PP Living Hinge Process Parameters

[ShiranS]

Hi everyone,

First thank you to all of you who posts questions and replies to them. I've learnt a great deal from Eng-tips.

My question today is on PP living hinges. Production guys at my company are having difficulty in molding a living hinge that can be flexed more than a couple times.

The material we are using is SRM 100NC by Reliance. In April we had this same problem and after investigating I found that we were running the mould with normal water not chilled. (The mold temperature would have been high) once we ran chilled water through the mold the hinge quality was sufficient. But now again the problem has come back.

In one hinge production tells me that the hinge only fails after a couple days. Another hinge the product fails after a couple hours of molding.

I don't have a lot of data to share. If you could tell me what parameters I should pay attention to when molding a living hinge with PP it would be very helpful.

The chief operating officer does not want to put a guy to flex every cap that is molded. It goes against his lean principles. How important is this?
Also I noticed that the product was under weight several items production had molded it. On average 2-3%. Does this have a huge effect?

My conclusion is that the melt temperature is too low and cooling is not sufficient to freeze the polymer chains.

One hinge std 7.8g - cycle time 14s - fails after several days. Molded-7.6g. Melt -210C
Second hinge std 4.2g - fails couple hours after. Molded 4.1g. Melt -190-200C
Thank you in advance.

 

[Paulkuklych]

Since you were able to get sufficient hinge quality earlier in the year & now quality is a problem again it means you have too much variation in the process. The variation could be in the material ( it can happen from batch to batch) or it could be in the process parameters (injection time, hold times, barrel temps, mold temp etc).

What I recommend is go back to the exact conditions you had before (someone did make a copy of them didnt they?) & if its still an issue then the mould & the machine condition needs to be checked.

Paul Kuklych

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

 

[ShiranS]

Thank you Paul for the reply.

Unfortunately the machine parameters are no where to be found. I recently joined the company at an entry level position and I've been trying to get the production to keep track of the settings. After months of trying now we are keeping track of some parameters.

And you are correct in saying the process is not stable. I'm assuming it is our fault because the mold when sampled with the same material in China the hinge was good. When we process the hinge fails after a couple flexes.

I should add not all hinges fail.

What I'm hoping for is a general guide to keep in mind when we are sampling the hinge/cap.

Thank you

 

[Pud]

I agree with the OP that the melt temperature is too low. We use around 240ºC for PP.
There may still be unmelted particles in the hinge.
Note that melt temperature makes little difference to cycle time compared with tool temperature (which imho should be around 40ºC for PPCP)

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[ShiranS]

Thank you for those figures.

I'm hoping to run a trial tomorrow to get all the data. I will keep these values at hand.

 

[rickfischer51]

When you say a hinge fails after a couple hours or a couple days, are you saying that the hinge is flexed once and left static and it breaks on its own? Or is the hinge being flexed multiple times during that time? How many flexing cycles are the hinges actually seeing?

Is this material a homopolymer? PP homopolymers are generally best for living hinges.

The best living hinges are flexed once immediately after demolding while the part is still hot. This is important to good living hinge performance. Also, make sure that your flow front does not hesitate when it hits the hinge. If the gate location is not right and the cavity on the gate side of the hinge is not completely filled when the flow front hits the hinge, the flow will hesitate. This is not good for living hinge performance.

Regarding the comment above about unmelted particles, the rule of thumb is that the shot size should be between 30% and 70% of the screw capacity. Less than 30% can result in long residence times and material degradation, greater than 70% can result in incomplete plastication of the resin.

Get a molding guide from the material supplier and start with that. Also, see what the resin supplier says about the suitability of this resin foe a plastic hinge.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[ShiranS]

Thank you rick for the reply.

When I say fail, I mean the hinge breaks upon flexing once or twice.

A few more details.
The lid is fine right after molding. 1 day after 4% fails. 2-3 days after 20% fails. (upon flexing once or twice)

What is the reason for this? I assume it is because aligned molecules relax and lose its orientation. Am I right in this assumption? How do i prevent this?
Injection is set at 80. I could go higher but I was told flash becomes a problem at higher speeds. How important is injection pressure for a good quality hinge?

SRM 100NC is a clarified random copolymer from reliance India (MFI 12). We've been told in India they use this material for hinges.

The biggest problem is that the hinge fails upon the first flex or second. This lid goes on top of a cream bottle that is desposed after use. We don't need a million flex life time but definitely more than a couple.

There is no regrind. Molding without color hinge breaks as well

 

[rickfischer51]

This appears to be creep rupture, not fatigue. You need adequate cooling, especially at the hinge. You want to freeze in the orientation and reduce crystalinity as much as possible.

http://congdonplasticsconsulting.com/ts/misc/Living Hinge.pdf

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[ShiranS]

Thank you rick again for the reply.

At least I know I'm thinking in the right direction.

I hope to run a trial soon. I will report back the results so this maybe helpful to others in a similar position.