The characteristics of commonly used transparent raw materials
Due to the high light transmittance of transparent plastics, the surface quality of plastic products must be strict, and there must be no spots, pores, whitening, haze, black spots, discoloration, poor gloss, and other defects.
Therefore, in the entire characteristics of commonly used transparent raw materials, we must pay great attention to and put forward strict or even special requirements for the design of raw materials, equipment, molds, and even products.
Secondly, because transparent plastics mostly have a high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to process parameters such as higher temperature, injection pressure, and injection speed so that the mold can be filled without being filled with plastic and produce internal stress and cause product deformation and cracking.
Because transparent plastics mostly have high melting points and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to process parameters such as higher temperature, injection pressure, and injection speed, so that the mold can be filled without generating internal stress and cause product deformation and cracking.
Problems that should be paid attention to in transparent plastic injection molding process
In order to reduce internal stress and surface quality defects, the following issues should be paid attention to in the transparent plastic injection molding process.
1) The injection temperature should be higher when the plastic resin does not decompose;
2) Injection pressure, generally higher to overcome the defect of high melt viscosity, but too high pressure will cause internal stress and cause difficult demoulding and deformation;
3) Injection speed, In the case of satisfying mold filling, generally it should be low, and it is better to use slow-fast-slow multi-stage injection;
4) Holding pressure time and molding cycle, under the condition that the product is filled with molds and does not produce dents or bubbles; it should be as short as possible to minimize the residence time of the melt in the barrel;
5) Screw speed and back pressure, on the premise of meeting the plasticization quality, it should be as low as possible to prevent the possibility of degrading;
6) Mold temperature, the cooling of the product 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.
7) In order to prevent the deterioration of the surface quality, generally use as little release agent as possible during injection molding; when using recycled materials, it should not be more than 20%.
Injection molding process of commonly used transparent raw materials
In addition to the above common problems that should be paid attention to in the transparent plastic injection molding process, transparent plastics also have some technological characteristics, which are described as follows:
1) PMMA has a high viscosity and poor fluidity, so high material temperature, and high injection pressure must be used for injection molding. The influence of injection temperature is greater than injection pressure, but the increase in injection pressure will help improve the shrinkage 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 manufacturability is better.
To improve fluidity, start from the injection temperature. The impact resistance is poor, the wear resistance is not good, it is easy to scratch, and it is easy to be brittle. It is necessary to increase the mold temperature and improve the condensation process to overcome these defects.
2) PC has a high viscosity, high melting temperature, and poor fluidity. It must be injected at a higher temperature (between 270-320T). Relatively speaking, the temperature adjustment range of the material is narrow, and the manufacturability 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 higher. Correspondingly, in order to prevent internal stress, the pressure holding time should be as short as possible.
The shrinkage rate is large, the size is stable, but the product has high internal stress and is easy to crack. Therefore, it is better to increase the temperature instead of pressure to improve the fluidity, and increase the mold temperature, improve the mold structure and post-processing to reduce the possibility of cracking.
When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the radiating nozzle must be controlled separately, the temperature of the mold must be high, and the resistance of the runner and gate should be small.
3) PET molding temperature is high, and the material temperature adjustment range is narrow (260-300℃), but after melting, the fluidity is good, so the manufacturability is poor, and anti-spreading devices are often added 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 control of mold temperature is to prevent warpage.
It is an important factor of deformation, so it is recommended to use hot runner molds. The mold temperature is high, otherwise, it will cause poor surface gloss and difficulty in demolding.
4) Defects and solutions of transparent plastic parts
Influenced by the internal stress anisotropy during the filling and condensation process, the stress generated in the vertical direction causes the resin to flow upward orientation, and the refractive index is different from the non-flow orientation. The product has cracks. In addition to paying attention to the injection molding process and the mold, it is best to make the product annealed. For example, the PC material can be heated to above 160°C for 3-5 minutes and then cooled naturally.
Because the water vapor and other gases in the resin cannot be discharged (during the mold condensation process) or due to insufficient mold filling, the condensation surface condenses too quickly to form a vacuum bubble.
Poor surface gloss:
The main reason is that the roughness of the mold is large, and on the other hand, the resin cannot copy the state of the mold surface due to premature condensation. All these make the surface produce small irregularities and make the product lose its luster.
It refers to the dense ripples formed from the straight gate as the center. The reason is that the melt viscosity is too large, and the front end material has condensed in the cavity, and then the material breaks through the condensing surface, causing shock lines on the surface.
It is mainly caused by dust falling into the raw materials in the air or too much water content in the raw materials.
White smoke, black spots:
It is mainly formed by the decomposition or deterioration of the resin in the barrel due to partial overheating of the plastic in the barrel.