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The injection molding process to consider seven factors

First, the shrinkage

    Impact thermoplastic molding shrinkage factor is as follows:

    1.1 varieties of plastic molding process of the thermoplastic also shaped from crystallized volume change exists within should be strong, the freezing of the residual stresses in the plastic parts within strong molecular orientation, and other factors, so compared with the thermosetting plastic shrinkage than large, wide range of shrinkage, clear direction, another shrinkage after molding, annealing or humidity conditioning treatment after shrinkage generally than the thermosetting plastic.

1.2 Plastic characteristic molding molten material and the cavity surface contact with the outer layer is immediately cooled to form a low-density solid-state shell. The plastic parts due to the poor thermal conductivity of plastic, the inner layer is slowly cooled and the solid layer to form a large shrinkage of high density. Wall, slow cooling, high density layer thickness shrinkage. In addition, without insert and insert layout, the number of direct impact on the direction of the material flow, density distribution and the size of the contraction resistance characteristics of plastic parts to shrink the size and directional greater impact.

1.3 feed port form, size, and distribution of these factors directly affects the material flow direction, density distribution, security pressure feeding effect and molding time. Direct feed inlet and the feed port section the (particularly sectional thicker) contraction is small but large directivity, the feed inlet width and short length of the directivity small. Parallel to the direction of or with the feed stream from the feed port near the large shrinkage.

   1.4 molding conditions mold temperature is high, the slow cooling of the molten material, and high density, shrinkage, in particular, due to high crystallinity of crystalline material, the large volume change and therefore contraction greater. Mold temperature distribution inside and outside with plastic cooling and density uniformity also directly affect the size and direction of the amount of the various parts of contraction. In addition, to maintain the pressure and time of contraction greater impact, pressure, long time shrink small but directional. High injection pressure, the melt viscosity difference is small, and the interlaminar shear stress is small, the elastic rebound after demolding large and therefore shrinkage can also be an amount of decrease, the feed temperature is high, shrinkage, but directionality small. Adjust mold temperature molding pressure, injection speed and the cooling time and other factors may also be appropriate to change the contraction of the plastic parts.

Mold design according to the contraction range of a variety of plastics, the wall thickness of the plastic parts, the shape, the form of the feed inlet size and distribution, the shrinkage of the various parts of the plastic parts is determined by experience, again calculated cavity size. Precision plastic parts and difficult to grasp shrinkage generally suitably designed mold as follows:

① whichever is less shrinkage of plastic parts outside diameter, inside diameter, whichever is greater shrinkage to leave tryout after amendment leeway.

② test mode to determine the form, size, and molding conditions of the gating system.

Plastic parts by ③ to post-processing, post-processing to determine the size of the change (measured after 24 hours after ejection).

(4) actual contraction correction mold.

⑤ try molding process conditions can be appropriately changed slightly correct shrinkage values ​​in order to meet the requirements of the plastic parts.

 Second, the fluidity

2.1 thermoplastics mobility size, generally from the molecular weight, melt index, Archimedes spiral flow length, and performance analysis of viscosity and flow ratio (flow length / wall thickness of the plastic parts) and a series of index. Small molecular weight and wide molecular weight distribution, differential molecular structure tacticity, high melt index, spiral flow length, the performance of a small viscosity, flow than large fluidity like, the plastic must be of the same model name, check the manual to determine its flowability is applicable At the time of injection molding. Requested by the mold design commonly used plastic flow can be broadly divided into three categories:

 ① flow good PA, PE, PS, PP, Ca, poly (4)-methyl pentene;

② fluidity medium polystyrene series resin (such as ABS, AS), PMMA, POM, polyphenylene ether;

③ poor flowability PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, a fluorine plastic.

2.2 various plastics mobility is also due to the molded factors change, the main factors affecting the following points:

    ① high temperature material temperature increases fluidity, but different plastics also vary, PS (especially impact resistance and type of higher MFR values), PP, PA, PMMA, modified polystyrene (such as ABS, AS) , the flow of plastic such as PC, CA, with the temperature change. PE, POM, the temperature increase or decrease its liquidity. So the former in the molding temperature is preferably adjusted to control the flow.

 ② pressure injection pressure increases the melt by the shearing action, fluidity is also increased, in particular PE, POM is more sensitive, so an injection pressure molding is preferably adjusted to control the flow.

 ③ mold structure in the form of the gating system, the size, arrangement, cooling system design, the flow resistance of the molten material (such as type surface finish, the cross-sectional thickness of the material passage, and the cavity shape, the exhaust system), and other factors have a direct impact to the melt in the cavity within the actual flow, where procure the melt to reduce the temperature, increasing the resistance of liquidity of the fluidity is lowered. Mold design should be based on the fluidity of the plastic used in the selection of reasonable structure. Molding while the material temperature, mold temperature and injection pressure, injection speed, and other factors may also be controlled to appropriately adjusting the padding to meet the molding requires.

 Third, the crystalline

No crystallization can be divided into two categories of crystalline plastics and non-crystalline (also known as amorphous) plastic the thermoplastics their condensation.

So-called crystallization phenomenon is the condensation of the plastic from the molten state to the molecule by an independent movement, is completely unordered state, into molecular stop freedom movement, by slightly fixed position, and has a molecular arrangement of a tendency to become a formal model of a kinds of phenomena.

Visual plastic thick-walled plastic parts in transparency as the discrimination of these two types of plastic appearance standards, generally the crystalline material is opaque or translucent (such as POM), the amorphous material is transparent (such as PMMA, etc.). But there are exceptions, such as poly (4) methyl pentene as there is a high transparency of the crystalline plastics, ABS amorphous material, but not transparent.

Injection molding machine should pay attention to the following requirements and considerations of crystalline plastic mold design and select:

 ① material temperature rise to heat molding temperature required equipment use plasticizing capacity.

② the cooling return of heat release, to be sufficiently cooled.

 ③ molten state and solid state, the proportion of poor, mold shrinkage, prone to shrinkage, porosity.

 ④ fast cooling, low crystallinity, small contraction, a high degree of transparency. The crystallization degree is related to the wall thickness of the plastic parts, the wall thickness of slow cooling, high crystallinity, and large shrinkage, good physical properties. Crystalline material should be required to have to control mold temperature.

 The ⑤ anisotropic significant internal stress. Not crystallized molecules after ejection continued tendency to crystallize in the state of energy imbalance, prone to distortion, warping.

⑥ crystallization temperature range is narrow, prone to not melt is injected into the mold at the end or blockage of the feed inlet.

Fourth, the heat-sensitive plastic and easily hydrolyzed plastic

4.1 thermosensitive means certain plastics are more sensitive to heat, at high temperatures the heating time is longer, or feed inlet cross-section is too small, the shearing action is large, the higher temperature materials prone to discoloration, degradation, decomposition of the tendency, having such characteristics plastic called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate ethylene copolymer, POM, polytrifluoroethylene chloride, etc.. The heat-sensitive plastic decomposition byproducts such as monomers, gas, solid, especially some decomposition gas on the human body, equipment, molds are stimulated, the corrosion effects or toxicity. , Mold design, select injection molding machines and molding should pay attention to, should be used in the injection molding machine screw should be large cross-section of the gating system, mold and barrel chrome not * angle hysteresis materials, molding temperature must be strictly controlled in plastic Add stabilizer, to weakened its thermal performance.

4.2 even if some plastic (such as PC) containing a small amount of water, but in the high temperature and pressure will decompose, such performance is called easily hydrolyzable, which must be pre-heated and dried.

Stress cracking and melt fracture

5.1, some plastic stress sensitive molding is easy to produce internal stress and brittle and easy to crack the plastic parts in the external force or solvent effect that cracking phenomenon. To this end, in addition to the additives are added to improve the open crack resistance in the raw material, the raw materials should be noted sulfate, a reasonable choice of the molding conditions, in order to reduce the internal stresses and increased crack resistance. Should choose a reasonable shape of the plastic parts, inserts, such measures should not be set to minimize stress concentration. Mold design should increase the stripping slope, use reasonable feed inlet and ejector mechanism, molding should be appropriate to adjust the melt temperature, mold temperature, injection pressure and cooling time, try to avoid too cold and brittle plastic parts, mold release post-processing after molding of plastic parts may also wish to increase the resistance to cracking, the elimination of internal stress and prohibits contact with the solvent.

5.2 when the polymer melt flow rate of a certain melting body, the flow rate through the nozzle hole at a constant temperature exceeds a certain value, the melt surface the obvious transverse cracks called melt fracture, the detrimental plastic appearance and physical properties. Therefore, in the selection of high melt flow rate polymers, etc., should be increased by the nozzle, runner, the cross section of the feed port, to reduce the injection speed, to raise the material temperature.

6, thermal performance and the cooling rate

 6.1 various plastics have different specific heat, thermal conductivity, heat distortion temperature and thermal performance. High specific heat plasticizing require heat, should be used in the injection molding machine plasticizing capacity. High heat distortion temperature plastic cooling time can be as short as early as stripping, but to prevent cooling deformation after ejection. A low thermal conductivity plastic cooling speed is slow (such as the very slow the cooling rate of the ionic polymer, etc.), it must be cooled sufficiently to enhance the effect of mold cooling. Hot runner mold applicable to the low specific heat and high thermal conductivity of the plastic. Large specific heat, low thermal conductivity, low thermal deformation temperature, and slow cooling of the plastic is not conducive to high-speed molding, and must use the appropriate injection molding machine and strengthen the mold cooling.

6.2 The variety of plastics according to their characteristics of the types of plastic parts shape, requirements must maintain proper cooling rate. Mold must molding requirements set heating and cooling system to maintain a certain mold temperature. When the material temperature elevated mold temperature should be cooled to prevent deformation of plastic parts after ejection, shorten the molding cycle, reduce the degree of crystallinity. When the plastic waste heat is insufficient to maintain a certain temperature so that the mold, the mold should be provided with a heating system, so that the mold is maintained at a certain temperature, to control the cooling rate, to ensure the fluidity, and to improve the filling conditions, or for the control of the plastic parts to slow cooling, prevent unevenness and improve the degree of crystallinity and the internal and external cooling of the thick-walled plastic parts. Good liquidity, forming large, uneven material temperature press plastic molding sometimes need to alternate heating or cooling or local heating and cooling and This end, the mold should be provided with the corresponding cooling or heating system.

Seven hygroscopicity

Plastics due to a variety of additives to make it moisture different affinity degree, the plastic can be broadly divided into the absorbent, adhesion, water and non-absorbent is also difficult to adhere to the water of the two the compound moisture content must be controlled within the allowable range within or under high temperature, high pressure water becomes gas or hydrolysis occurs so that the resin foaming, decreased mobility, appearance and poor mechanical properties. Hygroscopic plastic suitable heating method and norms must be required to be preheated, when in use to prevent the absorbent.