Introduction: As a transparent plastic, we must first ensure high transparency, followed by strength, impact resistance, abrasion resistance, heat resistance, chemical resistance, and low water absorption rate. Only in this way can we meet the transparency requirements in use, and cannot There are any markings, stomata, whitening. Due to defects such as haze, black spots, discoloration, and poor gloss, the design of raw materials, equipment, molds, and even products must be paid great attention to and strict or special requirements in the entire injection process. This article shares the injection characteristics of transparent plastics And precautions, the content is for the reference of friends:
Most transparent plastics have high melting point and poor fluidity. Therefore, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments in the process parameters such as high temperature, injection pressure, injection speed, etc., so that when the plastic is injected, the mold can be filled without generating Internal stress causes product deformation and cracking. This article discusses the matters that should be paid attention to in terms of raw material preparation, equipment and mold requirements, injection molding process and raw material processing of products.
Three common transparent plastics pay attention to process characteristics
Process characteristics of PMMA
PMMA has high viscosity and slightly poor fluidity. It must have high material temperature and high injection pressure. The influence of injection temperature is greater than injection pressure. Increased injection pressure is beneficial to improve the shrinkage rate of the product.
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 manufacturability is better. Therefore, to improve the fluidity, we can start from the injection temperature.
The impact is poor, the wear resistance is not good, easy to scratch, easy to crack, so the mold temperature should be increased, the condensation process should be improved to overcome these defects.
Process characteristics of PC
PC has high viscosity, high melting temperature, and poor fluidity. It must be injection molded at a higher temperature (between 270 and 320°C). The range of material temperature adjustment is relatively narrow, and the processability is not as good as PMMA.
The injection pressure has little effect on the fluidity, but due to the high viscosity, the injection pressure is still greater. In order to prevent the generation of internal stress, the holding time should be as short as possible.
The shrinkage rate is large and the size is stable, but the product has large internal stress and is easy to crack. Therefore, it is better to increase the temperature rather than the pressure to improve the fluidity, and reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the radiation nozzle should be controlled separately, the temperature of the mold should be high, and the resistance of the runner and gate should be small.
Process characteristics of PET
PET molding temperature is high, the material temperature adjustment range is narrow (260-300 ℃), but after melting, the fluidity is good, so the processability is poor, and often need to add anti-rolling device in the nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through the stretching process and modification. Accurate mold temperature control is an important factor to prevent warpage and deformation, and hot runner molds are recommended.
Raw material preparation and drying
Since any impurities contained in the plastic may affect the transparency of the product, so it is stored and transported. During the feeding process, seals must be taken to ensure that the raw materials are clean.
In particular, the raw material contains moisture, which will cause the raw material to deteriorate after heating, so it must be dried, and when injection molding, a dry hopper must be used for feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it will not pollute the raw materials.
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 accessory depressions, especially resins with poor thermal stability exist, so before use and after shutdown, all parts should be cleaned with a screw cleaning agent to prevent impurities from sticking. When there is no screw cleaning agent, resins such as pe and ps can be used to clean the screw.
When temporarily shut down, in order to prevent the raw material staying at high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be reduced, such as pc, pmma and other barrel temperatures should be lower than 160 ℃. (The hopper temperature should be lower than 100℃ for pc)
Mold design (including product design)
In order to prevent poor return flow, or uneven cooling, which results in poor plastic molding, surface defects and deterioration, generally the following points should be noted when designing the mold:
a) The wall thickness should be as uniform as possible, and the draft angle should be large enough;
b) The transition part should be gradually. Smooth transition to prevent sharp corners. Sharp edges, especially pc products must not have gaps;
c) Gate. The flow channel should be as wide and thick as possible, and the gate position should be set according to the shrinking and condensing process, and a cold material well should be added if necessary;
d) The surface of the mold should be smooth and low in roughness (preferably less than 0.8);
e) Exhaust holes. The tank must be sufficient to exhaust air and melt gas in a timely manner;
f) Except for pets, the wall thickness should not be too thin, generally not less than 1 mm.
Injection molding process and injection molding machine
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process:
a) Should use special screw, injection molding machine with separate temperature control nozzle;
b) The injection temperature should be higher if the plastic resin does not decompose;
c) Injection pressure: generally higher to overcome the defect of high melt viscosity, but too high pressure will cause internal stress and cause difficulty in mold release and deformation;
d) Injection speed: in the case of filling the mold, it is generally low, and it is best to use slow-fast-slow-multi-stage injection;
e) Pressure holding time and molding cycle: in the case of satisfying the product filling mold without generating depressions or bubbles; it should be as short as possible to minimize the residence time of the melt in the barrel;
f) Screw rotation speed and back pressure: Under the premise of satisfying the plasticizing quality, it should be as low as possible to prevent the possibility of solution drop;
g) Mold temperature: the cooling of the product is good or bad, which 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.