7 factors to consider in injection molding process.

1. Shrinkage.

The form and calculation of the shrinkage of thermoplastic plastic are as follows:

1. The varieties of plastics thermoplastic plastic molding process because there are crystal taxiing change with the volume of the internal stress is strong, frozen inside the plastic residual stress is large, molecular orientation factors such as the gender is strong, so compared with thermosetting plastic shrinkage is bigger, wide range of shrinkage and clear direction, also after molding shrinkage, shrinkage rate after annealing or wet processing generally are larger than the thermosetting plastic.

2. When the plastic parts are formed, the molten material and the surface of the cavity contact the outer layer immediately to cool down to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic parts is slowly cooled to form a high density solid layer. So the wall thickness, slow cooling, thick layer thickness shrinkage. In addition, there are no embedded parts and the layout of embedded parts, and the quantity is directly influencing the direction of material flow, density distribution and shrinkage resistance, etc., so the characteristics of plastic parts have a great influence on shrinkage and direction.

3. The form, size and distribution of the feed mouth directly affect the direction of the material flow, the distribution of density, the effect of maintaining pressure and the forming time. Direct feed inlet, large section of feed inlet section (especially the thicker section) is smaller, but the direction is large, and the width and length of feed mouth are small. The contraction is larger when the inlet is near or parallel to the flow direction.

4. The mold temperature of molding conditions is high, and the molten material cools slowly, and the density is high and the shrinkage is large. In particular, the crystallinity is high and the volume changes greatly, so the shrinkage is larger. The temperature distribution is also related to the cooling and density uniformity of the plastic parts. In addition, maintaining pressure and time has a great impact on the contraction, while the pressure is large and the time is long, but the direction is large. The injection pressure is high, the viscosity difference of the melt material is small, the shear stress is small, and the elastic rebound is large after stripping. Therefore, the shrinkage can be reduced in a moderate amount, the material temperature is high and the shrinkage is large, but the direction is small. Therefore, it is necessary to adjust mold temperature, pressure, injection speed and cooling time during molding to change the shrinkage of plastic parts. Mold design according to the scope of all kinds of plastic shrinkage, wall thickness and shape, feed port, size and distribution of the form according to the experience to determine the shrinkage rate of plastic pieces of each part, and then to calculate the cavity size. For high precision plastic parts and difficult to master shrinkage rate, it is advisable to design the mold as follows:

The small shrinkage rate of the outside diameter of the plastic parts was obtained, and the large shrinkage rate was obtained, so as to leave the room for modification after the test mode.

The shape, size and molding conditions of the casting system are determined.

The size variation of the molded parts after treatment is determined by post-processing (the measurement must be 24 hours after the release).

Adjust the mold according to the actual shrinkage.

It can modify the shrinkage value slightly to meet the requirements of plastic parts.

Ii. Liquidity

1. Thermoplastic liquidity size, general but from the size of molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length/plastic pieces of wall thickness) and a series of index are analyzed. Small molecular weight, broad molecular weight distribution, the regularity of the molecular structure is poor, high melt index, long length of spiral flow, low viscosity, flow ratio of the liquidity is good, for the same commodity plastics must check the specifications to judge whether its liquidity is suitable for injection molding. According to the mold design requirements, the fluidity of commonly used plastics can be divided into three types:

The liquid is good for PA, PE, PS, PP, CA, poly (4) methyl hexene;

Liquid medium polystyrene resin (ABS, AS), PMMA, POM, polyphenyl ether;

It is a liquid PC, hard PVC, polyphenyl ether, polysulfone, polyaromatic sulfoxide, fluorine plastic.

2. The liquidity of all kinds of plastics has also changed due to various forming factors. The main factors are as follows:

(1) high temperature is expected to increase liquidity, but different plastic also vary, PS (especially impact resistance and higher MFR), PP, PA, PMMA, modified polystyrene (such AS ABS, AS), PC, such AS CA plastic liquidity larger changes with temperature. For PE, POM, temperature increase and decrease have little effect on its fluidity. So the former adjusts the temperature to control the fluidity.

As the pressure increases, the melt material is greatly affected by shear, and the liquidity is also increased, especially for PE and POM. Therefore, it is timely to adjust the injection pressure to control the liquidity.

(3) the mould structure of gating system form, size, layout, design of the cooling system, the molten material flow resistance (such as surface roughness, the feeding section thickness, cavity shape, exhaust system) factors directly affect the molten material within the cavity of the real liquidity, that prompted the molten material to reduce the temperature, increase liquidity resistance of the liquid is reduced. The mould design should be based on the fluidity of the plastic used, and choose the reasonable structure. When forming, it can also control the material temperature, mold temperature and injection pressure, injection speed and other factors to properly adjust the filling conditions to meet the molding needs.

Iii. Crystallization

Thermoplastic plastic can be divided into two categories: crystalline plastic and amorphous (amorphous) plastic according to its condensation.

Crystal phenomenon is the so-called plastic by molten state to condensation, the molecules move by the independent, complete in the order status, become a molecular stop freedom of movement, according to the fixed position slightly, and there is a tendency to make molecular arrangement formal models of a phenomenon.

As judging the appearance of these two types of plastic standard visual transparency of the thick wall of plastic parts, general crystalline material is not transparent or translucent (e.g., POM, etc.), amorphous material is transparent, such as PMMA, etc.). However, there are exceptions, such as poly (4), which is crystalline plastic but has high transparency, ABS is amorphous but is not transparent.

Attention should be paid to the following requirements and precautions in mold design and selection of injection molding machine:

The quantity of heat needed to rise to the molding temperature is much more than that.

When cooling back, the heat release is large enough to be cooled sufficiently.

The proportion difference between the melting state and the solid state is large, the forming shrinkage is large, and the shrinkage cavity and porosity are easy to occur.

Fast cooling, low crystallinity, close reduction, high transparency. The crystallinity is related to the wall thickness of the plastic part, the wall thickness is slow, the crystallinity is high, the shrinkage is large and the property is good. So the crystal material should be controlled according to the requirement.

The anisotropy is significant and the internal stress is large. After the molds, the uncrystallized molecules have the tendency of continuous crystallization, in the state of energy imbalance, which is prone to deformation and warping.

The crystallization temperature range is narrow, and it is easy to inject the mold or plug the feeding port.

4. Thermal sensitive plastic and hydrolyzed plastic.

1. Heat-sensitive refers to some plastic on the thermal sensitive, heated at high temperature for a long time or inlet cross section is too small, the shear effect, higher temperature prone to change color, degradation, the tendency of decomposition, the characteristics of plastic called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytrifluorochloroethylene, etc. Thermal sensitive plastics produce monomers, gases, solids and other byproducts during decomposition, especially some of the decomposition gases have stimulation, corrosion or toxicity to human body, equipment and molds. Therefore, mold design, injection molding machine and molding, should be paid attention to when should choose screw injection molding machine, pouring system section should be big, mold and cylinder should be chrome plated, can not have * Angle hysteresis, must strictly control the molding temperature, the plastic adding stabilizer, reduce its thermal performance.

2. Some plastics (such as PC), even with a small amount of water, can be decomposed under high temperature and pressure. This performance is called hydrolysis, which must be heated and dried in advance.

Stress cracking and melt fracture.

1. Some plastics are sensitive to stress, which is easy to produce internal stress and brittle when forming, and cracking occurs under the action of external forces or under the action of solvent. For this reason, in addition to the addition of additives in the raw materials to improve the crack resistance, the raw materials should be dry and reasonable to select the forming conditions to reduce the internal stress and increase the crack resistance. It is necessary to choose reasonable shape of plastic parts, not to set the insert and other measures to minimize stress concentration. When the mould design should increase draft, choose reasonable inlet and ejection mechanism, should the appropriate adjustment of material molding temperature and cooling time, mold temperature, injection pressure, avoid too cold short from plastic, therefore after molding plastic parts is also appropriate for reprocessing increase cracking resistance, stress relief and prevent contact with the solvent.

2. When a certain blend melt flow rate of polymer melt under constant temperature through the nozzle hole when its velocity exceeds a certain value, the melt apparent transverse cracks, known as the melt fracture surface, the parts surface and damage to property. Therefore, in the selection of polymer with high melt flow rate, the nozzle, runner and inlet section should be enlarged to reduce the injection speed and improve the material temperature.

Thermal performance and cooling rate.

1. Various kinds of plastics have different thermal properties, such as specific heat, thermal conductivity and thermal deformation temperature. It is necessary to use plastic injection molding machine. Hot deformation temperature high plastic cooling time can be short, stripping early, but after stripping to prevent cooling deformation. The cooling rate of the plastic with low heat conductivity is slow (such as the cooling rate of ionic polymer is extremely slow), so it must be fully cooled and the mold cooling effect should be strengthened. Hot runner mould is suitable for plastics with low specific heat and high heat conductivity. With low heat transfer rate and low heat transfer rate, plastic with low thermal deformation temperature and slow cooling rate is not conducive to high speed molding. Proper injection molding machine and cooling of mold are necessary.

2. All kinds of plastics are required to maintain proper cooling rate according to their type and shape. Therefore, the mold must be set according to the molding requirement to set up the heating and cooling system to maintain certain mold temperature. When the temperature is high, the mould should be cooled to prevent deformation after mold release, shorten the molding cycle and reduce the crystallinity. When plastic waste heat is not enough to make the mold to keep a certain temperature, the mould should be equipped with heating system, to keep the mold in a certain temperature, cooling rate to control, ensure the liquidity, improve filling conditions or to control parts make its slow cooling, prevent the thick wall parts inside and outside the uneven cooling and improve the degree of crystallinity, etc. For good fluidity, the molding area is large and the material temperature is not uniform. The molding condition of the plastic parts sometimes needs to be heated or cooled alternately, and the heating and cooling are used. The mould should be equipped with corresponding cooling or heating system.

7. Hygroscopic property

With various additives in plastic, make its in water have different degree of personal relationship, so the plastic can be roughly divided into hygroscopic, adhesion, moisture and water absorption and water are difficult to stick two, moisture content in material must be controlled within the scope of the permit, or under high temperature and high pressure water into gas or hydrolysis, the resin foam, less liquidity, poor appearance and mechanics performance. Therefore, the absorbent plastic must be preheated according to the proper heating method and the specification, and prevent the moisture absorption when used.