Shrinkage
The factors affecting the shrinkage of thermoplastics are as follows:
In the molding process of plastics thermoplastics, due to the volume change of crystallization, the internal stress is strong, the residual stress in the plastic parts is large, and the molecular orientation is strong, so the shrinkage rate is larger than that of the thermosetting plastic. The shrinkage rate is wide and the directivity is obvious. In addition, the shrinkage after shrinkage, annealing or humidity control after molding is generally larger than that of thermosetting plastic.
When the plastic part is formed, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid outer casing. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with a large shrinkage. Therefore, the wall thickness, the slow cooling, and the high density layer thickness are large. In addition, the presence or absence of inserts and insert layouts and quantities directly affect the flow direction, density distribution and shrinkage resistance, so the characteristics of the plastic parts have a great influence on the shrinkage size and directionality.
The factors such as the form, size and distribution of the feed port directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time. The direct feed port and the feed port have a large cross section (especially thicker cross section), but the shrinkage is small but the directivity is large, and the feed port width and the short length are small. The shrinkage is large near the feed port or parallel to the flow direction.
Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystallized material, the crystallinity is high and the volume changes greatly, so the shrinkage is larger. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the amount and direction of shrinkage of each part. In addition, maintaining pressure and time also has a large effect on shrinkage. When the pressure is large and the time is long, the shrinkage is small but the direction is large. The injection pressure is high, the viscosity difference of the melt is small, the interlaminar shear stress is small, and the elastic rebound is large after demolding, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the directionality is small. Therefore, factors such as adjustment of mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.
According to the shrinkage range of various plastics, the thickness and shape of the plastic part, the size and distribution of the feed port, the shrinkage rate of each part of the plastic part is determined empirically, and then the cavity size is calculated. For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally advisable to design the mold by the following method:
1 The outer diameter of the plastic part is taken to be smaller, and the inner diameter is taken to a larger shrinkage rate to leave room for correction after the test.
2 The test mold determines the form, size and molding conditions of the casting system.
3 The post-treatment plastic parts are post-treated to determine the dimensional change (measured must be 24 hours after demolding).
4 Correct the mold according to the actual shrinkage.
5 Re-test the mold and adjust the process conditions slightly to correct the shrinkage value to meet the requirements of the plastic parts.
fluidity
a) The fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length / plastic wall thickness). The molecular weight is small, the molecular weight distribution is wide, the molecular structure is poor in regularity, the melt index is high, the snail flow length is long, the viscosity is small, and the flow ratio is good. For the plastic of the same product name, the specification must be checked to determine whether the fluidity is applicable. For injection molding. According to the mold design requirements, the fluidity of common plastics can be roughly divided into three categories:
Good fluidity Pa, PE, PS, PP, Ca, poly(4) methyl decene;
Medium-sized polystyrene resin (such as aBS, aS), PMMa, POM, polyphenylene ether;
Poor flowability of PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastic.
b) The fluidity of various plastics also changes due to various molding factors. The main factors affecting are as follows:
When the temperature of the material is high, the fluidity increases, but different plastics also have different characteristics, PS (especially high impact resistance and MFR value), PP, Pa, PMMa, modified polystyrene (such as aBS, aS), The fluidity of plastics such as PC and Ca varies greatly with temperature. For PE and POM, the temperature increase and decrease has little effect on the fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.
When the pressure injection pressure is increased, the molten material is subjected to large shearing action and fluidity is also increased, especially PE and POM are sensitive, so the injection pressure should be adjusted during molding to control the fluidity.
The form, size, arrangement, cooling system design, melt flow resistance (such as profile finish, channel section thickness, cavity shape, exhaust system) of the mold structure casting system directly affect the molten material in the cavity. The actual fluidity, if the melt is reduced in temperature and the fluidity resistance is increased, the fluidity is lowered. The mold should be designed according to the fluidity of the plastic used, and a reasonable structure should be selected. When molding, it can also control the material temperature, mold temperature and injection pressure, injection speed and Other factors to properly adjust the filling to meet the molding needs.
Crystallinity
Thermoplastics can be divided into two types: crystalline plastics and amorphous (also known as amorphous) plastics. The so-called crystallization phenomenon is that when the plastic is from the molten state to the condensation, the molecules move independently, completely in a disordered state, and the molecules stop moving freely, at a slightly fixed position, and have a tendency to make the molecules arranged into a regular model. a phenomenon. As a criterion for judging the appearance of these two types of plastics, it is possible to visually recognize the transparency of thick plastic parts of plastics. Generally, the crystalline material is opaque or translucent (such as POM), and the amorphous material is transparent (such as PMMa, etc.). However, there are exceptions. For example, poly(4) methyl decene is a crystalline plastic with high transparency, and aBS is amorphous but not transparent. In the mold design and selection of injection molding machines, attention should be paid to the following requirements and precautions for crystalline plastics:
The heat required to raise the temperature to the molding temperature is high, and equipment with a large plasticizing capacity is used.
When cooling back, the heat is released and it needs to be cooled sufficiently.
The specific gravity difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage cavities and pores are likely to occur.
Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part, the wall thickness is slow to cool, the crystallinity is high, the shrinkage is large, and the physical properties are good. Therefore, the crystalline material should control the mold temperature as required.
The anisotropy is remarkable and the internal stress is large. Molecules that have not crystallized after demolding tend to continue to crystallize, are in an energy-unbalanced state, and are prone to deformation and warpage.
The crystallization temperature range is narrow, and it is easy to inject the unmelted material into the mold or block the feed port.
Thermal plastics and easily hydrolyzed plastics
Thermosensitive means that some plastics are sensitive to heat, when the heating time is long at high temperature or the cross section of the feed port is too small, and when the shearing effect is large, the temperature of the material is increased, and the tendency of discoloration, degradation and decomposition is prone to occur. Plastics are called heat sensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene and so on. The heat-sensitive plastics produce by-products such as monomers, gases, solids, etc. during decomposition, and in particular, 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, should use screw injection molding machine, the casting system should have a large section, mold and barrel should be chrome-plated, no * angle stagnation, must strictly control molding temperature, plastic Stabilizer is added to reduce its thermal performance.
Some plastics (such as PC), even if they contain a small amount of water, decompose under high temperature and high pressure. This property is called easy hydrolysis, and it must be preheated and dried.
Stress cracking and melt fracture
Some plastics are sensitive to stress, and are prone to internal stress during molding and are brittle and fragile. The plastic parts are cracked under the action of external force or under the action of solvent. To this end, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should be dry, reasonable selection of molding conditions to reduce internal stress and increase crack resistance. And should choose a reasonable plastic shape, it is not appropriate to set up inserts and other measures to minimize stress concentration. When designing the mold, the draft should be increased, and a reasonable feeding port and ejection mechanism should be selected. The material temperature, mold temperature, injection pressure and cooling time should be properly adjusted during molding to avoid mold release when the plastic parts are too cold and brittle. After molding, the plastic parts should also be post-treated to improve the crack resistance, eliminate the internal stress and prohibit contact with the solvent.
When the polymer melt of a certain melt flow rate exceeds a certain value when passing through the nozzle hole at a constant temperature, a significant lateral crack occurs on the surface of the melt, which is called melt fracture, which impairs the appearance and physical properties of the plastic part. Therefore, in the selection of polymers with high melt flow rate, the nozzle, runner, and feed port sections should be increased to reduce the injection speed and increase the material temperature.
Thermal performance and cooling rate
Various plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. When the plasticization is higher than the heat, it requires a large amount of heat, and an injection molding machine with a large plasticizing capacity should be used. High heat distortion temperature The plastic can be cooled for a short period of time, and the mold release is early, but it is prevented from cooling deformation after demolding. Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers and other cooling rates are extremely slow), so they must be sufficiently cooled to enhance the mold cooling effect. Hot runner molds are suitable for plastics with lower heat and high thermal conductivity. Plastics with higher specific heat, lower thermal conductivity, lower heat distortion temperature and slower cooling rate are not conducive to high-speed molding, and proper injection molding machines and mold cooling must be used.
Various plastics are required to maintain an appropriate cooling rate according to their type characteristics and shape of the plastic part. Therefore, the mold must be set to the heating and cooling system according to the molding requirements to maintain a certain mold temperature. When the material temperature raises the mold temperature, it should be cooled to prevent deformation of the plastic parts after demolding, shorten the molding cycle, and reduce the crystallinity. When the residual heat of the plastic is not enough to keep the mold at a certain temperature, the mold should be provided with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure the fluidity, improve the filling conditions or control the plastic parts to cool slowly. Prevent uneven cooling inside and outside of thick-walled plastic parts and improve crystallinity. For good fluidity, large forming area and uneven material temperature, it is sometimes necessary to use heating or cooling alternately or local heating and cooling according to the molding condition of the plastic parts. For this purpose, the mould should be equipped with a corresponding cooling or heating system.
Hygroscopicity
Plastics have various additives to make them have different degrees of moisture. Therefore, plastics can be roughly divided into two types: moisture absorption, adhesion moisture, non-absorbent water and non-stick moisture. The water content in the material must be controlled within the allowable range. In the case, the water becomes gas or hydrolyzes under high temperature and high pressure, causing foaming of the resin, deterioration of fluidity, and poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated as required by appropriate heating methods and specifications to prevent re-absorption during use.
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