Injection Molding - PBT Living Hinge - Hinge Breakage 1

[LivingHinge]

Hi all,

We are facing a problem with an injected molded part which includes a hinge. The main issue is that we are having a quite amount of hinge breakages. The material used is PBT. I know that ideally living hinges should be injected by PP, however, due to other issues we can only inject this part by PBT.

We have done several checks and although the number of hinge breakages have been decreased the problem isn't completely solved yet. Mainly, we have done the following checks:
- Increased the injection speed in which it has been resulted into less breakages, however still having a number of breakages. Additionally, we are quite limited how much we can increase the injection speed as other issues such as the dimensions will be effected.
- Checked the moisture content and the dew point of the material.
- Checked the injection cylinder temperatures and it's according the data sheet of the material.
- Have done a filling analysis to check if there's any weld line on the hinge and from the results it has been shown that it's not the case.
- Increased the orifice diameter of the nozzle the same as the diameter of the hot sprue bush of the hot runner system to have a better melt flow. Please not that the mold is a hot runner mold which this makes it impossible to change the position of the gate.
- Hinge thickness has been measured and all cavities found in order. Please note that the mold is a 4 cavity mold.
- Material checks have been done to check if the material is getting degraded or not and the results showed that it's not the case.

Additionally, the hinge is being bent twice (180 deg) immediately after the parts are ejected. Another check is being done after production where the hinge is being bent one more time prior the part being used in assembly.

Could anyone please recommend what should be done more to reduce/eliminate the breakages ?

Thank you in advance.

 

[Pud]

Has it ever been ok?

www.tynevalleyplastics.co.uk

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

 

[hendersdc]

I agree with Pud, we can't make suggestions until we know the design and material choices were sound to begin with.

Also, can you give more detail about the failure? Does it snap off on the first try, crack, thin, does bend angle matter, etc? Any pictures?

 

[Paulkuklych]

Since you are having a number of hinge breakages that means the rest of them are good parts. Correct?

Sounds like you are having variation in the process.

I suggest keep records of the process parameters such as injection time, hold time, pressures, material and mould temp for every shift. Does your moulding machine keep a log of the variation? This makes it easy to identify.

If you are getting large variation and a lot of breakages then you are more likely to find the cause since there are several reasons for variation: material, mould, machine or human being.







Paul Kuklych

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

 

[Demon3]

This thread shows a nice discussion on PBT and nylon for living hinges. Apparently it can work.

https://www.mappinc.com/plastic-industry-forum/article572.htm

Some great advice from Gary Casterline, I met him once so if you can't track him down I should have a business card here somewhere in a pile.

This looks good too.

http://www.plasticstoday.com/imm/ar...good-living-hinge-materials-have-follow-rules

Dr. Chris DeArmitt
President

Expert plastic materials help

http://www.phantomplastics.com

 

[LivingHinge]

Hi all,

Thanks for your feedback.

@Pud - No actually hinge breakage has always been an issue.

@hendersdc - Mainly after few bends some cracks start to show up and eventually the hinge breaks off.

The above is a cross section view of the hinge. All the dimensions are in range, however, I've suggested to add extra thickness at the ends of the hinge to increase tear resistance and improve the melt flow of the material.

@Paulkuklych - Yes you are right. We have done several analysis and since it's a 4 cavity tool we found out that cavity 4 is the least one which with hinge breakages. We have checked what it has different from the other cavities and everything seems that is in order, such as hinge thickness and injectors of the hot runner system.

Any suggestions please ?

Thank you advance.

 

[hendersdc]

If you have been having problems with the hinge breaking from the beginning it is likely a design and materials issue and unlikely you can fix with just process changes. Demon3 provided a good link for a potential materials source.

Here is a guide for dimensioning of living hinges, there are no dimensions on your cross section but my guess is that the plastic in your hinge is too thick. [URL unfurl="true"]http://machinedesign.com/fasteners/care-and-feeding-living-hinges[/url]

 

[LivingHinge]

That's right the main issues are the design of the tool (since it's a hot runner) and the material itself (PBT), however, it's quite impossible to tackle these two main issues for several reasons so the only way forward is to figure out what can be done in regards to process and maybe slight modification to the tool.

I have checked the referred link and checked the dimensions of the hinge as well and they are within the limits. Furthermore, another factor which it might causing these breakages to the hinge might be that the gate is quite close to the hinge, therefore the last part which is being filled is the hinge area. As I mentioned already it's a hot runner tool so the position of the gate cannot be changed. A filling study has been done to check for any weld lines if they are present on the hinge and it appeared that there's no weld line on the hinge. Moreover, each part is being flexed twice after ejection by an operator to maximize the strength of the hinge.

What's your feedback about this guys? Are we missing something to keep on having these breakages ?

 

[Pud]

I assume this hinge has actually been designed for PBT? It's not for some reason changed to PBT after tool has been made?

I also am tempted to suggest the hinge is too thick.

Have you tried running at max tool temperature? PBT needs a high tool temp to achieve max crystallinity. e.g. 90°C+

How are you checking the moisture content of the material? It's just that PBT (as with all polyesters) requires a vanishingly small water content to affect the properties.






www.tynevalleyplastics.co.uk

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

 

[Paulkuklych]

Along with the suggestions above, do a short shot test eg 25%,50% and 75% full part.

Test for cavity balance and flow hesitation thru the hinge. Flow hesitation can cause quality issues. Post pics.

By the way, gate positions can be changed in hot runners its just at matter of cost & mould design.

Paul Kuklych

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

 

[Paulkuklych]

Also, did you find any variation in the process from shot to shot? For example does the fill time remaining constant over a 24 hour period? If it has variation how much is the variation?

Paul Kuklych

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

 

[LivingHinge]

Thanks once again guys for your feedback.

@Pud - Originally the hinge was designed to work with another material, however, due to other issues there was a change in material to PBT. Actually we have never tried to run tool on high tool temperatures. The engineer of the material's supplier has recommended to run tool at <45°C. Are you sure that ideally PBT should run on a tool temperature above 90°C for maximum crystallinity? Regarding the moisture content, we are checking it through Sartorius equipment and values are within the limit i.e. <0.02%.

Lately we have decided to reduce the thickness of the hinge and do some trials. Since it's a 4 cavity tool and we have to modify the inserts of the hinge we have concluded to modify only one insert for the trial. Do you recommend to do all the inserts or it's enough to try with one cavity only?

@Paulkuklych - I have done a short shot test already. All cavities are balanced and filled without any hesitation. Regarding the gate position the only issue is money so we are trying to find a way cheaper to solve this hinge breakage issue. Process parameters are being monitored and even tolerances are set to avoid any variations in the process. Basically, there's no variations in the process, they are quite constant.

Thanks in advance.

 

[hendersdc]

Instead of modifying the tool you might consider just modifying the parts that come off the existing tool as a quick way to see if a thinner hinge works better. This can be done a few different ways based on the geometry of your part. The most accurate would be to use a mill which I assume you have access to but you could also do it by hand with some sandpaper if you needed to.

 

[Paulkuklych]

Variation is not always a bad thing.
For example, its normal to have variation in the injection pressure so that fill time can be maintained. Slight changes in material viscosity or temp can cause the machine to use a different injection pressure in order to maintain injection speed and therefore injection time. It can vary by 10 to 15% so be careful about limiting injection pressure. A change in injection time is one of the parameters that will effect part quality.

Paul Kuklych

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

 

[Pud]

Mould temperature: see here as an example

Crastin PBT

Imho, 45°C is a much too low tool temp. I prefer 60°c to 80°C as a minimum.

A high tool temp also decreases moulded-in stresses.

H

www.tynevalleyplastics.co.uk

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

 

[Carlos Gomes]

Hi.

One of the rules of thumbs for project with polymeric materials is to use PP or POM to mold a living hinge.
Unless that fact, the PBT have a similar behaviour to PP or to POM, semi-cristaline and ductile mechanical behaviour. One of my advices is to increase the cristalinity degree of the plastic part, first of all, increase the melt temperature, but only a small level, and then increase the mold temperature, but only when the injection temperature was increased. First, the melt temperature and only if this dont help you, you go to mold temperature.
The project of the living hinges is a narrow thickness compared with the rest of the component, and you can put a shear rate less pronunced on that zone, to do this you should decrease the flow rate to put less orientation on living higes. When you change this parameter care should be taken with mold temperature. Choose a correct graphic on filling speed.
Related with the plastic material, you can choose a PBT with high molecular weight (high viscosity), to better resist to the open/close movements. On the cavity, you can increase the thickness of the living hinges, cutting metal on that zone.

Hope i could help.