Precision injection molding technology

Precision injection molding generally refers to the precision and surface quality of forming parts are demanding process, there are also people who believe that precision of the precision injection refers to the size of the plastic injection molding technology of 0.01 ~ 0.001 mm.In recent years, with electronic, medical, communications, automotive and other industries, the rapid development of high precision, high performance requirements of plastic products is growing, the traditional plastic molding process has been difficult to meet the requirements of the current high standards, at the same time, the injection molding is one of the most important plastic molding method, therefore, precision injection molding technology is getting more and more get the attention of the people, make the precision injection molding technology constantly progress, new technology constantly emerging.Factors influencing the precision of injection molding precision injection molding for products not only has high dimensional precision, lower buckling deformation, good transfer characteristics, but also should have excellent optical performance and so on, therefore, it is for injection molding machine, mold, injection molding technology, performance and the formula of raw material and so on all have special requirements, there are many factors which can affect the precision injection molding.
According to the actual production, the influencing factors of precision injection molding mainly has the following several aspects:
(1) design of precision plastic products, plastic shrinkage rate should take into account the shrinkage rate of plastic.Tolerance grade precision injection products can be according to the SJB72-7.8 - the two tolerance grade 1 and 2.
(2) products of plastic parts size and shape of the product size and shape accuracy have great influence to the production practice shows that the more products the more complex shape and size, its precision is low.
(3) the precision of plastic parts and mould structure mold cavity number and production batch is closely related.Also shows that the precision of the single cavity mould structure is higher than multi-cavity mould structure, the precision of the small batch production is higher than mass.
(4) mold machining accuracy due to the products must be within the cavity injection molding, therefore, the precision of the products no matter how to also can't more than the precision of the cavity.Under normal circumstances, low accuracy than the mould precision injection molding products one or two tolerance grade.In terms of mould manufacturing technology at present, most of the mold cavity and high-speed milling, grinding, polishing processing methods such as manufacturing or electricity, the processing method can achieve the highest accuracy required.
(5) injection molding machine for precision injection molding of products is of high accuracy, so they usually require for special precision injection molding machine.Precision injection molding machine injection power is larger, higher control accuracy and response speed of the hydraulic system is faster, clamping system to have enough rigidity.
When the choice of precision plastic injection molding plastic products molding shrinkage rate fluctuation is bigger, in addition, different varieties of plastic, due to adopt the kinds and ratio of polymer and additives is different, their performance in injection molding flow properties and forming performance will have a big difference;Even for plastic components and the ratio of the same, because manufacturers, factory season and environment condition factors, such as injection molding of products with them also exist between the question of whether or not to stable size and shape.Therefore, precision injection molding plastic in addition to the requirements of good flow property and formability, want use them to produce the products to the stability of the shape and size.Otherwise, it is difficult to guarantee the precision of the products.Therefore, precision injection molding plastic products, must be in plastic varieties and its formability was born in consideration, as far as possible choose the material shrinkage of small, long-term stability.Usually, PS, ABS, AS, PMMA, PC, PPO, PSU can molding grade 3, 4, the precision of products;PA series can be formed 4 precision products;And POM, PP, PE, TPE is not suitable for forming accuracy of 4 class above products.
In addition, because of the inorganic filler or low glass fiber enhancement of plastic shrinkage rate, high dimensional stability, reason often is used for forming of high precision products.Precision injection molding process characteristics of precision injection molding process characteristics are mainly high injection pressure, injection speed and the temperature control must be accurate.
(1) high pressure injection test show that, for different plastic, variety, shape and size precision injection molding of the pressure needed for 180-250 mpa (injection mold injection pressure is commonly used in the 40-200 mpa), in certain special circumstances require higher.The highest has about 450 mpa.
(2) the size of the injection speed injection speed, significantly influence the productivity of the products not only, more important is to reduce the size of the products of tolerance.In order to improve the accuracy of injection molded parts, especially for complex shape product molding, in recent years the development of the injection speed, injection velocity is changing, its change law according to the shape of the structure of the parts and plastic performance decisions.
(3) must be precise temperature control temperature has great influence on the quality of products molding, injection molding is one of the three process conditions.For precision injection, not only the problem of high and low temperature, but also the precision of temperature control problems.Obviously, in the process of precision injection molding, if not accurate temperature control, the liquidity of plastic melt and formability and shrinkage rate is not stable, so it won't be able to guarantee the precise of products.From this perspective, the use of precision injection molding, no matter for the barrel and nozzle, or for injection mold, must be kept under strict control their temperature range.For example, the precision injection molding, the barrel and the PID controller, the temperature of the nozzle temperature range of control within 1 ℃, and for ordinary injection molding machine, its temperature is 20 ℃ 300 ℃.For precision injection molding production, in order to guarantee the precision of the products, in addition to strictly control the temperature of the barrel, nozzle and mould, also notice the influence of ambient temperature on the product accuracy after demoulding.Design precision injection note precision injection mold design, in addition to the ordinary design method and requirements of injection mold, because of the strict requirements in accuracy, some also pay special attention to the following items:
1.Mold design should have a higher precision mold precision though is closely related to the processing and assembly technology, but haven't come up with appropriate technical requirements, when the design or the mould structure design is unreasonable, so no matter how high the processing and assembly technology, mould precision still cannot be guaranteed.In order to ensure accuracy of precision injection mold don't impact due to design problems, must pay attention to the following four points:
(1) the design precision of spare parts and technical requirements should be compatible with precision products to make the mold to ensure accuracy of products, the first requirement precision cavity and parting surface precision must be compatible with products accuracy.Generally speaking, the dimensional tolerance of precision injection mold cavity should be less than a third of the products of tolerance, and needs to be in accordance with the actual conditions of products specific to determine.Parting surface, parting surface precision parallel degree) is mainly used to ensure accuracy of cavity.For small precision injection mold, parting surface parallel technology requirements of about 0.005 mm.Die structure of parts, although not directly involved in the injection molding, but effect the precision of mould cavity, and, in turn, affects the precision of products, so all deal with their structure parts put forward appropriate and reasonable accuracy.
(2) determine the move, with the fixed mold precision under normal circumstances, the general injection mold relies mainly on the guide pin guide institutions to ensure the accuracy of involution.However, because of the clearance fit of guide pillar and guide hole, big or small there are a certain gap between, the gap is often the shadow
The installation precision of mold in injection molding machine, a dynamic model and the fixed mould leads to wrong move, so it is difficult to used for injection molding precision products.In addition, under the condition of high temperature injection, dynamic and thermal expansion of the fixed template will sometimes move occurred between the wrong and finally change to products accuracy.In precision injection mold, shall die by the move as possible as you can, move between fault, managed to ensure the involution precision of mould and mould, can design cone positioning or cylindrical guide are in guide pin guide pin positioning of blending.
(3) the mould structure should have enough stiffness of precision injection mold must have enough stiffness, otherwise, they are on the injection pressure and mould clamping force under the action of large elastic deformation will happen, causing change mould precision, and thus affect the precision of products.For monolithic convex and concave die, its structure stiffness by its shape size and mould materials are needed to ensure, for the insert or spell convex, concave die, its structure stiffness is often associated with fastening frame used in the insert.The material of the frame should use 40 cr alloy structural steel, heat treatment hardness requirements HRC30 around.
(4) activity in mould movement should be accurate precision injection mold parts, often with some of the forming block or core activities.Spare sports inaccurate if these activities, namely after every movement can't return to the original position accurately, so no matter how much mold parts processing precision high accuracy, precision and the structure of the mould itself products will appear a large fluctuations.In order to solve this problem, need to adopt some special motion positioning structure.
2.Avoid bad mold design and make the products appear uneven contraction design precision injection, if improper or temperature control system design of die structure and is easy to make the products appear uneven shrinkage phenomenon, the phenomenon of products precision and the stability of the products precision will produce bad influence.In order to avoid this kind of problem, need to pay attention to the following points:
(1) gating system should try to make the material flow balance.
(2) the cavity of the cloth row should try to keep the mold temperature heat balance.
(3) temperature control system is better to separate each cavity temperature regulation.
(4) best punch and the die set up cooling water.
3.Avoid the mold problem make products appear demoulding deformation current precision injection products is generally not a very big size, wall thickness is thin, some still with many thin ribs, so it is easy to deformation when parting, the deformation will inevitably cause the loss of precision products.In order to avoid products appear demoulding deformation, the following three points should be paid attention to.
1) mould besides should have enough stiffness, its structure and suitable products demoulding;
(2) generally speaking, it is best to precision injection products to take off the template (push board) release, this is beneficial to prevent products demoulding deformation;
(3) precision injection products draft are generally smaller, but not easy to release.
4.For complex shape of products should be framed structure of the mould to make products can achieve higher precision, must to grinding cavity, but the cavity can be grinding, will be restricted by the complexity of the product shape.In order to solve this problem, for complex shape precision injection molding products, the best cavity design into framed structure.Precision forming technology.
5.Application in the mould injection compression molding (ICM) ICM technology refers to the mold under the condition of open distance began taking, injection to a certain amount, after mold closing within the cavity on the melt into effect compression, injection mold completely closed end, then pressure maintaining and cooling until the take out the products.Injection compression molding are compacted products through compression behavior, make the surface of products have uniform pressure distribution, the compaction of products of high dimensional accuracy and stability, the deformation is small.It is in the case of mold opening will melt into the cavity
And flow channel, the injection pressure is very low, and reduce or eliminate the caused by the holding the internal stress of resin molecular orientation and products, so as to improve the dimension stability of products.ICM technology of flexible and control ability are markedly improved compared with general of the injection mould.So the technology can be used to produce higher precision parts, especially high precision cylindrical parts.
6.High speed injection molding high speed injection molding method of melt filling speed than traditional 10 to 100 times faster, makes the melt to produce high shear flow in the cavity, the viscosity decreased, mold filling speed, plastic surface hardening slowing down, and the limit of the thin wall products molding thickness, inhibit excessive molding pressure, at the same time it is due to the low pressure in the mold flow, reduce internal stress.For thin-wall precision products, can use when we do the injection screw forward cause melt absorb energy, after moving screw stops by melt inflation high speed full of cavity.
7.Without pressure of injection molding pressure injection molding technology refers to the plastic melt at high speed, high pressure filling into the mold, then close to the nozzle of the needle valve, plastics melt in die cavity automatic offset the contraction of the parts, so can reduce the buckling deformation of the products.But this method need to estimate in advance the holding of the shrinkage added part, so need to have higher in injection mold cavity pressure value, need the clamping force is high also.
8.Other intelligent control precision injection molding technology of the processing conditions in the process of continuous monitoring and precise control is very important.With the development of computer technology, computerized injection molding has been widely used.There are statistical process control (SPC)
Technology, P ID technology, fuzzy logic control (FCC), central network control (NNC) method and the model based on reverse processing backbone network size control, etc.Conclusion as a new technology, new materials, new technology and new equipment appeared, especially the computer is widely used in plastic processing, created good conditions for precision injection technology, will continue to ultra high speed, super precision, miniaturization, and intelligent direction of development and perfection.