Delamination of Living Hinge
Delamination of Living Hinge
(OP)
We experience severe living hinge delamination problem. We suspect excessive 1.Frozen Skin formation and the consequential 2.high Shear rate/Shear stress during filling due to hesitation flow in the hinge caused by apparently improper gate location. The latter is too close to the hinge resulting in the following filling dynamics: while the gated side of the box fills relatively easily, the flow is slowly crossing through or even stopping in the hinge; the resultant, significant Frozen Skin restricts the flow channel. Due to the latter event and the sudden climb of injection flow rate through the hinge (after the completion of filling of the gated box half) the Shear rate rises remarkably causing high Shear stresses among the layers of laminar flow leading to delamination. We will run a precision Flow analysis to test this hypothesis. Please post another possibilities, help is appreciated in advance.
G. Markus
www.robustmolding.com






RE: Delamination of Living Hinge
The flow of the material needs to be right. In other words the cavity needs to completely fill before the flow reaches the hinge area so there is no hesitation effect.
Try injecting the material faster and also make sure the venting is adequate before changing the gate location.
Hope this helps.
Paul Kuklych
http://www.improve-your-injection-molding.com
RE: Delamination of Living Hinge
Have you a picture of the problem you could share with us?
www.tynevalleyplastics.co.uk
It's ok to soar like an eagle, but weasels don't get sucked into jet engines.
RE: Delamination of Living Hinge
Pud, as to reactive v. proactive analysis/optimization, we unfortunately heard of the issue after the mold was built. I would not have dared posting this issue if the situation were other way around...
G. Markus
www.robustmolding.com
RE: Delamination of Living Hinge
Anyway,best of luck with the project.
Paul Kuklych
http://www.improve-your-injection-molding.com
RE: Delamination of Living Hinge
i've also read somewhere (uni course, presentation summarised from "Konstruiren mit Kunstoffen", german book about design with plastics) that if you open and close the living hinge while it's still warm, the fibers orient such that it (l.h.) can sustain many more cycles.
does anyone know if that is true and is it used in the industry? i imagine there are some difficulties (if you have automated handling of newly molded products) to close and open the product.
RE: Delamination of Living Hinge
This makes it cheap and easy to handle the caps after molding. It would be very expensive to open and close every single cap especially when production quantities are millions per month.
Paul Kuklych
http://www.improve-your-injection-molding.com
RE: Delamination of Living Hinge
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Delamination of Living Hinge
Thank you for your continuing feedback. We folded a few times the living hinge (attached to apx. 7*8*2.5 inch tall box halves) as soon as it came out from the mold causing high level of advantageous molecular orientation and it worked flawlessly without delamination. One time folding was OK, multiple ones gave significantly better results. However, it required an operator, which adversely affected the profitability of the project. Since then we ran an initial Flow analysis and found horrible shear rates and shear stresses in the hinge, way exceeding the allowable percentage (apx. 1) of room temperature tensile strength of the resin. Interestingly the level of Frozen Skin formation at the instant of cavity fill was not excessive despite the early hesitation flow in the hinge - probably the speeding up flow remelted it, which could be another culprit for delamination - please comment. It seems we actually have to slow down the flow, permitted by the low Frozen Skin level, in order to improve shear conditions. The plan is to compute optimal filling conditions (inj. time/melt temp./mold temp.) and run a designed OFAT (One Factor at a Time) molding trial or better yet, Designed Experiments (DOE or DOX) to see whether we can avoid the cost/time of relocating the hot gate requiring even the addition of a hot manifold into mold. Thickening of the hinge is another option; your comments are greatly appreciated.
G. Markus
www.robustmolding.com
RE: Delamination of Living Hinge
is there an example of this, an image preferrably, somewhere on the internet?
RE: Delamination of Living Hinge
Here is a link to a flip top cap mould with in mold closing
http://skeng.com.au/images/video/Clip5.wmv
Paul Kuklych
http://www.improve-your-injection-molding.com