About molding of thermoplastics

- Jul 11, 2017-

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Shrinkage

It is generally appropriate to design the mold as follows:

① on the plastic parts to take a smaller shrinkage, diameter to take a larger shrinkage, to leave the test after the correction of the room.

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

③ After the post-processing of plastic parts to determine the size of the post-processing changes (measurement must be demolished after 24 hours).

④ according to the actual shrinkage correction mold.

⑤ and then try to mold and may be appropriate to change the process conditions slightly corrected shrinkage to meet the requirements of plastic parts.

 

fluidity

According to the mold design requirements can be commonly used plastic flow is divided into three categories:

① good mobility: nylon PA, polyethylene PE, polystyrene PS, polypropylene PP;

② medium flow: polystyrene series resin (such as ABS, AS), plexiglass PMMA, POM, polyphenylene oxide PPO;

③ poor mobility: polycarbonate PC, polyphenylene sulfide PPS, polysulfone PSF, poly aryl sulfone PSU, fluorine plastic PTFE.

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Various plastic flow is also due to the formation of factors and changes, the main factors are the following:

① temperature: high temperature of the material is increased, but different plastic are also different, polystyrene (especially impact-resistant HIPS), polypropylene, nylon, plexiglass, modified polystyrene (such as ABS, AS ), Polycarbonate and other plastic flow with the temperature changes. For polyethylene, POM, then the temperature increase or decrease on its mobility is less affected. So the former in the molding should adjust the temperature to control the flow.

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② pressure: injection pressure increases the melting of the material by the shear, the mobility is also increased, especially polyethylene, POM more sensitive, so the molding should adjust the injection pressure to control the flow.

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③ mold structure: the form of pouring system, size, layout, cooling system design, melt flow resistance (such as: surface finish, material section thickness, cavity shape, exhaust system) and other factors have a direct impact on the melt in the The actual flow in the cavity, where the melt to reduce the temperature, increase the mobility of the flow is reduced.


Therefore, the mold design should be based on the plasticity of the plastic used, choose a reasonable structure. Molding 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 according to their condensation when the crystallization phenomenon can be divided into crystalline plastic and non-crystalline (also known as amorphous) plastic two categories.

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The so-called crystallization phenomenon is the plastic from the molten state to the condensation, the molecules moved by the independent, completely in a non-order state, into a molecular stop free movement, according to a slightly fixed position, and there is a tendency to make the molecules into a regular model of a tendency Phenomenon.

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As a measure of the appearance of these two types of plastic plastic depending on the transparency of the general crystal material is opaque or translucent (such as polyoxymethylene, etc.), amorphous material is transparent (such as plexiglass, etc.). But there are exceptions, such as ABS for the amorphous material but not transparent.

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In the mold design and selection of injection molding machine should pay attention to the following requirements and precautions for crystalline plastic:

① the temperature rise to the molding temperature of the heat required to use large plastic equipment.

② cooling heat release when the heat, to fully cool.

③ molten state and the proportion of large difference between solid, forming shrinkage, prone to shrinkage, pores.

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

⑤ anisotropy significant, large internal stress. After the demoulding of non-crystalline molecules continue to crystallize the tendency, in the energy imbalance, prone to deformation, warping.


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