摘要
选择性激光烧结(Selective Laser Sintering,简称SLS)是快速成形的重要分支,其最初目的是制造用于验证的原型件,随着技术的不断发展,制造能够直接运用的塑料功能件,以及与铸造技术相结合制造复杂的铸件,已成为SLS的两个重要发展方向,并有着广阔的应用前景。为此,本文深入研究了间接法制造塑料功能件的增强材料和工艺,直接法制造塑料功能件的材料和SLS成形工艺,复杂铸件的SLS熔模铸造工艺,覆膜砂的激光烧结机理以及砂型(芯)的SLS成形与浇注工艺。
对热塑性聚合物激光烧结热力学的进行了研究,得出激光烧结与表面张力有关,粉末粒径,几何形貌等影响表面张力的因素都会对SLS成形过程产生影响。建立了热塑性聚合物激光烧结的三个子模型:光学模型、热学模型和烧结模型,为SLS材料和成形工艺的研究提供了理论支持。
通过比较无定型聚合物:聚苯乙烯(PS)、高抗冲聚苯乙烯(HIPS)、聚碳酸酯(PC)的SLS成形性能和原型件的力学性能,得出PS和HIPS的激光烧结性能好,但原型件为多孔结构,力学性能差,需要对其进行增强后处理才能达到塑料功能件的要求。本文选择以环氧树脂作为后处理增强树脂,通过调节相容性,渗透性和固化速度,最终获得了强度和精度满足普通塑料功能件要求的增强制件。
采用溶剂沉淀法制备了尼龙12粉末,研究了溶剂、搅拌速度、溶解温度、降温方法及工艺等对粉末几何形貌、粉末粒径及其分布的影响,得出了尼龙12粉末的制备工艺,提出了在降温过程中增加一个成核阶段,同时在体系中加入0.1%的气相二氧化硅作为异相成核剂,从而制备粒度分布窄、近球形的尼龙12粉末,该粉末无需分级就可直接用于SLS成形。尼龙12 SLS成形时预热温度高,为减缓尼龙12粉末在激光烧结过程中的热氧老化,对尼龙12粉末的防老化方法及配方进行了研究,得出在制粉时加入KI/K_3P_2O_6/1098/168四组份复合抗氧剂可有效地减缓尼龙12粉末的热氧老化。
对自制尼龙12粉末的SLS成形工艺进行了研究,虽然尼龙12熔体的结晶收缩是SLS成形时收缩的主要来源,但本文的研究表明翘曲与熔体的粘度相关而不由尼龙12的结晶决定。粉末的几何形貌、粉末的粒径及其分布、老化、成核剂、成形工艺等都会对尼龙12粉末的SLS成形产生影响。但在最理想情况下的预热温度窗口仅1-2℃。通过优化成形工艺,严格控制预温度,利用自制的尼龙12制备了密度为0.96 g/cm~3、拉伸强度为41 MPa、弯曲强度为47.8 Mpa,弯曲模量为1.30 GPa和冲击强度为39.2MPa的尼龙12 SLS制件,满足作为普通塑料功能件的要求。
为提高尼龙12 SLS制件的力学性能,并避免增强填料对激光烧结性能的不利影响,提出了用溶剂沉淀法制备适用于SLS成形的尼龙复合材料粉末的方法,成功制备了尼龙12包覆钛酸钾晶须的复合材料粉末,粉末的几何形貌规则、成形性能良好。复合材料的力学性能较纯尼龙12有大幅提高。其拉伸强度、冲击强度、弯曲强度和弯曲模量分别为68.3MPa、31.2MPa、110.9 MPa和2.833 GPa。
由于直接烧结尼龙12的预热温度高,预热温度窗口窄,成形工艺复杂,难以推广,更不能在国产大台面SLS设备上应用。为此本文从理论上提出并证明了实现降低尼龙12预热温度,扩大预热温度窗口的可行性,实验研究结果表明:加入全反射玻璃微珠可提高工作腔的空气温度,有利于扩大预热温度窗口;增塑剂在一定程度上可降低成形时的预热温度,但增塑剂在高温下容易挥发,使得成形工艺不稳定;用热塑性弹性体改性的尼龙12合金可大幅降低预热温度。本研究用溶剂沉淀法制备合金粉末,并用水和丙酮作为置换溶剂,成功制备了分散良好的合金粉末,其预热温度为123～160℃,预热温度窗口达到37℃。而制件的拉伸强度,模量等较尼龙12有所降低,但冲击强度上升。通过加入成核剂,可使拉伸强度,模量等上升到与纯尼龙12同等水平。运用扫描电镜(SEM),X射线衍射(XRD)对其微观结构进行了分析,提出了合金粉末的激光烧结机理。
深入研究了复杂铸件的SLS熔模铸造工艺。在研究了SLS模料特性的基础上,提出了复杂熔模模料的脱出工艺,并且通过增加排气冒口、脱蜡口和蜡模通气道和辅助浇道等措施,获得了内在质量好、无夹渣的优质铸件。
SLS烧结覆膜砂可以直接成形用于铸造的砂型(芯),特别是对于一些空间曲面或流道十分复杂的铸件,有着显著的优势,但覆膜砂的SLS成形与热塑性聚合物材料的成形有着显著的区别。因此,本文从热固性树脂的角度,运用差热扫描分析(DSC),热失重分析(TG),红外分析(IR)等手段对覆膜砂烧结的物理和化学特征进行了研究。研究表明树脂固化反应分两步进行,激光烧结工艺对树脂的固化有着显著的影响,固化动力学的研究结果显示覆膜砂在水平方向和深度方向的固化差异十分显著,深度固化将产生大量的气体并伴随着玻璃化温度的大幅升高和烧结体的翘曲变形。
研究了复杂砂型(芯)的SLS成形工艺,提出通过提高粘砂深度来防止细小结构被推动,通过优化覆膜砂的性能,降低预热温度和减小输入能量来提高烧结砂型(芯)的精度。为进一步提高砂型(芯)的强度,还对砂型(芯)的后固化工艺进行了研究。针对激光烧结覆膜砂型(芯)透气性差,发气量大的特点,对SLS砂型(芯)的铸造工艺进行了研究,并浇注成功了液压阀体等一批复杂结构的铸件。
本研究的内容满足了国内快速成形对制备塑料功能件的需求,并与铸造技术相结合,显示了在SLS制备复杂铸件方面广泛的应用前景。更重要是通过这些进步推动我国快速成形技术的整体进步,对加快我国SLS技术的发展和应用,扩展快速成形在各个行业中的应用有重大的意义。
Selective laser sintering(SLS) is a rapid prototyping technology.Traditionally,these techniques have been focused on areas where the primary interest was in generating a physical model of a component or system for the purpose of visualization.More recently, the most two important applications for SIS are manufacture of plastic functional components with sufficient mechanical properties and dimensional accuracy,and making complex castings combining with foundry technology.Some foreign companies have developed their own SIS materials.But,in China,as the research began later,the only commercial material is polystyrene(PS).The foreign materials are not only expensive, but not external sell.More important,as the laser sintering technology is so complex and strict,it can not be used in homemade SLS equipment in China.In this paper,aiming at developing the high-quality SLS materials and expanding the applications of SLS, indirect and direct preparation plastic functional components,investment casting of complex cast using SLS mould,laser sintering of pre-coated sand and casting technology are studied.
The laser sintering thermodynamics behavior of thermoplastics is investigated and it is found that laser sintering is a spontaneous process inclining to decrease surface area driven by surface tension.The factor in which affect the surface tension such as particle size,particle geometric pattern etc.,would also affect the laser sintering process.The three sub-model of laser sintering are developed:optical model,thermal model and sintering model.It would provide therory sustaining to develop materials and laser sinering technology.
The laser sintering behavior and mechanical properties of amorphous polymers are studied,containing polystyrene(PS),high-impact polystyrene and polycarbonate(PC). In order to further improve the properties of the prototype,the post-processing—infiltrating with reinforcing resin—must be implemented.The compatibility and permeability of the reinforcing resin and prototype are studied.The ideal curing model is also developed.Through reinforce,the mechanical properties of the prototype are much improved with good precision.
The method of preparing nylon12 powder by using a dissolution-precipitation process was investigated in this paper,the factors that affect the process of solvent,stir speed,dissolution temperature,cooling method and technology to particle geometric pattern,particle size and its distribution were studied.In order to obtain narrow particle size distribution and near spheric shape powder,this paper proposed that adding a nucleation stage at cooling process and using 0.1%SiO_2 as nucleating agent.The powder can be used in SLS need not classify.In the presence of air,nylon 12 is liable to thermo-oxidafive decomposition,the result shows that adding compound antioxidants in the process of prepare nylon 12 powder are effective methods to inhibit oxidation.
The laser sintering behavior of the nylon 12 powder prepared by dissolution-precipitation is studied.The result shows that although the main shrinkage in SLS process is crystal shrinkage,the warp is relative to melt viscosity but not crystallization temperature.The SLS process is greatly affected by particle geometric pattern,particle size and its distribution,oxidation,nucleating agent,and laser sintering technology.At ideal conditions,the sintering window is only 1～2℃.With strictly control of the part bed temperature and laser sintering technology,the SLS nylon 12 prototype with 41 MPa of tensile strength,47.8 MPa of flexural strength,39.2 MPa of impact strength,1.30GPa of flexural modulus have been fabricated.
In order to improve the mechanical properties of SLS nylon 12 parts and avoid the disadvantages of enhancer to laser sintering process.Dissolution-precipitation was introduced to prepare SLS nylon 12/potassium titanium whiskers(PTW) composites powder.The results indicate that when the PTW content of the composites is small,the shape of the powder becomes more regular and the particle diameter distribution becomes narrower.The mechanical properties are greatly improved by PTW.The tensile strength,bending strength,and bending modulus of the composites containing 20 wt% PTW are 68.3 MPa,110.9 MPa,and 2.83 GPa,respectively,which are much higher than those of nylon-12 and glass-filled nylon-12.But it can not be used to prepare nylon 12 nanocomposite of nano-SiO_2 and clay,as the low density and high viscosity of the powder.
As the high pre-heated temperature and narrow sintering window,the SLS process is strict and complex for nylon 12,more important it can not be used in SLS equipment with large working face.This paper proposed and proved that it is possible to achieve low temperature sintering for SLS in theory.The experiments results indicate that:high refractive glass bead can increase air temperature of working chamber,as a result it have a wide sintering window;plasticizing agent can decrease the pre-heated temperature in SLS process,but the plasticizing agent tend to volatilize,as a result,the SLS technology is not stability;,thermoplastic elastomer modified nylon 12 alloy can deeply decrease the part-bed temperature.This paper prepared the alloy powder by dissolution-precipitation. In order to solve the conglobation of the alloy powder,solvent substitute is used.The sintering window is 37℃at the range of 123～160℃.The structure of sintered nylon 12 alloy was characterized by(X-ray diffraction) XRD、and SEM,transmission electron microscopy(TEM),the mechanism of low temperature sintering was proposed.The nylon 12 alloy can be used to fabricate large scale prototype in SLS equipment with large working face.
As high melting temperatures and melting viscosities,it is very difficult to dewax completely for the SLS pattern.In order to avoid carbonization residues,the proper dewaxing technology is developed in this study:(1) the wax is removed first at low temperature,(2) the temperature is then increased to 200-250℃to allow most of the HIPS to flow out and finally(3) baking is performed at a temperature higher than 500℃. Furthermore,examples are used to illustrate the settings of slag extraction risers,riser vents and associated running channels for producing complex parts.
The precoated foundry sands can be used to make the complex sand model and sand core.This paper researched the sintering mechanism of precoated foundry sands as thermosetting.The result indicate that the curing of the resin is notably affected the sintering characteristics.The post curing technology was also studied.At last the eligible casting part is obtained.
The study in this paper,on many aspects,supplies gaps of SLS field in China.The development of plastic function parts and casting complex cast related process for SLS will meet the requirement of the market in making,and will promote the applications and development of SLS.
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