Due to the advantages of light weight, good toughness, easy molding and high cost, more and more plastics are used to replace glass in modern industry and daily products, especially in the optical instruments and packaging industry, which is developing especially rapidly. However, because it requires good transparency, high wear resistance and good impact resistance, a lot of tasks have to be done in the composition, process, configuration, and mold of plastic throughout the injection molding process to ensure that these are used instead of glass. This plastic (hereinafter referred to as transparent plastic) has excellent surface quality to meet the requirements of use. Compared with reinforced nylon.
The transparent plastics currently used in the market are polymethyl methacrylate (PMMA, commonly known as acrylic or plexiglass), polycarbonate (PC), polyethylene terephthalate (PET), and transparent. Nylon, acrylonitrile-styrene copolymer (AS), polysulfone (PSF), etc., of which we are most exposed to PMMA, PC and PET plastics. Let's take these three plastics as examples to talk about transparent plastics. Features and injection molding process.
I. Performance of transparent plastics
Transparent plastics should first of all have high transparency. They mainly have certain strength and wear resistance, can withstand impact, have good heat resistance, and strengthen nylon. It has excellent chemical resistance and low water absorption. Only in this way can it meet the requirements of transparency and remain unchanged for a long time.
Injection molding process of transparent plastic. However, the product has high internal stress and is prone to cracking. Therefore, it is better to increase the temperature rather than pressure to improve the flow, and to raise the mold temperature and improve the mold structure and post-treatment to reduce the possibility of cracking. When the injection speed is low, the gate is prone to ripples and other defects, and the temperature of the radiation nozzle should be controlled separately. When the mold temperature is high, the runner and gate resistance is small.
Second, the common problems that should be noted in the process of transparent plastic injection molding
Due to the high transmittance of transparent plastic, the surface quality of plastic products is strictly required, and there should not be defects such as streaks, porosity, whitening, halo, black spots, discoloration and poor gloss. Configuration, mold, and even product design must pay great attention and put forward strict or even special requirements.
Secondly, due to the high melting point and poor flowability of transparent plastics, process parameters such as higher temperature, injection pressure and injection speed often have to be fine-tuned in order to ensure the surface quality of the product so that the plastic can be filled to the mold and reinforced nylon. There is no internal stress that causes deformation and cracking of the product. In the following, from the raw material preparation, configuration and mold requirements, injection molding process and raw material handling of the product, transparent nylon, talk about the matters that should be noted.
(1) Preparation and drying of raw materials Since any impurities contained in the plastic may affect the transparency of the product. Therefore, in the process of stacking, transportation and feeding, pay attention to sealing and ensure the cleanliness of raw materials. Especially, the raw materials contain moisture, which will lead to deterioration of the raw materials and enhance nylon after heating.
(2) Clean the barrel, screw and its accessories to prevent raw material purification and the presence of old materials or impurities in the grooves of the screw and accessories.
In particular, the presence of poorly heat-stable resin. Therefore, before use and after shutdown, clean the parts with a screw cleaner so that they do not adhere to impurities. When there is no screw cleaner, the screw can be cleaned with PE, PS and other resins. When the machine is temporarily stopped, in order to prevent the raw material from stopping at high temperature for a long time, causing the plastic to drop. The temperature of the dryer and the barrel should be reduced. For example, the temperature of the barrel of PC and PMMA etc. should be lowered to below 160°C. (The hopper temperature of PC should be reduced to below 100℃)
(3) Issues to be noted in mold design (including product design) In order to prevent the occurrence of poor reflow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, the following points should generally be noted in the mold design.
a) The wall thickness should be as uniform as possible. The draft angle should be large enough.
b) Transition should be gradual. Smooth transition to prevent sharp corners. Produce sharp edges, especially PC products should not have gaps.
c) Gates. The runner should be as wide and thick as possible, the gate position should be set according to the shrinkage condensation process, and cold wells should be added if necessary.
d) The surface of the mold should be smooth and nylon 66. low roughness (preferably below 0.8).
e) Ventilation holes. The tank must have enough space to discharge air and gas from the melt.
f) Except for PET, the wall thickness should not be too thin, generally not less than lmm.
(4) Issues to be noted in the injection process (including the requirements for injection molding machine) In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection process.
a) Special screws and injection machines with independent temperature-controlled nozzles should be used.
b) Injection temperature should be higher than the temperature at which the plastic resin does not decompose.
c) Injection pressure: generally higher to overcome the disadvantages of high melt viscosity, nylon 66. However, if the pressure is too high, the internal stress will cause difficulties in demoulding and deformation.
d) Injection speed: Generally low in the case of filling the mold, preferably slow-fast-slow multi-stage injection.
e) Holding time and molding cycle: In the case of satisfying the product filling, there should be no dents and bubbles; it should be as short as possible to reduce the stopping time of the melt in the barrel.
f) Screw speed and back pressure: the strengthening of nylon should be reduced as much as possible under the premise of satisfying the plasticization quality. Prevent the possibility of decline.
g) Mold temperature. The cooling of the product is good or bad, nylon 66. has a great influence on the quality, so the mold temperature must be able to control the process accurately. If possible, the mold temperature should be higher.
(e) Other issues Because in order to prevent the deterioration of the upper surface quality, the release agent should be used as little as possible during the injection process; when used, it should not be greater than 20%. For products other than PET, post-treatment should be carried out to eliminate internal stress. PMMA should be dried in hot air at 70-80T for 4 hours; PC should be cleaned with air and glycerin. Liquid paraffin, etc. is heated 110-135C for up to 10 hours or more, depending on the product.
PET must go through a biaxial stretching process to obtain excellent mechanical properties. Third, the characteristics of transparent plastic injection molding process. In addition to the above common problems, transparent plastics also have some process characteristics of nylon 66.
They are described as follows.
1. Process characteristics of PMMA PMMA has high viscosity, slightly poor flowability and reinforced nylon. Therefore, it should have high material temperature and high injection pressure for injection molding, in which the influence of injection temperature is greater than injection pressure, but the increase of injection pressure is beneficial to improve the shrinkage of products. Injection temperature range is wide, melting temperature is 160℃, decomposition temperature is 270℃, so the temperature adjustment range is wide and the processability is strong. The processability is good. Therefore, the improvement of fluidity can start from the injection temperature. Poor impact, poor wear resistance, easy to scratch, easy to brittle, so the mold temperature should be increased, improve the condensation process to overcome these defects.
2, PC process characteristics PC viscosity, high melting temperature, poor fluidity, back to the capital must be injected at a higher temperature (between 270-320T), relatively speaking, the material temperature adjustment range is narrow, the process is not as good as PMMA. injection pressure has little effect on fluidity, but due to the high viscosity, injection pressure is still very large, in order to prevent the internal stress of nylon, to strengthen the nylon. Try to shorten the pressure.
3, PET process characteristics PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ℃), but good fluidity after melting, so the processability is poor, often add anti-induction device at the nozzle. After injection mechanical strength and performance are not high, in fact, reinforced nylon. The performance must be improved by stretching procedure and modification. Accurate control of mold temperature to prevent warpage. The important identity of deformation, so it is recommended to use hot runner molds. The mold temperature should be high, otherwise it will cause poor surface gloss and difficult to release the mold.
Fourth, the defects of transparent plastic parts
Only the defects that affect the transparency of the product are mentioned here. These defects are about the following kinds.
(1) Silver pattern. In the process of filling and condensation, the internal stress anisotropy is affected, said to be transparent. The stress generated in the vertical direction orients the resin on the flow, and the non-flow orientation produces a difference in refractive index, producing shiny filaments. When it is expanded, the product may crack. In addition to attention to injection molding process and mold
(2) Air bubbles. Water vapor and other gases in the resin are not expelled, (during the mold condensation process) or "vacuum bubbles" are formed when the condensation surface condenses too quickly due to insufficient filling. Injection molding. The method of overcoming is shown in the table.
(3) Poor surface gloss. Mainly due to the roughness of the mold, on the other hand, condensation too early, so that the resin can not copy to the mold surface, these make the surface of the cattle produce small unevenness, so that the product discoloration.
(4) Shock ripple: It is a dense ripple formed from the center of the gate. The reason is that the melt viscosity is too large, the front-end material has condensed in the cavity, and the condensed surface is broken by the back-end material, strengthening the nylon. And the impact pattern appears on the surface.
(5) Whitening. Haze: It is mainly caused by the dust in the air falling into the raw material or the excessive water content of the raw material.
(6)White smoke. Black spot: mainly due to the plastic in the barrel, due to local overheating, the resin in the barrel is decomposed or deteriorated.
