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 http://en.wikipedia.org/wiki/Heat_shrink_tubing :

"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.

Trouble with Extruded and Injection Molded Parts

Skippy: Hey Buzz - here was someone who needed help – “I have developed a new product for the construction industry. All was good while in u.s. except tooling costs. We decided to outsource to china to try to resolve this problem but in doing so we set ourselves back (over a year now) from taking this to market. ... (Product includes) both extrusion and injection molding ... with tight tolerances as profile extrusion marries to injection components. Also seeking advice on injection molds as currently mold maker claims to be doing "adjustments" to molds for about three weeks now. I am leery as to the quality of molds loss of temper etc. What should I look for in pics before final payment is made to ensure I am receiving a quality mold?”

Buzz: Any serious plastics professional will tell you that there are three major considerations in assemblies of injection molded and extruded parts - particularly when they are going to be used together with other mating parts, and sold anonymously through distribution into a vague category like 'Construction' - especially with warranty related questions in the background. The inquiry didn't mention number of cavities and general mold design criteria so without specifics it would be difficult to 'direct' them on the injection molding portion at this early point.

Skippy: What is needed?

Buzz: Consistent, clear and written communication should be the watchwords. Depending on which of these following items you NEED, it will change the way one would think about design and tool up of either plastic process, and the resulting 'tweaks' needed on tooling -

1) Dimensions (critical, control, reference)
2) Aesthetics (critical, control, reference surfaces)
3) Physicals (flammability, electrical, impact, UV, reactivity with other parts etc)

Skippy: So what thinking should be included in the ' big picture '?

Buzz: Here are a few general thoughts to consider (assuming the DESIGN issues are taken care of:)

a) Photostereolithography - for $1500 or so you can have sample parts made to dimension for critical assembly fits, including undercuts, blind holes etc. Bet on the final injection molded parts being closer to print than the extrusions (caveat - a poorly or improperly packed, voided, or stress-molded-in part can 'hit the numbers' and still be a poorly performing part in application - see Aesthetics and Physicals) and then have some examples of the extrusion produced with high and low critical tolerances produced by p- as above and submitted by your extruder. In many years, we've not found more than about a half dozen with more than 3-4 CRITICAL dimensions for assembly between parts. On the other hand - a major item to think about during the contract review process would be what OTHER mating parts does your system need to match up with? It may be that one of the parts is not yet fully developed in how it relates to them . . .

b) Understand what you NEED in terms of the big three stated above - one of the three is always easy to produce at 'rate and weight'; two of three is more difficult and probably will affect production rates, all three will nearly always affect production and discard rates and therefore your final delivered price.

c) Insist that your sample parts be produced from the actual production tooling at PRODUCTION rates from PRODUCTION material - nothing creates more ' availability to market headaches ' than to submit your sample assemblies for further consideration (aka engineering evaluations - electrical, UV, or smoke/flame) only to find out that the sample parts were made from a general purpose or utility grade of material that does not include any additives addressing your special needs - self extinguishing characteristics, dielectric properties, impact or notched IZOD capability, UV characteristics, static dissipative qualities, biocides, fungicides etc.

d) Work with a house that has their own in-house wire capability for the extrusion tooling. It is much less costly to open up or remake the extrusion tooling than the injection molding tooling - probably on the order of 6-10 to one. Ask to see examples of their tooling and their tool storage area. Notice how they keep the unused tooling on the shelf - cleaned and measured, shiny and production ready or rusty, filled with plastic and unkempt against the day someone might reorder? A number of houses will quote inexpensive flat plate tooling, but be wary of the how these tools work over multiple lots of raw materials which can and do change - particularly if your spec a 'utility grade' of material.

e) Be at least a little paranoid; Trust, but Verify. For every one of the good plastic outfits out there, there are some who won't measure up beyond the sample parts. Look for a house that has some sort of quality system/policy (with or without the ISO moniker), with sample retains on less than perfect quality parts used year to year to maintain quality checks on output, and written records of past production runs including retained information back to incoming raw materials and supplier raw materials certifications if possible etc.

f) While working with your Product Manager, get it in writing; material specs, quality and production records - ask to attend and help fill out a 'Contract Review' - answering the couple dozen critical questions that a good processor needs answers to to ' help them help you ' is a critical path to success. Some sales people can actually act in this ' Product Management ' role; and unfortunately many can't.

g) Ask for and be prepared to get enough samples to have engineering testing done on the resulting samples - and do it.

A well run product development process has additional nuances of course, but these should give you a good grounding.