Sunday, February 17, 2008

Plastic 'Shrinks' When Heated (?)

Buzz – say Skippy, I’m always amazed at the long list of things to consider when developing a product. I ran into what seemed to be a routine question about ‘plastic product shrinkage’. The situation was that product was produced as usual, sold into the distribution channels and then made its way into various final applications in different areas of the country. After some time in installation, apparently some product was exposed to elevated heat(s) and ended up shrinking, leaving gaps between product pieces. A light examination up the food chain back to the material supplier was done, and the information passed back down was that Plastic Shrinks When Heated’ – what’s up with that?

Skippy - I always liked that line in Star Wars - "Every thing is true; from a certain point of view." I'm pretty sure that the technical folks would not try to have you believe that plastic simply by design "shrinks when heated" - the reverse is actually true - something is happening here, but it is something else.

Buzz – Well there are products that do exhibit ‘shrink’ when heated – what’s the difference with what is happening here?

Skippy – What will we use for the foundational thinking?

Buzz – Lets use these as our working assumptions -

a) Thermoplastic is composed of molecules in polymer chains tangled up together
b) When heat energy is incorporated into a mass of plastic, additional energy is imparted to the molecules in these chains
c) The molecules get excited and the chains begin to move apart from one another and the mass "grows / occupies more space" - more open space is left between the finite numbers of molecules in the chains (the net amount of plastic isn't changing) as the chains untangle.
d) If no other additional source of energy is expended to lock the molecules in an orientation of "apartness" (Read: Stress) while 'cooling', then as the plastic cools and gives up the heat energy, the molecule chains will re-entangle, nestling back together more closely and the air (if that’s what is occupying the space between them) will return to other areas of the environment.

Skippy – Good; now then, here are two examples where ADDITIONAL energy is ADDED beyond the energy to "melt" the plastic and the result is to ENCOURAGE a part being "larger" than what the apparent volume would be -

Example one -

Sheet for thermoforming is extruded and pulled through a roll stack, and as it leaves the stack, the pulling device continues to stretch and "orient" the plastic during the cooling stage.

Buzz - Isn’t thick gage Polyethylene sheet famous for "orientation"? Even to the extent that you specify the amount of "orientation" you expect in the sheet as it is produced so that it will have predictable "sag" during forming?.

Skippy - Yes, the short version on the way to check for correct orientation is to cut some sheet into known dimensional pieces (2" x 10"), lay them on a tray with a little talc on the tray to allow the sheet to slide around easily, and put the tray into a lab oven at a known temperature (different for different types of plastic) and heat the sheet up. Then without adding mechanical stress allow it to cool. This process will allow the sheet to heat up enough to RELEASE the processed in orientational STRESS (additional mechanical energy that is not apparent to the eye) and the sheet will 'shrink' in one or more directions. Repeated re-heatings once the stress is all released will not make the plastic shrink further. It is not the heating that "shrinks the plastic".

Buzz – So the heating releases the STRESS that is holding it bigger.

Skippy – Right. You measure the sheet before and after the heating and cooling and do some calculations and you see the amount of "orientation" that was present between being extruded and after releasing the stretched in orientation.

Example two –

The other more obvious example is "Heat Shrink Tubing" - I'm sure you know what this is - it is extruded tubing of various materials including Polyolefins or PVC that you put over wire connections, add a little heat with a heat gun and PRESTO the tube shrinks into intimate contact to cover the electrical connection. This Tubing is made by extruding and adding additional MECHANICAL STRESS into the part - orientation if you will that once the tubing is heated, will release the mechanical stress - it will not change the mass of plastic, only allow this built in energy to be released - so by design it is to 'shrink when heated'.

Buzz – aha, here’s an excerpt from :

"Heat shrink can also be expansion-based. This process involves producing the tubing as normal, heating it to just above the polymer's crystalline melting point and mechanically stretching the tubing (often by inflating it with a gas) finally it is rapidly cooled. Later when heated, the tubing will "relax" back to the un-expanded size."

Skippy – Spot on. Of course, by design, the product we opened the discussion with is not being designed to 'shrink when heated' and a complete understanding of the temperatures the product sees - in manufacture, in warehousing, in transportation, in job site storage and in application is in order. It seems likely that orientation exists in the product (as it does in most plastic processes) and something between the time it is a plastic product coming out of the line and it is in final installation is allowing the product to anneal and lose some of this orientation. So this much is true -The part is 'shrinking', and it is 'due to heat', but it is probably changing in size by releasing trapped in as produced stresses.

Buzz – Well another thing that was mentioned was that most of the material is shipped in over the road trucks to warehouses all over.

Skippy: As indicated, a holistic examination of the entire delivery system is in order - perhaps some or most of your product is getting warm enough to be self annealing in the over-the-road trucks passing through very hot conditions during the delivery phase, and has released its orientation while in boxes - these then if brought above the stress releasing temp and re-cooled in application can't release any more stress - it's already gone. On the other hand, if there are other parts that are not annealed in the delivery system and installed and they are heated up later (after installation?) it is possible they could be releasing molded in stress or orientation at that time. Look at the system variables just in our own US – products are produced in every corner and shipped coast to coast. Much of these products if routed to or through hot locations can get a good healthy annealing in hot trucks - the unknown to be studied during the product management cycle is for just how hot and for how long.

Buzz – So, another note to the product managers out there; an important item to be considered while developing plastic products is to pay attention to it all the way through to its final use(s). The development cycle should include the entire system from design considerations to installation (in the full range of environmental conditions) including the delivery methods and path of distribution.

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