Shanghai Daoqin Plastics Co., Ltd.

plastics products quality 取决于材 material (resin, processing additives and other) selection and injection molding processing conditions. plastics molding processing is 一门科学 and engineering 紧密结 combined 交叉学科, its is: understanding material features, most processing conditions, made most property can product. thermoplastic property plastics injection molding products molding shrinkage rate and crystallization degree and large small related, and polymerization and additives has 关, crystallization not only 取决于 chemical structure but also also to processing process in cooling speed rate, melt temperature degree, mold temperature degree, products thick degree and other effect/influence, to mold design type dimensions and control products dimensions degree belt/tape difficult. production in need understanding injection molding process for various plastics shrinkage rate effect/influence 规律. polypropylene (PP) tool has relatively high crystallization property, to product belt/tape relatively good rigid property, 但也决定 its relatively large products molding shrinkage, to its in injection molding field/area application belt/tape less made, usually, PP crystallization degree usually is 40～70％, its it is non 定形区, it molding shrinkage 会出现 in 1~2.5％ scope/range inner. in and lower application 厂家 交流 in 经常碰 to need solution reduce PP molding shrinkage issues. therefore less research for in PP production and processing process in how to reduce its shrinkage rate and other issues research.

2 commonly used polypropylene (PP) processing technology

    polypropylene (PP) as is growth most is strong/high general purpose plastics, and its tool has relatively wide molding processing property has very large 关系. 如PP can injection molding, blow molding, true hot molding, coating, molding, can 熔接, machine processing, electroplating and foaming and other, can in metal surface spray coating. currently, commonly used polypropylene (PP) molding processing technology has injection molding, 挤塑, film, foaming and other.

    commonly used polypropylene (PP) polymerization methods and its structure different, can separation is homopolymer, random copolymer and copolymer, usually injection molding using/adopting homopolymer and copolymer PP, 挤塑三 types can use, blow molding and film (如BOPP) can using/adopting homopolymer and random copolymer PP. currently market upper injection molding and extrusion PP use amount relatively large. generally injection molding temperature degree in 180~210℃ between, injection molding pressure in 65~140MPa, mold temperature degree is 40~70℃. drying temperature degree in 80℃ left right [1].

3 polypropylene (PP) molding shrinkage property and injection molding process

    polypropylene (PP) molding shrinkage is injection molding 厂家 关心 indicators, molding shrinkage crystallization shrinkage, orientation shrinkage, hot shrinkage, negative shrinkage and rear shrinkage. usually, PP from injection temperature degree reduce to room temperature 时, shrinkage relatively large, shrinkage rate generally in 1％~2.5％, tool has toward property, in shrinkage 宰10％-15％ rear shrinkage [2].

    molding shrinkage large small and resin property can, injection molding process parameters, products thick and is has material in has 关. its in injection pressure and resin temperature degree improve/increase for reduce molding shrinkage has 利, for product its it property can 无不利 effect/influence. therefore in injection molding process in, selection relatively high injection pressure, anti material material in 充模时 cooling to flow property belt/tape effect/influence. below/following separation is injection pressure, injection time, mold temperature degree and resin temperature degree for molding shrinkage effect/influence [3].

3.1 injection pressure

    injection pressure is screw 端面处 as used for plastics material mono 位面积 upper force, can injection 时 充模 pressure, 随 充模 pressure improve/increase, products molding shrinkage decrease small. 如进行塑 bottle production 时, 当充模 pressure from 5MPa increase large to 10DMPa, bottle shrinkage rate 由1.66％ reduce to 1.62%. therefore using/adopting high pressure injection make pressure increase has reduce products shrinkage rate, its reduce shrinkage rate causes is high pressure injection 时, plastics separation 子间受 to pressure shrink degree increase large, separation and separation between combined more 紧密, products shrinkage degree versus small.
    pressure is melt in temperature degree and pressure lower mold cavity 并一直持续 to gate this pressure. pressure for products shrinkage 起决定 property function, pressure high, products shrinkage small; if injection process conditions and equipment can ensure stability pressure, make products generate shrinkage by 动, effect/influence products dimensional stability. 封口之 front mold cavity pressure 取决于 holding pressure pressure, holding pressure pressure upper can as holding pressure stage/phase injection molding pressure, if gate time is 一定, that improve/increase holding pressure pressure must make gate pressure improve/increase, holding pressure pressure for gate pressure effect/influence upper 就反映 for shrinkage rate effect/influence, holding pressure pressure upper is injection molding pressure 直接反映. 图3显示 injection pressure molding shrinkage 关系.

 充模 speed rate is 指充模时 线 speed degree, speed rate for PP products shrinkage rate effect/influence relatively complex. from material temperature and pressure degree 说, improve/increase material speed rate has pressure 传导, make shrinkage decline. injection speed degree large, scratch hot large, products internal stress increase large, increase toward property, products shrinkage rate increase large.
充模 speed rate is injection speed degree 确定. another mold structure effect/influence (如 gate high degree and mold cavity deep degree ratio value). speed rate for molding shrinkage effect/influence tool has di heavy property, if from separation structure 方面 看, improve/increase speed rate will make crystallization and orientation as add strong/high, crystallization shrinkage and orientation function increase large, molding shrinkage tool has increase large trend; is speed rate increase large rear, has 助于克服 melt film 腔时 困 difficult, if injection time 不变, mold cavity inner material amount 必然会 increase, this 意味 补 shrink time versus for long, is molding shrinkage will decline. 图4显示 injection time and molding shrinkage between 关系.
　

             injection time /s
图4  injection time and molding shrinkage 关系(沿 material direction)
3.3 mold temperature degree    
mold temperature degree is control products cooling type main/primary factors, it for molding shrinkage effect/influence main/primary performance in gate rear products demolding front this process. gate 发生冻结 rear, mold temperature degree for gate rear PP products shrinkage 起主导 function. it through separation 子 冻结 orientation effect/influence type table/sheet 层冻结层 thick degree and formed speed degree, injection pressure and holding pressure force effect/influence will 会消失. mold temperature rise high, cooling type time will long, demolding rear products shrinkage rate generally increase large, this is is mold temperature high make plastics crystallization solid growth speed degree decrease slow, temperature degree and ring temperature degree add large, products hot cold shrink function versus for increase large, shrinkage rate versus increase large. reduce mold temperature can make 冻结层迅 speed thick, decrease small products shrinkage rate. 图s示出 mold temperature degree and shrinkage rate 关系.

图5  mold temperature degree and molding shrinkage 关系(沿 material direction)
3.4 melt temperature degree
PP products in holding pressure flow and cooling type stage/phase shrinkage melt temperature degree rise high increase large, its in material direction shrinkage rate reduce is obvious, melt temperature degree relatively low 时, material direction shrinkage rate large 于垂直 direction shrinkage rate, melt temperature degree rise high, shrinkage direction property relatively small, 并出现一拐点, injection molding temperature degree high temperature degree is 宜. in process in, has machine add hot temperature degree improve/increase, its shrinkage rate has reduce 情形. 图6示出 machine temperature degree and products shrinkage rate 关系, this can understanding is melt temperature degree rise high viscosity reduce, maintain injection molding pressure and holding pressure pressure 不变, 则传递 to mold cavity pressure versus increase high, due to gate temperature degree relatively high, gate time can can long, this has 利于补充 material material and increase 密实程 degree, can decrease/reduce shrinkage.
但 is, in experiment/testing process in also 发现, melt temperature degree high need versus improve/increase 锁模功 rate; this increase mold and ejection machine negative 担, meanwhile long products cooling cycle, production rate reduce, therefore in production most good need using/adopting improve/increase machine temperature degree and melt temperature degree method improve/increase can force, amount use low temperature 模塑.

      
           machine temperature degree （ melt temperature degree /℃）
图6  machine 简（ melt ） temperature degree and molding shrinkage 关系
3.5 PP crystallization property for molding shrinkage effect/influence [2, 4]
PP crystallization type polymer, its crystallization degree most high can to 70%, PP melt in mold in cooling 时伴随 crystallization, most large crystallization speed degree in 120℃ below/following 出现, 并放出 a large amount of crystallization hot, therefore mold cooling control very important. cooling process also has obvious shrinkage （ high melting point 30℃ lower melt, rear cooling to room temperature 25℃时 ratio 容变化 is 16%）, molding shrinkage relatively large. PP injection molding products, due to surface and mold 接触, cooling relatively fast, temperature degree relatively low, 晶核形 formed easy, difficult long large; internal cooling relatively slow, temperature degree relatively high, 晶核形 formed relatively difficult, density small, can long very large. therefore surface small internal large, large small for PP material mechanical properties effect/influence very large. generally 而言, can reduce crystallization long speed rate factors, can decrease small products in crystallization degree, has decrease small shrinkage rate, reduce mold temperature, improve/increase melt temperature degree and other.
another, PP crystallization rear can can 会出现二次 crystallization, main/primary distribution in non and crystallization 不完善 部 separation inner, is one-time crystallization 延续, therefore crystallization speed degree very slow. another, has products molding in also 会发生一 types rear crystallization phenomenon, this crystallization is in molding front or molding process in formed, 但不完 full, in molding rear 球晶进一步 long large, this is one-time crystallization 延续. crystallization and rear crystallization make products property can and dimensions in use and process in 发生变化, usually in thick and cold easy 出现. solutions is 退火处理, temperature degree generally control in Tg～Tm scope/range inner, most temperature degree is in most large crystallization speed degree temperature degree 附近, is 一 接近于 and other temperature and crystallization process.
3.6 its it factors
    has research table/sheet 明[5], mixed material shrinkage relatively polymerization grade material decline 50％, is value is, polymerization grade material shrinkage rate in 1.3％至2.2% between, mixed grade material material shrinkage rate in 0.5%至1.0% between. meanwhile elastomer use amount improve/increase,材 material shrinkage rate decline. for/regarding large type also is small type 模, shrinkage rate type pressure increase decrease small, and large small 无关.
    equipment add material machine and ejection machine 构也会 effect/influence products shrinkage rate, ejection defect, add material positive 常, screw 回位不充 separation, screw speed unstable, back pressure 不均, liquid pressure system 止回阀工 as positive 常, hot temperature parts control 不灵, cooling and other, can can effect/influence products shrinkage rate.
    from material 本身考虑, can can has 几方面 causes effect/influence products shrinkage rate. most Common is material pellets 不均, water amount separation, screw material formed separation has and other. has add color agent 也会引起 products shrinkage, is has material system has effect/influence dimensions stability formed separation, but also this material make non micro amount (如0.001％), effect/influence molding shrinkage 宰, make shrinkage rate increase large, make shrinkage 出现各 toward property, 垂直于 material direction shrinkage rate large flow direction shrinkage rate. another, has research table/sheet material concentrated degree for molding shrinkage effect/influence large.
4 summary    
    PP molding shrinkage and raw materials and processing conditions related. from raw materials degree 讲, can optimization PP crystallization 形态, adjustment polymerization process or is add combined nucleating agent, make fine 化, to improvement molding shrinkage 目: from processing conditions 讲, can from gate dimensions, products thick, injection pressure, speed rate, machine and mold temperature degree and other adjustment, 当然, this its in factors more, versus between effect/influence 关系也 very complex, very more issues also has 待于进一步 in-depth research.