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 spots, discoloration, poor gloss and other defects, so in the entire injection molding process of raw materials, equipment.
Strict and even special requirements should be paid attention to and put forward in the design of molds and even products. Secondly, due to the high melting point and poor fluidity of transparent plastics, in order to ensure the surface quality of products, it is often necessary to adjust the process parameters such as high temperature, injection pressure, injection speed, etc., so that the injection of plastic can not only fill the mold, but also will not produce internal stress, which will cause deformation and cracking of the product.
The following is the preparation of raw materials. This paper discusses the matters needing attention in the aspects of equipment and mold requirements, injection molding process and raw material treatment of products.
Preparation and drying of raw materials
As any impurity in the plastic may affect the transparency of the product, therefore, and storage, transportation. 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 during injection molding, the drying hopper must be used for feeding. In order to ensure that the air will not be polluted during the drying process.
Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and the existence of old materials or impurities in the screw and accessories depression, especially the existence of resin with poor thermal stability, screw cleaner should be used to clean all parts before use and after shutdown, so that no impurities can be stuck. 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, the temperature of dryer and barrel should be reduced, such as PC, PMMA, etc., to below 160 ℃. (hopper temperature should be lower than 100 ℃ for PC)
Mold design (including product design)
In order to prevent poor backflow, surface defects and deterioration caused by poor backflow or uneven cooling, the following points should be paid attention to in mold design:
The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge, especially PC products must not have a gap;
Gate. The runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, the cold material well should be added;
Die surface should be smooth and smooth, with low roughness (preferably less than 0.8);
Vent hole. The tank must be sufficient to discharge air and gas in melt in time;
Except for pet, the wall thickness should not be too thin, generally not less than 1 mm.
Injection molding technology and 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
The injection molding machine with special screw and separate temperature control nozzle should be selected;
On the premise that the plastic resin does not decompose, the injection temperature should be higher;
However, due to the high melt force and high pressure, it is difficult to overcome the internal defects;
Injection speed: generally, it should be low when the mold filling is satisfied. It is better to adopt multi-stage injection of slow fast slow;
Holding time and molding cycle: under the condition that the product filling is satisfied and there is no depression and bubble, it should be as short as possible to reduce the residence time of molten materials in the barrel;
Screw speed and back pressure: on the premise of meeting the plasticizing quality, it should be 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 the process, if possible, the mold temperature should be higher.
Other issues
In order to prevent the deterioration of the upper surface quality, the release agent should be used as little as possible during injection molding; when the recycled material is used, it should not be more than 20%.
All products except pet should be treated to eliminate internal stress. PMMA should be dried in 70-80t hot air circulation for 4 hours; PC should be dried in clean air and glycerin. The heating time of liquid paraffin wax at 110-135 ℃ depends on the product, and the maximum time is more than 10 hours. In order to obtain good mechanical properties, pet must go through biaxial stretching processWhat problems should be paid attention to in transparent plastic injection molding?
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 spots, discoloration, poor gloss and other defects, so in the entire injection molding process of raw materials, equipment.
Strict and even special requirements should be paid attention to and put forward in the design of molds and even products. Secondly, due to the high melting point and poor fluidity of transparent plastics, in order to ensure the surface quality of products, it is often necessary to adjust the process parameters such as high temperature, injection pressure, injection speed, etc., so that the injection of plastic can not only fill the mold, but also will not produce internal stress, which will cause deformation and cracking of the product.
The following is the preparation of raw materials. This paper discusses the matters needing attention in the aspects of equipment and mold requirements, injection molding process and raw material treatment of products.
Preparation and drying of raw materials
As any impurity in the plastic may affect the transparency of the product, therefore, and storage, transportation. 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 during injection molding, the drying hopper must be used for feeding. In order to ensure that the air will not be polluted during the drying process.
Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and the existence of old materials or impurities in the screw and accessories depression, especially the existence of resin with poor thermal stability, screw cleaner should be used to clean all parts before use and after shutdown, so that no impurities can be stuck. 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, the temperature of dryer and barrel should be reduced, such as PC, PMMA, etc., to below 160 ℃. (hopper temperature should be lower than 100 ℃ for PC)
Mold design (including product design)
In order to prevent poor backflow, surface defects and deterioration caused by poor backflow or uneven cooling, the following points should be paid attention to in mold design:
The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge, especially PC products must not have a gap;
Gate. The runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, the cold material well should be added;
Die surface should be smooth and smooth, with low roughness (preferably less than 0.8);
Vent hole. The tank must be sufficient to discharge air and gas in melt in time;
Except for pet, the wall thickness should not be too thin, generally not less than 1 mm.
Injection molding technology and 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
The injection molding machine with special screw and separate temperature control nozzle should be selected;
On the premise that the plastic resin does not decompose, the injection temperature should be higher;
However, due to the high melt force and high pressure, it is difficult to overcome the internal defects;
Injection speed: generally, it should be low when the mold filling is satisfied. It is better to adopt multi-stage injection of slow fast slow;
Holding time and molding cycle: under the condition that the product filling is satisfied and there is no depression and bubble, it should be as short as possible to reduce the residence time of molten materials in the barrel;
Screw speed and back pressure: on the premise of meeting the plasticizing quality, it should be 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 the process, if possible, the mold temperature should be higher.
Other issues
In order to prevent the deterioration of the upper surface quality, the release agent should be used as little as possible during injection molding; when the recycled material is used, it should not be more than 20%.
All products except pet should be treated to eliminate internal stress. PMMA should be dried in 70-80t hot air circulation for 4 hours; PC should be dried in clean air and glycerin. The heating time of liquid paraffin wax at 110-135 ℃ depends on the product, and the maximum time is more than 10 hours. In order to obtain good mechanical properties, pet must go through biaxial stretching process
