The Properties of Pmma and Its Molding Process Characteristics

PMMA is commonly known as polymethyl methacrylate. It is a non-toxic and environmentally friendly material. It is often used for acrylic sheets, acrylic plastic pellets, acrylic light boxes, signboards, acrylic bathtubs, car taillights, signal lights and instrument panels in the automotive field, blood storage containers in the pharmaceutical industry, video discs and light diffuser for industrial applications, electronic product buttons especially transparent buttons and daily consumer goods such as beverage cups & stationery.
 
Process characteristics of PMMA
PMMA is an amorphous polymer with a Tg of 105°C, a melting temperature of more than 160°C and a decomposition temperature of 270°C. Therefore, the molding temperature range is wide.
 
PMMA in the molten state has high melt viscosity and poor fluidity, and the melt viscosity is sensitive to temperature changes. The change of injection temperature has a more significant effect on the melt flow length than the injection pressure, and is more obvious than the injection rate and the mold temperature. Therefore, changing the fluidity of PMMA molding mainly starts from the injection temperature.
 
PMMA has a certain degree of hydrophilicity. The water absorption of the PMMA particles reaches 0.3% to 0.4%. The presence of moisture causes bubbles to appear in the melt, and the obtained product has silver wires and transparency is also reduced. Therefore, it is required to dry the resin before injection.
 
Since high transparency is the characteristic of PMMA, any impurities will be exposed on the product due to the light refraction relationship. Therefore, it is required that the environment must be cleaned before processing and molding.
 
PMMA is hard, brittle and easy to break. Therefore, it is necessary to choose the molding, shrinkage rate and demolding slope of the product. Common plunger injection molding machines and screw injection molding machines can be used as long as the product consumption does not exceed 70% to 80% of the maximum injection volume. Use open nozzles with heating and temperature control devices.
 
1. Processing of raw materials
PMMA has a certain degree of water absorption, and its water absorption rates reach 0.3% to 0.4%. However, injection molding must be performed at a temperature below 0.1%, usually 0.04%. The presence of water causes bubbles, gas lines and reduces transparency in the melt. Therefore, PMMA needs to be dried. The drying temperature is 80℃ to 90℃, and the time is more than 3 hours.
 
In the drying process, in addition to strictly controlling the drying process to prevent the resin particles from sticking to agglomerates, attention should also be paid to the cleanliness of the drying equipment and containers to prevent impurities and dust from mixing in and affecting the quality of the products.
 
The dried resin particles should be processed in time, and the resin particles which are not used should be kept properly. Use a closed container to keep the resin particles to prevent the resin particles from re-absorbing moisture when they are in equilibrium with the ambient temperature and humidity.
 
Recycled materials can be used 100% in some cases. The actual amount depends on the quality requirements, usually over 30%. Recycled materials should avoid pollution, otherwise the transparency and properties of the finished product will be affected.
 
2. Selection of injection molding machines
PMMA has no special requirements for injection molding machines. Because PMMA has high melt viscosity, a deep screw groove and a nozzle hole with a big diameter are required. If the  product is required to have high strength, a screw with a bigger length to diameter ratio should be used for low temperature plasticization. In addition, PMMA must be stored in a dry hopper.
 
3. Mould and gate design
The mold temperature is an important factor in PMMA molding. The mold temperature can be 60℃ to 80℃. The diameter of the main runner should match the inner taper. The best angle is 5° to 7°. If you want to inject products with a thickness of 4mm or over 4mm, the angle should be 7°. The diameter of the runner should reach 8 to 10mm, and the overall length of the gate should not exceed 50mm.
 
For products with a wall thickness of less than 4mm, the runner diameter should be 6 to 8mm; for products with a wall thickness of more than 4mm, the runner diameter should be 8 to 12mm; the depth of fan-shaped and tab-shaped gates should be 0.7 to 0.9t (t is the thickness of the product wall); the diameter of the needle gate should be 0.8 to 2mm; the smaller size should be used for low viscosity. Common vent holes have the length of 0.05 to 0.07mm and width of 6mm. the demolding slope is between 30'-1°and 35'-1°30°in the cavity part.
 
4. Melting temperature
It can be measured by air injection method: ranging from 210℃ to 270℃, depending on the information provided by the supplier.
 
5. Injection temperature
PMMA is an amorphous polymer with no obvious melting point temperature. There is a wide selection range in molding, which can be selected between 160 to 270℃. Fast injection can be used, but high internal stress should be avoided. Multi-stage injection should be used, for example, slow-fast-slow. When thick parts are injected, the slow speed should be used.
 
The appropriateness of the injection temperature can be judged by the melt to air injection method, that is, whether the melt flowing out from the nozzle at a low speed is bright, transparent and bubble free. If it is an opaque, fuzzy, bubble, silver wire expansion body, it can be considered that the temperature is too high or the water content of the resin is too high. Generally speaking, under the condition of ensuring that the melt fills the mold cavity, the injection temperature can be low to reduce possible discoloration and impacts on performance.
 
6. Injection pressure
The influence of injection pressure on the fluidity of PMMA melt is not as obvious as that of temperature. Because of the high melt viscosity and poor fluidity, a higher injection pressure is still required, especially for products with complex shapes and thick walls, in order to overcome problems such as melt flow resistance and reducing shrinkage dents and bubbles. It should still be noted that the increase in pressure will cause the product to easily produce internal stress.
 
The selection of PMMA injection pressure is based on the wall thickness of the product. Then, make adjustments according to actual problems in molding. Generally, the injection pressure for thick walled products with wide gates and easy flows are between 80 to 100MPa, while the pressure required for products with more difficult melt flows is greater than 140MPa. And 110MPa to 140MPa is suitable for molding most products.
 
7. Holding time
If the temperature is 260°C, the holding time should not exceed 10 minutes. If the temperature is 270°C, the holding time should not exceed 8 minutes.
 
8. Molding cycles
The molding cycle is often related to the wall thickness of the product. Because the PMMA melt begins to solidify at about 140°C and reaches Tg at 105°C, it can be demolded at a high temperature with little deformation. Therefore, the molding cycle is generally short. Only when the mold temperature is too high, the cycle is slightly extended.