julyxiv

大開眼界, 真係睇到眼都突埋.  我double confirm google神, 佢話鋁的溶點係660.3 °C....40W heater去到660.3 °C..... 佢又真係有video喎.  但點解....6百幾度高溫金屬仲係灰色而唔係紅色?! 而上網再亂check, 表面溫度在581度時的color係"Red heat, visible in the sunlight" (source: http://www.zianet.com/ebear/metal/heattreat4.html)

頂.. 40W heat 谷大一倍都冇100W, 我個heatbed 140W..... 我居然仲可以生勾勾係度吹水

hon2013

Took so long using hand writing pad, so I'll just type quick reply here.

I think you don't see the glow when the sun light is strong. A heater alone could get red hot:
http://forums.reprap.org/read.php?1,314104,330127#msg-330127

But you don't see glow in this casting video (1:47):
https://www.youtube.com/watch?v=VhII8q4apiA

Think about energy conservation. The heated bed have a big surface that lost a lot of heat through convection & radiation. So some c hing even have difficulty reaching 100 degree. When all you power is lost to the surrounding environment, the temperature cannot raise further. And with a 2000W heater in a small room, could still feel cold in winter.

julyxiv

回覆 82# hon2013

感謝C兄花咁多時間找出証明, 長智識了.

睇咗咁多proof都仲要拗就係我唔啱. Heater overheat 我是相信的, 所以marlin入面有fail save 的setting (要多1隻thermo).  但我好難接受用一個幾十W heater可以溶鋁呢個事實 (我唔係拗), 所以又死去搵google神到參拜下...

鋁嘅heat capacity係0.9/g/degree....咁我目測個video個heater, 淨係個heater block 11g (20x20x10mm, 6061T), 咁, 就算在真空下(冇heat lost), 要60秒內(Video所見)到達660度都已經要100W.  將12V變19V就咁大威力, 好難信報.

吖...計到呢到我知係有可能溶咗舊鋁, 但又同時好奇怪, 因為, 推到佢變100W已經難, 假設冇heat lost仲難, 佢樓上仲有一舊鋁散熱, 體積重量都不少於heat block, 雖然中間有支不锈鋼分隔, 但660度高溫都唔傳熱就唔使用散熱了.  Draw 幾百W電, heat block唔溶塊版都頂唔順了.

只能說, 我相信師兄的例証是真的有可能, 不過要好多嘢配合先可能出現.

另外, 風扇同暖爐引發火警 (唔計漏電, 另一議題) 已經好耐冇見了. 家電咁多年歷史, 好多fail safe嘢已不知不覺間做了很多 (不是燎交哎, 不要多心)

絕對好奇

雖然諗唔明點解可以熱到鋁方會溶，不過玩到著火小弟都試過！
係因為裝發熱管時裝得唔好，攪到包住發熱管嘅絕緣膠套走位，
令到兩條電缐短路，睇片應該係高溫將絕緣膠套溶化，令到電缐短路因而起火！

hon2013

回覆 83# julyxiv

This is interesting, never thought about calculation, it's complicated anyway. But your post prompts me to verify. Not to disprove you, just that you lay down the info for easy follow up.

Specific heat capacity of aluminum: 0.91 (kJ/kg K)
Aluminum melting point: 660°
Heater block: 0.011kg
Energy needed to raise temp to 660° (assume 20° ambient)
= 0.91 x 0.011 x (660-20) = 6.4 kJ

Actual power of 40W heater driven at 19V = 40 *19/12 = 63.3 W (J/s)
Time to raise temp (assume no heat lost) = 6400 / 63.3 = 101s

In actual case, there are heat lost. The higher the temp -> the quicker the lost.
When the lost is more than 40W (or 63W in this case), all input energy is lost, and the temp cannot rise further. Don't know how to calculate this.

hon2013

雖然諗唔明點解可以熱到鋁方會溶，不過玩到著火小弟都試過！
係因為裝發熱管時裝得唔好，攪到包住發 ...
絕對好奇 發表於 2015-1-15 20:56

This is another fire hazard, care must be taken on the bed connection (proper wire & quality connector needed). And of cause no shorts, taobao heater are usually well insulated nowadays.

ma_hty

回覆 85# hon2013

> Actual power of 40W heater driven at 19V = 40 *19/12 = 63.3 W (J/s)

哎呀呀，這是在計什麼?? 錯得這麼清脆。

Actual power ... at 19V = 40 * (19/12)^2 = ...

hon2013

You are absolutely right, that why I don't like calculation, hehe.

Correction:
Actual power of 40W heater driven at 19V = 40 *(19/12)^2 = 100.278 W (J/s)
Time to raise temp (assume no heat lost) = 6400 / 100.278 = 63.82s

Make sense.  Heat lost has its effect.

高登阿伯

回覆 81# julyxiv

唔好當google係神，好多嘢google教唔到你，溶咗鋁係可以睇唔到紅色，唔信youtube睇下人地屋企後院玩鑄鋁就知。

你引條link係鋼料至成立，鋁廠係唔會睇顏色估溫度。

點解會咁，因為有樣叫光澤嘅嘢，溶咗嘅鋁如果溫度唔夠高，發出嘅紅光會比光澤掩蓋，肉眼睇唔到。

ma_hty

回覆 88# hon2013

> Time to raise temp (assume no heat lost) = 6400 / 100.278 = 63.82s

...

熱力散失到環境這一個因素, 是一個影響很大的因素, 不可以只為了方便計算 隨隨便便就當它不存在的.  你貼的視頻的作者就小心得多了, 這視頻 https://www.youtube.com/watch?v=vVRwHwxNy7s , 也是同一作者貼的, 只有heatsink 沒風扇 12V電源.  330度 就不能再上.  這個溫度當然是不能正常打印 (^^" 廢話, 沒風扇 膠料都推不動了, 如何印?), 但... 要燒著什麼嘛, 大概不能.

另外, 視頻中的 "heatsink", 算不上是個 heatsink, heatsink 應該是這樣的 http://www.google.com/search?q=heatsink .

我想... 這些視頻比較明確的訊息 應該是... "不清楚自己在做什麼, 就不要隨便改動設計".    如果算術不好 就更不應輕舉妄動.