Characteristics and injection molding process of transparent materials commonly used in plastics

Because of the light weight, good toughness and easy to form. The cost is low. In modern industry and daily products, more and more plastics are used instead of glass, especially in optical instruments and packaging industries, and it has developed rapidly. But because of the requirements of its transparency, wear-resistant parts to be high, impact toughness is better, back to the plastic composition, injection molding process, equipment. 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. At present, the transparent plastics commonly used in the market include polymethyl methacrylate (commonly known as the acrylic or plexiglass, code PMMA), polycarbonate (Code PC). Polyethylene terephthalate (PET), transparent nylon. Among them, 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

Transparent plastics, due to high transmittance, must require strict surface quality requirements of plastic products, and there should be no spots, pores, and whitening. Fog halo, black spot, discoloration, poor luster and other defects, so in the whole injection process on raw materials, equipment. The design of mould, even product, should pay attention to and put forward strict even special requirements. Secondly, 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 apintejinusted 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. Especially, the raw material contains water, which will cause the raw material to deteriorate after heating, so it must be dried, 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. When there is no screw cleaner, 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, such as PC, 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 (including product design)

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.

a) The wall thickness shall be as uniform as possible and the release angle shall be large enough;

b) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge, especially PC products must not have gaps;

c) Gate. The runner should be as wide and short as possible, and the gate position should be set according to the shrinkage and condensation process. If necessary, the cold charging well should be added; d) The surface of the mould shall be smooth and low in roughness (preferably less than 0.8);

e) Exhaust holes. The tank must be sufficient to discharge the air and the gas in the melt in time;

f) Except pet, the wall thickness should not be too thin, generally not less than 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.

a) Special screw and injection molding machine with independent temperature control nozzle shall be selected;

b) The injection temperature should be high injection humidity on the premise that the plastic resin does not decompose;

c) Injection pressure: 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;

d) Injection speed: in the case of satisfying the filling of the mold, it is generally advisable to lower, the best can be used slow, fast, slow, multi-stage injection;

e) 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) Screw speed and back pressure: on the premise of meeting the plasticizing quality, it is necessary to minimize the possibility of reducing;

g) Mold temperature: the cooling quality of the products has a great impact on the quality, so the mold temperature must be able to accurately control its process, if possible, the mold temperature should be higher.

（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 hours; PC should be in clean air, glycerin. Liquid wax and other heating 110-135 ℃, time according to the product, the maximum need more than 10 hours. 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. Therefore, 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. Therefore, 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. Therefore, 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. In order to prevent the internal stress, the pressure holding time should be as short as possible. The shrinkage rate is large and the size is stable, but the internal stress of the product is large and easy to crack. Therefore, 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. When the injection speed is low, the defects such as ripple are easy to occur at the gate. The temperature of the radial nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.

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 ℃ for 3-5 minutes, then cool naturally.

（2） Bubble: water gas and other gases mainly in the resin cannot be discharged (during the condensation process of the mold) or “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, on the other hand, 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.

About PTJMould Industrial Company Limited

PTJmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.