Polymethyl methacrylate (PMMA) resin is the most important kind of acrylic resin, and it is called acrylic resin together with polymethyl methacrylate, ethyl ester, butyl ester, polymethyl methacrylate, polyacrylic acid and its polymer, polyacrylamide and methyl cyanoacrylate. Polymethyl methacrylate (PMMA) resin has the best performance, the largest output and the largest application in this kind of resin.
physicochemical property
PMMA is colorless and transparent, and its light transmittance is the best among plastics, which is higher than that of glass and has a wide light transmittance range. The reflectivity changes with the incident angle, and the light absorption rate is small, so it can be totally reflected. When the curvature of PMMA carrier (plate, rod)
The Tg of PMMA amorphous polymer is 105℃, the melting temperature is higher than 160℃, and the decomposition temperature is higher than 270℃, so the molding temperature range is wide.
The characteristics of molten PMMA are high melt viscosity and poor fluidity, and the melt viscosity is sensitive to temperature changes. The influence of injection temperature on melt flow length is more significant than that of injection pressure, injection rate and mold temperature. Therefore, changing the fluidity of PMMA molding mainly starts from the injection temperature.
PMMA has certain hydrophilicity, and the water absorption of its particles reaches 0.3% ~ 0.4%. The existence of moisture makes bubbles appear in the melt, and the obtained products have silver lines, and the transparency is also reduced. Therefore, it is necessary to dry the resin before injection molding.
Because high transparency is the characteristic of PMMA, any impurities will be exposed on the product because of the refraction of light, so it is required to clean the environment before processing and molding.
PMMA is hard, brittle and easy to break, so it is necessary to choose the molding, shrinkage and demoulding inclination of the product.
Ordinary plunger injection molding machines and screw injection molding machines can be processed as long as the product consumption does not exceed 70% ~ 80% of the maximum injection quantity. Choose an open nozzle with heating and temperature control devices.
Shooting skill
(1) cleanliness is one of the basic conditions for the performance of PMMA injection molding products, otherwise the products will be scrapped due to poor light transmittance. To this end, we should pay attention to the following points.
① Pay special attention to cleaning the hopper and barrel of injection molding machine. If there are other material residues in the barrel, clean the barrel first (the cleaning material can be recycled PMMA). If there are PVC, POM and other materials residues in the barrel, it is not allowed to clean it directly with recycled PMMA. This must be noted.
(2) When opening the package, clean the granular materials before opening the package. Care should be taken to properly preserve the remaining resin particles to prevent them from being contaminated by static electricity or mixed with other resin particles.
Special composite material
Typical injection
Forming condition
General injection molding conditions
English
British system
international system of units (SI)
metric system
Temperature temperature
350-390 f 177- 199 c is at the back of the back area.
360-400 degrees Fahrenheit 182-204 degrees Celsius in the middle of the central area.
370-410 f188-210 c in the anterior region.
The melting temperature of the melt is 360-425 F182-218 C.
Mold temperature is175-225 f 79-107 C.
Pressure pressure
Injection pressure10000-15000 psi 69-103 MPa.
Keep the pressure at 5000- 10000 psi 34-69 MPa.
Back pressure 50- 100 psi 0.34-0.69 MPa
Speed speed
The filling speed is 1-2 inches/sec 25-5 1 mm/sec.
The screw speed is 60-90 rpm.
Drying conditions
Time and space. temperature
Time and temperature: 4 hours at 200 degrees Fahrenheit and 4 hours at 93 degrees Celsius.
Dew point 0.0 degrees Fahrenheit -65438 degrees Celsius +08 degrees Celsius
Moisture content
Humidity content 0.02% 0.02%
Permanent antistatic composite material
Typical injection
Forming condition
General injection molding conditions
English
British system
international system of units (SI)
metric system
Temperature temperature
390-420 F 199-2 16 C is in the back of the back area.
410-440f210-227b in the middle of the central area.
430-460 f22 1-238 C in front of the anterior region.
The melting temperature of the melt is 415-470 f 213-243 C.
Mold temperature is 100- 150 F 38-66 C.
Pressure pressure
Injection pressure10000-15000 psi 69-103 MPa.
Keep the pressure at 5000- 10000 psi 34-69 MPa.
Back pressure 50- 100 psi 0.34-0.69 MPa
Speed speed
The filling speed is 1-2 inches/sec 25-5 1 mm/sec.
The screw speed is 60-90 rpm.
Drying conditions
Time and space. temperature
Time and temperature: 4 hours at 200 degrees Fahrenheit and 4 hours at 93 degrees Celsius.
Dew point 0.0 degrees Fahrenheit -65438 degrees Celsius +08 degrees Celsius
Moisture content
Humidity content 0.02% 0.02%
(3) Environmental remediation In addition to the operator's own cleanliness, the operator should also pay attention to the cleanliness of the operating environment, such as around the injection molding machine, operating platform, product stacking and particle storage. During the operation, attention should also be paid to capping the hopper.
(4) Renovation of mold oil (such as antirust oil, cutting oil, lubricating oil, etc.). ) is not allowed to exist in the mold to avoid cracking and sticking of the product. It is generally not recommended to use release agent.
(2) The use of recycled materials is consistent with the color of PMMA recycled materials, and there is no impurity. It can be used after being mixed with new materials in products with low requirements, and the mixing ratio is 1/4 of new particles. The mixed materials must be pre-dried before molding, and the drying process is the same as that of new materials. For products with high light transmittance requirements, recycled materials are not allowed in principle.
(3) Post-treatment of products
Like PS and other plastics, PMMA products will inevitably remain some internal stresses for various reasons, which will lead to cracks in the product during use. In order to reduce or eliminate these stresses, it is necessary to post-treat the obtained products.
The post-treatment of PMMA products is carried out in a hot air circulating drying oven at 70 ~ 80℃. The treatment time depends on the wall thickness of the product, and the longer the thickness increases, the shorter it will be, usually about 4 hours, and then slowly cool to room temperature. PMMA resin particles absorb moisture easily. Because of the existence of moisture, they are heated and evaporated during the molding process, which leads to the foaming and expansion of the melt, and some problems such as silver lines, bubbles, poor transparency and paste spots appear. Therefore, drying should be carried out before molding, especially for particles with poor packaging conditions or long packaging opening time. PMMA is dried by hot air circulating drying equipment, and its drying process parameters are: temperature 70 ~ 80℃, time 2 ~ 4h, and material layer thickness 30 mm..
And in the drying process, besides strictly controlling the drying process to prevent resin particles from sticking into blocks, we should also pay attention to the cleaning of drying equipment and containers to avoid the mixing of impurities and dust and affect the product quality.
The dried resin particles should be treated in time, and the resin particles that are not used for the time being should be properly kept. It is packed in a closed container to prevent resin particles from reabsorbing water in the process of balancing with ambient temperature and humidity.
4 molding process parameters
(1) Injection temperature PMMA is an amorphous polymer with no obvious melting point. The range of molding choice is wide, which can be selected between 160 ~ 270℃; At the same time, the change of temperature has obvious influence on the fluidity of melt. With the increase of material temperature, the flowing length of melt increases obviously, which is very important for the filling of thin-walled, long-flowing and complex products. However, it must be noted that when selecting high material temperature, it is easily influenced by other process parameters, which will bring discoloration to the product surface.
Whether the injection temperature is appropriate can be judged by observing the injection method of melt into air, that is, whether the melt flows out of the nozzle at a low speed is bright, transparent and bubble-free. If it is opaque, fuzzy, with bubbles and silver lines, it can be considered that the temperature is too high or the water content of the resin is too high.
Generally speaking, the injection temperature can be lower when the melt fills the mold cavity, so as to reduce the possible discoloration and influence on the performance.
(2) Injection pressure The influence of injection pressure on the fluidity of PMMA melt is not as obvious as that of temperature, but due to high melt viscosity and poor fluidity, higher injection pressure is still needed. Especially for products with complex shape and large wall thickness, in order to overcome the melt flow resistance and reduce shrinkage cavity, porosity and bubbles. It should also be noted that the increase of pressure will lead to internal stress in the product.
The selection of PMMA injection pressure is first based on the wall thickness of the product. Then adjust according to the actual problems in molding. Generally, the injection pressure of thick-walled products with wide gates is between 80 ~ 100 MPa, and the pressure of products with difficult melt flow is greater than 140MPa.10 ~140 MPa is suitable for the molding of most products.
(3) The increase of injection rate is beneficial to the filling of PMMA melt, but high-speed injection often affects the quality of molded products, especially around the gate, which greatly reduces the light transmittance of products and increases the internal stress of products. In general, it is best not to use high-speed injection. Only when the gate is small (mostly needle gate) and it is not easy to fill the mold or the weld mark is obvious, choose a higher injection rate. It is best to use multi-level injection.
(4) Mold temperature Mold temperature is an important factor in PMMA molding, and the control of mold temperature is related to the structure and shape of the product, the use requirements and the material itself. High mold temperature is beneficial to melt filling, which can reduce the appearance of weld marks, improve the transparency of products, especially reduce the internal stress of products. Higher mold temperature will increase the shrinkage of products, easily cause dents, and take a long cooling time, which will affect the whole molding cycle.
In actual molding, it is difficult to fill the mold, but also it is necessary to increase the mold temperature (generally 40 ~ 65℃) to improve the weld line and prevent shrinkage cavity. Most of them use water cooling to control the mold temperature.
(5) Molding cycle The molding cycle is often related to the wall thickness of the product. Since PMMA melt begins to solidify at about 140℃ and reaches Tg at 105℃, it can be demoulded at a higher temperature with little deformation, so the molding cycle is generally short, and it will be slightly extended only when the mold temperature is too high.
In addition, PMMA has strict processing requirements and is sensitive to humidity and temperature. It should be fully dried before processing, and its melt viscosity is high, so it needs to be molded at a higher temperature (225-245℃) and pressure, and the appropriate mold temperature is 65-80℃. The stability of PMMA is not very good, and it will degrade at high temperature or if it stays at high temperature for too long. The screw speed should not be too high (about 60%), and thick PMMA parts are prone to "holes", so it is necessary to adopt the method of "low material temperature, high mold temperature and slow injection" for processing.
applied range
PMMA can be used in bulletproof glass, window glass, instrument accessories, engine room covers, signal lights, indicator lights and optical instruments such as glasses, magnifying glasses and various lenses for aviation, aerospace, automobiles and ships. Color plates, high-grade decorative materials, furniture, windows, partitions and other building materials. Plastic surgery prosthesis, nose prosthesis, eye prosthesis, medical light guide tube, denture powder and other medical devices. Stationery, daily necessities, such as pens, drawing tools, teaching models, specimens, lamps, buttons, hairpins, candy plates, etc.
PMMA is processed by injection molding, extrusion, casting, tape casting, coating, bonding and other molding processes.