Specification for film shrinking?

[bee-man]

Once upon a time, people used hair dryers and multiple splices to apply film to windows that were not flat. But somewhere in time, "shrinking" became the preferred method - was this accidentally discovered by installers, or was this initiated by manufactures? In any case, manufactures must be aware and continue to design/engineer film to shrink since it is important for automotive application... but to my limited knowledge, there is no specification. Has any company or standards committee (ANSI?) attempted to drive a valid and meaningful way to test and measure film shrinkability? It would be nice to have a number to compare for handling/installation, and may give film engineers/manufactures something to hold or improve.

[Felixbiel]

Once upon a time, people used hair dryers and multiple splices to apply film to windows that were not flat. But somewhere in time, "shrinking" became the preferred method - was this accidentally discovered by installers, or was this initiated by manufactures? In any case, manufactures must be aware and continue to design/engineer film to shrink since it is important for automotive application... but to my limited knowledge, there is no specification. Has any company or standards committee (ANSI?) attempted to drive a valid and meaningful way to test and measure film shrinkability? It would be nice to have a number to compare for handling/installation, and may give film engineers/manufactures something to hold or improve.

Some kind of specification would a lot better than a sales rep. stating that it shrinks like butter and you can do a Beetle in no time. It's a good idea then we would have some kind of gauge to go by.

[EWF]

Wow !! What a superb question !!!

First, some basics to help in the understanding of this - then we address the specific question.

Window films shrink in two directions:

MD (Machine Direction) is the direction that the film moves in as it passes through the various coating and processing steps

TD (Transverse Direction) is perpendicular to Machine Direction

Films always shrink easier in MD vs TD. This is actually a desirable attribute. It was once believed (on my end) that a film that shrank equally well MD/TD would be optimal. I was provided with test rolls where MD/TD shrink rates were engineered to be roughly equal. We invited in local dealers to try it. It was a complete failure. Everyone hated it.

It was subsequently discovered that there is an optimal MD/TD "shrink ratio". I do not know what that is - it's a closely guarded secret.

Too much shrink is also a problem. You may have read posts that refer to phenomena along defroster wires called "maggots" (we call them puckers). These are often attributable to films with too much shrink.

As to your specific question, Although shrink can be quantified:

Too little shrink is a problem.

Too much shrink is a problem

The right MD/TD ratio is just as critical

There is NO optimal value for these parameters because all of you do things a bit differently. No single window film company has a lock on this, however there are brands out there which seem to have a reputation for preferential handling characteristics. Based on what I hear and have seen during our own in-house trials, Global, Llumar and SunTek consistently rate very high. No surprise there. Still, there are plenty of dealers that like the way Johnson, Solar-Gard and Madico shrink. Installer preference plays a critical role.

There probably is a "sweet spot" which combines the right MD/TD ratio, as well as the total amount of shrink. But don't expect anything to be published along these lines. Such closely guarded trade secrets are the valuble intellectual property of the top-tier major manufacturers.

It's probably not necessary anyway: Each installer is his/her own "shrink meter". If you like the way the film handles, like the price and the service from the supplier - that's really what counts!

-Howard

[Tint_Guru]

Wow !! What a superb question !!!

First, some basics to help in the understanding of this - then we address the specific question.

Window films shrink in two directions:

MD (Machine Direction) is the direction that the film moves in as it passes through the various coating and processing steps

TD (Transverse Direction) is perpendicular to Machine Direction

Films always shrink easier in MD vs TD. This is actually a desirable attribute. It was once believed (on my end) that a film that shrank equally well MD/TD would be optimal. I was provided with test rolls where MD/TD shrink rates were engineered to be roughly equal. We invited in local dealers to try it. It was a complete failure. Everyone hated it.

It was subsequently discovered that there is an optimal MD/TD "shrink ratio". I do not know what that is - it's a closely guarded secret.

Too much shrink is also a problem. You may have read posts that refer to phenomena along defroster wires called "maggots" (we call them puckers). These are often attributable to films with too much shrink.

As to your specific question, Although shrink can be quantified:

Too little shrink is a problem.

Too much shrink is a problem

The right MD/TD ratio is just as critical

There is NO optimal value for these parameters because all of you do things a bit differently. No single window film company has a lock on this, however there are brands out there which seem to have a reputation for preferential handling characteristics. Based on what I hear and have seen during our own in-house trials, Global, Llumar and SunTek consistently rate very high. No surprise there. Still, there are plenty of dealers that like the way Johnson, Solar-Gard and Madico shrink. Installer preference plays a critical role.

There probably is a "sweet spot" which combines the right MD/TD ratio, as well as the total amount of shrink. But don't expect anything to be published along these lines. Such closely guarded trade secrets are the valuble intellectual property of the top-tier major manufacturers.

It's probably not necessary anyway: Each installer is his/her own "shrink meter". If you like the way the film handles, like the price and the service from the supplier - that's really what counts!

-Howard

That was a great answer to a great question.

[SuperSetKing]

Just to add my 2 cents on the subject

Different films shrink better than others. Better meaning to most people, they shrink faster. All film shrinks around 3% per sq inch. At what tempature determines how fast it shrinks. For example, Llumar, solargard and Johnson all shrink at about 120 degrees Fahrenheit from my research. Global starts to shrink around 160. So, when working with this film, to some people it may seem that the llumar film shrinks better than Global due to the fact you seem to be moving a little faster and you can visually see the film reacting. In reality its doing the same thing its just taken a moment longer for the film to reach that temperature.

So for the explanation to why films shrink at different temperatures. Its really pretty simple, at the time of manufacturing, what ever the polyester was heated to while initially being stretched is its residual memory. Thus the film react and shrink when it reaches that temperature again.

Now that you know a little more about polyester, you can compare films when shrinking and get a good idea who is the original mfg of the polyester is since you can count how many mfg in the world on about one hand.

[bee-man]

Wow, Howard Thanks for the information - very enlightening. I was wondering if a film that was engineered to shrink equally in both directions could make the Beetle back glass as easy as the shrink wrap easter egg kits, but I guess not. Aside from installation technique, I'm a little surprised that the industry does not demand this spec for other reasons like:

Quality:

Better insures that multiple manufacturing lines are yielding a similar product (I'm sure this must be an internal spec).

Liability:

Protects the manufacture and customer from questionable material.

Sales:

May help to mitigate installation concerns on new products.

Competition:

May encourage more competition and better products.

Anyway, now I better understand the sensitivity and mechanics with shrinking. Thanks for everyone's input!

[Guest rondy_]

Thanks Howard

[EWF]

I got quite a few questions on my post, so I thought I would follow up:

There were several questions about why a film with equal MD/TD shrink would be a bad thing. It seems to defy logic. Why would such a film not be optimal?

This is why:

It would work out OK if you were tinting a basketball instead of a rear window. A basket ball is a sphere (curved equally in both dimensions). A rear window is not. The rear window of a vehicle is curved unequally along two axes.

The Longitudinal Axis:

Draw an imaginary line from the front bumper to the rear. That’s the longitudinal axis of the vehicle (front to rear). The glass is curved along this dimension.

The Lateral Axis:

That would be side to side (perpendicular to the longitudinal axis). The glass is also curved along this dimension as well.

Generally speaking, the glass is always curved to a greater degree along the lateral axis then it is along the longitudinal axis. When shrinking a back window, the film is laid onto the glass MD parallel to the lateral axis. Since the film shrinks better in MD and the glass is curved to a greater degree along the lateral axis, things work out OK.

I hope this clarifies things!!

-Howard

[Booms]