The molding process of general screw injection molding […]
The molding process of general screw injection molding machine is as follows: firstly, the granular or powdery plastic is added into the barrel, and the plastic becomes molten after the rotation of the screw and the outer wall of the barrel is heated. Then, the machine closes the mold and moves the injection seat forward to make the nozzle close to the gate of the mold, and then the pressure oil is injected into the injection cylinder to push the screw forward with high pressure and speed After a certain time and pressure (also known as pressure holding) and cooling, the mold can be opened to take out the product (the intention of pressure maintaining is to avoid the reverse flow of molten material in the mold cavity, make up the material in the mold cavity, and ensure that the product has a certain density and scale service). The basic requirements of injection molding are plasticization, injection and molding. Plasticization is the condition to complete and ensure the quality of molded products. In order to meet the requirements of molding, it is necessary to ensure the satisfactory pressure and speed of injection. Because of the high injection pressure, a high pressure occurs in the mold cavity (the uniform pressure in the mold cavity is generally between 20 and 45 MPa), so it is necessary to have a satisfactory large clamping force. Thus, the injection equipment and clamping equipment are the key parts of the injection molding machine.
What factors affect the injection process of screw injection molding machine?
1、 Shortening rate
The method and calculation of thermoplastic molding shortening are as mentioned above. The factors influencing the shortening of thermoplastic molding are as follows:
1.1 plastics in the molding process of thermoplastic plastics, there are still some factors such as volume change from crystallization, strong internal stress, large residual stress frozen in the plastic parts and strong molecular orientation. Therefore, compared with thermosetting plastics, the shortening rate is larger, the scale of shortening rate is wide, and the direction is obvious. The shortening rate after other molding, annealing or humidity control treatment is generally more than heat Solid plastic is big.
1.2 during molding, the molten material and the outer surface of the mold cavity touch the outer layer immediately to form a low-density solid shell. Because of the poor thermal conductivity of the plastic, the inner layer of the plastic part is cooled slowly to form a shortened high-density solid layer. Therefore, the wall thickness, slow cooling and high density layer thickness will shorten greatly. In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the direction of material flow, density dispersion and reduction of resistance, so the characteristics of plastic parts have great influence on shortening size and direction.
1.3 the feed inlet mode, size and dispersion directly affect the material flow direction, density dispersion, pressure maintaining and feeding function and molding time. If the cross section of the direct feed port and the feed inlet is large (especially the thicker section), the shortening is small but the directionality is large, and the directionality is small if the feed port width and length are short. If it is close to the feeding port or parallel to the direction of the material flow, it will shorten greatly.
1.4 molding conditions: mold temperature is high, melting material cooling is slow, density is high and shortening is large, especially for crystalline material, because of high crystallinity and large volume change, the shortening is greater. Mold temperature dispersion is also related to the surface cooling and density uniformity of plastic parts, and directly affects the shortening amount and direction of each part. In addition, sticking to pressure and time also has a greater impact on shortening, while those with higher pressure and longer duration will shorten less but have greater directionality. The injection pressure is high, the viscosity difference of the melt is small, the shear stress between layers is small, and the elastic rebound after demoulding is large, so the shortening can be appropriately reduced, the material temperature is high, the shortening is large, but the directivity is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time can also change the shortening condition of plastic parts. In mold planning, according to the shortening scale of various plastics, the wall thickness and shape of plastic parts, the mode, scale and distribution of feeding port, the shortening rate of each part of plastic parts is confirmed according to experience, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp the shortening rate, the following methods should be used for mold planning:
① The outer diameter and inner diameter of the plastic parts should be shortened at a smaller rate, so as to reserve the extent of correcting after the test.
② The mode, scale and molding conditions of the pouring system were confirmed by mold test.
③ The plastic parts to be post-treatment shall be confirmed by post-treatment (it is necessary to measure 24 hours after demoulding).
④ Correct the mould according to the practical shortening condition.
⑤ Try the mold again and modify the process conditions appropriately. Modify the shortening value slightly to meet the requirements of the plastic part.
2.1 the activity size of thermoplastic can be analyzed from a series of indexes, such as molecular weight, melt index, active length of Archimedes spiral, embodied viscosity and activity ratio (process length / plastic part wall thickness). The products with small molecular weight, wide molecular weight dispersion, poor molecular structure regularity, high melt index, long screw activity length, low viscosity, and high activity ratio make the activity better. It is necessary to check the instructions of plastics with the same product name to determine whether its activity is suitable for injection molding. According to the basic requirements of mold planning, the activities of commonly used plastics can be roughly divided into three categories
① The activity of PA, PE, PS, PP, Ca and poly (4) methylstilene was good;
② A series of middle active polystyrene resins (such as ABS, as), PMMA, POM and polyphenylene oxide (PPO) were prepared;
③ Poor activity of PC, rigid PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.
2.2 the activity of various plastics also varies with each molding element, and the primary influencing factors are as follows:
① The activity of PS (especially impact resistant and high MFR), PP, PA, PMMA, modified polystyrene (such as ABS, as), PC, Ca and other plastics changed greatly with temperature. For PE, POM, temperature has little effect on its activity. Therefore, the former should adjust the temperature to control the activity.
② When the injection pressure is increased, the melt will be subject to shear action, and the mobility will also increase. Especially, PE and POM are more sensitive. Therefore, the injection pressure should be adjusted to control the activity.
③ The mold structure, pouring system mode, scale, placement, cooling system planning, molten material movement resistance (such as mold surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the practical activity of molten material in the mold cavity. When the temperature of molten material is promoted to drop and the activity resistance is added, the activity will decrease. Mold planning should be based on the activity of the plastic used, select a reasonable structure. During molding, the material temperature, mold temperature, injection pressure and injection speed can also be controlled. The filling condition should be adjusted properly to meet the molding requirements.
Thermoplastic plastics can be divided into crystalline plastics and amorphous plastics according to their non crystallization phenomenon during condensation.
The so-called crystallization phenomenon refers to the phenomenon that the molecules move from independent to disordered state when the plastics are from melting state to condensation state. The molecules stop their free movement and move in a slightly fixed position, and there is a tendency to make the molecular placement a regular model.
As a criterion for judging the appearance of these two types of plastics, the transparency of thick wall plastic parts can be determined. Generally, crystalline materials are opaque or semi transparent (such as POM), while amorphous materials are transparent (such as PMMA). However, there are some exceptions, such as poly (4) methylene is crystalline plastic but has high transparency, ABS is amorphous but not transparent.
In mold planning and selection of injection molding machine, the following requirements and precautions for crystalline plastics should be noted:
① It takes a lot of heat to raise the material temperature to the molding temperature, so it needs plasticizing equipment.
② When cooling and recycling, it gives out a lot of heat and needs sufficient cooling.
③ The difference of specific gravity between molten state and solid state is large, forming shortening is large, and shrinkage and porosity are easy to occur.
④ It has the advantages of fast cooling, low crystallinity, small shortening and high transparency. The crystallinity is related to the wall thickness of the plastic part, while the wall thickness has slow cooling, high crystallinity, large shortening and good physical properties. Therefore, it is necessary to control the mold temperature according to the requirements.
⑤ The anisotropy is obvious and the internal stress is large. After demoulding, the non crystallized molecules have the tendency of continuous crystallization, which is in the energy imbalance state, prone to deformation and warpage.
⑥ The crystallizing temperature scale is narrow, and it is easy to inject unfused material into the mold or block the feed port.
4、 Thermosensitive plastics and hydrolyzable plastics
4.1 thermal sensitivity refers to the fact that some plastics are more sensitive to heat. When the heating time is long at high temperature, or the cross section of the feed inlet is too small, and the shear effect is large, the material temperature increases and is easy to change color, degrade and differentiate. The plastics with this characteristic are called heat sensitive plastics. Such as hard PVC, poly (vinylidene chloride), vinyl acetate copolymer, POM, poly (trifluoroethylene), etc. During the differentiation of thermosensitive plastics, monomers, gases, solids and other by-products occur, especially some differentiation gases have influence, corrosion or toxicity on human body, equipment and mold. Therefore, attention should be paid to mold planning, selection of injection molding machine and molding. Screw type injection molding machine should be selected. The cross section of pouring system should be large. The mold and barrel should be chrome plated. There should be no * angle lag. It is necessary to strictly control the molding temperature and take part in the stabilizer in the plastic guess, so as to weaken its thermal sensitivity function.
4.2 some plastics (e.g. PC) may differentiate under high temperature and high pressure even if they contain a small amount of water. This function is called easy hydrolysis. Therefore, it is necessary to preheat and dry them.
5、 Stress cracking and melt cracking
5.1 some plastics are sensitive to stress and easy to crack due to internal stress during molding. Plastic parts crack under external force or solvent. Therefore, in addition to adding additives to improve the crack resistance of the material, attention should be paid to the dullness of the material and reasonable selection of molding conditions to reduce the internal stress and add crack resistance. The reasonable shape of plastic parts should be selected instead of setting inserts to reduce stress concentration. Mold planning should increase the demoulding slope, select reasonable feed inlet and ejection structure, properly adjust material temperature, mold temperature, injection pressure and cooling time during molding to avoid demoulding when plastic parts are too cold and brittle. After molding, plastic parts should be treated to improve cracking resistance, eliminate internal stress and prevent contact with solvent.
5.2 when the polymer melt with a certain melt activity rate passes through the nozzle hole at constant temperature, when the flow rate exceeds a certain value, a significant transverse crack occurs on the surface of the melt, which is called melt cracking, which damages the appearance and physical properties of the plastic part. Therefore, in the selection of polymer with high melt activity rate, the cross-sections of nozzle, runner and feed inlet should be increased, injection speed should be reduced and material temperature should be improved.
6、 Thermal function and cooling rate
6.1 various plastics have different specific heat, thermal conductivity, thermal deformation temperature and other thermal functions. The plasticizing machine with high specific heat needs more heat, so the injection molding machine with high plasticizing ability should be selected. The cooling time of plastics can be short and the demoulding is early, but the cooling deformation should be avoided after demoulding. The cooling rate of plastics with low thermal conductivity is slow (such as ionic polymer, etc.), so it is necessary to cool sufficiently and strengthen the cooling effect of mould. Hot runner mold is suitable for plastics with low specific heat and high heat conductivity. The plastics with high specific heat, low thermal conductivity, low thermal deformation temperature and slow cooling rate are not conducive to high-speed molding. It is necessary to select appropriate injection molding machine and strengthen mold cooling.
6.2 it is necessary to adhere to appropriate cooling rate for various plastics according to their variety characteristics and shape of plastic parts. Therefore, it is necessary to set the heating and cooling system according to the molding requirements to adhere to a certain mold temperature. When the material temperature increases the mold temperature, it should be cooled to avoid deformation after demoulding, shorten the molding cycle and reduce the crystallinity. When the plastic waste heat is insufficient to make the mold adhere to a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature, so as to control the cooling rate, ensure the mobility, improve the filling conditions or control the plastic parts to cool slowly, so as to avoid uneven cooling on the surface and improve the crystallinity of the thick wall plastic parts. For those with good activity, large forming area and uneven material temperature, it is necessary to heat or cool for replacement or partial heating and cooling according to the molding condition of plastic parts. Therefore, the mould should be equipped with corresponding cooling or heating system.
Because of various additives, plastic has different degrees of affinity to moisture, so plastics can be roughly divided into two types: moisture absorption, water adhesion and non water absorption, which are not easy to adhere to water. To guess the water content, it is necessary to control the water content within the allowable scale. Otherwise, in high temperature and high pressure, water will change into gas or hydrolysis, which will make the resin foaming, activity decline, appearance and mechanical function not Good. Therefore, it is necessary for hygroscopic plastics to select appropriate heating methods and standards for preheating according to the requirements, so as to avoid moisture absorption again when using.