【正文】 nal cavities or passages, such as those found in an automotive engine block or a valve body, cores are utilized. Cores are placed in the mold cavity to form the interior surfaces of the casting and are removed from the finished part during shakeout and further processing, like molds, cores must posse strength, permeability, ability to withstand heat, and collapsibility。 in the United States alone, about 15 million tons of metal are cast by this method each year. Typical applications of sand casting include machine bases, large turbine impellers, propellers, plumbing fixtures, and numerous ponents for agricultural and railroad equipment. The capabilities of sand casting are given in Table .Basically, sand casting consists of (a) placing a pattern (having the shape of the desired casting) in sand to make an imprint, (b) incorporating a gating system, (c) removing the pattern and filling the mold cavity with molten metal, (d) allowing the metal to cool until it solidifies, (e) breaking away the sand mold, and (f) removing the casting().Sands. Most sandcasting operations use silica sand (SiO2) as mold material. Sand is inexpensive and is suitable as mold material because of its hightemperature characteristics and high melting point. There are two general types of sand: naturally bonded (bank sand) and synthetic (lake sand). Because its position can be controlled more accurately, synthetic sand is preferred by most foundries. For proper functioning, mold sand must be clean and preferably new.Several factors are important in the selection of sand for molds, and it involves certain tradeoffs with respect to properties. Sand having fine, round grains can be packed closely and, thus, forms a smooth mold surface. Although finegrained sand through pores. Good permeability of molds and cores allows gases and steam evolved during the casting to escape easily. The mold also should have good collapsibility to allow for the casting to shrink while cooling and, thus, to avoid defects in the casting, such as hot tearing and cracking (see ).Types of sand molds. Sand molds () are characterized by the types of sand that prise them and by the methods used to produce them. There are three basic types of sand molds: greensand, coldbox, and nobake molds. The most mon mold material is green molding sand, which is a mixture of sand, clay, and water. The term “green” refers to the fact that the sand in the mold is moist or damp while the metal is being poured into it. Greensand molding is the least expensive method of making molds, and the sand is recycled easily for subsequent use. In the skindried method, the mold surfaces are dried, either by storing the mold in air or by drying it with torches, Because of their higher strength, these molds generally are used for large castings.In the coldbox mold process, various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These molds are more accurate dimensionally than greensand molds but are more expensive. In the nobake mold process, a synthetic liquid resin is mixed with the sand, and the mixture hardens at room temperature. Because the bonding of the mold in this and in the coldbox process takes place without heat, they are called coldsetting processes.Sand molds are oven dried (baked) prior to pouring the molten metal。另外，在模具零件制作的工藝方面選擇材料和加工方法都必須根據(jù)生產(chǎn)條件，在滿足加工精度的條件下選擇經(jīng)濟的加工方法。Smax=SKHM式中SK為注射機動模固定板和定模固定板的最大間距（mm）； HM為模具厚度（mm） H1+H2+（5～10）+HM=397～402＜SmaxXSZY250注射機的最大開模行程為500mm，因此開模行程足夠。式中H1為制品所用的脫模距離，H2為制品高度，注系統(tǒng)凝料必需的長度，單位均為mm。注射機的開模行程應(yīng)大于脫模取出塑件所需的開模距離。Vi=（cm）V澆=（cm）kV=250=200 ≥（cm） 因此，此注射機注射量足夠。根據(jù)生產(chǎn)經(jīng)驗，制品注射量一般不超過注射機最大注射量的80%。因此，可用四個定距螺釘同時實現(xiàn)上述兩種功能，既起到推板導(dǎo)向作用，又對推板復(fù)位后進行限位。因為斜導(dǎo)柱安裝在定模板上，滑塊安裝在動模的側(cè)向抽芯機構(gòu)，同時，模具還采用推桿脫模機構(gòu)，并依靠復(fù)位桿使推桿復(fù)位，側(cè)型芯的水平投影面積與推桿相重合或推桿推出距離大于側(cè)型芯的底面時，則可能會產(chǎn)生推桿和側(cè)型芯互相干擾的現(xiàn)象。推出脫模機構(gòu)的推出零件種類也很多，這里用推桿推出脫模，即用推桿將塑件從型芯上推出。錐面定位是用在注射大型、深腔、高精度制品或薄壁容器及偏心制品時，成形時往往有很大的側(cè)向作用力，因此不適合用在此次設(shè)計的模具當中，此次設(shè)計的模具采用導(dǎo)柱導(dǎo)向機構(gòu)。另外，合模完成后，為了防止兩個滑塊位置發(fā)生左右偏移，導(dǎo)致形腔結(jié)構(gòu)尺寸受到破壞，必須在滑塊合?；瑒咏K了位置處設(shè)置定位機構(gòu)，這里可以用兩個定位銷對滑塊進行定位,上下對稱各一個。=20176。再看形式（b），利用螺釘和楔緊塊右端面同時承受鎖緊力，但不易加工，螺釘能承受的鎖緊力很有限，所以不宜選用此形式。為了確保側(cè)型芯可靠地抽出和復(fù)位，保證滑塊在移動過程中平穩(wěn)，無上下竄動和卡死等現(xiàn)象，滑塊與滑槽必須很好地配合和導(dǎo)滑。斜導(dǎo)柱長度是根據(jù)抽芯距、固定端模板厚度，斜導(dǎo)柱直徑及傾角的大小等有關(guān)。斜導(dǎo)柱所受的彎曲力、抽芯力和開模力與傾角α的關(guān)系，如圖10所示，不計斜導(dǎo)柱與導(dǎo)滑孔間的摩擦力及滑塊與導(dǎo)滑槽間的摩擦力，其關(guān)系式為 。斜導(dǎo)柱傾角α不宜太大，常采用15176。為脫模斜度，因為制品是標準零件所以制件不允許斜度，此時脫模角為0176。所以將側(cè)型芯從制品中抽出所需的力叫抽芯力。此次設(shè)計采用斜導(dǎo)柱式抽芯機構(gòu)。（5）中心距尺寸 式中為中心距基本尺寸，是平均尺寸，其公差為，取為Δ/3，為塑料制品中心距的基本尺寸，也為平均尺寸，其公差為Δ。（1）型腔徑向尺寸計算式中為型腔徑向基本尺寸，且為最小尺寸，其公差為,取為Δ/3，為塑料制品的基本尺寸，且為最大尺寸，其公差Δ，為塑料的平均收縮率，這里材料是HPVC，因此=（+）/2=%。成形零部件工作尺寸的確定必須考慮塑料的成形收縮、成形零部件的制造偏差及成形零部件的磨損等各方面因素。由于塑料零件外形有凹陷部分，對應(yīng)成形模上必須有凸起部分，為便于加工起見，不把成形模設(shè)計成一個整體，而是采用嵌入式的成形模，即用型芯嵌入固定板中形成型腔結(jié)構(gòu)。澆口的形式很多，參考《實用模具設(shè)計與制造手冊》給出的澆口形式，根據(jù)塑料種類、塑料制品的形狀及分模落料形式，應(yīng)選側(cè)澆口，如圖8所示。又因其長度短，所以澆口內(nèi)可容納的塑料熔體體積很小，故很容易冷卻固化，從而有助于防止保壓力不足或保壓時間過短而引起的倒流現(xiàn)象。 圖6在容易修磨情況下，分流道的長度應(yīng)盡可能短，以減少壓力損失及廢料，因此兩個型腔之間的間距也盡量小。分流道其截面形狀及尺寸主要取決于制品的大小、模具結(jié)構(gòu)以及所加工的塑料種類。其尺寸應(yīng)根據(jù)XSZY250注射機配套的定位圈尺寸及定模座板的厚度進行確定，澆口套與定模座板之間的配合采用H7/m6。由于注射成形時，注射機對模具施加的壓力很大，主要作用于澆口套上，且主流道在與高溫塑料熔體和注射機噴嘴反復(fù)接觸和碰撞，所以一般不將主流道直接開設(shè)在定模上，而是將它單獨開在一個澆口套中，通常在淬火后嵌入模具，這樣在損壞時便于更換或修磨。 主流道進口端與噴嘴頭部應(yīng)為球面接觸，在澆口套上銑出一淺的球形定位槽，將噴嘴的球形頭壓在澆口套凹槽內(nèi)。見圖4。為了使冷凝料能從主流道中順利拔出，將主流道設(shè)計成圓錐形。澆注系統(tǒng)由主流道、分流道、澆口和冷料穴等四部分組成。 圖3澆注系統(tǒng)是指塑料熔體從注射機噴嘴出來后，到達型腔之前在模具中所流經(jīng)的通道，其作用是將熔融狀態(tài)的塑料從噴嘴處平穩(wěn)的引入模具型腔，并在熔體填充和固化定形的過程中將注射壓力和保壓力傳遞到塑料制品各部位，以獲得組織致密、外形清晰、表面光潔和尺寸精確的塑料模具。而側(cè)向抽芯運動方向既可鉛垂（圖1），也可水平（圖2），但對兩者進行比較發(fā)現(xiàn)，前者上下兩個側(cè)型芯的自重會影響各自的抽芯力，導(dǎo)致兩個側(cè)型芯所用抽芯力的大小不同，破壞兩個斜導(dǎo)柱的受力平衡。另外，型腔越多，成形出的制品精度也就越低。由聚氯乙烯的成型條件工藝參數(shù)和注塑機型號參見《塑料成型工藝及模具簡明手冊》表3表313和表31318，選型號為XSZY250的注射機。聚氯乙烯是流動性、熱穩(wěn)定性都很差的塑料，因此不宜選用注塞式注射機?？墒惯M入料筒內(nèi)的塑料顆粒有一個預(yù)先塑化的過程，注射機內(nèi)的注射注塞用螺桿代替。塑料在料筒內(nèi)受到料筒壁和分流梭兩方面?zhèn)鱽淼臒崃慷芑扇廴跔顟B(tài)。（3）角式注塑機：注射裝置和合模裝置的軸線互成垂直排列。它的優(yōu)點是重心低、工作平穩(wěn)、模具安裝、操作及維修均較方便，模具開檔大，占用空間高度小；但占地面積大，大、中、小型機均有廣泛應(yīng)用。因為氯離子的存在，它會輕微吸水，生產(chǎn)時，需哄干；烘干溫度在75℃～90℃之間，～；聚氯乙烯的熔體粘度大，流長比小，薄壁制品可能缺膠，因此，澆口和流道相對較大。加工過程中，對熱敏感，熱穩(wěn)定性差，受熱時，引起不同的降解；對硬質(zhì)PVC，對應(yīng)變敏感，變形后不能完全復(fù)原；對軟質(zhì)PVC，還有增塑劑外遷