What are the factors that affect the setting of TPE and TPR injection molding? As a TPE manufacturer, plastic Bo technology to professional explanation: TPE, TPR injection molding when setting the important factors to consider!
A. Shrinkage rate
The form and calculation of shrinkage of thermoplastic molding as mentioned before, the factors affecting the shrinkage of thermoplastic molding are as follows.
1, plastic varieties of thermoplastics molding process because there is also crystallization of the volume change, internal stress, frozen in the plastic residual stress, molecular orientation and other factors, so compared with thermosetting plastics, the shrinkage rate is larger, a wide range of shrinkage, directional obvious, in addition to the shrinkage after molding, annealing or moisture treatment of the shrinkage rate is generally larger than thermosetting plastics.
2, plastic parts characteristics molding molten material and cavity surface contact with the outer layer immediately cooled to form a low density of 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, with or without inserts and insert layout, the number of direct impact on the direction of material flow, density distribution and shrinkage resistance size, so the characteristics of the plastic parts on the shrinkage size, directional impact.
3、Inlet form, size and distribution of these factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect and molding time. Direct feed port, feed port cross-section (especially thicker cross-section) is small shrinkage but directional, the feed port is wide and short length is small directional. Close to the inlet or parallel to the direction of the material flow is a large shrinkage.
4, molding conditions mold temperature is high, the molten material cooling slow, high density, shrinkage, especially for crystalline material because of 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, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also 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 difficult to grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold.
①Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter 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.
2.1 Thermoplastic fluidity size, generally from the molecular weight size, melt index, Archimedes spiral flow length, performance viscosity and flow ratio (process length / plastic wall thickness) and a series of indices to analyze. Small molecular weight, wide molecular weight distribution, poor molecular structure, high melt index, long spiral flow length, small performance viscosity, flow ratio is good, the same name of the plastic must check its instructions to determine whether its liquidity is suitable for injection molding. According to the requirements of mold design, the fluidity of commonly used plastics can be roughly 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.
2.2 The fluidity of various plastics also changes due to various molding factors, the main factors affecting the following.
① temperature material temperature increases the fluidity, but different plastics also vary, PS (especially impact-resistant type and MFR value higher), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics fluidity with the temperature changes more. 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, 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.
Third, stress cracking and melt rupture
3.1 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 dry, reasonable choice 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 appropriate to adjust the 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.
3.2 When a certain melt flow rate of polymer melt, at a constant temperature through the nozzle hole when its flow rate exceeds a certain value, the surface of the melt occurs obviously lateral cracks called melt rupture, which is detrimental to the appearance and physical properties of plastic parts. 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.
Fourth, the thermal properties and cooling speed
4.1 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 plastic with high specific heat, low thermal conductivity, low heat deflection temperature and slow cooling rate is not suitable for high speed molding, so appropriate injection molding machine must be used and cooling of the mold should be strengthened.
4.2 According to the characteristics of various plastics and the shape of plastic parts, it is necessary to maintain the appropriate cooling speed. Therefore, the mold must be set up with heating and cooling system according to the molding requirements 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 the fluidity, improve the 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 the 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. For this reason, the mold should be equipped with the corresponding cooling or heating system.
TPE, TPR injection molding setting needs to consider the important factors to share here. In terms of TPE, TPR elastomers, the above four factors such as shrinkage, fluidity, stress cracking and cooling rate are the key factors to be considered when designing and producing TPE, TPR material molds. For those who are new to TPE,TPR materials, you will need to consult your material supplier for information.