Enterprise name: Zongsheng Precision Mould Co., Ltd.
Contact: Mr. Nie 13923298099 (Wechat same number).
Miss Jia 15820657628 (Wechat same number).
Tel: 0757 2226 9630.
0757 color 22269276.
Fax: 0757 2226 9631 22269276.
Web site: en.fszongsheng.com.
Add: no. 1, Rongye 6 road, Fengxiang industrial zone, Daliang, Shunde district, Foshan city, Guangdong province
There are many kinds of thermoplastics, even the same kind because of resin.
The application and process characteristics are also different due to the different ratio of son and additive.
In addition, in order to change the characteristics of the original kind, various chemical methods such as copolymerization and cross-linking are commonly used to introduce a certain percentage of other monomers or polymers into the original resin structure. in order to change the structure of the original resin into a new modified product with improved physical and processing properties.
For example, ABS is a modified copolymer after the introduction of the second and third monomers such as acrylonitrile and butadiene in polystyrene molecules, which can be regarded as modified polystyrene, which has better inductive properties and technological properties than polystyrene.
Because there are many kinds of thermoplastics and their properties are messy, even the same kind of plastics can only be used for injection molding and extrusion, so this chapter mainly introduces all kinds of thermoplastics used for injection molding.
The method and accounting of thermoplastic molding shortening as mentioned earlier, the factors that affect thermoplastic molding shortening are as follows:
1.1 there are still some factors in the molding process of thermoplastics, such as the volume change of crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and so on, so the shortening rate is larger than that of thermosetting plastics. the shortening rate is wide and directional, and the shortening rate after molding, annealing or humidification treatment is generally higher than that of thermosetting plastics.
1.2 when the plastic parts are molded, the melting material and the outer layer of the cavity touch the outer layer immediately to cool to form a low-density solid shell.
Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is cooled slowly by petg embryo injection mold to form a high-density solid layer with large shortening.
Therefore, the wall thickness, slow cooling and high density layer thickness are shortened.
In addition, the presence or absence of inserts and the layout and number of inserts have a direct impact on the direction of material flow, density dispersion and shortening resistance, so the characteristics of plastic parts have a great influence on shortening size and directionality.
1.3 the way, scale and dispersion of the feed port directly affect the direction of material flow, density dispersion, compression and shrinkage use and forming time.
The direct feed port and the feed port with large cross-section (especially thick cross-section) are shortened but the direction is large, and the direction is small when the feed port width and length are short.
Those near to the feed port or parallel to the direction of the material flow are shortened greatly.
1.4 under the forming conditions, the mold temperature is high, the melting material cools slowly, the density is high, and the shortening is large, especially for the crystallized material because of its high crystallinity and large volume change, so the shortening is even greater.
Mold temperature dispersion is also related to the internal and external cooling and density uniformity of plastic parts, directly to the shortening size and directionality of each part.
The customized holding pressure and time of the oil bottle mold also have a great influence on the shortening, while the pressure is high and the time is long, the shortening 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 after demoulding is large, so the shortening can also be reduced appropriately, the material temperature is high, the shortening is large, but the direction is small.
Therefore, the shortening condition of plastic parts can also be changed by adjusting the mold temperature, pressure, injection speed and cooling time during molding.
In the mold planning, according to the shortening scale of all kinds of plastics, the wall thickness and shape of the plastic parts, the scale and dispersion of the feed port, the shortening rate of each part is confirmed according to the experience, and then the cavity scale is calculated.
For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, it is generally appropriate to use the following methods to plan the mold:
The main results are as follows: (1) the outer diameter of the plastic parts should be shortened with a smaller shortening rate and the inner diameter with a larger shortening rate, so as to keep the degree of correction after testing the mold.
(2) the mold test confirms the mode, scale and forming conditions of the gating system.
(3) the plastic parts to be processed after post-processing to confirm the change of scale (the measurement must be 24 hours after demoulding).
(4) correcting the mold according to the shortening condition of practice.
(5) try the mold again and change the process conditions appropriately and modify the shortening value slightly to meet the requirements of plastic parts.
2.1 the activity size of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melting index, Archimedes helix activity length, apparent viscosity and activity ratio (process length / plastic wall thickness).
The molecular weight is small, the molecular weight dispersion is wide, the molecular structure is poor, the melting index is high, the snail activity length is long, the apparent viscosity is low, and the activity ratio is large, the fluidity is good. For plastics of the same name, it is necessary to check their instructions to determine whether their activity is suitable for injection molding.
According to the requirements of mold planning, the activity of commonly used plastics can be roughly divided into three categories:
1 active nylon, polyethylene, polystyrene, polypropylene, cellulose acetate, poly (4) methyl garrison;
(2) active medium polystyrene series resins (such as ABS, AS), plexiglass, polyformaldehyde, polyphenylene ether;
(3) poor activity polycarbonate, rigid polyvinyl chloride, polyphenylene ether, polysulfone, polyaromatic sulfone, fluoroplastics.
2.2 the activity of all kinds of plastics also varies with each forming factor, and the primary influencing factors are as follows:
The main results are as follows: (1) the activity increases when the temperature is high, but there are differences among different plastics. The activity of polystyrene (especially impact resistant and high MFR), polypropylene, nylon, plexiglass, modified polystyrene (such as ABS, AS), polycarbonate, cellulose acetate and other plastics changes greatly with temperature.
It has little effect on the activity of polyethylene and polyformaldehyde, then the increase or decrease of temperature has little effect on its activity.
Therefore, the former should adjust the temperature to control the activity when forming.
(2) when the pressure injection pressure increases, the melt material is greatly affected by the shear effect, and the activity is also increased, especially polyethylene and polyformaldehyde are more sensitive, so it is appropriate to adjust the injection pressure to control the activity.
(3) the mold structure gating system mode, scale, placement, cooling system planning, melting material movement resistance (such as surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the practical activity of the melting material in the mold cavity. the activity is reduced if the melting material is reduced by lowering the temperature and increasing the active resistance. (3) the mold structure gating system, scale, placement, cooling system planning, melting material movement resistance (such as surface finish, material channel section thickness, cavity shape, exhaust system and so on.
The reasonable structure should be selected according to the activity of the plastic used in mold planning.
During molding, the material temperature can also be controlled, and the mold temperature, injection pressure and injection speed should always be properly adjusted to meet the molding needs.
Foshan Shunde Zongsheng Precision Mould Co., Ltd.
Tel: 0757 Tel: 22269630 / 13923298099 (Mr. Nie).
Web site: www.fszongsheng.com.
Add: no. 1, Rongye 6 road, Fengxiang industrial zone, Daliang, Shunde district, Foshan, Guangdong
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