Because plastics have the advantages of light weight, good toughness, easy molding, low cost, etc., modern plastics and daily-use products are more and more replaced with plastics, especially for optical instruments and packaging industries. The development is particularly rapid. Due to the requirement for better transparency, higher wear resistance and better impact toughness, a lot of work has to be done on plastic components, the process of injection molding, equipment, molds, etc. to ensure that these are used instead of glass. Plastic (hereinafter referred to as transparent plastic), the surface quality is good, so as to meet the requirements of use.
The commonly used transparent plastics on the market today are polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed PMMA), polycarbonate (codenamed PC), polyethylene terephthalate (codenamed PET). , transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code PSF), etc., among which we are the most contacted PMMA. PC and PET three plastics, due to limited space, here are the three plastics For example, discuss the characteristics of transparent plastic and injection molding process.
First, the performance of transparent plastic
Transparent plastic must first have high transparency, followed by a certain degree of strength and abrasion resistance, resistance to impact, heat resistant parts, good chemical resistance, low water absorption, and only in this way can it be transparent in use. The requirement is long-term. PC is an ideal choice, but it is mainly difficult to use expensive raw materials and injection molding process. Therefore, PMMA is still the main choice (for general requirements of products), and PPT can only be stretched. Get good mechanical properties, so it is used in packaging and containers.
Second, the common issues that should be noted during the injection molding of transparent plastics
Because of the high light transmittance of transparent plastics, the surface quality of plastic products must be required to be strict, there can be no markings, pores, whitening, haze, black spots, discoloration, poor luster and other defects, and thus raw materials throughout the injection molding process, Equipment, molds, and even the design of products, must pay great attention and put forward strict or even special requirements.
Secondly, because transparent plastics are mostly of high melting point and poor fluidity, in order to ensure the surface quality of products, it is often necessary to make fine adjustments in the process parameters such as barrel temperature, injection pressure, injection speed, etc., so that the plastic injection can be full of molds, and not Internal stress will cause product deformation and cracking.
For equipment and mold requirements, injection molding process and product raw material handling, talk about matters that should be noted:
●dry
Preparation and drying of raw materials Because of containing any impurities in the plastic, it may affect the transparency of the product, and therefore storage, transportation.
During the feeding process, we must pay attention to sealing to ensure that the raw materials are clean. In particular, moisture is contained in the raw materials, which will cause deterioration of the raw materials after heating. Therefore, it must be dried. During the injection molding, feeding must be performed using a drying hopper. Also note that the drying process is necessary. In this case, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the material.
●Cylinder, screw and accessories cleaning
In order to prevent contamination of raw materials and presence of old materials or impurities in the screw and attachment recesses, resins with poor thermal stability exist. Therefore, before use, all parts are cleaned with a screw cleaning agent to prevent them from being contaminated with impurities. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.
When temporary shutdown, in order to prevent the raw materials from staying at high temperature for a long time and cause the solution to drop, the temperature of dryer and barrel should be reduced. For example, PC, PMMA and other barrel temperatures should be reduced to below 160°C. (Hopper temperature for PC Should be reduced to below 100°C
Problems to be noted in the design of the mold (including the design of the product)
In order to prevent backflow, or uneven cooling, poor plastic molding results in surface defects and deterioration.
In the mold design, the following points should be noted:
● The wall thickness should be as uniform as possible, and the draft must be large enough;
● Transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
● Gates. Runners should be as wide and short as possible, and the gate location should be set according to the contraction condensation process. If necessary, coolers should be added.
● The mold surface should be smooth and clean, with a low roughness (preferably lower than 0.8);
● Exhaust vents. Grooves must be sufficient to discharge air and gas from the melt.
● Except for PET, the wall thickness should not be too thin, generally not less than lmm.
●Injection process should pay attention to issues (including injection molding machine requirements)
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process:
● The special screw should be used, injection molding machine with separate temperature control nozzle;
● The injection temperature should be higher than the injection temperature if the plastic resin does not decompose.
●Injection pressure: generally high, to overcome the defects of large melt viscosity, but the pressure is too high will cause internal stress caused by demoulding and deformation;
●Injection speed: In the case of filling the mold, it should be generally low, it is best to use slow-fast-slow multi-level injection;
●Pressure holding time and molding cycle: In the case of satisfying the mold filling of the product, no cavities or bubbles are generated, and it should be as short as possible to minimize the residence time of the melt in the barrel;
● Screw speed and back pressure: Under the premise of satisfying the quality of plasticization, it should be as low as possible to prevent the possibility of dropout;
● Mold temperature: The cooling of the product has a great influence on the quality. Therefore, the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.
Other issues
Since it is necessary to prevent the deterioration of the upper surface quality, the release agent should be used as little as possible during the injection molding; when using recycled materials, it should not be greater than 20%.
In addition to PET, the products should be post-processed to eliminate internal stress, PMMA should be dried in a hot air circulation at 70-80°C for 4 hours; PC should be in clean air, glycerin, liquid paraffin, etc. Heating 110-135°C, time by product However, it takes more than 10 hours. PET must undergo a biaxial stretching process to obtain good mechanical properties.
Third, transparent plastic injection molding process
Transparent plastic process characteristics
In addition to the above common problems, transparent plastics also have some technological features, which are described as follows:
1, Process characteristics of PMMA
PMMA has large viscosity and poor fluidity. Therefore, high material temperature must be used and high injection pressure must be used for injection molding. The influence of injection temperature is greater than the injection pressure, but the injection pressure is increased, which helps to improve the shrinkage rate of the product.
The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature is 270°C, so the material temperature adjustment range is wide and the processability is good. Therefore, the fluidity can be improved, and the injection temperature can be used. Poor impact, poor wear resistance , easy to draw flowers, easy to embrittlement, it should increase the mold temperature, improve the condensation process, to overcome these defects.
2, the technical characteristics of PC
PC has large viscosity, high melt temperature and poor fluidity. Therefore, it must be injected at a relatively high temperature (between 270-320°C). Relatively speaking, the material temperature adjustment range is narrow and the processability is not as good as that of PMMA. The injection pressure has a greater influence on the fluidity. Small, but due to the large viscosity, still have a large injection pressure, and in order to prevent internal stress, the holding time should be as short as possible.
High shrinkage rate, dimensional stability, but the internal stress of the product is large, easy to crack, so it is better to use temperature instead of pressure to improve the fluidity, and to improve the mold temperature, improve the mold structure and post-treatment to reduce the possibility of cracking. When the injection speed When it is low, the gate is prone to defects such as corrugation, the temperature of the radiation nozzle must be controlled separately, the temperature of the mold must be high, and the flow path and gate resistance should be small.
3, the process characteristics of PET
The PET molding temperature is high, and the material temperature adjustment range is narrow (260-300°C), but after melting, the fluidity is good, so the processability is poor, and the flow delay device is often added in the nozzle. Mechanical strength and performance after injection are not High, must be through the stretching process and modification to improve performance.
Accurate mold temperature control is an important factor in preventing warpage and deformation. Therefore, hot runner molds are recommended. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding.
Fourth, the defects and solutions of transparent plastic parts
The defects are probably the following:
Silver pattern
Due to the influence of the internal stress anisotropy during the filling and condensation process, the stress generated in the vertical direction causes the resin to be oriented on the flow, and the non-flow orientation produces a refractive index that is different from the refractive index, and when expanded, it may cause The product is cracked. In addition to the attention on the injection molding process and mold, the best product for annealing. If the PC material can be heated to above 160 °C for 3-5 minutes, and then can be naturally cooled.
bubble
Water vapor and other gases in the main resin are not discharged (in the process of mold condensing) or due to insufficient filling, the condensing surface condenses too quickly to form a vacuum bubble.
Surface gloss difference
Mainly in the mold roughness, on the other hand, premature condensation, so that the resin can not copy the state of the mold surface, all of which make its surface produces tiny bumps, leaving the product tarnished.
Shock pattern
Refers to the formation of dense ripples from the center of the sprue, the reason is because the melt viscosity is too large, the front material has condensed in the cavity, then the material will break through the condensation surface, and make the surface of the shock pattern.
White haze
Mainly due to the dust in the air falling into the raw material or raw water content caused by too much.
White smoke black spots
Mainly due to plastic in the barrel, due to local overheating caused by the resin decomposition or deterioration of the barrel formed.