7 factors to consider in the injection molding process

First, the shrinkage rate
The form and calculation of thermoplastic molding shrinkage As mentioned earlier, the factors affecting thermoplastic molding shrinkage are as follows:
1.1 plastic varieties thermoplastic molding process due to the volume change of crystallization shape, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, so compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, the directionality is obvious, and the shrinkage rate after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics.
1.2 Characteristics of plastic parts: When molding, the molten material is in contact with the cavity surface, and the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, and high-density layer thickness shrink greatly. In addition, the presence or absence of inserts, the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a greater impact on shrinkage size and directionality.
1.3 The form, size and distribution of the feed port directly affect the direction of material flow, density distribution, pressure holding and shrinkage and molding time. The direct inlet and the inlet section are large (especially the thick section) have small shrinkage but large directionality, and the wide and short inlet has little directionality. Those that are close to the feed port or parallel to the direction of the material flow are greatly contracted.
1.4 Molding conditionsThe mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large, especially for the crystalline material, because of the high crystallinity and large volume change, the shrinkage is greater. The mold temperature distribution is also related to the cooling and density uniformity inside and outside the plastic parts, which directly affects the shrinkage and directionality of each part. In addition, the holding pressure and time also have a great influence on the contraction, and the contraction is small but the directionality is large if the pressure is large and the time is long. The injection pressure is high, the viscosity difference of the molten material is small, the shear stress between layers is small, and the elastic jump after demolding is large, so the shrinkage can also be appropriately reduced, the material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.
In the mold design, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the form size and distribution of the feeding port, the shrinkage rate of each part of the plastic parts is determined empirically, and then the cavity size is calculated. For high-precision plastic parts and it is difficult to grasp the shrinkage, it is generally advisable to design the mold by the following methods:
(1) Take a smaller shrinkage rate for the outer diameter of the plastic parts and a larger shrinkage rate for the inner diameter to leave room for correction after mold testing.
(2) The mold trial determines the form, size and molding conditions of the pouring system.
(3) The plastic parts to be post-processed are post-processed to determine the size change (the measurement must be 24 hours after demolding).
(4) Modify the mold according to the actual shrinkage situation.
(5) Re-test the mold and appropriately change the process conditions, slightly modify the shrinkage value to meet the requirements of plastic parts.
2. Liquidity
2.1 The fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melting index, Archimedes spiral flow length, performance viscosity and flow ratio (process length / wall thickness of plastic parts). Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melting index, long spiral flow length, small viscosity, large flow ratio then the fluidity is good, the same name of the plastic must check its manual to determine whether its fluidity is suitable for injection molding. According to the mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:
(1) Good fluidity PA, PE, PS, PP, CA, poly(4) methylpentene;
(2) Medium fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether;
(3) Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyaryl sulfone, fluoroplastic.
2.2 The fluidity of various plastics also changes due to various molding factors, and the main factors affecting are as follows:
(1) The fluidity of high temperature material temperature increases, but different plastics also have differences, PS (especially impact resistant type and MFR value is high), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics fluidity changes greatly with temperature. For PE, POM, temperature increase and decrease have little effect on their fluidity. Therefore, the former should adjust the temperature to control the fluidity when molding.
(2) When the pressure injection pressure increases, the molten material is greatly affected by shear and the fluidity is also increased, especially PE and POM are more sensitive, so it is advisable to adjust the injection pressure to control the fluidity during molding.
(3) The form, size, arrangement, cooling system design of the mold structure pouring system, the flow resistance of the molten material (such as profile finish, forehearth section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of the molten material in the cavity, and the fluidity will be reduced if the molten material is reduced to reduce the temperature and increase the fluidity resistance. The mold design should be based on the fluidity of the plastic used, and a reasonable structure should be selected. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to properly adjust the filling situation to meet the molding needs.
Third, crystallinity
Thermoplastics can be divided into two categories: crystalline plastics and amorphous (also known as amorphous) plastics according to their lack of crystallization when condensing.
The so-called crystallization phenomenon is that when the plastic is condensed from the molten state, the molecules move independently, are completely in an unordered state, become molecules stop moving freely, according to a slightly fixed position, and there is a tendency to make the molecular arrangement a normal model.
As a criterion for judging the appearance of these two types of plastics, the transparency of the thick-walled plastic parts of the plastic can be determined, and the general crystalline material is opaque or translucent (such as POM, etc.), and the amorphous material is transparent (such as PMMA, etc.). However, there are exceptions, such as poly(4)methyl quinene is a crystalline plastic with high transparency, and ABS is amorphous but opaque.
In the mold design and selection of injection molding machine, attention should be paid to the following requirements and precautions for crystalline plastics:
(1) The material temperature rises to the molding temperature requires more heat, and equipment with large plasticizing capacity is used.
(2) When cooling and returning, the heat released is large, and it should be fully cooled.
(3) The specific gravity difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage pores and pores are easy to occur.
(4) Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic parts, and the wall thickness is slow to cool, with high crystallinity, large shrinkage and good physical properties. Therefore, the crystalline material should be controlled according to the requirements.
(5) The anisotropy is significant, and the internal stress is large. Molecules that are not crystallized after demolding have a tendency to continue crystallization, are in an energy imbalance state, and are prone to deformation and warpage.
(6) The crystallization temperature range is narrow, and it is easy to inject the unmelted material into the mold or block the feeding port.
4. Heat-sensitive plastics and easily hydrolyzed plastics
4.1 Thermal sensitivity refers to some plastics are more sensitive to heat, in high temperature heating time is longer or the feed port section is too small, when the shear effect is large, the material temperature is easy to occur discoloration, degradation, decomposition tendency, plastics with this characteristic are called heat-sensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytrifluoroethylene, etc. Heat-sensitive plastics produce monomers, gases, solids and other by-products during decomposition, especially some decomposition gases have irritating, corrosive or toxic effects on the human body, equipment and molds. Therefore, mold design, selection of injection molding machine and molding should pay attention to the use of screw injection molding machine, pouring system section should be large, mold and barrel should be chrome-plated, there should be no * angular lag material, must strictly control the molding temperature, add stabilizer to the plastic, weaken its thermal performance.
4.2 Some plastics (such as PC) even if they contain a small amount of moisture, but at high temperature and high pressure will decompose, this performance is called easy hydrolysis, which must be preheated and dried.
5. Stress cracking and melt rupture
5.1 Some plastics are sensitive to stress, easy to produce internal stress and brittle and crack during molding, and cracking occurs under the action of external force or solvent. To this end, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should pay attention to drying, reasonable selection of molding conditions to reduce internal stress and increase crack resistance. And should choose a reasonable shape of plastic parts, should not set up inserts and other measures to minimize stress concentration. Mold design should increase the demolding slope, choose a reasonable feed port and ejection mechanism, molding should be appropriate adjustment of material temperature, mold temperature, injection pressure and cooling time, try to avoid plastic parts too cold and brittle when demolding, after molding plastic parts should also be post-treated to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
5.2 When a polymer melt with a certain melt flow rate, after its flow rate exceeds a certain value when passing through the nozzle hole at a constant temperature, obvious transverse cracks occur on the melt surface called melt rupture, which damages the appearance and physical properties of plastic parts. Therefore, in the selection of polymers with high melt flow rate, the nozzle, runner, and feed section should be increased, the injection speed should be reduced, and the material temperature should be increased.
6. Thermal performance and cooling rate
6.1 Various plastics have different specific heat, thermal conductivity, heat deflection temperature and other thermal properties. When plasticizing with high specific heat, the heat is large, and an injection molding machine with large plasticizing capacity should be selected. The cooling time of plastics with high heat deflection temperature can be short, and the demolding is early, but cooling deformation should be prevented after demolding. Plastics with low thermal conductivity have a slow cooling rate (such as ionopolymers, etc.), so they must be fully cooled to strengthen the mold cooling effect. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with large specific heat, low thermal conductivity, low heat deflection temperature and slow cooling rate are not conducive to high-speed molding, and appropriate injection molding machines must be selected and mold cooling must be strengthened.
6.2 All kinds of plastics according to their type characteristics and plastic parts shape, requirements must maintain an appropriate cooling rate. Therefore, the mold must be set up with heating and cooling systems according to the molding requirements to maintain a certain mold temperature. When the material temperature increases the mold temperature, it should be cooled to prevent the deformation of plastic parts after demolding, shorten the molding cycle, and reduce the crystallinity. When the plastic waste heat is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve the filling conditions or control the plastic parts to cool them slowly, prevent uneven cooling inside and outside the thick-walled plastic parts and improve the crystallinity. For good fluidity, large molding area and uneven material temperature, it is sometimes necessary to heat or cool alternately or use local heating and cooling together according to the molding situation of plastic parts. For this purpose, the mold should be equipped with a corresponding cooling or heating system.
7. Hygroscopicity
Plastics because of a variety of additives, so that it has different degrees of affinity to moisture, so plastics can be roughly divided into moisture absorption, adhesion of moisture and non-absorbent and not easy to adhere to moisture two, the water content in the material must be controlled within the allowable range, otherwise at high temperature, high pressure moisture into gas or hydrolysis, so that the resin blistering, fluidity decrease, appearance and mechanical properties are poor. Therefore, hygroscopic plastics must be preheated according to appropriate heating methods and specifications as required, and absorb moisture when used.