بسبب خفة الوزن, صلابة جيدة وسهل التشكيل. التكلفة منخفضة. في الصناعة الحديثة والمنتجات اليومية, يتم استخدام المزيد والمزيد من البلاستيك بدلاً من الزجاج, خاصة في صناعة الأدوات البصرية والتعبئة والتغليف, وقد تطورت بسرعة. But because of the requirements of its transparency, wear-resistant parts to be high, impact toughness is better, back to the plastic composition, عملية القولبة بالحقن, معدات. A lot of work should be done to ensure that the plastic used in place of glass (hereinafter referred to as transparent plastic) has good surface quality, so as to meet the requirements of use. يجب الانتباه إلى هذه النقاط عند اختبار موردي القوالب, the transparent plastics commonly used in the market include polymethyl methacrylate (commonly known as the acrylic or plexiglass, code PMMA), البولي (Code PC). Polyethylene terephthalate (حيث تلتقي التدفقات المتعددة أو عندما يحدث النفث أثناء ملء القالب), transparent nylon. بينهم, PMMA is the most common contact with as (acrylic styrene copolymer), polysulfone (PSF). PC and pet three kinds of plastics, because of the limited amplitude, the following three plastics are taken as an example to discuss the characteristics and injection molding process of transparent plastics.
1、 Properties of transparent plastics
Transparent plastics must have high transparency first, then have certain strength and wear resistance, can resist impact, heat-resistant parts are better, chemical resistance is better, water absorption is small, only in this way can they meet the requirements of transparency and remain unchanged for a long time. The following table is listed to compare the performance of PMMA, PC and pet.
(1) Because of the variety, this is just the average value, and the actual data of different varieties are different.
(2) PET data (mechanical aspect) is the stretched data.
2、 Common problems in the injection process of transparent plastics
بلاستيك شفاف, due to high transmittance, must require strict surface quality requirements of plastic products, and there should be no spots, المسام, and whitening. Fog halo, بقعة سوداء, تلون, poor luster and other defects, so in the whole injection process on raw materials, معدات. The design of mould, even product, should pay attention to and put forward strict even special requirements. ثانيا, because transparent plastics are mostly melting point high and poor fluidity, in order to ensure the surface quality of products, the process parameters such as high temperature, injection pressure and injection speed are often adjusted to make plastic injection not only fill the mold, but also do not produce internal stress, which will cause product deformation and cracking.
Here is the preparation of raw materials. The article discusses the requirements of equipment and mould, injection molding process and raw material treatment of products.
（1） The preparation and drying of raw materials may affect the transparency of the products due to any impurity in the plastic, so it is stored and transported. During the process of feeding, we must pay attention to sealing to ensure the raw materials are clean. خصوصاً, the raw material contains water, which will cause the raw material to deteriorate after heating, لذلك يجب تجفيفها, and in the injection molding, the feeding must use the drying hopper. It is also important to note that during drying, the air input should be filtered and dehumidified to ensure that the raw materials will not be polluted.
（2） Cleaning of barrel, screw and its accessories
In order to prevent raw material pollution and the existence of old materials or impurities in the screw and accessory depression, especially resin with poor thermal stability, screw cleaning agent shall be used to clean all parts before and after the machine is shut down so that they can not be stuck with impurities. عندما لا يكون هناك منظف لولبي, PE, PS and other resins can be used to clean the screw. When the temporary shutdown is in order to prevent the long stay time of raw materials at high temperature and cause the degradation, the temperature of dryer and barrel should be reduced, مثل جهاز الكمبيوتر, PMMA and other barrel temperatures should be reduced to below 160 ℃（ Hopper temperature shall be reduced to below 100 ℃ for PC)
（3） Problems to be paid attention to in mold design (بما في ذلك تصميم المنتج)
In order to prevent the backflow from flowing smoothly, or the uneven cooling, the plastic forming is not good, surface defects and deterioration occur, the following points should be paid attention to in the design of the mold.
أ) The wall thickness shall be as uniform as possible and the release angle shall be large enough;
ب) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge, especially PC products must not have gaps;
ج) Gate. The runner should be as wide and short as possible, ويجب ضبط موضع البوابة وفقًا لعملية الانكماش والتكثيف. اذا كان ضروري, the cold charging well should be added; د) The surface of the mould shall be smooth and low in roughness (preferably less than 0.8);
ه) Exhaust holes. يجب أن يكون الخزان كافيًا لتصريف الهواء والغاز في الذوبان في الوقت المناسب;
F) Except pet, يجب ألا يكون سمك الجدار رقيقًا جدًا, عموما لا تقل عن LMM.
（4） Problems in injection molding process (including requirements of injection molding machine)
In order to reduce the internal stress and surface quality defects, the following problems should be paid attention to in the injection molding process.
أ) Special screw and injection molding machine with independent temperature control nozzle shall be selected;
ب) The injection temperature should be high injection humidity on the premise that the plastic resin does not decompose;
ج) ضغط الحقن: generally high to overcome the defect of high viscosity of molten material, but too high pressure will cause internal stress, which will cause demoulding due to difficulty and deformation;
د) سرعة الحقن: in the case of satisfying the filling of the mold, it is generally advisable to lower, the best can be used slow, سريع, slow, multi-stage injection;
ه) Holding time and forming period: when the product is filled with mould, no depression or bubble is produced; It is advisable to try to reduce the retention time of the molten material in the barrel as soon as possible;
F) سرعة المسمار والضغط الخلفي: on the premise of meeting the plasticizing quality, it is necessary to minimize the possibility of reducing;
g) درجة حرارة العفن: جودة تبريد المنتجات لها تأثير كبير على الجودة, لذلك يجب أن تكون درجة حرارة القالب قادرة على التحكم بدقة في عمليتها, اذا كان ممكنا, يجب أن تكون درجة حرارة القالب أعلى.
（5） Other issues
In order to prevent the deterioration of the surface quality, the mold release agent is used as little as possible in general injection molding; When using recycled materials, it shall not be more than 20%.
For the products except pet, the products should be treated after treatment to eliminate the internal stress. PMMA should be dried by 70-80t hot air circulation for 4 ساعات; يجب أن يكون الكمبيوتر في الهواء النظيف, جلسيرين. Liquid wax and other heating 110-135 ℃, الوقت حسب المنتج, the maximum need more than 10 ساعات. Pet must be biaxial tensile process to obtain good mechanical properties.
3、 Injection molding technology of transparent plastics
（1） Process characteristics of transparent plastics: in addition to the above common problems, transparent plastics also have some technological characteristics, and are now described as follows:
1. The process characteristic of PMMA is high viscosity and poor fluidity. لذلك, it is necessary to inject the injection with high material temperature and high injection pressure. The effect of injection temperature is greater than the injection pressure, but the injection pressure is improved, which is conducive to improving the shrinkage of the product. The injection temperature range is wide, the melting temperature is 160 ℃, and the decomposition temperature is 270 ℃, so the temperature of the material is wide and the process is good. لذلك, the fluidity can be improved from injection temperature. The impact is poor, the wear resistance is not good, easy to scratch, easy to crack, so the mold temperature should be improved, the condensation process should be improved to overcome these defects.
2. The process characteristics of PC
PC has high viscosity, high melt temperature and poor fluidity. لذلك, injection molding must be carried out at a higher temperature (between 270-320t). The temperature regulation range is relatively narrow and the process property is not as good as PMMA. The injection pressure has little influence on the fluidity, but due to the high viscosity, the injection pressure should be larger. من أجل منع التوتر الداخلي, يجب أن يكون وقت الاحتفاظ بالضغط قصيرًا قدر الإمكان. معدل الانكماش كبير والحجم ثابت, لكن الضغط الداخلي للمنتج كبير وسهل التشقق. لذلك, it is advisable to improve the fluidity by increasing the temperature of the mold rather than the pressure, and reducing the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. عندما تكون سرعة الحقن منخفضة, the defects such as ripple are easy to occur at the gate. يجب التحكم في درجة حرارة الفوهة الشعاعية بشكل منفصل, يجب أن تكون درجة حرارة القالب مرتفعة, ويجب أن تكون مقاومة العداء والبوابة صغيرة.
3. The process characteristics of pet
Pet has high forming temperature and narrow material temperature regulation range (260-300 ℃), but after melting, it has good fluidity, so it is poor in process, and it is often necessary to add a flow preventing device in nozzle. Mechanical strength and performance are not high after injection, and the performance can be improved only through the stretching process and modification. The accurate control of die temperature is to prevent warping. The important return element of deformation is recommended to adopt hot runner mould. The temperature of the mould is high, otherwise the surface luster is poor and the demoulding is difficult.
4、 Defects and solutions of transparent plastic parts
Due to the space relationship, only the defects affecting the transparency of the products are discussed. Please refer to the product specification or other materials of our company for other defects?. There are some defects as follows:
（1） Silver grain: the stress produced in vertical direction, which is influenced by the anisotropy of internal stress during mold filling and condensation, causes the resin to flow upward orientation, but produces flash silk grain due to different refractive index compared with non flow orientation. When it expands, cracks may occur in the product. In addition to attention in injection molding process and mold (annealing treatment is preferred for products. If the PC can be heated to 160 ℃ لـ 3-5 الدقائق, then cool naturally.
（2） Bubble: water gas and other gases mainly in the resin cannot be discharged (during the condensation process of the mold) أو “vacuum bubble” is formed due to insufficient mold filling and too fast condensation on the condensation surface.
（3） Poor surface gloss: mainly due to the large roughness of the mold, على الجانب الآخر, the condensation is too early, so that the resin can not copy the mold surface state, all of which make the surface produce cattle small uneven, and make the product lose luster.
（4） Vibration pattern: refers to the dense ripple formed from the center of the straight gate. The reason is that the front end material has condensed in the cavity due to the excessive viscosity of the melt, and then the material breaks through the condensation surface, which causes the surface to appear the vibration pattern.
（5） White. Fog halo: mainly caused by dust falling into the raw material or the water content of raw materials is too large in the air.
（6） White smoke. Black spot: it is mainly formed by the decomposition or deterioration of resin in the cylinder due to local overheating. In order to make clear the measures taken to overcome these deficiencies.
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اتصال:
شخص: جيمس يوان
شركة: شركة JasonMould الصناعية المحدودة
يضيف: قرية لونجانج,مدينة LongXi,مقاطعة BoLuo,مدينة هويتشو,مقاطعة غانج دونج, الصين
هاتف: 86-752-6682869
بريد إلكتروني: [email protected]
موقع: https://www.jasonmolding.com/custom-injection-molding/
