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Water bottle mould is customized for long heating time or feed port section at high temperature.

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Water bottle mould is customized for long heating time or feed port section at high temperature.

Release date:2019-05-14 Author: Click:

     

Thermoplastics can be divided into two categories: crystalline plastics and amorphous (amorphous) plastics according to the absence of crystallization during condensation. 


The so-called crystallization phenomenon is that when the plastic changes from melting to condensation, the molecules move independently and are completely out of order, so that the molecules stop moving freely and move in a slightly fixed azimuth, and there is a tendency to make the molecules arrange into a normal model. 


As the standard for judging the appearance of these two types of plastics depends on the transparency of thick-walled plastic parts, water bottle moulds are generally made of opaque or translucent materials (such as polyformaldehyde, etc.) and amorphous materials are transparent (such as plexiglass, etc.). 


However, there are exceptions, such as poly (4) methyl garrison is a crystalline plastic with high transparency, and ABS is amorphous but not transparent.

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In mold design and selection of injection molding machine, attention should be paid to the following requirements and matters needing attention for crystalline plastics: 

The main results are as follows: (1) it takes a lot of heat for the material temperature to rise to the molding temperature, and the PET bottle embryo mold needs plasticizing equipment. 


(2) there is a large amount of heat released during cooling and recovery, so it should be fully cooled. 


(3) the difference of specific gravity between molten state and solid state is large, the forming is shortened, and it is easy to occur shrinkage and stomata. 


(4) Fast cooling, low crystallinity, small shortening and high transparency. 

The crystallinity is related to the wall thickness of the plastic parts, which has the advantages of slow cooling, high crystallinity, large shortening and good physical properties. 

Therefore, it is necessary to control the mold temperature of the crystalline material according to the requirements. 


(5) the anisotropy is remarkable and the internal stress is large. 

After demoulding, the uncrystallized molecules have a tendency of continuous crystallization, which is in a state of energy imbalance and is easy to deform and warp. 


(6) the crystallization temperature range is narrow, and it is easy to inject unmelted powder into the mold or block the feed port.


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4. Thermosensitive plastics and hydrolytic plastics. 


4.1 Thermosensitivity refers to the tendency of some plastics to change color, degrade and differentiate when they are heated for a long time at high temperature or the cross section of the feed port is too small, when the shearing effect is large. Plastics with this property are called thermosensitive plastics. 


Such as hard polyvinyl chloride, polyvinylidene chloride, vinyl acetate copolymer, polyformaldehyde, polytrifluoroethylene and so on. 

During the differentiation of thermosensitive plastics, monomers, gases, solids and other by-products occur, especially some differentiated gases have an impact on the human body, equipment, high-quality bottle blowing moulds, corrosion or toxicity. 

Therefore, attention should be paid to the design of the mold, the selection of the injection molding machine and the molding, the screw injection molding machine should be selected, the cross section of the pouring system should be large, the mold and barrel should be plated with chromium, and there should be no dead angle stagnant materials. it is necessary to strictly control the molding temperature and join the stabilizer in the plastic. weaken its heat-sensitive function. 


4.2 even if some plastics (such as polycarbonate) contain a small amount of water, they will differentiate under high temperature and high pressure. This function is called hydrolyzability, and it is necessary to heat it up in advance. 

5. Stress cracking and melt fracture. 


5.1 some plastics are sensitive to stress, and they are prone to internal stress and brittleness during molding, and the plastic parts crack under the action of external force or solvent. 


For this reason, in addition to participating in additives to improve crack resistance in raw materials, attention should be paid to boring raw materials and reasonable selection of molding conditions in order to reduce internal stress and increase crack resistance. 

In addition, the reasonable shape of plastic parts should be selected, and the inserts should not be set up to reduce the stress concentration as much as possible. 


In the mold design, the demoulding angle should be increased, the reasonable feed inlet and ejection mechanism should be selected, and the material temperature, mold temperature, injection pressure and cooling time should be properly adjusted during molding, so as to avoid demoulding when the plastic parts are too cold and brittle. After molding, the plastic parts should also be post-treated to improve cracking resistance, eliminate internal stress and stop touching with the solvent. 


5.2 when the flow velocity of the polymer melt with a certain melt activity rate exceeds a certain value when it passes through the nozzle hole at a constant temperature, the appearance of the melt has a significant transverse crack, which is called melt fracture, which damages the appearance and physical properties of the plastic parts. 


Therefore, in the selection of polymers with high melt activity rate, the cross sections of nozzles, sprue and feed port should be increased, the injection speed should be reduced and the material temperature should be improved.

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6. Thermal function and cooling rate. 


6.1 all kinds of plastics have different specific heat, thermal conductivity, thermal deformation temperature and other thermal functions. 

Plasticizing with high specific heat requires a large amount of heat, so we should choose an injection molding machine with high plasticizing ability. 

The cooling time of plastics with high hot deformation temperature can be short and demoulding early, but it is necessary to prevent cooling deformation after demoulding. The cooling rate of plastics with low thermal conductivity is slow (for example, the cooling rate of ionic polymers is very slow), so it is necessary to be full of cooling, so it is necessary to strengthen the cooling effect of the mold. The hot sprue mold is suitable for plastics with low specific heat and high thermal conductivity. 


The plastics with high specific heat, low thermal conductivity, low thermal deformation temperature and slow cooling rate are not conducive to high-speed molding, so it is necessary to select appropriate injection molding machine and strengthen mold cooling. 

6.2 according to the characteristics of various kinds of plastics and the shape of plastic parts, it is necessary to adhere to the appropriate cooling rate. 

Therefore, it is necessary for the mold to set up a heating and cooling system according to the forming requirements in order to maintain a certain mold temperature. 


When the material temperature increases, the mold temperature should be cooled to prevent the plastic parts from deformation after demoulding, shorten the molding cycle and reduce the crystallinity. 


When the plastic waste heat is insufficient to make the mold adhere to a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature in order to control the cooling rate, ensure activity, improve filling conditions or be used to manipulate plastic parts to make them cool slowly, prevent uneven internal and external cooling of thick-walled plastic parts and improve crystallinity. 

For good activity, large forming area and uneven material temperature, the plastic parts sometimes need to be replaced by heating or cooling or local heating and cooling. 


For this reason, the mold should be equipped with a corresponding cooling or heating system. 

The mold temperature and thermal functions required for all kinds of plastics are shown in Table 1 Mel 4 and Table 1 Mel 5. 


7. Hygroscopicity. 

Because there are all kinds of additives in plastics, they have different degrees of closeness to moisture, so plastics can be roughly divided into two types: moisture absorption, adhesion moisture and non-water absorption, which is not easy to adhere to moisture. It is necessary to control the water content in the material within the allowable range. However, water turns into gas or hydrolysis under high temperature and high pressure, resulting in foaming, decreased activity, poor appearance and mechanical function of the resin. 

Therefore, it is necessary to choose appropriate heating methods and standards for preheating according to the requirements, and infrared radiation should be used to prevent moisture absorption.


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Foshan Shunde Zongsheng Precision Mould Co., Ltd. 

Tel: 0757 Tel: 22269630 / 13923298099 (Mr. Nie). 

Web site: www.fszongsheng.com. 

Add: no. 1, Rongye 6 road, Fengxiang industrial zone, Daliang, Shunde district, Foshan, Guangdong


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