Thermoplastic Injection Moulding Research Articles

Oil Bleeding Behaviour of Self-Lubricating Liquid Silicone Rubbers

The term liquid silicone rubber (LSR) covers a whole range of two-pack, heat-curing products, which may be processed using liquid injection moulding (LIM), a technique similar to thermoplastic injection moulding. Although this product group only accounts for about one thousandth of the world rubber market (corresponding to approximately 18 000 t/a) and is therefore a speciality rubber (ref. 1), due to their excellent properties, such as thermal stability, low-temperature flexibility, constantly good mechanical properties over a wide temperature range and good physiological compatibility to name but a few, products made of LSR are used in a wide variety of applications in many different fields. The most important fields of application include household appliances and the food industry, electronics and electrical engineering, sports and leisure activities and the automotive and aerospace industries (ref. 2). As a result of their low-temperature flexibility and heat stability, silicone rubbers are used for numerous applications in the automotive sector. Examples include spark-plug socket jackets, radiator seals, exhaust mountings, O-rings, seals and membranes. Nowadays, it is standard practice to use weather packs made of special silicone rubbers to which has been added a silicone oil incompatible with rubber and which exudes slowly after the product has cured for sealing cable connectors for wiring harnesses.

On the Simulation of Thermoplastic Injection Moulding Process

Abstract The relevance of enhancement of crystallisation kinetics by effect of shear flow and rheology during polymer solidification to the phenomena taking place during the injection moulding process has been shown by means of simulations performed on the basis of Lord and Williams [1, 2] model and its recent extensions [3 to 5]. A key point of the simulation was the solidification criterion based on a critical crystallization index; on the basis of simple calorimetric and rheological tests in the limit of zero shear rate, values of a few percent were given to the solidification crystallinity value. Many experimental features of pressure history both in the runner and in the cavity are recovered by model predictions, if both effects mentioned above are properly accounted for. However, predictions for gate sealing time, although improved, still have a sensitivity to gate thickness lower than that shown by the experiments. Similar conclusions, obviously, regard also the mass entering in the mould during holding. Another mechanism which sums up to shear stresses to accelerate thin gate solidification has still to be identified.

On the Simulation of Thermoplastic Injection Moulding Process

Abstract Numerical simulation of the injection moulding process of thermoplastic polymers has been performed on the basis of the Williams and Lord model and its recent extensions. The aim of the simulation was to identify material properties which play a relevant role to process modelling, including post filling stages. Predictions improve significantly by taking into account the effect of pressure on both viscosity and crystallisation kinetics. Several features, however, are not satisfactorily predicted. Also the relevance of convection during packing has been evidenced especially with reference to gate sealing time. As highly oriented crystalline structures are present in the final moulded object, the enhancement of crystallisation kinetics by the effect of flow is suggested as the phenomenon to be accounted for in order to gain a quantitative description of the whole process.

Predicting product quality with backpropagation: A thermoplastic injection moulding case study

This application uses live data from a thermoplastic injection moulding manufacturer to examine the feasibility and effectiveness of using backpropagation artificial neural networks for predictive quality control. Preprocessing and post processing of live data, formulating neural predictive strategies, selecting architecture and parameters, and handling of temporal aspects are topics. Performance of the neural networks are compared to other quality control methods, including control charts and statistical techniques. This case study demonstrates that even manufacturers who have modest expertise in computing and limited hardware and software availability can successfully use neural networks for data analysis and modelling.

An investigation of injection pressure predictions in flow simulation packages

This paper discusses the pressures predicted by three popular commercial flow packages during the injection phase of a thermoplastic injection moulding operation. The project required designing and building an instrumented mould. The mould geometry and its dimensions were then verified by a coordinate measuring machine. These dimensions were then entered into the three simulation packages. An experiment was designed to collect data on injection pressures at various locations in the flow channels and cavities of the mould during injection. These experimental data were then compared to pressures predicted by the simulations.

Optimal injection velocity profiling

Recently, manufacturers of thermoplastic injection moulding machines have developed controllers with programmable velocity control. The speed of the ram can typically be varied in as many as ten zones during the injection process to establish a velocity profile. This velocity profile however, is determined by trial and error. Some controller manufacturers have made claims about injection velocity profiling, however, actual experimental data is not available to confirm them. In this paper, some experiments were conducted in order to verify the effectiveness of these claims. The results of these experiments indicate that injection velocity control has little effect on reducing or eliminating the problems for which the claims are made.

Polyurethane Elastomers as Hydraulic Seal Materials

The availability of the diisocyanate paraphenylene diisocyanate (PPDI) and cyclohexyl diisocyanate (CHDI) has made possible the synthesis of thermoplastic polyurethanes (TPLTs) which have greater thermal stability than previously obtainable as predicted from their DMTA thermograms. The ability to combine elevated temperature service with the unique strength and wear resistant properties of polyurethane, together with the automation made possible by thermoplastic injection moulding is considered to represent an advance in current seal materials. This paper describes the special synthesis and formulation of TPU's based on PPDI and CHDI together with a spectrum of the strength properties used to judge them as potential seal elastomers. Some of these TPU elastomers possess tensile strengths of 50 MPa and compression set values of <25% obtained by using special post-curing techniques. DMTA data is used to predict these TPU's responses to elevated temperatures.

Freezing-off in disc cavities

A semi-quantitative analysis is presented of freezing-off in a disc cavity during the injection moulding of thermoplastics. A criterion is obtained which enables the occurrence of freezing-off to be predicted, at least crudely. The form of the criterion is found to depend on the direction of flow (radially outward or inward) in the disc.

Injection moulding of thermoplastics: Freezing of variable-viscosity fluids.

Injection moulding of thermoplastics: Freezing of variable-viscosity fluids.

Injection moulding of thermoplastics: Freezing of variable-viscosity fluids.

Injection moulding of thermoplastics: Freezing of variable-viscosity fluids.

Injection moulding of thermoplastics: Freezing of variable-viscosity fluids.

The injection moulding of thermoplastics involves, during mould filling, flows of hot polymer melts into mould networks, the walls of which are so cold that frozen layers form on them. An analytical study of such flows is presented here for the case when the Graetz and Nahme numbers are large and the Pearson number is small. Thus the flows are developing and temperature differences due to heat generation by viscous dissipation are sufficiently large to cause significant variations in viscosity (but the difference between the entry temperature of the polymer to a specific part of the mould network and the melting temperature of the polymer is not).

Injection moulding of thermoplastics. I. Freezing-off at gates

The injection moulding of thermoplastics involves, during mould filling, flow of a hot molten polymer into a mould network, the walls of which are so cold that the polymer freezes on them. During the constant pressure drop part of the filling stage, but not during the preceding constant flow-rate part, freezing-off, that is premature blockage of the mould network by frozen polymer, is possible. A semi-quantitative analysis of such freezing-off at a gate is presented here. The length-scales and time-scales of all the relevant physical processes occurring during freezing-off are identified and a criterion is obtained which enables the occurrence of freezing-off to be predicted, at least crudely.

Injection moulding of thermoplastics. II. Freezing-off in cavities

The injection moulding of thermoplastics involves, during mould filling, flow of a hot molten polymer into a mould network, the walls of which are so cold that the polymer freezes on them. During the constant pressure drop part of the filling stage, but not during the preceding constant flow-rate part, freezing-off, that is premature blockage of the mould network by frozen polymer, is possible. A semi-quantitative analysis of such freezing-off in a mould cavity is presented here. The length-scales and time-scales of all the relevant physical processes occurring during freezing-off are identified and a criterion is obtained which enables the occurrence of freezing-off to be predicted, at least crudely. This criterion is shown to be corroborated by experimental data for the filling of a spiral mould cavity by three different polymers under a range of different operating conditions.

Injection moulding of thermoplastics: Freezing during mould filling

The injection moulding of thermoplastic polymers involves, during mould filling, flows of hot melts into mould networks, the walls of which are so cold that frozen layers form on them. Theoretical analyses of such flows are presented here.

Process of Filling in Injection Molding of Thermoplastics

A theory was proposed to predict the process of unidirectional filling of a uniformly elongated flat cavity with a thickness, 2Y, and a width, 2Z, by a non-crystallizable thermoplastic at a constant temperature, TI, under a constant injection pressure, PI. The filled length, Λ, was related to the filling time, t, by an equation, Λ=ΛKJ(t/tK), where tK=tE/M and ΛK=(tKDINK)1/(n+1)Y. J(σ) evolved from a set of equations, J(σ)=∫σσI=0j(σI)dσI, and (j(σ))-1/n=γ(σ)+eσ∫σσI=0e-σIj(σI)dσI. In the scheme: tE=ρωY2/4κ, where ρ, ω, or κ is the density, the specific heat, or the thermal conductivity, respectively, of the cooling layer of the thermoplastic in the cavity; M=2(n+2)θ2/n where θ=(TII-TM)/(TI-TM); n or Ψ(T) is the non-Newtonian index or the fluidity, respectively, of the melt, where T specifies the temperature; TII is the temperature at which Ψ(TII)=EFF. Ψ(TI), where EFF is a constant (≤1) taken somewhat arbitrarily to be 0.5 for schematic division of the thermoplastic in the cavity into the flowing layer and the solidified layer; TM is the temperature of the wall of the cavity; DI=D0Ψ(TI)(PD/τ0)n, provided D0 or τ0 is a unit shear rate or a unit shear stress, respectively, for expression of the power law, D=εD0Ψ(T)(ετ/τ0)n, relating the shear rate of the melt, D, with the shear stress, τ, where ε=-1 for τ<0 and +1 for τ>0; PD=PI-PM, where. PM is the atmospheric pressure; NK=Nγn+2 where N=(n+2)-1 and γ=(1/2) exp (1/2); and γ(σ)≡(QM(σtK)/QK)-1/n provided that QK=2Z.2Y.ΛK/tK, where QM(t) is the flow rate through the channels preceding the cavity when the end be opened freely into the air without the cavity. If all the channels preceding the cavity are isothermal, γ(σ)≡a constant, γi, which vanishes if the melt enters from the cylinder directly into the cavity. Law of R. L. Ballman et al. of exponential decrease in speed of flow front is proven for the case where γ(σ)≡γi≤(n+1)1/(n+1), giving the time constant, B, equal to tK/n(1-n(γin+1/(n+1)n+1)1/n). Calculated fill-outs and calculated time constants agree with the experimental results of H. L. Toor et al. The present theory presupposed that the thermoplastic in the cavity can be divided into the inner layer, in which the temperature is TI, and the outer layer, in which the temperature falls down to TM following such a linear profile like that in the case of stationary conduction. The thickness of the inner layer, 2YI, was equated, by a set of assumptions, to (1-(tU/tE)1/2)