Curling Cartons

[mjs016]

We process recycled paper into paper egg cartons and then bake them in a 15 stage oven. When I first walked into the plant about 3 months ago they were having severe trouble with cartons that curled while in the ovens. I fixed most of the problem by tweaking the control temperatures but now I'm about to propose that we increase the weights of the cartons to further prevent curling.

THE REASON: Potato chips. I look in a bag of potato chips and realize that the thinner the chip is the more curled it is. I also realize that the reason why you have to cut a piece of bologna before you fry it is because if you don't then the outside cooks faster and contracts while the middle is still full of oils that hasn't cooked as quickly and it bowls in or out to make room. So I think that since they are too thin, or light. They are cooking too fast and curling. Thicker cartons would seem to trend towards not curling like thicker potato chips. Unfortunately, there is not much information available on this that I can find and I wonder if my theory is true.

Any suggestions, clarifications, or comments? I tried posting this once but couldn't find it after I did. So I'm trying it again.

MJS016

 

[HamishMcTavish]

Hi mjs016,

Just a few questions that you should ask yourself

How much do you know about the temperature profile in the oven? Are all cartons curling or just those in certain areas? Do you know where in the oven the curling occurs?

Presumably they didn't have the problem until recently (otherwise it wouldn't be a problem...) so
Has any recent maintenance on the oven been done?
Has any part of the process changed?
Has the material changed or been sourced from a different supplier? Is it within the bounds of the material specification?

Is seems to me that you are treating the symptoms but may not know what is causing the problem. If you recommend changing to a thicker material it may alleviate the problem but that would not prevent it from occuring again e.g. if the material is supplied to you at the thinner end of the specification.

Regards, HM

No more things should be presumed to exist than are absolutely necessary - William of Occam

 

[bartholomeus]

Hi mjs016,

Your lead appears a good one, and with that you have given yourself already the answer. Curling could occur if outer/inner parts dry more quickly then other parts. Thus, non-homogeneuous heatening is most probably the key to curling. The answer to your problem, therefore, might not be in your carton, but in your ovens.
By thickening your carton you will increase the carton stiffness, and in that way reduce curling. However, what is the ideal thickness?? You might end up with unacceptably thick material. Some pilot scale experiments should help you with that.
The other option (changes to the oven) might be a difficult one. You'll probably have to work with a heated moulds like in plastics manufacturing, which could be a pretty expensive solution.
So, to my opinion the answer is in your oven.

Cheers, matthew

 

[JKEngineer]

Moisture content, and uneven moisture content, of pulp products will cause curl. TAPPI will be a resource on the technical side of this.

As the others have suggested, the oven and its profile and uniformity may be driving factors. The response of the pulp will also be a factor. If your furnish has changed, it may be the cause of curling where none had existed before. Making the carton thicker may actually make the problem worse by increasing the side to side moisture variation of the carton.

Monitoring the product's temperature in and through the oven should be considered. Infrared thermography, which I do, is one way to approach this. I would be glad to discuss it further with you if it is of interest.



Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[unclesyd]

Can you flip the product in the oven?