Delamination of Living Hinge

[APDplastmold]

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

http://www.robustmolding.com

 

[Paulkuklych]

Indeed, it sounds like you have a gate location issue.

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

 

[Pud]

I'm surprised that with all the services you advertise on your website you did not work out the correct gate position and carry out a "precision flow analysis" before the tool was made.
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.

 

[APDplastmold]

Paul, thank you for your feedback, we plan to 1.compute/try faster injection, 2.increase lining hinge thickness while computing the major principal strain in it to stay as far away as possible from the material's elongation(strain) at break value and 3.relocate the gate as the last resort. How does this plan sound to you? Thank you in advance.

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

http://www.robustmolding.com

 

[Paulkuklych]

That sounds like a good plan. Did you also consider increasing the back pressure during plasticizing to help the flow?
Anyway,best of luck with the project.

Paul Kuklych

http://www.improve-your-injection-molding.com

 

[loki3000]

gate location is most likely the reason yes.

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.

 

[Paulkuklych]

With flip top caps, the mold usually has an in mold closing mechanism.
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

 

[MacGyverS2000]

My understanding (and this is from an outsiders point of view) is they close the lid while still warm, which helps with strand orientation, but I always understood it to be a one-time operation. I do not know if multiple open/close sequences would help more and is simply not done because of the extra complicated steps to do so... Pud?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[APDplastmold]

All,

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

http://www.robustmolding.com

 

[loki3000]

--With flip top caps, the mold usually has an in mold closing mechanism.

is there an example of this, an image preferrably, somewhere on the internet?

 

[Paulkuklych]

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