How to improve the transparency of plastic products

How to improve the transparency of plastic products?

Because of its light weight, good toughness and easy forming. With the advantages of low cost, more and more plastics are used to replace glass in modern industry and daily products, especially in optical instruments and packaging industry. However, due to the requirements of better transparency, wear resistance and impact toughness, the composition of plastic, the whole process of injection molding process, equipment. A lot of work should be done to ensure that the plastic used to replace 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 are polymethylmethacrylate (commonly known as acrylic or plexiglass, code PMMA) and polycarbonate (Code PC). Polyethylene terephthalate (PET), transparent nylon. As (acrylonitrile styrene copolymer), polysulfone (PSF) and so on, PMMA is the most exposed one. Due to the limited space of PC and pet, we will take these three plastics as an example to discuss the characteristics and injection process of transparent plastics.
1、 Properties of transparent plastics
Transparent plastics must first have high transparency, then have certain strength and wear resistance, can resist impact, heat-resistant parts are better, chemical resistance is better, water absorption is smaller, only in this way can they meet the requirements of transparency and remain unchanged for a long time. PC is an ideal choice, but due to its expensive raw materials and difficult injection molding process, PMMA is still the main choice (for products with general requirements), while PPT is mostly used in packaging and containers because it needs to be stretched to get good mechanical properties.
2、 Common problems in the injection process of transparent plastics
Due to the high transmittance of transparent plastics, the surface quality of plastic products is required to be strict, without any stripes, pores and whitening. Fog halo, black spot, discoloration, poor gloss and other defects, so in the whole injection process of raw materials, equipment. Mold, even product design, should pay attention to and put forward strict or even special requirements.
Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of products, it is often necessary to adjust the process parameters, such as barrel temperature, injection pressure and injection speed, so that the injection of plastics can not only fill the mold, but also produce internal stress and cause deformation and cracking of products.
In terms of equipment and mold requirements, injection molding process and raw material handling of products, the following points should be noted:
dry
The preparation and drying of raw materials may affect the transparency of the product due to any impurity in the plastic. Therefore, they are stored and transported.
In the process of feeding, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw material contains moisture, which will cause deterioration of the raw material after heating, so it must be dried, and when injection molding, the feeding must use a drying hopper. It should also be noted that in the drying process, the input air should be filtered and dehumidified to ensure that the raw materials will not be polluted.
Cleaning of barrel, screw and its accessories
In order to prevent the pollution of raw materials and the existence of old materials or impurities in the screw and its accessories, especially the resin with poor thermal stability, the screw cleaning agent shall be used to clean all parts before use and after shutdown, so that no impurities can be adhered to them. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.
When the machine is shut down temporarily, in order to prevent the raw materials from staying at high temperature for a long time, which will lead to the degradation, the temperature of the dryer and the barrel should be reduced, such as the temperature of PC, PMMA and other barrels should be reduced to below 160 ℃. (hopper temperature shall be below 100 ℃ for PC)
Problems in mold design (including product design)
In order to prevent backflow or uneven cooling from causing poor plastic forming, surface defects and deterioration;
Generally, the following points should be paid attention to in the mold design:
The wall thickness shall be uniform as far as possible, and the angle of demoulding shall be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
Gate. The flow passage shall be as wide and short as possible, and the gate position shall be set according to the contraction and condensation process, and the cold charging well shall be added if necessary;
The mould surface should be smooth and low roughness (preferably less than 0.8);
Vent. The tank must be sufficient to discharge the air and the gas in the melt in time;
Except pet, the wall thickness should not be too thin, generally not less than LMM.
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:
Special screw and injection molding machine with separate temperature control nozzle shall be selected;
The injection temperature should be higher when the plastic resin does not decompose;
Injection pressure: generally high, to overcome the defects of high viscosity of the melt, but too high pressure will produce internal stress, resulting in the difficulty and deformation of demoulding;
Injection speed: in the case of mold filling, it should be generally low, and it is better to adopt slow fast slow multi-stage injection;
Pressure holding time and forming cycle: under the condition that the product is filled with mould and there is no depression or bubble, it should be as short as possible to minimize the residence time of molten material in the barrel;
Screw speed and back pressure: on the premise of meeting the plasticizing quality, they should be kept as low as possible to prevent the possibility of falling off;

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, and the mold temperature should be higher if possible.

Other issues
In order to prevent the deterioration of the upper surface quality, the mold release agent shall be used as less as possible in general injection molding; when the recycled material is used, it shall not be more than 20%.
Except for pet, all products shall be post treated to eliminate internal stress. PMMA shall be dried for 4 hours in hot air circulation at 70-80 ℃; PC shall be dried in clean air and glycerin. The heating temperature of liquid paraffin is 110-135 ℃, the time depends on the product, and the maximum time is more than 10 hours. Pet must go through the process of biaxial drawing to get good mechanical properties.
3、 Injection molding technology of transparent plastics
Process characteristics of transparent plastics
In addition to the above common problems, transparent plastics also have some process characteristics, which are described as follows:
1. Process characteristics of PMMA
PMMA has high viscosity and poor fluidity, so it must be injected with high material temperature and high injection pressure, in which the effect of injection temperature is greater than that of injection pressure, but the increase of injection pressure is conducive to the improvement of product shrinkage.
The injection temperature range is wide, the melting temperature is 160 ℃, and the decomposition temperature is 270 ℃, so the material temperature adjustment range is wide and the processability is good. Therefore, to improve the fluidity, we can start from the injection temperature. Poor impact, poor wear resistance, easy to scratch, easy to crack, so we should improve the mold temperature, improve the condensation process, to overcome these defects.
2. Process characteristics of PC
PC has high viscosity, high melt temperature and poor fluidity, so it must be injected at a high temperature (between 270-320 ℃). The range of material temperature regulation is relatively narrow, and the processability is inferior to PMMA. The injection pressure has little effect on the fluidity, but because of the high viscosity, the injection pressure still needs to be higher. 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, so it is better to improve the fluidity by increasing the temperature rather than the pressure, and reduce the possibility of cracking by increasing the temperature of the mold, improving the structure of the mold and post-treatment. When the injection speed is low, the gate is prone to corrugation and other defects. 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. Process characteristics of pet
Pet has a high molding temperature and a narrow range of material temperature adjustment (260-300 ℃), but after melting, it has a good fluidity, so its processability is poor, and it is often necessary to add a delay prevention device in the nozzle. The mechanical strength and properties are not high after injection, and the properties can only be improved by drawing process and modification.
The accurate control of mold temperature is to prevent warpage. Therefore, it is suggested to use hot runner die. Mold temperature should be high, otherwise it will cause poor surface gloss and difficulty of demoulding.
4、 Defects and solutions of transparent plastic parts
The defects are as follows:
Craze
Due to the influence of anisotropy of internal stress during filling and condensation, the stress generated in the vertical direction makes the resin flow upward oriented, while the refractive index of the resin is different from that of the resin in the non flow direction, resulting in the flash filament. When it expands, the product may crack. In addition to the injection molding process and mold attention, the best products for annealing. If the PC material can be heated to above 160 ℃ for 3-5 minutes, and then cooled naturally.
Bubble
The main reason is that the water and other gases in the resin can't be discharged, (in the process of mold condensation) or because of the insufficient filling of the mold, the condensation surface condenses too fast and forms a vacuum bubble.
Poor surface gloss
On the other hand, the resin can't copy the surface state of the mold due to the large roughness of the mold and the premature condensation. All of these make the surface produce small unevenness and make the product lose luster.
Shockwave
It refers to the dense corrugation formed from the center of the straight gate. The reason is that the front-end material has condensed in the mold cavity due to the excessive viscosity of the melt, and then the material breaks through the condensed surface, resulting in the surface shaking.
White haze
It is mainly caused by the dust falling into the raw material in the air or the moisture content of the raw material is too large.
White smoke and black spot
It is mainly caused by the decomposition or deterioration of the resin in the barrel due to the local overheating of the plastic in the barrel.