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南京工程學(xué)院高等教育自學(xué)考試
畢業(yè)生學(xué)士學(xué)位申請表
承辦學(xué)院 江蘇財經(jīng)職業(yè)技術(shù)學(xué)院 專業(yè) 數(shù)控加工與模具設(shè)計
姓 名
吳英
性 別
女
身份證號
321283198902231021
聯(lián)系電話
18936383021
準(zhǔn)考證號
088810202012
班 級
10模具（本）
入學(xué)年月
年 月
畢業(yè)年月
年 月
外 語
情 況
□國家大學(xué)英語四級考試成績達(dá)到授予標(biāo)準(zhǔn)
t國家組織的本科段自學(xué)考試英語成績達(dá)70分及以上
本科畢業(yè)
證書編號
擬申報學(xué)士學(xué)位科類名稱
學(xué)學(xué)士
申報材料目錄
1、學(xué)習(xí)成績單
2、“外語情況”欄目中材料原件及復(fù)印件
3、身份證復(fù)印件
4、在校期間無違紀(jì)（如考試作弊等）行為
承辦學(xué)院蓋章 年 月 日
南京工程學(xué)院自考辦推薦意見：
（蓋章） 年 月 日
學(xué)位證書號碼：
汽車開發(fā)周期從40個月縮短到4個月。
一、
1、模具在工業(yè)發(fā)展中的地位
采用模具生產(chǎn)零部件，具有生產(chǎn)效率高、質(zhì)量好、成本低、節(jié)約能源和原材料等一系列優(yōu)點，用模具生產(chǎn)制件所具備的高精度、高復(fù)雜程度、高一致性、高生產(chǎn)率和低消耗，是其他加工制造方法所不能比擬的。已成為當(dāng)代工業(yè)生產(chǎn)的重要手段和工藝發(fā)展方向?，F(xiàn)代經(jīng)濟的基礎(chǔ)工業(yè)。現(xiàn)代工業(yè)品的發(fā)展和技術(shù)水平的提高，很大程度上取決于模具工業(yè)的發(fā)展水平，因此模具工業(yè)對國民經(jīng)濟和社會發(fā)展將起越來越大的作用。1989年3月國務(wù)院頒布的《關(guān)于當(dāng)前產(chǎn)業(yè)政策要點的決定》中，把模具列為機械工業(yè)技術(shù)改造序列的第一位、生產(chǎn)和基本建設(shè)序列的第二位(僅次于大型發(fā)電設(shè)備及相應(yīng)的輸變電設(shè)備)，確立模具工業(yè)在國民經(jīng)濟中的重要地位。1997年以來，又相繼把模具及其加工技術(shù)和設(shè)備列入了《當(dāng)前國家重點鼓勵發(fā)展的產(chǎn)業(yè)、產(chǎn)品和技術(shù)目錄》和《鼓勵外商投資產(chǎn)業(yè)目錄》。經(jīng)國務(wù)院批準(zhǔn)，從1997年到2000年，對80多家國有專業(yè)模具廠實行增值稅返還70%的優(yōu)惠政策，以扶植模具工業(yè)的發(fā)展。所有這些，都充分體現(xiàn)了國務(wù)院和國家有關(guān)部門對發(fā)展模具工業(yè)的重視和支持。目前全世界模具年產(chǎn)值約為600億美元，日、美等工業(yè)發(fā)達(dá)國家的模具工業(yè)產(chǎn)值已超過機床工業(yè)，從1997年開始，我國模具工業(yè)產(chǎn)值也超過了機床工業(yè)產(chǎn)值。
模具是大批量生產(chǎn)同形產(chǎn)品的工具，是工業(yè)生產(chǎn)的主要工藝裝備。
據(jù)統(tǒng)計，在家電、玩具等輕工行業(yè)，近90％的零件是綜筷具生產(chǎn)的；在飛機、汽車、農(nóng)機和無線電行業(yè)，這個比例也超過60％。例如飛機制造業(yè)，某型戰(zhàn)斗機模具使用量超過三萬套，其中主機八千套、發(fā)動機二千套、輔機二萬套。從產(chǎn)值看，80年代以來，美、日等工業(yè)發(fā)達(dá)國家模具行業(yè)的產(chǎn)值已超過機床行業(yè)，并又有繼續(xù)增長的趨勢。據(jù)國際生產(chǎn)技術(shù)協(xié)會預(yù)測，到2000年，產(chǎn)品盡件粗加工的75%、精加工的50％將由模具完成；金屬、塑料、陶瓷、橡膠、建材等工業(yè)制品大部分將由模具完成，50％以上的金屬板材、80％以上的塑料都特通過模具轉(zhuǎn)化成制品。
2、模具的歷史發(fā)展
模具的出現(xiàn)可以追溯到幾千年前的陶器和青銅器鑄造，但其大規(guī)模使用卻是隨著現(xiàn)代工業(yè)的掘起而發(fā)展起來的。
19世紀(jì)，隨著軍火工業(yè)(槍炮的彈殼)、鐘表工業(yè)、無線電工業(yè)的發(fā)展，沖模得到廣泛使用。二次大戰(zhàn)后，隨著世界經(jīng)濟的飛速發(fā)展，它又成了大量生產(chǎn)家用電器、汽車、電子儀器、照相機、鐘表等零件的最佳方式。從世界范圍看，當(dāng)時美國的沖壓技術(shù)走在前列——許多模具先進技術(shù)，如簡易模具、高效率模具、高壽命模具和沖壓自動化技術(shù)，大多起源于美國；而瑞士的精沖、德國的冷擠壓技術(shù)，蘇聯(lián)對塑性加工的研究也處于世界先進行列。50年代，模具行業(yè)工作重點是根據(jù)訂戶的要求，制作能滿足產(chǎn)品要求的模具。模具設(shè)計多憑經(jīng)驗，參考已有圖紙和感性認(rèn)識，對所設(shè)計模具零件的機能缺乏真切了解。從1955年到1965年，是壓力加工的探索和開發(fā)時代——對模具主要零部件的機能和受力狀態(tài)進行了數(shù)學(xué)分橋，并把這些知識不斷應(yīng)用于現(xiàn)場實際，使得沖壓技術(shù)在各方面有飛躍的發(fā)展。其結(jié)果是歸納出模具設(shè)計原則，并使得壓力機械、沖壓材料、加工方法、梅具結(jié)構(gòu)、模具材料、模具制造方法、自動化裝置等領(lǐng)域面貌一新，并向?qū)嵱没姆较蛲七M，從而使沖壓加工從儀能生產(chǎn)優(yōu)良產(chǎn)品的第一階段。
進入70年代向高速化、啟動化、精密化、安全化發(fā)展的第二階段。在這個過程中不斷涌現(xiàn)各種高效率、商壽命、高精度助多功能自動校具。其代表是多達(dá)別多個工位的級進模和十幾個工位的多工位傳遞模。在此基礎(chǔ)上又發(fā)展出既有連續(xù)沖壓工位又有多滑塊成形工位的壓力機—彎曲機。在此期間，日本站到了世界最前列——其模具加工精度進入了微米級，模具壽命，合金工具鋼制造的模具達(dá)到了幾千萬次，硬質(zhì)合金鋼制造的模具達(dá)到了幾億次p每分鐘沖壓次數(shù)，小型壓力機通常為200至300次，最高為1200次至1500次。在此期間，為了適應(yīng)產(chǎn)品更新快、用期短(如汽車改型、玩具翻新等)的需要，各種經(jīng)濟型模具，如鋅落合金模具、聚氨酯橡膠模具、鋼皮沖模等也得到了很大發(fā)展。
從70年代中期至今可以說是計算機輔助設(shè)計、輔助制造技術(shù)不斷發(fā)展的時代。隨著模具加工精度與復(fù)雜性不斷提高，生產(chǎn)周期不斷加快，模具業(yè)對設(shè)備和人員素質(zhì)的要求也不斷提高。依靠普通加工設(shè)備，憑經(jīng)驗和手藝越來越不能滿足模具生產(chǎn)的需要。90年代以來，機械技術(shù)和電子技術(shù)緊密結(jié)合，發(fā)展了NC機床，如數(shù)控線切割機床、數(shù)控電火花機床、數(shù)控銑床、數(shù)控坐標(biāo)磨床等。而采用電子計算機自動編程、控制的CNC機床提高了數(shù)控機床的使用效率和范圍。近年來又發(fā)展出由一臺計算機以分時的方式直接管理和控制一群數(shù)控機床的NNC系統(tǒng)。
隨著計算機技術(shù)的發(fā)展，計算機也逐步進入模具生產(chǎn)的各個領(lǐng)域，包括設(shè)計、制造、管理等。國際生產(chǎn)研究協(xié)會預(yù)測，到2000年，作為設(shè)計和制造之間聯(lián)系手段的圖紙將失去其主要作用。模具自動設(shè)計的最根本點是必須確立模具零件標(biāo)準(zhǔn)及設(shè)計標(biāo)準(zhǔn)。要擺脫過去以人的思考判斷和實際經(jīng)驗為中心所組成的設(shè)計方法，就必須把過去的經(jīng)驗和思考方法，進行系列化、數(shù)值化、數(shù)式化，作為設(shè)計準(zhǔn)則儲存到計算機中。因為模具構(gòu)成元件也干差萬別，要搞出一個能適應(yīng)各種零件的設(shè)計軟件幾乎不可能。但是有些產(chǎn)品的零件形狀變化不大，模具結(jié)構(gòu)有一定的規(guī)律，放可總結(jié)歸納，為自動設(shè)計提供軟件。如日本某公司的CDM系統(tǒng)用于級進模設(shè)計與制造，其中包括零件圖形輸入、毛坯展開、條料排樣、確定模板尺寸和標(biāo)準(zhǔn)、繪制裝配圖和零件圖、輸出NC程序(為數(shù)控加工中心和線切割編程)等，所用時間由手工的20%、工時減少到35小時；從80年代初日本就將三維的CAD／CAM系統(tǒng)用于汽車覆蓋件模具。目前，在實體件的掃描輸入，圖線和數(shù)據(jù)輸入，幾何造形、顯示、繪圖、標(biāo)注以及對數(shù)據(jù)的自動編程，產(chǎn)生效控機床控制系統(tǒng)的后置處理文件等方面已達(dá)到較高水平；計算機仿真(CAE)技術(shù)也取得了一定成果。在高層次上，CAD／CAM／CAE集成的，即數(shù)據(jù)是統(tǒng)一的，可以互相直接傳輸信息．實現(xiàn)網(wǎng)絡(luò)化。目前．國外僅有少數(shù)廠家能夠做到。
3、模具的發(fā)展趨勢
（1）模具軟件功能集成化
模具軟件功能的集成化要求軟件的功能模塊比較齊全，同時各功能模塊采用同一數(shù)據(jù)模型，以實現(xiàn)信息的綜合管理與共享，從而支持模具設(shè)計、制造、裝配、檢驗、測試及生產(chǎn)管理的全過程，達(dá)到實現(xiàn)最佳效益的目的。如英國Delcam公司的系列化軟件就包括了曲面/實體幾何造型、復(fù)雜形體工程制圖、工業(yè)設(shè)計高級渲染、塑料模設(shè)計專家系統(tǒng)、復(fù)雜形體CAM、藝術(shù)造型及雕刻自動編程系統(tǒng)、逆向工程系統(tǒng)及復(fù)雜形體在線測量系統(tǒng)等。集成化程度較高的軟件另外還包括：Pro/ENGINEER、UG和CATIA等。國內(nèi)有上海交通大學(xué)金屬塑性成型有限元分析系統(tǒng)和沖裁模CAD/CAM系統(tǒng)；北京北航海爾軟件有限公司的CAXA系列軟件；吉林金網(wǎng)格模具工程研究中心的沖壓模CAD/CAE/CAM系統(tǒng)等。
（2）模具設(shè)計、分析及制造的三維化
傳統(tǒng)的二維模具結(jié)構(gòu)設(shè)計已越來越不適應(yīng)現(xiàn)代化生產(chǎn)和集成化技術(shù)要求。模具設(shè)計、分析、制造的三維化、無紙化要求新一代模具軟件以立體的、直觀的感覺來設(shè)計模具，所采用的三維數(shù)字化模型能方便地用于產(chǎn)品結(jié)構(gòu)的CAE分析、模具可制造性評價和數(shù)控加工、成形過程模擬及信息的管理與共享。舉個例子如Pro/ENGINEER、UG和CATIA等軟件具備參數(shù)化、基于特征、全相關(guān)等特點，從而使模具并行工程成為可能。另外，Cimatran公司的Moldexpert，Delcam公司的Ps-mold及日立造船的Space-E/mold均是3D專業(yè)注塑模設(shè)計軟件，可進行交互式3D型腔、型芯設(shè)計、模架配置及典型結(jié)構(gòu)設(shè)計。澳大利亞Moldflow公司的三維真實感流動模擬軟件MoldflowAdvisers已經(jīng)受到用戶廣泛的好評和應(yīng)用。國內(nèi)有華中理工大學(xué)研制的同類軟件HSC3D4.5F及鄭州工業(yè)大學(xué)的Z-mold軟件。面向制造、基于知識的智能化功能是衡量模具軟件先進性和實用性的重要標(biāo)志之一。如Cimatron公司的注塑模專家軟件能根據(jù)脫模方向自動產(chǎn)生分型線和分型面，生成與制品相對應(yīng)的型芯和型腔，實現(xiàn)模架零件的全相關(guān)，自動產(chǎn)生材料明細(xì)表和供NC加工的鉆孔表格，并能進行智能化加工參數(shù)設(shè)定、加工結(jié)果校驗等。
（3）模具軟件應(yīng)用的網(wǎng)絡(luò)化趨勢
隨著模具在企業(yè)競爭、合作、生產(chǎn)和管理等方面的全球化、國際化，以及計算機軟硬件技術(shù)的迅速發(fā)展，網(wǎng)絡(luò)使得在模具行業(yè)應(yīng)用虛擬設(shè)計、敏捷制造技術(shù)既有必要，也有可能。美國在其《21世紀(jì)制造企業(yè)戰(zhàn)略》中指出，到2006年要實現(xiàn)汽車工業(yè)敏捷生產(chǎn)/虛擬工程方案，使汽車開發(fā)周期從40個月縮短到4個月。AA
南京工程學(xué)院
畢業(yè)設(shè)計任務(wù)書
江蘇財經(jīng) 學(xué)院 數(shù)控加工與模具設(shè)計 專業(yè)
設(shè) 計 題 目 驅(qū)動橋鎖沖壓模具設(shè)計
學(xué) 生 姓 名 吳英
班 級 10模具（本）
起 止 日 期 2012.02.06-04.26
指 導(dǎo) 教 師 林偉
教研室主任 赫英岐
發(fā)任務(wù)書日期 2012年2月6日
1.畢業(yè)設(shè)計的原始數(shù)據(jù)：
(1)產(chǎn)品圖及其技術(shù)要求；
(2)生產(chǎn)批量：大批量。
2.畢業(yè)設(shè)計(論文)的內(nèi)容和要求(包括技術(shù)要求、圖表要求以及工作要求等)：
(1)繪制產(chǎn)品零件圖；
(2)查閱資料并就自己承當(dāng)?shù)恼n題方向?qū)懗鲩_題報告（嚴(yán)格按統(tǒng)一格式）；
(3)完成產(chǎn)品的工藝設(shè)計及模具設(shè)計并繪制模具及模具零件的工程圖；
(4)按規(guī)定格式和要求用Word打印設(shè)計說明書一份；
(5)用PowerPoint或其它軟件制作答辯提綱一份。
3.畢業(yè)設(shè)計應(yīng)完成的技術(shù)文件：
(1)產(chǎn)品零件圖；
(2)開題報告（格式見校園網(wǎng)）；
(3)模具裝配圖及各零件圖；
(4)設(shè)計計算說明書；
(5)上述所有文檔的刻錄光盤一張。
4.主要參考文獻(xiàn)：
(1) 徐政坤.沖壓模具設(shè)計與制造.北京：化學(xué)工業(yè)出版社，2006
(2) 郭鐵良. 模具制造工藝學(xué).北京:高等教育出社，2002
(3) 盧險峰.沖壓工藝模具學(xué).北京：機械工業(yè)出版社，2006
(4) 雜志期刊：沖壓成形工藝，沖壓模具手冊，模具制造工藝學(xué)等
(5) 電子資料查詢：Google，Baidu，學(xué)校圖書館、閱覽室。
5.畢業(yè)設(shè)計(論文)進度計劃(以周為單位)：
起 止 日 期
工 作 內(nèi) 容
備 注
第1-2周
（02.06-02.19）
1、熟悉課題，查閱并收集有關(guān)資料，為寫開題報告做準(zhǔn)備；
2、繪制產(chǎn)品零件圖；
3．完成工藝性分析，提出設(shè)計方案；
檢查零件圖
和方案
第3-4周
（02.20-03.4）
1、寫出開題報告初稿。
2、完成工藝計算；
3、完成模具零件結(jié)構(gòu)形式確定及設(shè)計，選擇設(shè)備。
檢查開題報告
第5-6周
（03.5-03.18）
1、第8周完成開題報告的最后定稿；
2、繪制模具草圖
檢查開題報告與模具結(jié)構(gòu)草圖
第7-9周
（03.18-04.08）
繪制模具裝配圖及各設(shè)計件零件圖
檢查圖（電子版）
第10-11周
（04.09-04.22）
編寫設(shè)計說明書
檢查設(shè)計說明書
第12周
（04.23-04.26）
打印畢業(yè)設(shè)計資料并準(zhǔn)備答辯
檢查答辯PPT
教研室審查意見：
室主任
2012年2月8 日
學(xué)院審查意見：
教學(xué)院長
2012年2月12日
二、The injection and Compression Molding
Injection molding si principally used for the production of the thermoplastic parts, although some progress has been made in developing a method for injection molding some thermosetting materials. The problem of injecting a melted plastic into a mold cavity form a reservoir of melted material has been extremely difficult to solve for thermosetting plastics which cure and harden under such conditions within a few minutes. The principle of injection molding is quite similar to that of die-casting. The process consists of feeding a plastic compound in powdered or granular form from a hopper through metering and melting stages and then injecting it into a mold. After a brief coolling period, the mold is opened and the solidified part ejected. Injection-molding machines can be arranged for manual operation, automatic single-cucle operation, and full automatic operation. The advantage of injection molding are:
(i) a high molding speed adapted for mass production is possible;
(ii) there is a wide choice of thermoplastic materials providing a variety of usefull properties;
(iii) it is possible to mold threads, undercuts, side holes, and large thin sections.
Several methods are used to force or inject the melted plastic into the mold. The most commonly used system in the larger machines is the in-line reciprocating screw.The screw acts as a combination and plasticizing unit.As the plastic is fed to the rotating screw,it passes through three zones as shown: feed,compression, and metering. After the feed zone, the screw-flight depth is gradually reduced,forcing the plastic to compress. The work is converted to heat by shearing the plastic, making it a semifluid mass. In the metering zone, additional heat is applied by conduction from the barrel surface. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold.An antiflowback valve prevents plastic under pressure from escaping back into the screw flights.
The clamping force that a machine is capable of exerting is part of the size designation and is measured in tons. A rule-of-thumb can be used to determine the tonnage required for a particular job. It is based on two tons of clamp force per square inch of projected area. If the flow pattern is difficult and the parts are thin,this may have to go to three or four tons.
Many reciprocating - screw machines are capable of handing thermosetting plastic materials.Previously these materials were handled by compression or transfer molding.Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 375℃～410℃.Thermoplastic parts must be allowed to cool in the mold in order to remove them without distortion.Thus thermosetting cycles can be faster.Of course the mold must be heated rather than chilled,as with thermoplastics.
The importance of Injecting the mold are :
⑴、Plastics have the density small, the quality light, the specific tenacity big, the insulating property good, the dielectric loss low, the chemical stability strong, the formation productivity high and the price inexpensive and so on the merits, obtained day by day the widespread application in the national economy and people's daily life each domain, as early as in the beginning of 1990s, the plastic annual output already surpassed the steel and iron and the non-ferrous metal annual output sum total according to the volume computation.In mechanical and electrical (for example so-called black electrical appliances), domains and so on measuring appliance, chemical, the automobile and astronautics aviation, the plastic has become the metal the good substitution material, had the metal material plastic tendency.
⑵、Take the automobile industry as the example , as a result of the automobile lightweight, the low energy consumption development request, the automobile spare part material constitution occurred obviously has modelled the band steel the change, at present our country automobile plastic accounts for 5% which the automobile was self-possessed to 6%, but overseas has reached 13%, forecast according to the expert, the automobile plastic bicycle amount used will also be able further to increase.On modern vehicles, regardless of is outside installs the assorted items, the internal installation assorted items, the function and the structural element, all may use the plastic material, outside installs the assorted items to have the bumper, the fender, the wheel hub cap, the air deflector and so on; After the internal installation assorted items have in the display board, the vehicle door the board, the vice-display board, the sundry goods box lid, the chair, the guard shield and so on; The function and the structural element have the fuel tank, the radiator header, the spatial filter hood, the fan blade and so on.Statistics have indicated, our country in 2000 automobile output more than 200 ten thousand, the vehicle amounted to 1,380,000 tons with the plastic.Looked from the domestic and foreign automobile plastic application situation that, the automobile plastic amount used already became one of weight automobile production technical level symbols.
⑶、Injection of a molding formation as plastic workpiece most effective formation methods because may by one time take shape each kind of structure complex, the size precise and has the metal to inlay a product, and the formation cycle is short, may by mold multi-cavities, the productivity be high, when mass productions the cost isvery inexpensive, easy to realize the automated production, therefore holds the extremely important status in the plastic processing profession.Statistics have indicated, plastic mold composition all molds (including metal pattern) 38.2%, the plastic product gross weight about 32% is uses in injecting the formation, 80% above engineering plastics product all must use the injection formation way production. 4. counts according to the customs, our country in 2000 altogether imported mold 977,000,000 US dollars, in which plastic molding forms altogether 550,000,000 US dollars, occupied for 56.3%,2001 years altogether to import mold 1,112,000,000 US dollars, in which plastic molding forms altogether 616,000,000 US dollars, accounted for 55.4%.From the variety, the import volume biggest is the plastic molding forms.
⑷、Counts according to the customs, our country in 2000 altogether imported mold 977,000,000 US dollars, in which plastic molding forms altogether 550,000,000 US dollars, occupied for 56.3%, 2001 years altogether to import mold 1,112,000,000 US dollars, in which plastic molding forms altogether 616,000,000 US dollars, accounted for 55.4%.From the variety, the import volume biggest is the plastic molding forms.
In compression molding the palstic material as powder or preforms is placed into a heated steel mold cavity,Since the parting surface is in a horizontal plane ,the upper half of the mold descends vertically.It closes the mold cavity and pressures for a predetermined period.A pressure of from 2 to 3 tons square inch and a temperaure at approximately 350F converts the plastic to a semiliquid which flows to all parts of the mold cavity.Usually from 1 to 15 minutes is required for curing,altough a recently developed alkyd plastic will cure in less than 25 secends. The mold is then opended and the molded part removed.If metal insers are desired in the parts,they should be placed in the mold cavity on pins or in the holes before the plastic is loaded.Also, the preforms should be preheated before loading into the mold cavity to eliminate gases,inprove flow,and decrease curing time.Dieletric heating is a convenient method of heating the preforms.
Since the plastic material is placed directly into the mold cavity,the mold itself can be simpler than those used for other molding precesses.Gates and sprues are unnecessary.This also results in a saving in material,because trimmed-off gates and sprues would be a complete loss of the thermosetting plastic.The press require the full attention of one operator.However,several smaller presses can be operated by one operator. The presses are conveniently located so the operator can easilymove from one to the next.By the time he gets around to a particular press again,that mold will be ready to open.the thermosetting plastics which harden under heat and pressure are suitable for compression molding and transfer molding.It is not practical to moid shermoplastic materials by these methods,since the molds would have to be alternately heated and cooled.In order to harden and eject thermoplastic parts form the mold,cooling would be necessary.
Types of molds for compression molding.The molds used for compression molding are classified into four basic types, namely ,positive molds,landed positive mold,flash-type molds,and semipositive molds.In a positive mold the plunger on the upper mold enters the lower mold cavity.since there are no lands or stops on the lower die ,the plunger completely trap the plastic material and descends with full pressure on the charge.A dense part with good electrical and physical properties is produced.The amount of plastic placed in the die cavity must be accurately measured,since it determines the thickness of the part .A landed positive mold is similar to a positive mold except that lands are added to stop the travel of the plunger at predetermined point.In this case,the lands absorb some of the pressure that should be exerted on the parts.The thickness of the parts will be accurately controlled,but the density may vary cansideraby.In a flash-type mold,flash redges are added ti the top and bottom molds.As the upper mold exerts pressure on the plastic,excess material is forced out between the flash ridges where it forms flash.This flash is further compressed.becomes hardened,and finally stops the downard thavel of the upper mold.A slight excess of the plastic material is always chared to ensure sufficient pressurs to produce a dense molded part.This type of mold is widely used because it is comparatively easy to construct and it controls thickness and density within colse limits.The semipositive mold is a combination od the flash type and landed posive molds.In addition to the flash ridges,a land is employed to restrict the travel of the upper mold.