Micromoulding: process characteristics and product properties

Abstract

Key aspects of the technology and challenges associated with the micromoulding process are discussed. The apparent shear and extensional viscosity behaviour of a polyacetal at high wall shear rates have been measured using inline capillary rheometry on a commercial micromoulding machine and a larger servoelectric injection moulding machine; the polymer behaved predictably at shear rates in excess of 10⁶ s^-1. Initial moulding trials indicated that a stepped plaque and 0·25 mm thick rectangular plaque mouldings filled in a satisfactory manner, but a thicker plaque cavity exhibited a jetting flow into the cavity.

A data capture system capable of measuring multiple process dynamics at high sampling rates (up to 50 kHz per channel) allowed detailed process measurements taken during moulding of the stepped plaque moulding. Atomic force microscopy of the moulded products showed different surface finishes on each step of the stepped plaque moulding. Topography scans of the 0·25 mm thick rectangular plaque moulding showed that mould surface features with length scales of the order of a few micrometres were well replicated on the moulded product and the quality of the surface finish is dependent on the melt pressure during moulding.     

基于注塑CAE的管件反变形设计

利用注塑CAE软件对聚乙烯管件进行变形分析,发现管件孔口沿熔接线方向发生椭圆形变,然后结合实测不圆度数据,确定反变形型腔尺寸,对管件模具在型腔、型芯上的分型面方向和开模方向采用不同的收缩率进行实际注射实验.结果表明,对圆筒状管件采用反变形,使其孔口不圆度有很大改善.     

Relaxation patterns in powder injection mouldings made with repeated flow Reversals

Injection moulding suspensions based on both high and low molecular weight organic vehicles and incorporating both spherical and anisotropically shaped particles were prepared. A double end-gated rectangular bar cavity was used for injection moulding. Conventional mouldings were made by injection from one gate followed by static hold pressure. Modulated pressure mouldings were made in the same way but reciprocating flow was induced between the gates during solidification. The mouldings were sectioned, polished and heat treated whereupon a pattern of rings emerged in relief preserving the history of flow reversals as the solid-liquid interface advanced from the mould walls.     

The Tensile Behaviour of an Injection-Moulded Propylene–Ethylene Copolymer: the Effect of the Local Thermomechanical Processing Conditions

In this paper the mechanical behaviour of a propylene–ethylene copolymer was characterised as a function of the local processing conditions using injection-moulded axisymmetrical dumbbell-like specimens of diameter 1·5mm. The moulding programme, designed according to the central composite face technique, included variation of the injection and mould temperatures and the flow rate. The local thermomechanical environment associated with each moulding condition was determined by filling simulations and quantified by the local bulk temperature and the maximum shear stress. Based on these parameters, thermal and thermomechanical indexes were defined to quantify the morphology of the mouldings, which was evaluated experimentally by the skin ratio. The tensile tests were carried out at two different constant velocities: 2 mm min^-1 (0·56×10^-6ms^-1) and 3 ms^-1, in order to assess the tangent modulus, the strain and the energy at rupture. The results concerning processing–structure and processing–properties relationships are presented as 3D plots, based on polynomial interpolations of the experimental data. © 1997 SCI.     

Numerical analysis of neck propagation in polymeric materials. Part 2 – Neck propagation behaviour of poly (butylene terephthalate) mouldings

Abstract

This paper examines the factors controlling the formation and propagation of a neck in poly (butylene terephthalate) (PBT) mouldings under tensile loading. Tensile tests were used to investigate the load–displacement and deformation behaviour of PBT and the accompanying changes in surface temperature. In parallel with this experimental study, a numerical model was developed for the deformation of PBT mouldings and neck formation under tensile loading analysed using finite element analysis (FEA). The calculated numerical results were compared with the experimental data. This work has shown that formation does not occur in PBT immediately after the yield point. Instead, plastic deformation first progresses homogeneously through the testpiece. Neck formation and propagation, accompanied by a rise in temperature, then follow. The load–displacement behaviour calculated using FEA could be approximated to the experimental data by adapting an elastic–plastic model at a stable temperature to the necking behaviour of PBT moulding. Furthermore, the dependence of neck formation on strain rate is related to the plastic instability, as demonstrated by the numerical results, and does not depend upon heating effects.     

Interfacial instabilities in multimaterial co-injection mouldings Part 1 – Background and initial experiments

Abstract

Interfacial adhesion between the skin and core is vital for successful co-injection moulding. This is the first paper in a series, which introduces and describes an in mould method of mixing that is applicable regardless of the compatibility of the materials. It works by inducing turbulent mixing at the interface between the skin and core materials. It makes use of the change that occurs from laminar to turbulent flow at high injection speeds in co-injection moulding. This novel approach takes advantage of the moulding parameters already available within the co-injection system to offer an expanded range of material combinations for multimaterial moulding. Comparisons are made between multimaterial mouldings made with miscible polymers, immiscible polymers with no compatibiliser, and immiscible polymers bonded by compatibilisers.     

Interfacial instabilities in multimaterial co-injection mouldings: Part 2 - Interfacial mixing in transparent mouldings

Abstract

Previous research in co-injection moulding has produced viscosity ratio guidelines for skin and core components, which must be followed if good core distribution is to be achieved. However, by examining two phase systems of PMMA-PC, which fall within the recommended viscosity range, this work shows that viscosity matching of materials is not a sufficient requirement for interfacial stability. The transparency of the materials allows areas of interfacial mixing to show up as streaks in the mouldings, so that factors affecting stability can be determined. One system is found to be more stable than the others. Explanations for such effects are given by consideration of interfacial mixing, viscosity, tooling geometry, injection speeds, interfacial stresses and shear in multilayered mouldings.     

Reducing performance variations of compression moulded long glass fibre reinforced thermoplastics by targeted reinforcement

Abstract

Long glass fibre compression mouldings are prone to significant variations in mechanical properties. Stiffness and strength tend to vary across an individual moulding and in addition there are also variations when different mouldings are compared. Compression mouldings in the form of a top hat section were produced. This component was first modified to have ribs, then samples of both batches were reinforced with a thin thermoplastic composite plate with continuous fibres. In this way, four different batches were produced, which were tested under 3-point-bending. Further, this work investigated the changes in mechanical properties across the manufactured top hat section mouldings. Tensile test results strongly indicate that the scatter of mechanical properties increases towards the edges of the compression moulding. When the mouldings were tested under 3-point-bending, the experimental results were prone to a large scatter and it was not possible to predict the position of failure or the failure mode. However, this work successfully demonstrated that a targeted reinforcement of the moulding with continuous fibres can significantly reduce the performance variations, particular under loading with large strain. The findings of this work could lead to the development of new lightweight structural components, where there is a need for integrating functions into the moulding in order to reduce weight and costs for mass production.     

Residual orientation in injection‐moulded plaques of atactic‐polystyrene I: The effect of processing conditions

Injection moulded polymer articles often have residual macromolecular or crystalline orientation which can have a significant impact on the optical and mechanical properties of the moulded article. Small angle neutron scattering (SANS) was used to measure the molecular shape and orientation of deuterated blends of injection moulded polystyrene. For ～1‐mm‐thick mouldings of uniform rectangular cross‐section, the eccentricity in the SANS pattern gave a direct measure of the residual molecular orientation over the length scale ～100–1,500 Å. The residual orientation was found to vary significantly with injection moulding conditions with comparative residual orientation decreasing with decreasing mould fill‐time, and increasing with mould thickness and moulding temperatures. The orientation was found to be a minimum in the centre of the mould and highest near the surface and the average orientation at a particular position in the mould was found to be strongly correlated with the volume of material deposited as a solid skin layer during injection moulding. POLYM. ENG. SCI., 58:1322–1331, 2018. © 2017 Society of Plastics Engineers     

车用纤维增强热塑性塑料技术进展

随着汽车工业的发展,纤维增强热塑性塑料的需求大幅增长,其制造工艺技术也进入了快速发展期。诸如直接(在线)配料长纤维增强热塑性塑料(D-LFT)、为改进长纤维增强热塑性塑料(LFT)模制品表面质量而采取的模具型腔感应加热、LFT母粒、基于环状齐聚酯原位聚合的纤维增强复合材料、LFT专用玻璃纤维与表面浸润剂等新技术不断涌现。对这些最新的工艺技术进展作一综述,为开发车用纤维增强热塑性塑料提供参考。     

Weld line improvement of short fiber reinforced thermoplastics with a movable flow obstacle

Short fiber reinforced (SFR) thermoplastics are ideal materials from which to manufacture complex technical parts in high volumes with low energy expenditure. The orientation of the fibers, and hence their reinforcing effect, depends strongly on the nature of the cavity and on the injection molding process. One disadvantage of SFR thermoplastics is a significant decrease in mechanical properties in the areas of the weld lines, due to subopt imal fiber orientation as the melt streams reunite at these points. Common mold‐based and process‐based optimization techniques alter the fiber orientation after the formation of the weld line. The mold‐based approach presented here, on the other hand, operates at the time the weld line is formed: by redirecting the melt streams, it moves the weld line and improves the fiber orientation. A prototype mold is described, and samples produced from it with both standard and modified weld lines are compared with flawless specimens. The new technique yields a large rise in flexural strength and a smaller but significant improvement in tensile properties. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42025.     

立式空调机底盘注塑模设计

分析了立式空调机底盘的工艺特点和工艺要求,说明了模具工作原理,着重阐述了模具的型腔布置、分型面选择、浇注系统、成型零部件、定距分型机构、侧向分型与抽芯机构、型腔锁块机构、型芯支承柱、顶出与复位机构及冷却系统的设计特点。试模结果表明,该模具结构合理、动作可靠,所成型的注塑件质量满足技术要求。     

Observations on the melt memory of high negative birefringence spherulites in polypropylene

Previously reported work showed that high negative birefringence Type III polypropylene spherulites can occur in the weld zone of butt-welded injection mouldings. Their occurrence under conditions not normally associated with their formation has been investigated further using a variety of thermal treatments. The behaviour proves to be complex but clear evidence emerges that β-form nuclei remain active in the material for thermal treatments up to 200°C, although this is time dependent and the number of Type III spherulites steadily reduces.     

吹风机注塑模具浇口的优化设计

浇口位置在注塑成型过程中起关键性作用。介绍专业注塑模分析软件MPI在吹风机注塑模具浇口位置设计的应用,从产品的填充时间、充模压力、流动前沿温度和熔接痕等方面对不同的浇口设计方案进行对比。分析表明,两浇口位置方案具有注射压力、锁模力小,熔接痕和气穴数量少的优点,是较佳的浇口位置方案。     

Laser welding of niobium to AISI 304 steel using a copper interlayer

Abstract

Cu interlayers with thicknesses of 1, 1.5, and 2?mm were used to join niobium and AISI 304 steel. Fractures occurred in the weld, the Nb base metal, and the unmelted Cu interlayer when the Cu interlayer thickness was 1, 1.5, and 2?mm, respectively. When the thickness of the Cu interlayer was 1?mm, the weld microstructure consisted of austenite with Cu-rich particles along the austenitic grain boundaries and within the austenitic grains, a composite-like structure (the Fe₂Nb lamellae and particles in a γ matrix) embedded with coarse Cu globules, and a mixture of bulk Fe₇Nb₆, Nb-rich dendrites, and Cu matrix. The bulk brittle Fe₇Nb₆ phase embrittled the joint. However, when the thickness of the Cu interlayer was 1.5?mm, the weld microstructure consisted of austenite with Cu-rich precipitates along the austenitic grain boundaries and a Cu-rich phase embedded with Nb-rich particles and dendrites. Solid-solution strengthening of Cu by Fe was responsible for the improved mechanical properties of the joint. The mixture of Nb-rich particles and dendrites in the Cu matrix was also helpful in enhancing the joint strength. Furthermore, when the thickness of the Cu interlayer was 2?mm, the weld microstructure consisted of austenite with Cu-rich precipitates along the austenitic grain boundaries and within the austenitic grains, an unmelted Cu interlayer, and Nb-rich particles and dendrites embedded in a Cu matrix. The unmelted Cu interlayer reduced the joint strength.     

四板式正-斜双联齿轮注塑模

正-斜双联齿轮塑件由直齿轮和螺旋斜齿轮两部分组成,两部分之间有一底径为12mm,宽5mm的环形凹槽。设计了正-斜双联齿轮注塑模,论述了模具结构特点和工作原理。该模具采用四板结构,三次分型,一模一腔,四点浇口进浇,斜销-哈夫滑块侧向分型抽芯,推管推件,型环旋转脱螺旋斜齿轮。模具结构紧凑,工作可靠,操作方便,成型塑件精度高。     

基于CAE分析的精密线圈骨架制品及模具的优化设计

针对线圈骨架制品表面质量差、强度低的问题，使用Moldilow软件对制品的充模流动过程进行了CAE分析，优化了制品尺寸，把两处筋的厚度从3mm和3．5mm分别调整到2．5mm和1．5mm，显著减少了工作表面的熔接痕数量，并确定了注射成型工艺条件。模具采用两点进浇的点浇口方式，浇注系统可自动脱落。采用独特的哈夫块锥面定位结构，冷却系统使制品的冷却充分、均匀，保证了制品的质量要求。     

Process improvement in preparation of ball valves

Abstract

The industrial application of ceramic injection moulding has been limited to small components because of the associated binder burnout problems. Injection moulding may improve the manufacture of alumina ball valves for the process industries by reducing costly and potentially damaging machining operations. However, these components are too large to be produced using conventional thermoplastic binders. Aqueous injection moulding allows most of the binder system to be driven off as water vapour, cutting process time, reducing burnout difficulties, and causing less environmental pollution. The manufacture of ball valve blanks from -alumina was carried out using a die cavity of 25·4 mm outside dia. Improved processing parameters were efficiently identified using orthogonal arrays as described by Taguchi. The following factors were evaluated for their influence on defects: binder content in solution, die temperature, injection velocity, injection time, barrel temperature, injection pressure, hold pressure, and residence time in the mould. Binder content in solution was found to be the most influential factor. The defects observed included cracking, poor make up, and distortion. Ball valve blanks of significantly improved quality were successfully manufactured under the preferred conditions.     

Preparation and properties of waterborne poly(urethane acrylate)/silica dispersions and hybrid composites

Abstract

Simulations of the isothermal and non-isothermal filling of a rectangular cavity were carried out for polystyrene using a Giesekus viscoelastic constitutive equation, whereby in the non-isothermal case the thermal resistance at the mould wall was modelled with different heat transfer coefficients to investigate the effect of the thermal resistance on the development of the molecular orientation. Results for stress development along the flow front and the cold wall were compared showing that the principal stress differences in the middle of the flow front are lower than those at the mould wall. In case of the non-isothermal filling, the latter one will increase further while the melt is gradually cooling down, which is especially true if the thermal resistance at the mould wall has been properly considered. Consequently, the high molecular orientation at the wall seems to be rather a result of the non-isothermal shear flow than of the extension at the advancing front as usually assumed.     

Real time diagnostics of gas/water assisted injection moulding using integrated ultrasonic sensors

Abstract

An ultrasound sensor system has been applied to the mould of both the water and gas assisted injection moulding processes. The mould has a cavity wall mounted pressure sensor and instrumentation to monitor the injection moulding machine. Two ultrasound sensors are used to monitor the arrival of the fluid (gas or water) bubble tip through the detection of reflected ultrasound energy from the fluid polymer boundary and the fluid bubble tip velocity through the polymer melt is estimated. The polymer contact with the cavity wall is observed through the reflected ultrasound energy from that boundary. A theoretically based estimation of the residual wall thickness is made using the ultrasound reflection from the fluid (gas or water) polymer boundary while the samples are still inside the mould and a good correlation with a physical measurement is observed.