Temperature & pressure setting for plastic injection moulds in production

Update Time:2015/10/1
Plastic Injection Mould is an engineering technology.The important process conditions for injection molding are the temperature, pressure and corresponding action time that affect plasticizing flow and cooling and forming

A.Temperature Control

1.Feed cylinder temperature: Temperatures that need be controlled during injection molding are feed cylinder temperature, nozzle temperature, and mold temperature. The first two temperatures mainly affect plastics’ plasticizing and flowing, and the third one mainly affects plastics’ flowing and cooling .Every kind of plastics has different flow temperature. Flow temperature and decomposition temperature for same kind of plastics with different origin or brand also have differences because of different average molecular weight andmolecular weight distribution. The plasticizing process of plastics indifferent types of injection machines is also different, so the correspondingcylinder temperatures are varied.

2.Nozzle temperature: Nozzle temperature usually should be slightlylower than the highest temperature of feed cylinder. This is for avoiding thatmelt in straight through nozzle may cause material drooling phenomenon. But thenozzle temperature cannot be too low. Otherwise it will lead to untimelycondensing of melt which will block nozzle, or that untimely condensingmaterial enters cavity affecting product properties.

3.Mold temperature: Mold temperature has a great affect on productintrinsic properties and apparent mass. Mold temperature depends on plastics’crystallinity, requirements on product size, structure and properties, andother process conditions (melt temperature, injection speed, injection pressureand molding cycle). B.Pressure Control: pressures during injection molding includeplasticizing pressure and injection pressure. They directly affect plastics’plasticizing and product quality.

1.Plasticizing pressure: (Back Pressure) While using screw injectionmolding machine, Pressure that melt on the top of screw experiences when screwrotates back. Is called plasticizing, also known as back pressure. It’sadjusted by the overflow valve of hydraulic system. During injection molding,Keeping the screw’s rotate speed same, increasing plasticizing pressure willraise melt temperature and lower plasticizing speed. Besides, increasingplasticizing pressure usually can keep melt temperature and color material’smixing uniform, and expel airs from melt. In general, plasticizing pressureshould be as low as possible under the precondition of making sure productqualified. Its specific value varies from the plastics’ types, but it oftendoesn’t exceed 20kg/cm2.

2.Injection pressure: In the current production, injection pressure for almostall injection molding machines is subject to the pressure (calculated from oilcircuit pressure) that plunger or screw top forces on plastics. The function ofinjection pressure in injection molding is to protect plastics from flowresistance from feed cylinder to cavity, offer the velocity for melt to fillmould, and compact melt

C.Forming CycleThe time that one injection molding process takes is forming cycle, also knownas molding cycle. Actually it includes the following parts:Forming cycle: forming cycle directly affects labor productivity and equipmentutilization rate. So, during production, when quality is ensured, try toshorten every related time in molding cycle. In whole molding cycle, injectingtime and cooling time are the most important ones. They have a decisive impacton product quality. Fill time in injecting time is directly inverselyproportional to fill rate. In production, mold filling usually takes 3-5s.Pressure-holding time:in injection time is the pressure time forplastics in cavity. It accounts for a large proportion in the whole injectiontime. It usually takes 20-120s (specially thick product can reach to 5 – 10minutes). Before the melt around gate freeze, the pressure holding time hasaffect on product dimensional accuracy. But no affect after that.Pressure-holding time also has a most favorable value. We already know itdepends on material temperature, mold temperature, and size of main runner andgate. If size of main runner and gate and process conditions are normal, it’susually subject to the pressure value with minimum fluctuation range of productshrinkage. Cooling time mainly depends on product’s thickness, plastics’thermal property, crystallization property, and mold temperature. The end ofcooling time should base on no changes while product demoulds. Cooling timelineis usually between 30s and 120s. Cooling time needn’t be too long. This notonly reduces production efficiency, but also leads to difficult demoulding.What’s worse, it may cause demoulding stress if forcibly demoulding. Other timein molding cycle is related to whether the production process is continuous andautomatic and its degree.

Above is the simple analysis and explanation about injection molding technology and they may vary for specified injection mould and material. If there are any other questions,pls call our phone +86-13732202800 for technical consulting.





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