Vertical injection molding machine shrinkage rate (2)

Date：2015-09-27 23:29:43

        The main components of the plastic is a polymer (referred to as polymers), so the basic properties of plastics from the physical and chemical properties of polymers. Polymer according to molecular structure into a linear polymer and the polymer body. Thermoplastic polymer is a linear structure, molecular chains composed of many identical structural units covalently connected together by a certain way of repeating, van der Waals forces between the molecular chains, or to attract the hydrogen phase, so both thermoplastics a certain degree of strength, but also elastic and plastic, the temperature can be elevated to melt; thermosetting plastic phase in the prepolymer is a linear polymer molecules, but a chemical reaction in the curing process, there are atoms in the molecular chain between covalent Kin combine to form a spatial network structure, that is, body structure, so that after curing thermoset macromolecular movement can not occur between each other, the whole matter is a huge giant molecules, so thermosetting plastics brittle, not flexible and plastic, after melting heat no longer, if the temperature is too high, only to decompose. Vertical injection molding machine is mainly used for injection molding of thermoplastic plastics molding I5], applied in recent years, thermosetting plastic vertical injection molding machine injection molding method is also increasing.
        This study is limited to conventional thermoplastic injection molding machine vertical injection molding products, namely, that there is no chemical reaction occurs in the vertical injection molding machine injection molding process, the vertical injection molding machine injection molding materials experienced only physical change process. Vertical injection molding machine injection molding material properties, vertical injection molding machine injection molding process conditions and structure: on many factors thermoplastic injection molding machine vertical injection molding shrinkage rate impact from three aspects. The following were elaborated.
        1.2.1 Vertical injection molding machine is characterized by the material properties of the linear polymer main chain is a covalent bond can be internal rotation l6], which is a linear polymer having a root cause flexible. But the polymer main chain of internal rotation is not free, polymer-Africa repulsion adjacent link and interaction between molecules between atoms or groups bonded to make polymers respond to external stimuli with the outside world length about the stimulating effect of time. Temperature changes of polymers and polymer external force is applied, all belong to external stimuli, volume changes caused by polymer polymer response to external stimuli. Polymers made void volume (also known as free volume) polymer inherent volume between the polymer composition.
        Volume shrinkage cyanide copolymer is narrow the gap between the molecules, the continuous exchange of position by the polymer chain segments and the gap, the gap is gradually transferred to the surface of plastic wells release achieved. Molecular polymers athletic ability is stronger, the shorter the time to reach the volume of external stimuli required for equilibrium, so all the molecular structure of the polymer molecule athletic ability factors affecting all affect the volume shrinkage of plastic products in the process:
        Main chain structure of the main chain structure (1) polymer a significant impact on the polymer athletic ability. If the main chain composed entirely of a single bond, since the bond can be internal rotation, so that the polymer movement ability. But the difference of a single bond, within which there is still the difference rotation capability. Such as carbon-heteroatom bond rotational activation energy is less than C-C single bond, so the main chain structure of a C-51 one, one-S-C, a C and a C eleven N-101 polymer exercise capacity larger than the main chain structure is a C-C polymer athletic one. Double bonds in the polymer main chain itself can not rotate within. However, if the non-conjugated double bonds, but will make the polymer increases exercise capacity, because the composition of the double bond of the two atoms each reduction of one group, so that rotation of the main chain by the resistance than the single bond is small, this resistance reduced double bond in itself enough to offset the effects due to the rotation can not be brought inside; if it is conjugated double bonds, the molecular chain was rigid. When the main chain containing benzene, aromatic heterocyclic ring Cai, since aromatic heterocyclic not rotating, so poor exercise capacity polymer molecular chain was rigid.
        (2) When the polymer side groups pendant polar groups, the inner polar side group rotation increases between the activation energy and the intermolecular forces, the molecule reduced exercise capacity. When the side of the base is non-polar group, if the rotating inner side of the base of the barrier is small, due to the volume increase side groups make molecular packing loose and have published a pan-moving small points, anger side six star spin car designed barrier compared Large increase pendant bamboo product will increase the rotation of the main chain steric hindrance, the molecule reduced exercise capacity.
        Two arrangement rules thermoplastics between long-chain molecules (3) polymer chain, sometimes there is a rule in nature, resulting in possible crystalline polymers, it is divided into crystalline thermoplastics and non-crystalline ( also known as amorphous) two. Crystalline plastic melt solidification process, cooling rate varies with different degrees of crystallization, macromolecular crystallization region were ordered to make the gap between the molecules is significantly reduced, resulting in volume contraction on macroscopic plastic products . When the thermoplastic polymer having a regular rules and geometry of the chemical structure, the molecular long chain is easy to arrange a three-dimensional long-range order in the crystal lattice, namely the formation of crystals.
        For example the chemical structure unbranched polyethylene simplest, most regular geometric structure, and thus the most likely to form crystals, crystallization fastest; polystyrene Although the chemical structure of the rule, but the side groups is to take an arbitrary configuration in space type, JL what regularity is poor, making it difficult to crystallize: PVC configuration chlorine atoms in the molecular chain arrangement is random, and the radius of a hydrogen atom and a chlorine atom has a larger difference, so the geometric regularity of the poor and difficult to crystallization . The volume of non-crystalline plastic vertical injection molding machine injection molded article shrinkage is entirely caused by the temperature drop, called thermal shrinkage; volume crystalline plastic vertical injection molding machine injection molded article shrinkage, including thermal shrinkage caused by temperature drop crystallization and crystal formation caused by contraction. Shrinkage crystalline vertical injection molding machine injection products is significantly greater than the shrinkage of the amorphous vertical injection molding machine injection molding products.
        (4) additive particles are made of a synthetic resin plastics (including polymers and prepolymers) addition of stabilizers, coloring agents, plasticizers, fillers and other additives, after kneading, cooling, pulverizing obtained. These additives contraction vertical injection molding machine injection products will be affected. Low molecular weight substances such as plasticizers are added to the polymer, because the plasticizer molecules and the polymer has a strong affinity between the polymer so that the force is weakened, thus increasing the flexibility of the polymer chain, molecular motion capacity increased; and fillers usually hinder the movement of the polymer, the polymer of reduced exercise capacity. Process Parameters 1.2.2 Vertical injection molding machine injection molding process conditions on the vertical injection molding machine injection molding machine control panel can be set are as follows: barrel temperature (including the nozzle temperature), injection pressure, injection speed, hold pressure, dwell time and in-mold cooling time. Injection machine barrel outer ring equipped with electric heating, in the vicinity of the cylinder wall with a thermocouple and temperature control instruments connected to the control panel so the temperature value is set by the temperature of the cylinder wall and the inner wall of the nozzle.
        Since plastic is a poor heat conductor, so the actual temperature of the plastic is not only the setting of the barrel and nozzle temperature dependent, but also with a plastic barrel in a high temperature heating time related. If the vertical injection molding machine is a thick-walled injection molded products, within the required mold cooling time is longer, with a corresponding plastic in the barrel heating time will be extended. In actual vertical injection molding machine injection molding, it requires the operator to observe the melt stream of air injection and pre-production exterior appearance of the article to determine the barrel and nozzle temperature is appropriate. When the operator found suitable for a given plastic barrel and nozzle temperature, usually no change. Study on the vertical injection molding machine cylinder temperature injection molding shrinkage rate affects the way more emphasis on the theoretical aspects of meaning.
        Vertical injection molding machine injection molding process parameters can vary over a wide range of injection pressure, injection speed, hold pressure, dwell time and in-mold cooling time, the operator is mainly to get the appearance of these items by calling the move a parameter value vertical injection molding machine injection molding products. Vertical injection molding machine injection mold temperature molding surface is not all the same, forming one of the process conditions as mold temperature just an abstract concept, but rarely injection machine operator to adjust the mold cooling (or heating) medium temperature, but to explore Effect of mold temperature on the vertical injection molding machine injection molding shrinkage rate of vertical injection molding machine for injection molding have some significance, so in the following discussion together. By analyzing the literature experimental data and views, the following conclusions:
        (1) For thin-walled plastic cylinder temperature rises the vertical injection molding machine injection molding shrinkage rate a "drop, because the plastic melt as the temperature increases, the melt viscosity reduction, the flow resistance is reduced, Compaction pressures in favor of forming the mold cavity with the transfer material, so that the melt temperature increasing amount of thermal shrinkage phenomenon generated relegated to a secondary position; but if the gate cross-sectional dimension smaller than the thickness or plastic Great, so the gate solidify early, cutting off the stream approach, the vertical injection molding machine injection molding shrinkage rate will decrease with the barrel temperature.
        (2) increase the injection pressure and injection speed can be reduced vertical injection molding machine injection molded article shrinkage. Because the filling process faster, on the one hand because of the plastic melt shear heating while improving the melt temperature, reducing the flow resistance, on the other hand can also melt temperature is still high, the smaller flow resistance state earlier feeding into the packing stage. Especially for thin-walled plastic products and small gate plastic products, since the cooling speed, but we should try to shorten the filling process.
        (3) holding pressure and dwell time on the vertical injection molding machine injection molded article shrinkage maximum. Increasing the holding pressure and the holding time can extend the mold cavity plastics compacting, shrinkage is reduced, because the molten resin is compressed under the effect of the molding pressure, the higher the pressure, the greater the amount of compression occurs, the pressure after releasing elastic recovery are greater, and therefore the smaller the shrinkage.
        (4) For crystalline plastics, mold temperature is high, the heat will melt the mold cavity to maintain a long time, the product is cooled slowly, although the cooling rate is reduced in favor of material compaction, but more conducive to the cooling process polymer sufficiently crystallized, crystallinity increase, the result is shrinkage increases. For non-crystalline plastics, when the mold temperature is in the low temperature range, the difference between the vertical injection molding machine injection molded article ejection temperature between the ambient temperature is small, so close to the die temperature rises brought about by the mold wall slowing the growth rate of the solidified layer, stream center of the effective flow cross-section increases in favor of the role of the compacted material is more obvious, so shrinkage increased with mold temperature decreases; when the mold temperature is higher than 50 "C, with mold temperature, the difference between the vertical injection molding machine injection molded article ejection temperature between ambient temperature significantly increased after the contraction over the role of the die temperature rises brought about by the compaction of the material, the result is a contraction rate with the mold temperature is higher. But for polycarbonate cool, because of its poor melt flow, increasing the mold temperature can effectively reduce the melt viscosity, reduce material flow resistance, and thus always with the mold shrinkage temperature but decreases.
        (5) When the vertical injection molding machine injection molding products can not be completely free shrinkage in the mold, with the extension of the mold cooling time, the mold shrinkage impeded shrinkage reduction in size, thus widening and mold the gap between the size of the contraction process unhindered shrinkage rate.
        1.2.3 molding structure
        Molding structure includes a shape and foot shape and size, gating system of the mold cavity, a distribution j, and mold cooling (or heating) circuit. Molding structure determines the fundamental process of filling the plastic melt flow front how to promote Africa 1221 will mark the weld location, and at the end of the temperature distribution within the mold cooling process and the shrinkage of the vertical injection molding machine injection products distribution.
        If the cavity or core to shrink vertical injection molding machine injection molding products, hinder, then vertical injection molding machine injection molding products in a release had previously been contracted state in a non-free, there are in-mold limited effect. Cavity or core of the vertical injection molding machine Injection Molding shrinkage hinder the process can lead to the contraction of various parts of the vertical injection molding machine injection molding products a huge difference.
        Shrink-mold process unhindered shrinkage size, can reach the inner mold shrinkage hampered by the size of the contraction rate of 2 times. Large parts of the mold cavity thickness, internal products of slow cooling, when the gate closed, the core temperature of the material here is still high, hematocrit greater after the gate closed, cooling shrinkage of the core not fill material, so that an increase in the amount of shrinkage here. Usually in the shape of the design of vertical injection molding machine injection molding products, avoiding parts appear greater thickness difference, but let the thickness of the entire injection molding products vertical injection molding machines are as equal as possible. Large gate cross-sectional size, is conducive to the delivery pressure, improve the mold cavity pressure and postponed gate freeze time, increased fill quantity, thus reducing shrinkage vertical injection molding machine injection molding products, but over the General Assembly at the gate vertical injection molding machine injection molding product surface leaving visible traces, it is generally choose a small size and arranged in the secluded gate position, can be improved by increasing the number of gates filling and packing cases.
        Mold cooling circuit of distribution, cavity surface temperature, thus affecting the cooling rate and contraction vertical injection molding machine injection molding products at various points. From the mold cavity surface cooling circuit closer to a place, by the influence of the cooling medium is strong, the plastic melt is cooled quickly here, in this regard shorten the temperature change member took} sentence, so that the actual specific volume of plastics B gap between the value of the hematocrit value and balance the state increased, on the other hand when the inner into the mold contraction phase, vertical injection molding machine where the surface layer of the injection molded article temperature has been low, so the degree of shrinkage that can occur small. In the three factors affecting shrinkage, the mold structure is to enable vertical injection molding machine Injection Molding shrinkage different situations around the main factors.

【Return】