Usually, the plastic materials are formed in the shape of pellets and sent from the raw material manufacturers in paper bags, etc.

Since these pellets would have absorbed the moisture in the atmosphere, if they are used for injection molding while they still contain a lot of moisture, depending on the type of material, they can undergo hydrolysis, or their physical properties can decrease. Further, it is possible for silver streaks to appear on the surface of the molded product, and it is also possible for short shots or burn due to gas to occur.

In view of this, in the case of most molding materials, before they are put into the hopper drier, it is necessary to carry out pre-drying in a box type drying oven.

In pre-drying it is recommended to observe the appropriate drying temperature and drying time. This is because, if the drying is done at a temperature less than the appropriate temperature, even if the drying is done for a long time, the moisture content cannot be removed as desired. A material whose pre-drying has been completed should be used as quickly as possible. When any left over material is to be used some days later, carry out its pre-drying again before use.

The pre-drying conditions of special plastics are listed in Table 1.

Table 1 Pre-drying temperatures of plastic molding materials
Material name Symbol Pre-drying temperature (°C) Drying time (H)
Liquid crystal polymer LCP 110〜150 4〜8
Polyether imide PEI 120〜150 2〜7
Polyamide imide PAI 150〜180 8〜16
Thermoplastic elastomer TPE 120 3〜4
Poly ether ether ketone PEEK 150 8
Poly phenylene sulfide PPS 140〜250 3〜6
Poly allylate PAR 120〜150 4〜8
Poly sulfone PSU 120〜150 3〜4
ABS ABS 70〜80 2〜3
Acrylic PMMA 70〜100 2〜6
Polycarbonate PC 120 4〜6
Nylon 6 PA6 80 8〜15
Nylon 66 PA66 80 8〜15
Nylon 11 PA11 70〜80 8〜15
Nylon 46 PA46 80 8〜10
Poly acetal POM 110 2〜3
PBT PBT 120 4〜5

Pellets of plastic molding materials generally absorb moisture from the atmosphere to a certain extent. If the quantity of absorbed moisture is large, the plastic can undergo hydrolysis (there are plastics that undergo chemical dissociation with water as the initiator) in the process of being melted and mixed in the cylinder of the injection molding machine, or, when molding is done, this can cause silver streaks, air bubbles, or glossiness defects on the surface of the molded product, or can cause copying defects, etc. Therefore, it is necessary beforehand to put the pellets of molding materials in a drying apparatus and remove the moisture content in them. If the pre-drying is not done appropriately, it can lead to variations in the fluidity, deterioration of physical characteristics, and molding defects.

The following are the main types of driers being used at present.

(1) Hot air drier
The hopper drier and the box type drier are the typical equipment used with this type. The drying method is that of blowing hot air at the pellets thereby evaporating the moisture in them. Although this is a common and simple drying method, this method is not suitable for completely removing the moisture content.

(2) Dehumidified hot air t drier
In this method, after first removing the moisture in air, that air is heated and blown on the pellets thereby evaporating the moisture content in them. Since the air used for drying is re-circulated and used again after being dehumidified, heat loss will be small, and it is possible to carry out rational drying. This method is suitable for drying PBT, etc.

(3) Reduced pressure heat transfer type drier
This is a method for evaporating the moisture content in the pellet by heat transfer in a reduced pressure environment. Drying at low temperatures becomes possible, and hence it is possible to prevent the oxidization of plastic and to reduce the effects of additives in the pellet. In addition, this is also a method in which thermal loss is also small. This type of drier is attracting a lot of attention as the drier of the future.

“铁树开花，马生角” —— 黑龙滩的传说

仁寿黑龙滩被誉为“川西第一海”，“成都后花园”，关于她，有一个凄美的传说。



传说一龙太子痴心于情爱，被老龙王度身为人到人间历练。在人间，他是一位贫困的少年，和一位积劳成疾的老婆婆相依为命，龙太子称婆婆为娘亲。



然而，十几年的历练不仅没有改变太子对真爱的追求，反而让他找到了龙宫里无法得到的亲情。老龙王惊恐万分，为了不彻底失去他最疼爱的儿子，他不得不把他变回蛇身，并用特别粗大的铁链锁在黑龙山下。



娘亲因为思念儿子整日以泪洗面，最后哭瞎了双眼。凭着儿子身上特有的味道，娘亲爬到了黑龙山下，爬到了困住太子的地方。



担心娘亲的身体，太子不得不问老龙王他何时才能够离开。“铁树开花，马生角”这便是老龙王的回答！



明明知道这是不可能的事情，但是太子还是每天含着眼泪痴痴地盯着那根系着粗铁链铁树，盯着路过的每匹马的头部。他祈求能够早点离开，让娘亲在他离开后还有力量坚持活下去！



这一天终于到来了。



一穷书生娶媳妇，因为没有足够的钱雇用马，他就雇了头驴。到铁树旁，穷书生恰恰累了，就把佩在胸前的大红花取下来挂在铁树上，他则靠在树旁休息！



“铁树终于开花，马终于生角”，太子一声怒吼，挣脱了铁链。在雷电和大雨的压迫下，龙太子不得不离开。



望着奄奄一息的娘亲，太子一步三回头。每一次回头，他都会用头撞山头，都会撕心裂肺地三呼娘亲。每撞一次山头，都会 让山裂开一个大缝！高高的黑龙山就这样被分裂成24个岛屿，变成了24个望娘滩。也就是当今仁寿人引以为荣的黑龙滩。





如今，你舟行碧波上，用心去闻，你还能够闻出当年太子和它娘亲泪水中那咸咸的苦涩味道；在盘龙岛上，你用心去看，你还能够看到锁在铁链下龙头腮边清晰的泪痕！