7 factors to consider for injection molding process

I. Shrinkage rate

Thermoplastic molding shrinkage form and calculation as described earlier, the factors affecting the shrinkage of thermoplastic molding are as follows.

1. plastic species thermoplastic molding process because there is also the crystallization of the volume change in the shape up, strong internal stress, frozen in the plastic parts of the residual stress, molecular orientation and other factors, so compared with thermosetting plastics then shrinkage rate is larger, a wide range of shrinkage rate, directional obvious, in addition to the molding.

2. The characteristics of plastic parts are molded when the molten material and the cavity surface contact outer layer immediately cools to form a low-density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part cools slowly and forms a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, high-density layer is thick shrinkage. 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 distribution and shrinkage resistance, so the characteristics of the plastic parts have a greater impact on the size and direction of shrinkage.

3. Inlet form, size, distribution of these factors directly affect the direction of material flow, density distribution, the role of pressure-holding shrinkage and molding time. Direct inlet, inlet cross-section is large (especially thicker cross-section), the shrinkage is small but directional, the inlet is wide and the length is short, the directionality is small. Close to the inlet or parallel to the direction of material flow, the shrinkage is large.

4. molding conditions mold temperature is high, the molten material cooling slow, high density, shrinkage, especially for crystalline materials due to high crystallinity, volume change, so shrinkage is greater. Mold temperature distribution and cooling inside and outside the plastic parts and density uniformity is also related, directly affecting the size and direction of the shrinkage of each part. In addition, the holding pressure and time also have a greater impact on the shrinkage, the pressure is large, the time is long shrinkage is small but directional.

Injection pressure is high, the molten material viscosity difference is small, the interlayer shear stress is small, the elasticity after the mold jump, so the shrinkage can also be moderately reduced, the material temperature is high, shrinkage is large, but the directionality is small. Therefore, when molding, adjusting the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the shrinkage of plastic parts.

When designing the mold, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the inlet, the shrinkage rate of each part of the plastic part is determined empirically, and then the cavity size 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.

Design mold.

①Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter, in order to leave room for correction after the trial mold.

②Test mold to determine the form, size and molding conditions of the pouring system.

③To, post-process the plastic parts by post-processing to determine the size change (measurement must be after 24 hours after demolding).

④Correct the mold according to the actual shrinkage.

⑤ Try the mold again and can change the process conditions to slightly correct the shrinkage value to meet the requirements of the plastic parts.

Second, fluidity

Flowability is divided into three categories.

① good fluidity PA, PE, PS, PP, CA, poly (4) methyl garlicene;

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

③ poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyaryl sulfone, fluoroplastics.

1. thermoplastic fluidity size, generally can be analyzed from the molecular weight size, melt index, Archimedes spiral line flow length, performance viscosity and flow ratio (process length / plastic wall thickness) and a series of indices.

Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small performance viscosity, flow ratio is good liquidity, the same name of the plastic must check its instructions to determine whether its liquidity is suitable for injection molding.

2. The fluidity of various plastics also changes due to various molding factors, the main factors affecting the following.

① temperature material temperature is high, the liquidity increases, but different plastics also have differences, PS (especially impact-resistant and MFR value higher), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics with the temperature of liquidity changes. For PE, POM, then the temperature increase or decrease on its liquidity is less. So the former in the molding of the temperature should be adjusted to control the fluidity.

② pressure injection pressure increases, the molten material is subject to shear, liquidity also increases, especially PE, POM is more sensitive, so it is appropriate to adjust the injection pressure to control the liquidity when molding.

③ mold structure pouring system form, size, layout, cooling system design, molten material flow resistance (such as surface finish, channel cross-section thickness, cavity shape, exhaust system) and other factors directly affect the actual liquidity of the molten material in the cavity, where the molten material to reduce the temperature and increase the flow resistance of the liquidity will be reduced.

Mold design should be based on the fluidity of the plastic used, choose a reasonable structure. When molding, we can also control the material temperature, mold temperature and injection pressure, injection speed and other factors to properly adjust the filling situation to meet the molding needs.

Crystallinity

Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also called amorphous) plastics according to their crystallization phenomenon when condensing.

The so-called crystallization phenomenon is the plastic from the molten state to condensation, molecules from independent movement, completely in a state of no order, into molecules to stop free movement, according to a slightly fixed position, and a tendency to make the molecular arrangement into a regular model of a phenomenon.

As a criterion to distinguish the appearance of these two types of plastics depending on the transparency of the thick-walled plastic parts, generally crystalline material is opaque or translucent (such as POM, etc.), and amorphous material is transparent (such as PMMA, etc.).

However, there are exceptions, such as poly (4) methyl garoulein is crystalline plastic but has high transparency, ABS is amorphous material but is not transparent.

In the mold design and selection of injection molding machines should pay attention to the following requirements and considerations for crystalline plastics.

①More heat is needed to raise the material temperature to the molding temperature, so use equipment with large plasticizing capacity.

②The heat released during cooling and tempering is large and should be fully cooled.

③The specific gravity difference between molten state and solid state is large, molding shrinkage is large, easy to occur shrinkage, porosity.

④Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of plastic parts, the wall thickness is slow cooling, high crystallinity, high shrinkage, good physical properties. So the crystalline material should be required to control the mold temperature.

⑤ Significant anisotropy and high internal stress. After demoulding, the uncrystallized molecules have the tendency to continue to crystallize, and are in a state of energy imbalance, and are prone to deformation and warpage.

⑥Crystallization temperature range is narrow, easy to occur at the end of the unmelted material into the mold or block the inlet.

Fourth, heat-sensitive plastics and easy hydrolysis plastic

1. heat-sensitive refers to some plastics are more sensitive to heat, at high temperatures for a long time or feed port cross-section is too small, shear effect is large, the material temperature is prone to discoloration, degradation, decomposition of the tendency, with this characteristic of plastic called heat-sensitive plastics.

Such as hard PVC, polyvinyl chloride, vinyl acetate copolymer, POM, polyvinyl trifluoride, etc. Thermosensitive plastics produce monomers, gases, solids and other by-products during decomposition, especially some decomposition gases are irritating, corrosive or toxic to the human body, equipment and molds.

Therefore, mold design, selection of injection molding machine and molding should pay attention to, should use screw injection molding machine, pouring system cross-section should be large, mold and barrel should be chrome-plated, there should be no * corner stagnant material, must strictly control the molding temperature, plastic to add stabilizers to weaken its heat-sensitive performance.

2. Some plastics (such as PC) will decompose under high temperature and pressure even if they contain a small amount of water, and this property is called easy hydrolysis, which must be heated and dried in advance.

V. Stress cracking and melt rupture

1. Some plastics are sensitive to stress, molding is easy to produce internal stress and brittle and easy to crack, plastic parts under the action of external forces or in the role of solvent that cracking phenomenon.

For this reason, in addition to adding additives in the raw materials to improve the crack resistance, the raw materials should pay attention to drying, reasonable selection of molding conditions to reduce internal stress and increase the crack resistance. And should choose a reasonable shape of the plastic parts, should not set the inserts and other measures to minimize the stress concentration.

The mold design should increase the slope of the mold release, choose a reasonable feed port and ejector mechanism, molding should be properly adjusted material temperature, mold temperature, injection pressure and cooling time, try to avoid the plastic parts are too cold and brittle when the mold release, molding plastic parts should also be post-treatment to improve anti-cracking, eliminate internal stress and prohibit contact with solvents.

2. When a certain melt flow rate of the polymer melt, at a constant temperature through the nozzle hole its flow rate exceeds a certain value, the surface of the melt occurs obviously lateral cracks called melt rupture, damage the appearance of plastic parts and physical properties.

Therefore, in the selection of high melt flow rate of polymer, etc., should increase the nozzle, sprue, inlet cross-section, reduce the injection speed, increase the material temperature.

Sixth, the thermal properties and cooling speed

1. Various plastics have different thermal properties such as specific heat, thermal conductivity and heat deflection temperature. High specific heat of plasticization requires a large amount of heat, should be selected plasticizing capacity of the injection molding machine. The cooling time of plastics with high heat deformation temperature can be short and the mold can be released early, but the cooling deformation should be prevented after the mold is released.

The cooling speed of plastics with low thermal conductivity is slow (such as ionic polymers, etc. cooling speed is extremely slow), so it must be fully cooled, and the cooling effect of the mold should be strengthened. Hot sprue molds are suitable for plastics with low specific heat and high thermal conductivity. The plastics with high specific heat, low thermal conductivity, low heat deflection temperature and slow cooling rate are not suitable for high-speed molding, so appropriate injection molding machines must be used and cooling of the mold should be strengthened.

2. According to the characteristics of various plastics and the shape of plastic parts, it is necessary to maintain a proper cooling speed. Therefore, the mold must be set up with a heating and cooling system according to the molding requirements in order to maintain a certain mold temperature. When the material temperature makes the mold temperature rise, it should be cooled to prevent the deformation of plastic parts after demolding, shorten the molding cycle and reduce the crystallinity.

When the residual heat of plastic is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve filling conditions or to control the plastic parts to make them cool slowly, prevent uneven cooling inside and outside the thick-walled plastic parts and improve crystallinity, etc.

For good fluidity, molding area, uneven material temperature is sometimes required to alternate heating or cooling or local heating and cooling according to the molding situation. This mold should be equipped with the corresponding cooling or heating system.

Seven, moisture absorption

There are various additives in the plastic so that it has different degrees of affinity to water, so the plastic can be broadly divided into moisture absorption, water adhesion and non-absorbent and not easy to adhere to the moisture of the two, the water content in the material must be controlled within the allowable range, otherwise, at high temperature, high-pressure moisture into gas or hydrolysis, so that the resin blistering, liquidity decline, appearance and mechanical properties of bad.

Therefore, moisture-absorbing plastics must be preheated according to the requirements of appropriate heating methods and specifications to prevent re-absorption of moisture when in use.