Blank ratings - IM?

[kruzenvax]

Inspired by the "New dropshot" topic. A very newbie question: the number after IM - does it mean the higher the number the better is the rod?

Say, the Nitro rods are built in IM8 - is this good or bad?

Thanks

[Cindy]

Interesting to see this question. Steve (BD rod designer) will answer this question later this evening. As Sean/Bear mentioned, many of you should get our Bass DVD as there is an important section on rods and blanks in the bonus section
(1/2hr) , and how they work. Things are not quite as you would expect.

If you would like to see a small clip of the DVD go to link:
http://www.blackdiamondrods.com/mediaarts/video.html
Places of purchase include BM, BD and other shops listed in that section.

Also, check our forum for more specials.

[DAGGA]

I very much dont know #$&@#$&@#$&@#$&@e about rods or reels as i always been a pure tackle rat but i have owned a IM8 in the past that was the cruddiest spinning rod i have ever felt. Really really flexy and huge guides although the IM8 has nothing to do with guides i know but still a IM8 rod doesnt mean its good.

[scott]

the IM rating refers to the amount of graphite in the blank. The higher the rating, the higher the amount of graphite. However, just like the number of ball bearing in a reel, the amount of graphite does not always result in a better rod

[Shortlite]

The IM is a scale referring to the modulus of the graphite used. I am not an expert by any means, but I take modulus to be a measure of tensile strength and elasticity.
It is not a measure of how high end the graphite is. It's just possibly stronger material, which means the rods can be thinner (less weight for the same strength), or stronger (thicker walls, more durable bit still light weight).

There's been a heap of discussion on graphite, IM numbers, moduli of graphite etc. on www.rodbuilding.org. Use the search option and search with IM numbers as your keyword. This will bring up a list of all the posts related to this discussion.

[Steve Ross]

Hi All,
Sorry for the delay in posting, but this is by nature a highly technical subject and I needed to do my research!

The term 'IM' stands for intermediate modulus and is used by the CF manufacturer Hexcel to designate fibres with certain qualites:

Hexcel IM6 Strength (ksi)760 Modulus (msi)40
Hexcel IM7 Strength (ksi)780 Modulus (msi)40
Hexcel IM8 Strength (ksi)790 Modulus (msi)44
Hexcel IM9 Strength (ksi)920 Modulus (msi)42

ksi= thousands of pounds per square inch.
msi= millions of pounds per square inch.

This term covers graphite (btw, properly called carbon fibre as distinct from naturally-occuring graphite, but with a similar atomic structure) that ranges from 16 to 60 msi and is the most common carbon fibre produced for industry.

This type of CF has the best compromise between elongation and failure. The higher you go up the modulus ladder, the less inherent strength there is in the fibre. The fibre is incredably strong in tension but trades off it's abilty to withstand other forces, particularly compression.

As you know, the fibres (calles tows) are laid down with an epoxy resin (generally with 60% fibre-to-epoxy ratio) and a scrim of lightweight woven material on the inner surface, usually fibreglass, to form the basis of the cloth used for rod blanks. This cloth, after it is cut to a flat pattern shape, is rolled under pressure around a 'former' known as a mandrel. The function of the scrim is to prevent the mandrel from migrating through the parallel running layers of CF, causing an 'off-centre' blank of inconsistant wall thickness. As you can appreciate this factor is magnified at the tip where the mandrel can be measure in thousands of an inch in diameter.

When a higher modulus fibre is used, the blank can indeed be made lighter than a lower modulus blank because you achieve the same overall action using less material, in other words you get the same flex pattern of a rod but lighter in weight.

BUT, here's the catch: although the rod is lighter, the fibres will not resist compression loads very well. As the rod bends under slightly excessive (ie, beyond design limits) load, the inner curve of the bend compresses the fibres, causing them to break inside the resin. This weakens the inner wall just enough to let the rod start to go into an oval-shape section, flattening the wall, making it far weaker and concentrating all the (compressive) bend loads in one spot, collapsing the wall inward and causing the failure of the rod on that side. Now the other side is caused to stetch beyond its designed limits and, having low strength snaps off along the bending plane very evenly, leaving the tip free from the rod.

So, how do we make the material sronger to stop the inner wall collapsing? Easy: make the material thicker!
But now you've lost the reason for making it from HM-CF. Not only is it heavier, all those extra fibres aren't sitting there doing nothing: they contribute to the stiffness of the blank, making the action too fast, resulting in a very expensive broomstick!

As a blank manufacturer, you have to make a blank with inherent properties that make it fishable, but also fishable by a wide variety of fisherfolk in a wide variety of potentially damaging situations. Getting a bad name and a shipload of warranty claims is not good for business!

Therefore, you've started to see the industry move away from the 'modulus chase' in recent years, going more toward rods for specific styles of fishing and branded as such. These rods combine lower (but still 'high') modulus CF along with very close-tolerance manufacturing techniques to achieve the best performance and longevity available.

So...to answer kruzenvax's question, high modulus is not a bad thing in itself; it all depends on the engineering that goes into making the blank. Usually, this is where the cost is: you need skilled operators to build the blanks and you have a higher rejection rate of product which fall below standard during testing (which you need to do more often to ensure consistant quality-more cost!).

As you know, a blank is not a rod, so my suggestion is if your looking at a new manufacturers' rod, take your reel and a practice plug and test it out before purchase: see if it has the performance/castability you're after. This is the only way you can tell if it's what you need.

For those of you with an interest in this subject, Hexcel have two excellent PDF's on composite materials (Prepreg Technology and HexPly Prepreg Selector) located here:

http://www.hexcel.com/Products/Matri...ducts/Prepregs

There is also a page by Advanced Composites that has a table comparing different manufacturers CF properties:

http://www.advancedcomposites.com/technology.htm

Regards,
Steve.

PS: If you think CF rods cost too much, here's another reason why!

http://www.lieffcabraser.com/carbon_fiber.htm

[Nath_18]

Heres something interesting for you guys. Its a great read that gives you a tour through the Sage and Tibor reel factory. It shows you how Sage fly rods (not exacly spinning reels but works the same way) are made.

Theres also a write up on how Peter Morse managed to litterly blow his fly rod up to

Sage Factory

cheers, Nathan