed part dur­ing prod­uct de­vel­op­ment, pre­dic­tions of the weld line strength are required.

Tensile tests and simulation model

Sim­u­la­tion tools for plas­tic in­jec­tion mold­ed parts are al­ready very good at rep­re­sent­ing the po­si­tion­ing and di­men­sions of a weld line. De­ter­min­ing the weld line strength is on­ly pos­si­ble in the ab­stract be­cause of the nu­mer­ous fac­tors in­volved. Ten­sile tests are used here. With these tests, the strength of the weld line un­der ten­sile stress can be mea­sured and eval­u­at­ed me­chan­i­cal­ly in sev­er­al runs. The re­sult is an ap­prox­i­ma­tion of the re­al strength of the weld line. 

Avoiding the weld line

The oc­cur­rence of weld lines is of­ten un­avoid­able in com­plex mold­ed parts. Es­pe­cial­ly if the part does not al­low much room for ad­just­ments. In the de­vel­op­ment phase, mold de­sign­ers can re­sort to com­mon prac­tices to avoid the weld line. How­ev­er, these in­voke oth­er dis­ad­van­tages in re­turn, such as longer cy­cle times or in­creased en­er­gy con­sump­tion. The use of dy­nam­ic high-speed tem­per­a­ture con­trol sys­tems in the in­jec­tion mold, which al­low ther­mal en­er­gy man­age­ment at spe­cif­ic points and close to the con­tour, makes it pos­si­ble to avoid weld lines. And this can be done with­out length­en­ing the en­tire process and al­so with­out in­creas­ing the en­er­gy used. In terms of sus­tain­abil­i­ty, this is an im­por­tant point. Qual­i­ty de­fects due to a lack of weld line strength can thus be ruled out in advance.

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