Purging is widely used as an easy way to clean out materials from the extruderand die and to avoid a time-consuming stripdown cleaning There are many typesof materials used for purging, both commercial purge products and "homeengineered" purges. These materials generally fall into two categories:chemical purges and mechanical abrasive purges. Many combine both chemical andabrasive actions.Chemical purges work by attacking the plastic residue like asolvent. Many of them also incorporate a foaming action and require a 5-min"soak" in tbe barrel to allow the chemical action to work.

Mechanicalabrasive purges work by scouring and scrubbing material off the metal surfaces,using friction and shear. Processors do a lot of trial-and-errorexperimentation to find the right purge material and technique. No"magic" purge material will work best for all materials. Commercialpurge products are generally more expensive than "home engineered"purges. But if you can use less material with the commercial purge, or get tbejob done quicker, the total cost may actually be less.

Processorshave run all sorts of materials through extruders, in the quest for a cheap andeffective purge material. For instance,in the U.S. Midwest many operators use cattle-feedcorn, because it's readily available and does a fairly good job of scouring outthe machine. Here are two effective homemade purges:
Mix #/; S0% HDPE (or another htgh-viscosity resin) +50%diatomaceous earth (DE): DE is finely milled fossil shells of tiny seaorganisms. It is readily available in garden supply stores. DE looks like afine powder, but the particles have very sharp, angular edges, which produce ascouring action. Processors who run sheet and film use this purge, as it cansometimes get rid of die lines, saving a lengthy die teardown.
*Mix #2; HDPE or other viscous resin with kitchen cleanser andwater: Mix a slurry of water and three or four cans of Comet or Ajax kitchen cleanser in abucket. Feed HDPE at a normal purging rate and temperature, and slowly pour theslurry into the main feed port. The water in the slurry will flash to steam,which gives a very good cleaning effect combined with the abrasive in thekitchen cleanser. (Safety note: Warn any operators nearby that steam maydischarge from extruder openings. Do not look into the die or vents unlesswearing a full polycarbonate face sbield andlong-sleeved shirt.) Probably themost important thing is to purge often. The object is to prevent old materialsfrom being "baked on" the metal surfaces.Once this happens, purgingwill generally not be able to remove thematerial, and a manual stripdown cleaningwill be the only remedy. One mistake many processors make is to run the purgeonly at a fixed screw rpm. It's much more effective to vary the speed, changingfrom low to medium to high rpm, running for perhaps30 sec at each speed. Thisinduces different shear rates against the metal surfaces, helping to dislodgeold material from the walls.

清機(jī)是把用于把擠出機(jī)和口模內(nèi)材料清除出來的簡(jiǎn)易而廣泛的方法，它避免了耗時(shí)的拆機(jī)清洗。有很多材料可用于清機(jī)。既有商用的清機(jī)產(chǎn)品，也有自個(gè)兒特制的清機(jī)產(chǎn)品。這些材料常分成兩類：化學(xué)清洗劑和機(jī)械摩擦清洗劑。許多清洗劑同時(shí)具有化學(xué)和摩擦功能。化學(xué)清洗劑象一種溶劑一樣進(jìn)攻塑料殘?jiān)ＴS多化學(xué)清洗劑還加入一種發(fā)泡功能，需要在機(jī)筒內(nèi)“浸泡”5分鐘，從而發(fā)揮化學(xué)作用。

機(jī)械摩擦清洗的原理是從金屬表面摩擦和沖刷掉殘留材料，使用的是摩擦和剪切的方式。加工者做了很多試錯(cuò)試驗(yàn)來找到合適的清洗材料和方法。沒有什么“神奇”的材料對(duì)所有材料都管用的材料。商用清洗材料一般比自己制造的清洗材料貴。但是，假如你使用較少的商用清洗劑材料能更快把活干好，那么實(shí)際成本可能更小。

加工者需要廉價(jià)而有效地清洗材料。比如，在美國中西部許多操作者使用喂牛的玉米，因?yàn)樗菀椎玫剑瑢?duì)機(jī)器有好的清潔作用。下面介紹兩種自己制造的有效地清洗劑：

---1#混合物：50%HDPE＋50%硅藻土

---2#混合物：HDPE＋水＋去污粉。將HDPE以通常速度在熔融溫度下加入螺桿。將去污粉用水在桶里攪拌成漿液，從主喂料口慢慢加入漿液。這些漿液因?yàn)榘l(fā)泡而沖擊、摩擦、清洗螺桿。

Color or material changeovers tend to create problems. Extrusion people wouldlike to always have nice long runs. But in the real world, demands fromcustomers mean frequent changeovers to make 80 Ib of Product A, followed by 200Ib of Product B, etc. So it is important to approach changeovers in a waytominimize operator effort and problems:
■Know howcritical is cleanliness to the next product to be run. There are anecdotesaround any extrusion shop of somebody purging for 16 hr to avoid pulling thescrews to brush them off. The point is, if you know the next product requires athorough manual brush cleaning,don't waste time and material trying to get bywith purging.
■Ifpossible, plan runs in order of decreasing cleanliness requirements. Forexample, this means going from light to dark colors. That way, you should faceonly one difficult changeover,when going from the dark color back to a lightcolor.
■Havewritten purging or strip-down procedures to run each product. This helps operatorsplan their time, and eliminates discussions later on about how many pounds ofproduct have to bescrapped because the operator didn't clean the machineproperly.

顏色和材料切換常弄出問題。搞擠出的人總是想把機(jī)器開得長一些時(shí)間。但是在現(xiàn)實(shí)中，由于客戶的需求，我們常常需要不斷切換，生產(chǎn)80磅產(chǎn)品A，然后再生產(chǎn)200磅的產(chǎn)品B，等等。重要的是要找到一種方法，減少操作人員的工作量和出現(xiàn)的問題:

---知道下一個(gè)產(chǎn)品的清機(jī)是如何重要。假如你知道下一個(gè)產(chǎn)品要求徹底的手工清機(jī)，那你就不要浪費(fèi)時(shí)間和材料來糊弄清機(jī)。

---假如可能，要按照減少清機(jī)的順序排產(chǎn)。這就意味著要從淺色做到深色。反之，你只有拆機(jī)。

---寫出生產(chǎn)每個(gè)產(chǎn)品的清機(jī)或拆機(jī)的程序，這有助于操作人員計(jì)劃時(shí)間，消除他們關(guān)于會(huì)產(chǎn)生多少機(jī)頭料的爭(zhēng)論。

---產(chǎn)品的批量要與機(jī)器大小相對(duì)應(yīng)。這似乎是不言而喻的。但是許多加工者排產(chǎn)無計(jì)劃，用75機(jī)生產(chǎn)500磅的單子，結(jié)果產(chǎn)生了300磅的廢料。

---幫助操作人員，把清潔的操作工具擺放在擠出機(jī)周圍，如果頻繁換料，那就在擠出機(jī)周圍搞一個(gè)工作臺(tái)，把需要的東西都準(zhǔn)備好。

---研究原材料和成品流進(jìn)流出操作現(xiàn)場(chǎng)的順序。安排好以使操作人員花費(fèi)較少的精力在移動(dòng)運(yùn)輸工具上。