塑料压缩模.doc

塑料压缩模 在压缩成型过程中，塑料原料是以粉状或锭料形式放置在加热的金属模具型腔中。因为分型面是水平面，上模就垂直下行。闭合模具后，预成型加压加热作用一段时间。在压力为23t/in和温度为华氏温度350的作用下，是塑料呈半液态，充满模具型腔。虽然近年来开发的酚醛树脂可在25S内塑化，但塑料通常的塑化需要115min。最后开模取出塑件。如果零件中含有金属嵌件，应在注入塑料前，讲嵌件放入型腔定位孔中。锭料在装入模腔前预热出去气体，增强流动性，以便于充满模具和缩短塑化时间。电介质加热时加热锭料的便利方法。 因为塑料是直接加入模具型腔中的，所以压缩模比其他模塑料工艺的模具简单，不需要浇道和浇口，可以节省原料，因为清理浇道和浇口，对热固性塑料来说是极大的浪费。用于压缩模塑的压力机通常是垂直液压机。较大的压力机要求操作者全神贯注地操作；然而，一个操作者可以同时操作几台小型压力机，因此压力机要合理放置以便操作者能够方便你从一台到另一台进行操作，操作者要保证当他再次绕道特定压力机前时，模具恰好准备开模。 热固性塑料在加热和加压作用下固化，这个特性决定其适合压缩模和传递模。因为压缩模要交替的加热和冷却，所以热塑性塑料实际上不是用于压缩成型。为了使热塑性塑料之间硬化并从型腔中顶出，就需要将塑件冷却。 压缩模的类型。 用于压缩的模具可分为四种基本类型，分别是:不溢式压缩模、挡环不溢式压缩模、溢式压缩模和半溢式压缩模。在不溢式压缩模中，柱塞式凸模随上模进入下模模腔成型，因为下模没有挡环或限位装置，柱塞式凸模继续下行以全部压力施加在塑料上，这样就成产出具有良好电性能和物理性能的致密塑件。加入模腔的塑料剂量须精确计量，以为其影响零件的壁厚。挡环不溢式压缩模与不溢式压缩模类似，只是在预定点处增加了挡环，使柱塞式凸模在预定点停止运动，在这种模具中挡环吸收了原应作用在零件上的部分压力。这种压缩模可精确控制零件的壁厚，但是塑件密度的变化是相当大的。在溢式压缩模中，溢料会增加上下模的尺寸。当上模对塑料施压时，多余的塑料会从分型面处溢出。对溢料进一步施压，他就硬化最后在上模形成飞边。生产致密塑件通常需要使用的计算量稍多的原料来增加压力。这种类型的模具之所以在生产中广泛采用，是因为它想对比较容易制造，并能够控制塑件的厚度和致密度，使其更接近要求。半溢式压缩模是溢式压缩模和不溢式压缩模的组合类型，除了飞边外，采用的挡环可限制上模的移动。注塑模重要性塑料具有密度小、质量轻、比强度大、绝缘性好、介电损耗低、化学稳定性强、成型生产率高和价格低廉等优点，在国民经济和人民日常生活的各个领域得到了日益广泛的应用，早在二十世纪九十年代初，塑料的年产量按体积计算已经超过钢铁和有色金属年产量的总和。在机电（如所谓的黑色家电）、仪表、化工、汽车和航天航空等领域，塑料已成为金属的良好代用材料，出现了金属材料塑料化的趋势。 Compression Molding for PlasticsIn 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.一、Injects the mold the importance1. 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 peoples 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 comp