Plastics are widely used in modern industry and daily products due to their many advantages, especially in the packaging industry, where they often replace glass. Among them, the transparency of plastics is crucial. This article focuses on three common plastics: PMMA, PET, and PC, and discusses in detail the methods of improving the transparency of plastics from the perspective of injection molding process, covering equipment and mold requirements, raw material processing, process parameter optimization, and common defect resolution, providing a comprehensive and practical reference for relevant practitioners.
In today's material application field, plastics have gradually become the preferred material for the manufacture of many products due to their many advantages such as light weight, good toughness, easy molding, and low cost. In the packaging industry, the trend of replacing glass with plastics is becoming increasingly obvious, and plastic transparency, as a key performance indicator, directly affects the appearance and market competitiveness of products. How to effectively improve the transparency of plastics through injection molding has become the focus of attention in the industry. Next, we will analyze in depth the specific improvement methods for three types of plastics: PMMA, PET, and PC.
Transparent plastics need to have a series of characteristics to meet the transparency requirements in actual use and maintain long-term stability. First, high transparency is the basis, which directly determines its applicability in scenarios such as optical applications. Secondly, it must have a certain strength and wear resistance to withstand friction and collision in daily use, and good impact resistance to cope with possible accidental impacts. Excellent heat resistance and chemical resistance can ensure stable performance under different environmental conditions. At the same time, the water absorption rate should be small to avoid the decrease in transparency or deterioration of material properties due to water absorption. Among many transparent plastics, PC has ideal comprehensive performance, but the raw materials are expensive and the injection molding process is difficult. Therefore, PMMA is more commonly used for products with general requirements. PET is mostly used in packaging and container fields because it needs to be stretched to obtain good mechanical properties.
Cleaning of barrels, screws and their accessories
Importance: Raw material contamination or old materials and impurities remaining in the depressions of screws and accessories, especially resins with poor thermal stability, will seriously affect the quality of plastic products. For example, if the residual impurities decompose during the injection molding process, it may cause defects such as black spots and discoloration on the product, greatly reducing transparency.
Cleaning method: Before use and after shutdown, use a screw cleaner to thoroughly clean all parts to ensure that there is no impurity adhesion. If there is no screw cleaner, PE, PS and other resins can be used to clean the screw. During temporary shutdown, in order to prevent the raw materials from staying at high temperatures for a long time and degrading, the barrel temperature of plastics such as PC and PMMA needs to be reduced to below 160°C (the PC hopper temperature is reduced to below 100°C).
Key points of mold design (including product design)
Uniform wall thickness and demoulding slope: Uniform wall thickness can ensure that the plastic flows and cools evenly in the mold, preventing defects such as deformation and shrinkage that affect transparency. The demoulding slope is large enough to facilitate smooth demoulding of the product, avoiding surface scratches or internal stress concentration caused by forced demoulding, which affects transparency. For example, when producing PMMA lampshades, if the wall thickness is uneven, different degrees of shrinkage will occur during the cooling process, resulting in local streaks on the product and reducing transparency.
Surface finish of mold: The surface roughness of the mold is low (preferably less than 0.8), which can make the surface of the plastic product smooth, reduce light scattering, and improve transparency. If the mold surface is rough, tiny bumps will form on the surface of the plastic product, and diffuse reflection will occur when light passes through, reducing transparency.
Wall thickness requirements: Except for PET, the wall thickness of plastics should generally not be too thin, and should not be less than 1mm. Too thin wall thickness may make it difficult to fill the plastic, resulting in problems such as lack of material and bubbles, which seriously affect transparency. For example, when injecting PC sheets, if the wall thickness is less than the standard, bubbles are likely to appear, which greatly reduces the transparency of the sheet.
Vent hole and runner design: The vent hole and groove are large enough to discharge air and gas in the melt in time to prevent gas from forming bubbles in the product and affecting transparency. The transition part should be gradual and smooth to avoid sharp corners and sharp edges, especially for PC products. Notches can easily cause stress concentration, leading to product cracking and reducing transparency. The gate and runner should be wide, thick and short, and the gate position should be reasonably set according to the shrinkage and condensation process. If necessary, add a cold well to ensure smooth plastic filling and reduce defects such as flow marks that affect transparency.
Necessity of drying
Influence of impurities and moisture: Any impurities in the plastic may affect transparency. For example, dust falling into the raw material will form black spots in the product and block the propagation of light. Moisture will deteriorate the raw material after heating. For example, if the water content of PET raw material is too high, hydrolysis will occur during the injection molding process, reducing the molecular weight, resulting in reduced strength and poor transparency of the product.
Drying measures: Raw materials must be strictly sealed during storage, transportation, and feeding to ensure cleanliness. A drying hopper is used during injection molding, and the air input during the drying process should be filtered and dehumidified to prevent contamination of the raw materials. Different plastics have different drying conditions. For example, PMMA is generally dried at 70-80℃ for about 4 hours, and PC needs to be dried at 110-135℃ (depending on the product, up to more than 10 hours) to remove moisture and ensure injection molding quality.
Injection temperature: Under the premise that the plastic resin does not decompose, a higher injection temperature is conducive to better flow of the plastic melt, filling the mold cavity, reducing defects such as weld marks, and improving transparency. For example, the injection temperature range of PMMA is relatively wide (melting temperature 160℃, decomposition temperature 270℃). Properly increasing the injection temperature can improve its fluidity, make the surface of the product smoother, and improve transparency. However, too high a temperature may also cause the plastic to decompose and affect product quality.
Injection pressure: Since transparent plastics have high melting points and poor fluidity, generally higher injection pressure is required to overcome the problem of high melt viscosity and ensure complete filling of the mold. However, too high pressure will generate internal stress, making it difficult to demold the product, causing deformation or even cracking, and reducing transparency. For example, when PC is injected, it is necessary to reasonably control the pressure while ensuring filling to avoid excessive internal stress.
Injection speed: Under the condition of satisfying mold filling, it is generally advisable to use a lower injection speed, and it is best to use slow-fast-slow multi-stage injection. Slow injection can reduce the impact of the melt on the mold and prevent turbulence; fast injection can quickly fill when the front end of the melt is not condensed; slow injection in the later stage can make the melt flow smoothly in the cavity, reduce gas entrapment and flow marks, and improve transparency. For example, when injecting PET preforms, a suitable multi-stage injection speed can effectively reduce the vibration marks on the surface of the preform and improve transparency.
Press holding time and molding cycle: Under the premise of ensuring product filling and no dents or bubbles, the press holding time and molding cycle should be as short as possible. Too long holding time and molding cycle will cause the melt to stay in the barrel for too long, which may cause plastic degradation and affect transparency. Taking PMMA products as an example, appropriately shortening the holding time and molding cycle can improve production efficiency and product transparency while ensuring quality.
Screw speed and back pressure: Under the premise of meeting the plasticization quality, the screw speed and back pressure should be as low as possible. Too high screw speed and back pressure may cause plastic degradation, affecting product performance and transparency. For example, when PC plastic is injected, too high screw speed will cause shear overheating, PC decomposition, and reduce transparency.
Mold temperature: Mold temperature has a great influence on the cooling effect of the product, and precise control of mold temperature is crucial. Higher mold temperature is conducive to better flow and filling of plastic melt in the mold, reducing internal stress and improving product surface quality and transparency. For example, when PET is injected, the mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding, reducing transparency.
Release agent use: To prevent surface quality deterioration, release agent should be used as little as possible during injection molding. If the release agent remains on the surface of the product, it may form a thin film, which will affect the light transmission and reduce the transparency. If it must be used, a release agent with less impact on transparency should be selected, and the amount should be strictly controlled.
Recycled material use: The proportion of recycled material used should generally not exceed 20%. After multiple processing, the molecular structure of recycled materials may change and the performance may decrease. Excessive use will affect the transparency and overall quality of the product. For example, in PMMA products, excessive use of recycled materials may cause defects such as white halo in the product, reducing transparency.
Post-processing: Except for PET, all products should be post-processed to eliminate internal stress. For example, PMMA should be dried in hot air circulation at 70-80℃ for 4 hours, and PC should be heated to 110-135℃ in clean air, glycerin, liquid paraffin and other media (the time depends on the product). Internal stress can cause cracks, deformation and other problems in the product during use, reducing transparency. Post-processing can effectively improve this situation. PET must undergo a biaxial stretching process to obtain good mechanical properties and transparency.
PET has a high molding temperature (260-300℃) and a narrow material temperature adjustment range, but has good fluidity after melting. Due to its high molding temperature, it has high requirements for equipment and poor processability. It is often necessary to add an anti-stagnation device in the nozzle to prevent the melt from dripping at the nozzle. The mechanical strength and performance after injection are not high, and the performance can only be improved through stretching and modification. Accurate control of mold temperature is the key to preventing warping and deformation. It is recommended to use a hot runner mold and the mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding, reducing transparency. For example, when producing PET transparent film, if the mold temperature is not properly controlled, wrinkles will appear on the surface of the film, affecting transparency and mechanical properties.
PMMA has high viscosity and slightly poor fluidity, so it requires high material temperature and high injection pressure for injection molding. The injection temperature has a greater impact on its fluidity than the injection pressure. Increasing the injection temperature can effectively improve fluidity, make the product surface smoother, and improve transparency. Its injection temperature range is wide (melting temperature 160℃, decomposition temperature 270℃), and its processability is good. However, PMMA has poor impact resistance, poor wear resistance, easy scratches and brittle cracks. By increasing the mold temperature and improving the condensation process, these defects can be overcome and transparency can be improved. For example, when manufacturing PMMA display racks, appropriate mold temperature and injection process can make the display rack surface smooth, highly transparent, and not prone to cracks.
PC has high viscosity, high melt temperature, poor fluidity, requires high temperature injection molding (between 270-320℃), and the material temperature adjustment range is narrow, and its processability is not as good as PMMA. The injection pressure has little effect on its fluidity, but due to its high viscosity, a high injection pressure is still required. To prevent the generation of internal stress, the holding time should be as short as possible. PC has a large shrinkage rate and stable dimensions, but the product has large internal stress and is prone to cracking. It is advisable to improve fluidity by increasing temperature rather than pressure, and reduce the possibility of cracking by increasing mold temperature, improving mold structure and post-processing. When the injection speed is low, defects such as ripples are easy to occur at the gate. The temperature of the nozzle should be controlled separately, the mold temperature should be high, and the runner and gate resistance should be small. For example, when injection molding PC optical lenses, precise control of the injection molding process parameters can effectively reduce the internal stress of the lens and improve transparency and optical performance.
Cause: Mainly dust in the air falls into the raw materials or the raw materials have too much water content. Dust will form tiny particles inside the plastic product, and light will scatter when passing through, resulting in white halo phenomenon and reducing transparency. Water at high temperature will cause plastic to undergo hydrolysis and other reactions, change the plastic structure, and affect transparency.
Solution: Strengthen the sealing measures during the storage, transportation and feeding of raw materials to prevent dust from entering. Fully dry the raw materials and control the moisture content within a reasonable range. For example, using efficient drying equipment to ensure that the moisture content of raw materials such as PMMA and PC is less than 0.05% can effectively reduce the white haze phenomenon and improve transparency.
Cause: The plastic is locally overheated in the barrel, causing the barrel resin to decompose or deteriorate. Overheating may be caused by reasons such as too fast screw speed and uneven barrel heating. The decomposed or deteriorated resin forms white smoke and black spots in the product, blocking the propagation of light and seriously reducing transparency.
Solution: Optimize the injection molding process parameters, such as reducing the screw speed and adjusting the barrel heating temperature distribution. Regularly check and maintain the equipment to ensure the normal operation of the barrel, screw and other components. For example, reducing the screw speed by 10-20%, checking the barrel heating elements at the same time, and repairing or replacing abnormal elements can effectively reduce the generation of white smoke and black spots and improve transparency.
Cause: 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 vacuum bubbles. Bubbles form gaps inside the product, and light is refracted and reflected at the interface between the bubbles and the plastic, reducing transparency.
Solution: Strengthen the drying of raw materials to remove moisture and gas. Optimize the moldHave an exhaust system to ensure that the gas can be discharged smoothly. Increase the injection pressure and speed to ensure complete mold filling. For example, increasing the number and size of exhaust grooves in the mold and appropriately increasing the injection pressure by 5-10MPa can effectively reduce the generation of bubbles and improve transparency.
Cause: The melt viscosity is too high, the front material condenses in the cavity, and the later material breaks through the condensation surface, causing chattering on the surface. Chattering will make the surface of the product uneven, scatter light, and reduce transparency.
Solution: Appropriately increase the injection temperature and speed to reduce the melt viscosity. Optimize the mold gate design to make the melt flow more evenly. For example, increasing the injection temperature by 10-20℃, improving the gate shape, and using a fan gate instead of a pin-point gate can effectively reduce chattering and improve transparency.
Cause: On the one hand, the mold is rough, and the surface of the plastic product cannot replicate the surface state of the mold well; on the other hand, the condensation is too early, and the resin cannot fully fill the fine structure of the mold surface. Minor unevenness on the surface can cause diffuse reflection of light and reduce transparency.
Solution: Polish the mold to reduce roughness. Increase the mold temperature and extend the flow time of the resin in the mold so that it can better fill the mold surface. For example, reducing the mold surface roughness to below 0.4 and increasing the mold temperature by 10-15℃ can significantly improve the surface gloss and transparency of the product.
Cause: During the filling and condensation process, the internal stress is anisotropic. The stress generated in the vertical direction causes the resin to flow in an orientation, which has a different refractive index from the non-flow orientation, forming a flash streak, which may cause cracks in the product after expansion. Silver streaks can cause complex refraction and scattering of light inside the product, reducing transparency.
Solution: Optimize the injection molding process parameters to reduce the generation of internal stress. Anneal the product to eliminate internal stress. For example, PC material can be heated to above 160℃ for 3-5 minutes and then cooled naturally. These measures can effectively reduce silver streaks and improve transparency.
Improving the transparency of plastics is a complex process involving multiple links. From the perspective of injection molding process, the reasonable design and maintenance of equipment and molds, the proper handling of raw materials, the precise optimization of process parameters, and the effective solution of common defects all have a crucial impact on the transparency of plastics. For different plastics such as PMMA, PET, PC, etc., due to their different process characteristics, the methods for improving transparency also have different focuses. In actual production, it is necessary to comprehensively apply the above methods according to the specific types of plastics and product requirements, and continuously optimize the injection molding process to produce high-transparency plastic products to meet the market demand for high-quality plastic products.
(Some pictures of injection molding equipment, molds, plastic products, etc. can be inserted here to enhance the intuitiveness of the article. Due to format restrictions, they cannot be actually inserted. They can be added during typesetting according to actual conditions)
Contact: Smile Kuan
Phone: +86 134 2472 9214
E-mail: [email protected]
Add: CB17 Building No. 25, No. 8 Changma Road, Changping Town, Dongguan City, Guangdong Province, China