【正文】 段，流道間隙高度與口模圖形尺寸一致。若要實現(xiàn)內(nèi)筋與外壁在口模出口處平均流速相等，只有縮短內(nèi)筋流道的成型段的長度，增加供料段的長度，即要能夠保證內(nèi)筋流道與外壁流道，在形同的平均流速下，各自的總壓力降相等。擁有型芯鑲塊是內(nèi)流道結(jié)構(gòu)模頭的一個特點。所以，目前內(nèi)筋外流道結(jié)構(gòu)已經(jīng)很少采用，只是對于很短小的內(nèi)筋，或者是因為內(nèi)筋的結(jié)構(gòu)空間無法設計內(nèi)流道的地方還需要使用。采用兩塊分流支架板，用螺栓和銷釘分別固定型芯和分流錐。型芯實際上是口模圖形的內(nèi)側(cè)邊界，在擠出方向投影所形成的柱體，高度等于分流支架之后的模頭長度，即口模板，成型板和收縮板加在一起的高度。在錐面與地面之間應該有2~3mm的一小段地面垂直面，以保證分流錐與分流支架流道內(nèi)側(cè)的平順鏈接。內(nèi)筋流道形成于模頭的分流錐和型芯零件的內(nèi)部。模腔流道的入口形狀由分流支架流道所決定，出口形狀與口模圖形一致。所以對于本設計的等壁厚異型材外壁流道，由分流支架流道等角度收縮至平直流道。收縮角設計只是對收縮段流道而言，在功能塊處，由于結(jié)構(gòu)復雜，往往涉及多個面的收縮，不是一個收縮角能決定的。單獨設置一塊收縮板的主要目的也是如此。這對保證口模出口的料流平穩(wěn)是很重要的，但是，這不是決定平直段流道長度的全部因素。在保證擠出制品所需要的塑化度的前提下，縮短模頭成型段平直流道長度應該是有利的，因為這樣做減小了模頭壓力降，減少了擠出機的動力消耗；而且可以縮短模塑料在高溫下的停留時間。第8章 模頭模板設計和強度校核 模頭模板的設計擠出機頭都是由多塊模板疊接形成的。為保證模板間的密合，安裝空間允許的情況下，需要盡量選用較大的預緊力，或多布置連接點。（1）分流筋強度的校核分流筋必須有足夠的強度支撐型芯和分流錐，不出現(xiàn)變形和損傷現(xiàn)象，然而從減少對熔體料流的粘彈性擾動方面考慮，希望分流筋盡可能的細小，數(shù)量上也盡可能的少一些。也要注意過薄過長的分流筋有可能會因為意外出現(xiàn)失穩(wěn)現(xiàn)象。實際上，定型模對于塑料異型材擠出成型起著十分重要的作用，它最終決定著異型材的尺寸精度，這包括異型材的尺寸、形狀、公差和表觀質(zhì)量，并且直接影響異型材的落錘沖擊，加熱后的尺寸變化等物理性能。其目的是使從最終定型段出來的制品在橫截面內(nèi)外各處溫度達到完全冷卻(14℃左右)。③真空吸附足夠且均勻。f——系數(shù)，取16～30，f與型材、壁厚、質(zhì)量、真空度有關M——定型模真空度，～ 冷卻水路設計定型模冷卻水路設計應注意：水孔孔壁與真空槽孔的應大于3mm，以防止距離過薄，加工時引起水氣相穿的現(xiàn)象。 定型模型腔尺寸計算③斷面寬度超過40mm，給反彈弧。進而，對生產(chǎn)窗框異型材的原材料配方進行了設計；2. 機頭設計從現(xiàn)行市面上廣泛生產(chǎn)和使用的80型推拉框的橫截面外形尺寸出發(fā)進行分析；3. 設計從根據(jù)產(chǎn)品截面形狀和尺寸來確定口模板尺寸的突破口來入手，然后是對收縮板，分流錐支架板的設計；4. 根據(jù)分流支架板的形狀，來進行對型芯和分流錐的外形和尺寸的設計，其中對內(nèi)筋的流道設計是設計中的難點，而對于功能塊則多采用經(jīng)驗數(shù)據(jù)；5. 對定型模熱力學長度的計算和定型模腔的設計；本次設計也存在不足之處，例如：在處理功能塊的流變學分析中沒有現(xiàn)成的數(shù)學模型，不能對其尺寸進行準確推導計算，只能采用經(jīng)驗數(shù)據(jù)，這樣就使得最后的修模工作量變得非常大；此外，關于模具設計的成本估測，由于閱歷資質(zhì)的短缺，沒有能力完成；還有就是模具的使用年限，模具在使用過程中可能出現(xiàn)的問題及針對其問題的具體修模方法，沒有做出明確的預防方案等等。本設計的題目和任務是由周思凱老師具體下達的，在設計過程中也得到了周老師的悉心指導。本設計能夠順利完成，同樣離不開諸位專業(yè)老師的教育培養(yǎng)和身邊同學的熱情幫助。Mur,M and can reach the end points of embrittlement and total disintegration.The degradation of polymers exposed to UV, often described as photodegradation and frequently identified as photooxidation, can follow various routes. By absorbing UV radiation directly, a polymer molecule can reach a highenergy excited state where it bees unstable. If the excess energy can be dissipated in a fashion that does not affect the molecule by making it phosphoresce or fluoresce, or by converting the energy to heat that can be carried away, or by transferring the energy to another molecule, photochemical reaction does not started and thus, polymer degradation will not happen. However, such actions occur only rarely, since most polymers cannot dispose of the excitation energy without undergoing a chemical reaction that sets off a degradative process.In theory, many pure polymers should not absorb UV radiation, and thus, not be subject to photodegradation. However, in practice the most polymers contain impurities such as carbonyl or carboxy groups or hydroperoxides that readily absorb radiation in the 290–400 nm range causing them to break down. Thus, generating sites within the polymer structure where chemical reactions can be initiated and propagated by free radicals. The active groups may be unavoidably present as a result of reactions that occur during polymerization. Similarly, metallic ions are present in most polymers as residues from polymerization catalysts, or as constituents of pounding additives such as heat stabilizers, antioxidants, colorants, fillers and others. The metal ions are highly receptive to the absorption of UV radiation, and are efficient in transferring the absorbed energy to the polymer molecules around them, thus, they act as photosensitizers and can promote degradation at the same time that they perform their desired functions.Another contributor to photodegradation of polymers is oxygen, which helps any free radicals that may be liberated by the UV to initiate and propagate oxidation of the polymer, hence, the term photooxidation.Polyvinyl chloride suffers from poor heat stability. Its degradation occurs by autocatalytic dehydrochlorination initiated at the labile sites in the polymer chains. This leads to severe discoloration and loss of mechanical properties. The dehydrochlorination most probably proceeds by a chain mechanism involving radical intermediates. Various defect sites in PVC are branching.Inorganic and organic thermal stabilizers are monly added to protect the polymer from heat degradation. Among the most widely used ultraviolet stabilizers is titanium dioxide pigment. Filling a polyvinyl chloride position with this pigment substantially reduces the effective depth of penetration of ultraviolet light into the surface of an article formed from such a position.Mohamed et al. pointed out that barbituric acid and thiobarbituric acid are nontoxic organics, thermally stable materials of high melting point. Both contain active methylene groups, and can act as Hdonor through their enolic hydrogen groups, which can intervene with the radical species derived from the thermal degradation of PVC. They investigated the possibility of using barbituric acid and its thioanalogue as thermal stabilizers for rigid PVC.The effective stabilization often requires a bination of antioxidant system in which plementary overlap of different mechanistic pathways involved. This act often referred to as synergism, is the motivation for the use of admixing position of dibutyltin maleate and trinitro and its ester homologues. The stabilization agents of dibutyltin maleate and trinitro esters could retard somewhat the photodegradation of PVC. It is hoped that the total stabilizing effect of this admixed system should be greater than the sum of the individual effects when PVC is subjected to an environment where the effects of heat and UV are bined. Turoti et al. investigated the effect of the stabilizing action of admixed mixtures of dibutyltin maleate and trinitro and its ester homologues on polyvinyl chloride exposed to natural atmosphere. In their study, the degradation and stabilization reactions were monitored by color formation, tensile strength and elongation at break, reduced viscosity as well as determination of time to embrittlement. It is observed that the stabilized PVC sample has an effective reduc