保证人工翻译,
16.空气间隙大小的控制
自挤出机机头出口处到冷却辊与压辊接触线之间的距离称为“空气间隙”.空气间隙大小对挤出薄膜表面氧化程度和粘合力大小有影响,这是因为从机头出来的高温薄膜表面与大气发生氧化作用生成活性羟基,它可增加薄膜与基材之间的粘合强度.但氧化作用却使不与基材复合一面的聚乙烯表面（通常为热封层）降低了热封强度.因而,应综合两方面因素确定空气间隙的长度.一般不宜太长,因空气间隙过长会降低薄膜温度,也会导致复合强度下降.
The “air gap” means the distance from the exit of the crosshead to the contact line of cooling roller and press roller.The size of the air gap has effect on the surface oxidization degree and adhesion force of the extruded film.The reason is that the high temperature surface of the film coming out from the crosshead is reacted with the atmosphere to form active hydroxyl,which will increase the adhesive strength between film and base material.On the other hand the oxidization reaction will decrease the heat sealing strength on the PE surface (normally the heat sealing layer) that is not composited with base material.Consequently,the length of the air gap should be considered comprehensively from two aspects.Too long is not desirable because it will decrease the temperature of the film thereby decreasing the composite strength.
17.挤出薄膜发烟与臭味
低密度聚乙烯树脂在螺杆高速剪切和高温作用下,其分子支链被切断、降解,生成低分子化合物.当薄膜挤出模口时,低分子物质挥发产生“发烟”现象,也是“臭味”的来源.它对包装食品的复合材料是极为不利的,除了应选用“臭味”较低的树脂品种外,应尽量控制机头温度不能偏高,最好不超过300～320℃,低温挤出是减少发烟和臭味最有效的方法.
17.The smoke emission and smell of the extruded film
The molecular branched-chains of low density polyethylene will be cut-off,degraded to form low molecular weight compound in the high speed shear and high temperature of the extruder screw.
When the film is extruded out from die exit,the low molecular weight mass will volatilize and emit smoke,this is the source of the “smell”.It is extremely unfavorable for the composite material used for packing foods.In this case low “smell” resin should be chosen,in addition,the temperature of the crosshead should be controlled not too high,preferably lower than 300~320℃.
Low temperature extrusion is the most efficient method to decrease smoke emission and smell.