This report summarizes the experimental works that was carried out for a one-year research project developed as the continuation of previous research projects on the subject of light weight hollow core sandwich panels. The experiment focused on the investigation of creep behavior of light weight hollow core panel under long term static loading and high humidity conditions and its correlation with short term properties. Five types of surface panels were used, namely, 3.2 mm thick high density fibreboard with birch veneer on both sides, two thicknesses of M2 grade particleboard (6.3 mm and 9.5 mm) and two thicknesses of medium density fibreboard (6.3 mm and 9.5 mm). All panels were fabricated to the same final sandwich thickness of 45 mm using cell size of 12.7 mm Kraft paper honeycomb.
The results of the experiment show that the strongest facing material used to make the sandwich panels was the 3.2 mm hardboard with wood veneer lamination on both sides running along the long axis of the panel and test specimen, followed by the 6.3 mm MDF and the 9.5 mm MDF. The experiment demonstrated that exposing the panels to high humidity could cause strength loss of up to half of the original strength. However, the result of the experiment also suggested that it would be difficult to accurately predict the long term creep behavior of the sandwich panels using their corresponding short term flexural properties as the correlation between creep deformation and flexural properties was rather weak under the testing procedure and condition used.
The main objectives of this report were 1) review an existing heat transfer model (FP LogconTM) developed to estimate required conditioning times for commercial log species according to diameter and environmental conditions (mainly ambient yard and water spray temperatures) and 2) compare theoretical conditioning behaviour with real data supplied by plywood mills and previous experiments on log conditioning. The main findings were:
Logcon(TM) underestimated actual log heating rates in larger diameter logs (>15”), at least when ambient temperature is above freezing and warmer water (160°C) is used. It predicts an exponential increase in core conditioning time with diameter and a much greater diameter effect in freezing conditions. Review of past measured heating rate data and mill data from water heating facilities point to a more linear relationship with diameter for logs up to 28”, and heavy confounding effects introducing enormous variability into the data.
Contrary to theory, mill data of recorded ribbon average and end (core) temperatures from two different mills and conditioning systems (water spray or wet steam) at different times of the year show no correlation between wood temperatures and log diameters. This suggests significant deviations from model due to factors like location in pile and the vat (front, middle or back).
Sampled ribbon temperature data in Summer showed a full edge-to-core temperature inversion by the time of peeling, with ribbon coolest at leading end (log periphery) and hottest at core (Summer sample). Whole ribbons were within optimal temperature window for Douglas fir.
Ribbon temperature profiles taken in winter with deficient water heating at the time showed highest temperature in the mid-length, colder cores, and almost no ribbons within the optimal temperature window. Short conditioning times, insufficient heat energy delivered by the water recirculation system combined with heat losses from outside conditioning chambers are the main causes of cold wood at peeling time.
Mill data suggest that the shorter period of log heating via wet steam followed by hold equalization may be significantly more effective, particularly in freezing conditions. However this mill recorded ribbon average rather than trailing end (core) temperatures making it difficult to ascertain if there were any frozen cores in the severe Winter samples. Nevertheless the higher and much more consistent ribbon temperature averages across season (summer, mild and severe winter conditions) associated with far more consistent veneer production indices seasonally (reported separately) at the mill suggest the significant advantages of that mill’s wet steam facility.
Further work is recommended to modify and update FPLogconTM by collecting real-time log core heating rate data under warm water and wet steam practices in a wider range of environmental conditions. Measured log cooling data is also required.
Given the suspected ‘door-cooling’ effect further collection of lathe shift records should aim to identify any patterns associated with where in the vat logs came from if possible, to determine whether better insulation of entrances would be warranted.
This study was conducted with the aim of assessing the effects of log storage time and conditions at a BC mill yard on veneer production under mill production conditions. The second objective was to validate the FPInnovations LogdryTM drying model for developed for wood piles in Eastern Canadian mills. The software was used to generate drying rate predictions under the BC mill’s prevailing weather conditions and storage times for comparison with some measured residual moisture contents of Douglas fir logs kept in storage at the mill for six and nine months, sampled and peeled in a laboratory trial in 2016.
The 2016 lab trials suggested little effect of lengthy (winter) storage up to 9 months but mill experience suggests this is excessively long and logs deteriorate in terms of veneer production and quality considerably earlier. Unfortunately due to experimental circumstances the mill peeling trials for the 9 month stored logs were unable to provide an accurate assessment of the true effect on production. Mills trials indicated % heavy sap had remained fairly stable largely within the mill target of 14% to 17% over the storage periods. During the mill trials there were unavoidable heavy confounding effects of different average diameter for log groups and peeler knife condition affecting the expected veneer production variables.
The trials also demonstrated how pile size and height play a major role in protecting logs from drying; with very dry logs having a deleterious effect on veneer production. Logs held in small piles for 12 months or more, even with artificial ‘drying retardants’ such as end sealant and tarping were too dry for reliable peeling, causing very rapid knife wear, spinouts, veneer break-up and line blockages and significant lost recovery. The % heavy sap offtakes from these trials were just 2% to 4%.
LogDryTM provides a fairly good estimate of likely drying rate trends of mid-sized (35 cm/14” to 41 cm/16” range) Douglas fir under the BC mills historic weather conditions over 6 and 9 months.
LogDryTM (Birch setting) was closest to measured log MC in large diameter (46 cm/18”) logs but the Aspen setting was closer to measured MC in small logs (<30 cm/12”). In the limited sample of logs available from the mill in 2016 the 12” logs were much drier after 9 months storage than the model predicted, even on the Aspen setting. Further sampling of piled logs in the small diameter range is needed to verify this observation.
LogDryTM was used to estimate drying rates of logs stored before or after Summer. Modelling indicated a shorter viable storage window for logs delivered before Summer compared to just before Winter, especially in the 6-month range. Residual log MCs were very similar after 12 months regardless of start time.
Further work is required to better calibrate LogdryTM for major Western Canadian species, particularly Douglas fir, Spruce and Lodgepole pine, and reduce the calculation time for simulations. Further adjustment may be needed for simulating real drying rates in very small logs. The model assumption of similar residual MC after 12 months regardless of start time also needs to be verified.
A total of 48 peeler blocks and 256 mini-billets were sampled from mills to investigate the effects of yard storage time, and artificial yard drying and sprinkling on residual moisture contents (MCs) and veneer quality. MC in fresh and stored log inventories varied greatly across mills according to geographic location of their wood supply zones, bark damage and loss, and storage time and conditions. The main findings were as follows:
1. DF logs supplied by three BC mills from the Cariboo, Thompson Okanagan, or Kootenay regions were highly variable in wood MC.
2. Winter-cut DF logs with high sapwood MC stored had good bark retention and high moisture retention over 6 and 9 winter-spring months. No effects on veneer peeling roughness from longer-term winter storage up to 9 months.
3. Summer-cut logs had little or no residual bark, or the bark slipped off very easily during debarking. Exposed, bark-free summer-cut logs can dry and crack on edges and ends very quickly, within a few weeks.
4. A marked decline in veneer quality with piling time in Summer for spruce and DF, suggesting an optimum window of processing of such exposed logs of about two weeks. Veneer quality and recovery suffered markedly once the logs had fully air dried mainly because of edge splits creating natural fragmentation of the ribbon.
5. Mills receiving dry-zone logs with much lower MC have a very limited storage window, especially over winter. As little as 2-3 weeks if bark is damaged or missing.
6. Veneer quality could not be definitively tied to log residual MC. Under the controlled laboratory conditions used here it was observed that peeling quality could still be good at low sapwood MC (35-40%) and or very high (MC>100%). Whether this is still the case in mill production is unknown.
7. Logs must never be allowed to fall below FSP and develop edge-checks or deep end checks.
8. Wax emulsion end sealants were effective at hampering drying and end checking on high MC logs, but not effective on low MC logs.
9. Sprinkling retained log freshness and peel quality in high MC DF for several months and prevented log drying and end splitting as well as inner log staining. Ends absorbed considerable extra moisture. Some variability in peel quality was noted.
10. The prototype EM1000 Ground Penetrating Radar could only be reliably used in log edge mode in DF. The unit would also require re-calibration for the very high sapwood MC in spruce and wet-zone DF logs.