Thermosensitive plastics and easily hydrolyzable plastics

- Jul 11, 2017-

Thermosensitive refers to some of the plastic is more sensitive to heat, heat at high temperatures for a long time or the inlet cross section is too small, the shear effect is large, the material temperature increases pr.jpg

Thermosensitive refers to some of the plastic is more sensitive to heat, heat at high temperatures for a long time or the inlet cross section is too small, the shear effect is large, the material temperature increases prone to discoloration, degradation, decomposition tendency, with this characteristic Plastic is called heat-sensitive plastic. Such as POM. Thermosensitive plastic in the decomposition of monomer, gas, solid and other by-products, in particular, some of the decomposition of the human body, equipment, molds have irritation, corrosion or toxicity. Therefore, the mold design, the choice of injection molding machine and molding should be noted that should use screw injection molding machine, pouring system cross section should be large, mold and barrel should be chrome, no dead angle stagnant material.

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Some plastic (such as polycarbonate PC) even if it contains a small amount of water, but at high temperatures, high pressure will also be broken, this performance is known as easy hydrolysis, which must be preheated and dried.

 

Stress cracking and melt fracture

Some of the plastic on the stress-sensitive, easy to produce internal stress and brittle crack, plastic parts in the external force or under the action of the solvent cracking phenomenon. To this end, in addition to adding additives in the raw materials to improve open crack resistance, the raw materials should pay attention to dry, reasonable choice of molding conditions to reduce the internal stress and increase crack resistance. And should choose a reasonable shape of plastic parts, should not be set to insert and other measures to minimize stress concentration.

 

Mold design should increase the stripping slope, the choice of a reasonable inlet and the top of the body, the molding should be appropriate to adjust the material temperature, mold temperature, injection pressure and cooling time, try to avoid plastic parts too cold when the mold release , After molding plastic parts should also be post-processing to improve anti-cracking, to eliminate internal stress and to prohibit contact with the solvent.

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When a certain melt flow rate of polymer melt, at constant temperature through the nozzle hole when its flow rate exceeds a certain value, the melt surface obvious transverse cracks called melt fracture, damage the appearance and physical properties of plastic parts. Therefore, in the choice of high melt flow rate of polymer, etc., should increase the nozzle, runner, inlet cross section, reduce injection speed, improve the material temperature.

 

Thermal performance and cooling rate

Various plastic has different heat, thermal conductivity, heat distortion temperature and other thermal properties. Specific heat plasticization needs high heat, should use the plasticizing ability of the injection molding machine. Hot deformation temperature High plastic cooling time can be short, stripping early, but after stripping to prevent cooling deformation. Low thermal conductivity of the plastic cooling rate is slow, it must be fully cooled, to strengthen the mold cooling effect. Hot runner mold for low specific heat, high thermal conductivity of plastic. Specific heat, low thermal conductivity, low heat distortion temperature, slow cooling of the plastic is not conducive to high-speed molding, you must use the appropriate injection molding machine and strengthen the mold cooling.

 

All kinds of plastic according to its type of characteristics and plastic parts shape, requires the need to maintain the appropriate cooling rate. So the mold must be set according to the requirements of heating and cooling system to maintain a certain mold temperature. When the material temperature to mold temperature should be cooled, to prevent deformation of plastic parts after stripping, shorten the molding cycle, reduce the degree of crystallinity. When the plastic waste heat 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, to ensure fluidity, to improve the filling conditions or to control the plastic parts to slow down, To prevent thick-walled plastic parts inside and outside the cooling uneven and improve the degree of crystallinity.

 

On the flow of good, forming area, the material temperature is uneven according to the shape of plastic parts sometimes need to heat or cooling alternate use or local heating and cooling and use. For this mold should be equipped with a corresponding cooling or heating system.

 

Hygroscopicity

Plastic because of a variety of additives, so that the water has a different degree of affinity, so the plastic can be divided into moisture, moisture and moisture is not easy to adhere to the two moisture, the material must be controlled in the allowable range , Or under high temperature, high pressure water into a gas or the role of hydrolysis, so that resin blistering, decreased mobility, appearance and mechanical properties.

 

So the hygroscopic plastic must be required to use the appropriate heating methods and norms for preheating, the use of infrared radiation also need to prevent reabsorption.

 

About reinforced plastic


In order to further improve the thermosetting and the mechanical properties of thermoplastics. Often in the plastic glass fiber (referred to as glass), talc, mica, calcium carbonate, kaolin, carbon fiber as a reinforcing material to resin as the mother and binder and the composition of new composite materials, known as reinforced plastic Oxygen resin for the mother resin plastic reinforced plastic, also known as fiberglass).

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As the glass mixed with glass varieties, length, content and other different, its process and physical properties are also different. The following describes the injection of thermoplastic reinforced plastic.

 

Thermoplastic reinforced plastics

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Thermoplastic reinforced plastics are generally composed of resin and reinforcing materials. Commonly used resins are mainly nylon, polystyrene, ABS, AS, polycarbonate, linear polyester, polyethylene, polypropylene, POM and so on. Reinforced materials are generally alkali-free glass fiber (with two long, long fiber material is generally consistent with the pellets, for 2 to 3 mm, short fiber material is generally less than 0.8 mm) after surface treatment with resin prepared. Fiberglass content should be selected according to the proportion of resin the most reasonable ratio, generally between 20% to 40%. As a result of a variety of enhanced plastic resin used in different glass fiber length, diameter, presence and absence of alkali and surface treatment agent different enhancement effect is different, forming characteristics are not different.

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As mentioned above, the reinforcing material can improve a series of mechanical properties, but there are also a series of shortcomings: the impact strength and impact fatigue strength is low (but the notched impact strength); transparency, welding point strength is also reduced, shrinkage, strength, thermal expansion coefficient , The anisotropy of the thermal conductivity increases. So the current plastic is mainly used for small, high strength, heat, poor working environment and high precision requirements of plastic parts.


Process characteristics


⑴ poor mobility: enhanced plastic melt index than the average material low 30% to 70% due to poor mobility, prone to poor filling, poor welding, uneven distribution of glass and other ills. Especially for long fiber material more prone to the above defects, and also easy to damage the fiber and affect the mechanical properties.

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⑵ forming shrinkage is small, the anisotropy is obvious, the shrinkage of the molding is smaller than that of the non-reinforcing material, but the anisotropy increases along the direction of the flow shrinkage, the vertical direction is large, the feed inlet is small, the distance is large, Occurrence of warping, deformation.

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⑶ bad mold, wear and tear easy to mold release, and wear on the mold, the injection of the material on the casting system, core wear is also large.

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⑷ prone to gas: the formation of fiber surface treatment agent is easy to evaporate into the gas, must be discharged, or prone to poor welding, lack of material and burns and other ills.

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Forming precautions

In order to solve the technical defects in the plastic process, in the molding should pay attention to the following matters:

⑴ should use high temperature, high pressure, high-speed injection.

⑵ mold temperature should be high (the crystalline material should be adjusted according to requirements), while preventing the resin, glass fiber separately, glass exposed and local burns.

⑶ pressure to fill should be sufficient.

⑷ plastic parts should be uniform cooling.

⑸ material temperature, mold temperature changes on the impact of plastic parts contraction, high temperature shrinkage, pressure and injection pressure increases, can shrink smaller but less affected.

⑹ due to enhanced material rigidity, high heat distortion temperature can be high temperature at the mold, but pay attention to uniform cooling after stripping.

(7) Appropriate release agent should be used.

⑻ It is appropriate to use screw-type injection molding machine.


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