Forming characteristics and process parameters of common plastics

Polyethylene PE

▶ 1, low moisture absorption (<0.01%), can not be dried before molding; if necessary, can be dried at 70 ~ 80 ° C for 1 ~ 2h;

▶ 2, melting point: HDPE, about 130 ~ 137 ° C; LDPE, about 108 ° C ~ 115 ° C; decomposition temperature above 300 ° C. The barrel temperature of the LDPE can be controlled at 140-180 ° C, and the HDPE is controlled at 180-220 ° C;

â–¶ 3, good melt fluidity, low viscosity, strong shear sensitivity of viscosity (especially LDPE), low injection pressure can be used during processing, generally 60 ~ 80MPa;

â–¶ 4, the injection rate of PE is medium or slow, and high-speed injection is not suitable because PE has a tendency to melt fracture during high-speed injection;

▶ 5, the choice of mold temperature during processing is related to the density of PE, usually, the mold temperature of LDPE is 35 ~ 60 ° C, HDPE is 50 ~ 80 ° C;

â–¶ 6. The molding shrinkage rate is large (1.5~3.5%). When the wall thickness of the product is uneven, the ribs are prone to scars. Uneven cooling is easy to cause warping deformation.

Polypropylene PP

Rigid PVC RPVC

▶ 1. RPVC is a typical heat sensitive plastic. Stabilized RPVC can cause severe decomposition of RPVC even at very low temperatures, such as 180 ° C, if the time is long (such as 40 min or more). Therefore, the molding temperature and the residence time of the material in the barrel should be strictly controlled. The molding temperature of RPVC ranges from 160 to 200 °C. Usually 160 ~ 190 ° C, the maximum does not exceed 200 ° C; barrel temperature distribution is usually set in a stepwise manner; nozzle temperature should be 10 ~ 20 ° C lower than the end temperature of the barrel;

▶ 2. For products with low requirements, the raw materials before molding may not be dried; however, the moisture content of the raw materials should be dried. Drying method is usually in a hot air circulating oven, drying at a temperature of 90 ~ 100 ° C for 1 ~ 2.5h;

â–¶ 3, RPVC is generally poorly fluid, and injection molding usually uses a higher injection pressure and a lower injection rate. The injection pressure is above 90 MPa, and the holding pressure is mostly 60-80 MPa. If the injection rate is too high, more frictional heat will be generated to cause the plastic to burn and cause discoloration. Therefore, a medium or low injection rate can be used when forming RPVC;

▶ 4, the mold temperature is generally below 40 ° C, the maximum does not exceed 60 ° C;

â–¶ 5, the screw speed is generally 20 ~ 50r / min; too fast screw speed will lead to excessive temperature rise, resulting in material decomposition.

POM POM

▶ 1. POM is also a typical heat-sensitive plastic, which will be seriously decomposed at 240 °C. At 210 ° C, the residence time should not exceed 20 min; even at 190 ° C, the residence time should preferably not exceed 1 h. Therefore, under the premise of ensuring the fluidity of the material during injection molding, the lower molding temperature and the shorter heating time should be selected as much as possible;

▶ 2. POM has a distinct melting point, homopolymer POM is 175 ° C, and copolymerized POM is 165 ° C. At the time of molding, the distribution of the barrel temperature is 190 to 200 ° C in the front stage, 180 to 190 ° C in the middle stage, 150 to 180 ° C in the rear stage, and the nozzle temperature is 170 to 180 ° C. For thin-walled products, the barrel temperature can be increased appropriately, but not more than 210 ° C;

▶ 3, POM has low hygroscopicity and the resin can be dried before processing. If necessary, it can be dried at 90 ~ 100 ° C for 2 ~ 4h;

â–¶ 4. The melt viscosity of POM is sensitive to shear rate. Therefore, in order to improve the melt fluidity, it is not possible to increase the temperature alone, but also to increase the injection rate and the injection pressure. For large gates, thick-walled short-flow processes, and small-area products, the injection pressure is 40-80 MPa; the general product is about 100 MPa. Small gate, thin wall long process, large area products, high injection pressure, 120 ~ 140MPa;

▶ 5, the mold temperature is usually controlled at 80 ~ 100 ° C, for thin-wall long process and complex shape of the product, the mold temperature can be increased to 120 ° C;

â–¶ 6. High crystallinity makes the molding shrinkage rate high (2~2.8%), and the wall thickness is prone to defects such as dents and shrinkage.

Polycarbonate PC

▶ 1, the aggregation state characteristics belong to amorphous plastic, Tg is 149 ~ 150 ° C; Tf is 215 ~ 225 ° C; molding temperature is 250 ~ 310 ° C;

â–¶ 2. The thermal stability is good and increases with the increase of molecular weight. However, PC is easily degraded when exposed to water at high temperatures, and the moisture content is required to be below 0.02% during molding. Moisture at high temperatures is particularly harmful to PCs. The PC resin must be sufficiently dried prior to molding (and sufficient care should be taken to prevent the dried material from absorbing moisture). The quick test of the drying effect is to use "air injection" on the injection molding machine;

â–¶ 3, high melt viscosity, poor fluidity, its flow characteristics are close to Newtonian fluid, melt viscosity is less affected by shear rate, and sensitive to temperature changes, adjust processing in the appropriate molding processing temperature range Temperature, can effectively control the viscosity of PC;

â–¶ 4, due to high viscosity, high injection pressure, generally controlled at 80 ~ 120MPa. For thin-walled long-flow processes, complex shapes, and small gate sizes, the injection pressure should be appropriately increased to 120-150 MPa in order to smooth and timely mold the melt. The holding pressure is 80-100 MPa;

▶ 5, when molding, cooling and solidification is fast, in order to delay material condensation, the mold temperature needs to be controlled to 80 ~ 120 ° C;

▶ 6. There are a large number of benzene rings in the main chain of PC. The rigidity of the molecular chain is large. It is easy to generate large internal stress during injection molding, which may cause cracking of the product or affect the dimensional stability of the product. (Long-term heat treatment at 100 °C or above, Its rigidity increases and internal stress decreases).

Nylon PA

â–¶ 1. Common varieties and their melting points:

Nylon n series: nylon 6-215 ~ 220 ° C; nylon 12-178 ° C;

Nylon m, n series: nylon-46 295 ° C; nylon-66 255 ~ 265 ° C; nylon - 610 215 ~ 223 ° C; nylon -1010 200 ° C;

Copolycondensed nylon: Due to the poor regularity of the molecular chain, the crystallinity and melting point are generally low. For example, nylon-6∕66∕1010 has a melting point of only 155-175 ° C, but it has good transparency and elasticity.

▶ 2, high melting point, narrow melting range (about 10 ° C). Considering that the PA has a high melting point and poor thermal stability, the processing temperature is generally higher than the melting point of about 30 ° C;

â–¶ 3, hygroscopicity is large, and the amide group is easy to be hydrolyzed at high temperature, causing a serious decrease in molecular weight; (must be strictly dried to a moisture content of less than 0.05%, especially when the material is used for recycling, it should be strictly dried, and if necessary, a tackifier can be added) ");

â–¶ 4, the melt viscosity is low, the apparent viscosity is sensitive to temperature, because the cooling rate of the melt is fast, it is necessary to prevent the plastic from clogging the nozzle hole, the runner, the gate, and the like. In order to prevent the counterflow of the melt, the screw head should be equipped with a check ring; in addition, in order to prevent the "flowing" phenomenon of the melt at the nozzle, a self-locking nozzle should be selected;

â–¶ 5, high injection pressure is not required when injecting PA, the general selection range is 70 ~ 100MPa, usually not more than 120MPa. The injection rate should be slightly faster, which can prevent the problems of ripple and insufficient filling due to the fast cooling rate;

▶ 6. The mold temperature is generally controlled at 40 to 90 °C. The mold temperature has a great influence on the performance of the product;

â–¶ 7. The amide group is sensitive to oxygen at high temperature, and it is prone to oxidative discoloration (if necessary, a special heat stabilizer for nylon can be added);

â–¶ 8, high crystallinity, large molding shrinkage, easy to produce crystal stress, and obviously increase with the thickness of the product;

▶ 9. The slow moisture absorption of the molded product is likely to cause a large change in dimensional accuracy. This point is also utilized for conditioning, usually in boiling water or potassium acetate aqueous solution (potassium acetate to water ratio of 1.25:1, boiling point of 121 ° C);

â–¶ 10. There are few varieties of organic pigments suitable for melt coloring (the amide group has a reducing property, and the molding temperature is high).

PMMA

▶ 1, PMMA amorphous polymer, Tg is 105 ° C, the melting temperature is greater than 160 ° C, and the decomposition temperature is up to 270 ° C or more, the molding temperature range is wider;

▶ 2, PMMA resin particles are easy to absorb water, and the presence of these moisture, due to heat volatilization during the molding process, causes the foam to foam, expand, and cause the product to appear silver, bubbles, transparency, and streaks. . PMMA drying on hot air circulation drying equipment, the drying process parameters: temperature is 70-80 ° C, time is 2 ~ 4h;

â–¶ 3, PMMA melt viscosity is sensitive to temperature changes. The effect of the change in injection temperature on the melt flow length is significantly greater than the injection pressure and injection rate, and much more than the mold temperature. Therefore, changing the fluidity of PMMA during molding is mainly from the injection temperature. However, when the high material temperature is selected, it is easily affected by other process parameters to bring discoloration to the surface of the product;

â–¶ 4, PMMA melt viscosity is relatively large, fluidity is relatively poor, therefore, requires a large injection pressure, usually a wide gate, easy to flow thick-walled products selected injection pressure is between 80 ~ 100MPa, and the melt The pressure required for products with more difficult flow is greater than 140 MPa, and 110-140 MPa is suitable for molding most products;

â–¶ 5. When injection molding PMMA products, high-speed injection often makes the periphery of the product unclear, which makes the light transmission of the product greatly reduced. Therefore, it is better not to use high-speed injection under normal circumstances.

â–¶ 6. Since the high transparency is the characteristic of PMMA, the presence of any impurities will be exposed on the product due to the light refraction relationship. Therefore, it is required to clean the environment when processing the material.

▶ 7, the temperature range is 40 ~ 60 ° C, the maximum should not exceed 80 ° C.

PBT

▶ 1. PBT is a crystalline material with obvious melting point, melting point is about 225 °C; PBT decomposition temperature is 280 °C; in actual production, the injection temperature is generally selected between 240 and 265 °C, and the unreinforced grade is lower. Temperature, enhanced grade with higher temperature;

▶ 2. PBT is easily degraded by water at high temperatures. Dry before injection molding, and control the moisture content below 0.02%. When using hot air circulation drying, when the temperature is 105 ° C, 120 ° C or 140 ° C, the corresponding time does not exceed 8h, 5h, 3h;

â–¶ 3, PBT has good fluidity in the molten state, low viscosity, only in nylon, it is easy to "cast" in the molding;

â–¶ 4, due to good fluidity, generally using a moderate to medium injection pressure, PBT injection pressure is generally 50 ~ 100MPa;

â–¶ 5, PBT cooling speed is fast, so use a faster injection rate;

â–¶ 6. When pre-plasticizing PBT, the screw speed is generally not more than 80r/min, generally between 25~60r/min. The back pressure is generally 10~15% of the injection pressure;

â–¶ 7 PBT is prone to anisotropy during the molding process;

▶ 8. The mold temperature is generally controlled at 70~80°C, and the temperature difference between each part does not exceed 10°C.