Replies

1. 
   StanleyNiemiec | Apr 02, 2002 12:47am | #1

   From my hardboard R&D days with Masonite, I seem to remember a humidification test standard that involved a 40-50% change (increase) in Relative humidity.  For the MDF project I was working on, I vaguely recollect the Linear Expansion (LE) data ranged from 0.2 to 0.5% change.  I forget what the actual change was in terms of panel moisture content.

   This is somewhat deceptive a number (actually range of numbers) because there was a difference in LE based on machine directions (length of panel vs across the panel), based on adhesive type/amount, based on wood species, based on board density, and based on additives.  I think if you need more accurate, exacting data you should trace back and contact the panel products manufacturer or for more general info, you might check out the American Panel Asssociation's (APA's) website.

   Expect manufacturers to be a bit gun shy about giving this data.  After all, LE is what the entire LP, Inc. siding failure/debacle was about.

2. 
   dondownunder | Apr 02, 2002 01:31pm | #2

   Jamie,

   Dimesional stability of the brand I use is:

   Length & width : 0.3% from 30-90% RH,

   Thickness :5% from 30-90%RH.

   I don't know if all brands are the same but this would be a guide FWW.

   Don

1. 
   Jamie_Buxton | Apr 02, 2002 09:25pm | #3

   Thanks, guys.   I've found similar numbers poking around the web after I posted the question.   What you're saying is that MDF, despite being a mushed-together collection of wood particles, has an expansion coefficient similar to lumber with the grain.   That's not the result I expected.

1. 
   StanleyNiemiec | Apr 03, 2002 12:22am | #4

   As far as I remember, the values are valid only within the constraints of the test procedure.  I do not recall MDF LE values ever directly correlated to actual Equilibrium Moisture Content (EMC) numbers.  Unless you have specific information that does, do not assume that the tables applicable to wood for temperature/humidity conditions (ie Relative Humidity) necessarily apply to fiber based composites.   The presence of resins and other additives to the furnish skew the EMC numbers.

   It is also essential that machine direction (that is how the mat was formed) be considered in applying LE values.   I seem to recall that when obtaining test samples, the test procedures require a specific orientation when cutting.   While most individuals assume that the orientation of the fibers is random, this is not the case.  Some alignment does exist and as such properties across the panel are different than those of the length of the panel.   I seem to recall that across the panel values are generally greaterer than the lengthwise values.