Injection Moulding Grade Research Articles

Characterising the linear viscoelastic behaviour of an injection moulding grade polypropylene polymer

The linear viscoelastic behaviour of an injection moulding grade polypropylene is studied using theoretical and computational methods. Polypropylene has a variety of engineering applications as a component. However, it commonly exhibits viscoelastic deformations. This paper analyses the creep and recovery responses of the BJ368MO polypropylene copolymer using the Burgers and generalised Maxwell models. Within the linear viscoelastic regime, an experimental creep strain at 20 text{MPa} is used to determine the rheological constants of the models. These constants (springs and dashpots) are determined using a nonlinear least-squares curve fitting of the experimental creep. Then they are used to predict the creep and recovery responses of the polymer at three different stresses, 10 text{MPa}, 12.5 text{MPa} and 15 text{MPa}. The experiments are made using tensile specimens designed according to the ASTM D638-14standard. The theoretical evaluations are made using the creep and recovery equations derived from their constitutive. Whereas COMSOL Multiphysics software is used during the finite element (FE) analyses. The results of the theoretical and FE calculations are verified using creep and recovery experiments. Based on the validation analyses, both viscoelastic models showed lower deviations from the experimental results when a computational approach is used. In addition, the viscoelastic models are compared by evaluating the residuals of the creep and recovery strain predictions. The theoretical analyses showed better predictions at 12.5 text{MPa} and 15 text{MPa} stresses when the generalised Maxwell model is used. However, the improvements are attributed to the recovery predictions. When FE is used, the Burgers model showed lower mean absolute percentage errors (MAPEs) in all creep and recovery predictions. The model has a minimum of 6.37% error at the 10 text{MPa} stress and a maximum of 8.23% error at the 15 text{MPa}. By comparison, the generalised Maxwell model showed a minimum of 9.24% error at 12.5 text{MPa} and a maximum of 12.8% error at 15 text{MPa} stresses. The novelty of this paper is on predicting the creep and recovery behaviour of the polymer using the FE and theoretical approaches in the linear viscoelastic regime. The findings suggest that the FE analyses using the Burgers viscoelastic material model provide better predictions, with all calculated errors falling below 10%.

Morphology, thermal and dynamic mechanical properties of poly(lactic acid)/expandable graphite (PLA/EG) flame retardant composites

This work reports on the effect of expandable graphite (EG) on the morphology, thermal and dynamic mechanical properties of flame retardant poly(lactic acid) (PLA)/EG composites. The composites were prepared by melt-mixing and their structure, morphology, melting and crystallization behaviour, as well as their dynamic mechanical properties, were investigated. It was found that graphite layers still existed in an aggregate structure with poor filler dispersion resulting in a lack of interfacial adhesion between EG and the PLA matrix. The presence of EG did not favour the crystallization of PLA, increased the glass transition temperature and showed a reduction in the crystallinity of the composites. The composites with higher filler contents showed enhanced storage and loss moduli. The glass transition temperatures from the loss modulus and damping factor curves varied inconsistently with EG content. The use of commercial EG as filler in PLA can preserve the thermal properties of injection moulding grade Cereplast PLA.

Influence of the Molar Mass on Long-Chain Branching of Polypropylene.

Long-chain branching (LCB) with peroxydicarbonates (PODIC) is known as a suitable post-reactor process to introduce strain-hardening behaviour and an increase of melt strength to a linear polypropylene (PP). This opens up new possibilities for processing and therefore application. Especially in the case of adding value to PP post-consumer waste, LCB is a promising approach. LCB takes place by a combination of chain scission and recombination after radical activation of the PP macromolecule. However, chemical modification of post-consumer waste is challenging because of the inhomogeneous composition and the manifold number of PP grades. The influence of the molar mass of the linear PP precursor on this reaction was studied with different PP grades ranging from extrusion grade to injection moulding grade. To exclude side effects, all PP grades had similar polydispersity indices. A PP with higher molar mass undergoes significant chain scission during the LCB process compared to a PP with low molar mass for injection moulding. Therefore, the two grades differ significantly in their branching number, which influences their behaviour in elongational flow.

Poly-para-phenylene-copolymers (PPPs). Part 3: scratch and wear resistance

Two types of poly-para-phenylene (PR-120 as an extrusion and PR-250 as an injection moulding grade) were previously investigated with regard to their thermal, rheological and mechanical properties. In the last part of this paper series, the tribological properties, in particular the scratch resistance and wear under abrasive, sliding and rolling conditions, were evaluated as a function of the materials’ molecular characteristics. In general, PR-120 exhibited a better scratch and wear resistance, which is expected due to its higher molecular weight. However, other factors also seem to play an important role, although convincing correlations between the scratch and wear behaviour and the morphological and mechanical properties could not be found so far. All the properties tested were mirrored to those of polyetheretherketone and other polymers tested under similar conditions.

Poly-para-phenylene-copolymers (PPP) for extrusion and injection moulding. Part 2: mechanical behaviour

Two types of poly-para-phenylene-copolymers (PR-120 as an extrusion and PR-250 as an injection moulding grade) are commercially available. In this paper, their mechanical properties were evaluated as a function of their molecular characteristics. The higher viscosity PR-120 possesses a higher strength, modulus and hardness, which is in agreement with its lower molecular flexibility compared to PR-250. On the other hand, the latter version shows a better fracture toughness. All tests were carried out at room temperature.

Poly-para-phenylene-copolymers (PPP) for extrusion and injection moulding Part 1 – molecular and rheological differences

Poly-para-phenylene (PPP) is at room temperature the stiffest and hardest thermoplastic polymer known. Two types of PPP (PR-120 as an extrusion and PR-250 as an injection moulding grade) are commercially available. In this paper, their thermal and rheological performance was evaluated as a function of their molecular characteristics. The lower viscosity of the PR-250 material is in agreement with its lower average molecular weight Mw, and this material exhibited a slightly higher glass transition temperature and thermal stability than material PR-120. The latter, on the other hand, possessed a much higher stiffness, as revealed by dynamic mechanical thermal analysis tests.

Crystalline structure of annealed polylactic acid and its relation to processing

This paper focuses on the crystalline structure of injection moulding grade poly(lactic acid) (PLA) and the effect of crystalline structure on the processing. The research is induced by the significant differences in crystallinity of the pure PLA resin, and the injection moulded product, and thus the reprocessing of PLA products. 2 mm thick PLA sheets were injection moulded and re-crystallized in a conventional oven at 60-140°C, for 10-60 minutes to achieve various crystalline contents. The properties of these sheets were investigated by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and wide angle X-ray diffraction (WAXD). In a processing plant the rejected parts are recycled and reused as raw material for further cycles, accordingly the various crystalline content PLA products were reprocessed as a resin, to investigate the processing itself. When PLA products are reprocessed, due to the adherent feature of amorphous PLA processing difficulties may occur. This adherent effect of the amorphous PLA was investigated and characterized.

Study of modified polypropylene for rotational moulding applications

Polypropylene is stiffer than polyethylene, having high heat distortion temperature. It is autoclaveable, offers excellent chemical resistance, environment stress-cracking resistance and surface hardness. It is not possible to produce rotomolded article from injection moulding polypropylene grade because melt flow index (MFI) required for polypropylene rotomoulding is higher. Also, injection moulding grade PP is not suitable to prevent the material from thermo oxidative degradation during long rotomoulding cycles. The reactive extrusion is a promising technique to get higher MFI, which can be used for rotomoulding process with proper additive package. In this paper the polypropylene grade is modified for rotational moulding applications with the help of reactive extrusion. It is done in order to get higher MFI polypropylene. Sintering is observed to be related to zero shear viscosity and directly to MFI. During the experimentation it was observed that, for higher MFI material sintering completed at lower temperature. It is found that as MFI increases the flexural properties of rotomoulded specimen increases, and notched izod impact, tensile strength, and elongation at break decreases. Fine spherulites were observed for low viscosity material. SEM images revealed the coarseness with increased MFI.

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 1 – Shear and extensional behaviour

The shear and extensional properties of injection moulding grade hydroxyapatite–polyethylene composites developed for orthopaedic applications have been studied. The composite was prepared without processing aids owing to concerns over the potential biological responses to such additives. The composite containing 20 vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron–Pierce equation was found to be useful in predicting the viscosities of the composite systems. The activation energy of the composite and the unfilled polymer were equal, indicating that the 20 vol.-% system exhibits the same flow mechanism as the unfilled polymer. A qualitative assessment of extensional properties was made following Cogswell's method. The extensional stress of the unfilled polymer decreases with increasing temperature whereas the composites behave in a complex manner. For all the systems the Trouton ratios tend to increase with apparent shear rates. The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.

Recycling of APC-2 offcuts

A method for reclaiming waste APC-2 offcuts from pattern cutting of prepregs is reported. The method entails cutting the scrap prepreg into small flakes and compounding them with additional poly(ether ether ketone) (PEEK) resin in a twin-screw extruder to produce a short fibre-reinforced injection moulding grade material. The tensile and flexural properties of mouldings made from the reclaimed material were measured and compared with those of mouldings made from a commercially available injection moulding grade short carbon fibre/PEEK. The molecular weight, degree of crystallinity and fibre length distribution of the various samples of PEEK and carbon fibre/PEEK at different stages of processing are reported. Specimens made of the recycled material were stiffer and stronger in tensile loading than the ones made from the commercial moulding compound. This was due to the presence of fibres with a longer average length and better interfacial adhesion between the fibres and matrix in the recycled material. The PEEK matrix in the recycled material had a higher crystallinity than that in the commercial material. A simple economic assessment has been performed showing that, in principle, the recycling route is viable.

Injection moulding grade of biodegradable polymers

La hidrocefalia es una dilatación patológica de los ventrículos cerebrales por un incremento del volumen del líquido cefalorraquídeo (LCR). Se produce generalmente por uno de dos mecanismos básicos fisiopatológicos: defecto de circulación o de reabsorción del LCR, dando lugar a los dos principales tipos de hidrocefalia: comunicante y no comunicante. Se puede producir tanto en el niño como en el adulto, predominando en el niño las causas genéticas y la patología perinatal. Las causas adquiridas se pueden producir en cualquier rango de edad. La hidrocefalia normotensiva o hidrocefalia crónica del adulto es una forma de hidrocefalia comunicante, en la cual hay un aumento patológico del volumen de los ventrículos cerebrales con presión de apertura del LCR normal. Se caracteriza por una tríada de síntomas: trastorno de la marcha, deterioro cognitivo e incontinencia de esfínteres y es potencialmente reversible mediante un shunt de derivación. La hipertensión intracraneal (HPTIC) benigna, llamada también pseudotumor cerebri, se define por unos criterios clínicos que se refieren a un cuadro de HPTIC con elevación de la presión de apertura del LCR en punción lumbar (PL), con una composición normal del mismo y descartadas otras causas de HPTIC con las pruebas oportunas. Si bien es considerado un cuadro benigno, hay riesgo de pérdida de visión permanente. El síndrome de hipotensión intracraneal tiene una etiología y presentación clínica variable que se produce, en la mayoría de casos, por fuga del LCR a través del saco tecal de forma espontánea o secundaria a PL, cirugía, traumatismos o sobredrenaje de un shunt de derivación. Cefalea ortostática, disminución de la presión de apertura del LCR y captación meníngea en resonancia magnética son las manifestaciones más características.Hydrocephalus is the pathological enlargement of the brain ventricles caused by accumulation of cerebrospinal fluid (CSF). Two basic pathophysiological mechanisms are involved: impairment of CSF circulation or resorption; causing the two main types of hydrocephalus: communicating and noncommunicating hydrocephalus. Appears both in childhood and adulthood. Genetic causes and perinatal pathology dominate in children. Acquired causes are involved in any age range. Normal pressure hydrocephalus or chronic adult hydrocephalus is a type of communicating hydrocephalus, in which ventricles of the brain enlarge with normal CSF opening pressure. Clinically is characterized by the triad of abnormal gait, sphincter incontinence and cognitive impairment. In most cases, the placement of a cerebrospinal fluid shunt revert it. Idiopathic intracranial hypertension (IIH) or pseudotumor cerebri, is clinically characterized by an increased CSF opening pressure in lumbar puncture (LP), the composition of the cerebrospinal fluid is normal, and no other cause of intracranial hypertension are identified. Although is considered benign, permanent blindness can occur. Intracranial hypotension syndrome has a diverse etiology and variable clinical presentation; usually, spontaneous or secondary (LP, surgery, trauma or shunt) CSF leaks through thecal sac causes it. The most characteristic symptoms are orthostatic headache, lower CSF opening pressure and meningeal contrast enhancement.