Mikael Rigdahl

Influence of high injection pressures on the internal stress level in injection moulded specimens

Abstract Internal stresses in high and low density polyethylene specimens, injection moulded at pressures ranging from 100 MPa to 450 MPa have been measured by a stress relaxation method. The internal stress parameter ( σ i ), which is an average value of the internal stress distribution in the samples, changes from a negative value (compressive stresses) at normal injection pressures to a small positive value (frozen-in tensile stresses) at the highest pressures used. The yield stress increases in approximately the same way with the pressure, while the mould shrinkage in the flow direction decreases. It is suggested that the decrease in the absolute value of the internal stress parameter originates from an increase of the melting temperature with pressure, resulting in a more homogeneous solidification.

Calculation of Residual Thermal Stresses in Injection Molded Amorphous Polymers by the Finite Element Method

Abstract The finite element method (FEM) is used to calculate the distribution of residual internal stresses in an injection molded plate-shaped polystyrene specimen. The first step of the calculating procedure is the determination of the temperature distributions in the plate and its variation with cooling time. The temperature distribution data are then used for the determination of the corresponding stress distribution. Also this latter step is accomplished using the FEM-technique. The residual stress distribution is obtained when the sample has been cooled to the temperature of the mold. The main result of the calculations is the finding that the surface layer of the plate is subject to compressive stresses, while the interior accommodates stresses of tensile type. The calculations relate to a polystyrene specimen. The results are shown to agree well with earlier data concerning internal stresses in injection molded objects. The possible influence of anisotropy and visco-elastic relaxation has been ne...

Influence of high injection moulding pressures on the engineering properties of linear polyethylene

Abstract Two grades of high density polyethylene, one injection moulding grade and another with a substantially higher molecular weight (melt index 0.1 g 10 min ) were injection moulded at pressures ranging from 100 to 500 MPa using a modified conventional injection moulding machine. For the high molecular weight grade, improvements were observed in the elastic modulus, the tensile strength at rupture measured in the flow direction, and the unnotched impact strength. These improvements were accompanied by a second high temperature (137°C) melting peak in d.s.c. diagrams. For both grades it was also found that the mould shrinkage decreased and the crystallinity increased with injection pressure.

The Assessment of Internal Stresses in Plastics by a Stress Relaxation Method

Abstract When plotting the slope of stress relaxation curves (stress vs. log time) against the initial stress, straight lines are obtained intersecting the axis at a stress value which appears to be associated with the internal stress level of the sample. For injection molded samples, the internal stresses are negative, i.e. frozen-in compressive stresses. They disappear on annealing. Both tensile and compressive stresses can be introduced into stress-free samples by suitable thermal treatment when in a state of compression or tensile strain.

Processing and Water Absorption Behavior of Foamed Potato Starch

Starch foams were prepared from four types of potato starch using two-step extrusion with an intermediate conditioning step at 53% relative humidity in order to control the moisture content. The moisture content was the driving force for the expansion during the second extrusion. The second extrusion was performed with two different dies in order to achieve differences in porosity of the materials. Glycerol in combination with water was used as plasticizer for the starches. The rheological properties of the melts and the moisture content of the starch materials prior to the expansion were determined. The porosity of the expanded structures was characterized using environmental scanning electron microscopy imaging and density measurements. The absorption capability of the starch-based foams was investigated using aqueous NaCl solutions, and the water uptake of thin starch-based films from humid air was also evaluated. Foams prepared from amylopectin potato starch were found to exhibit the highest porosity, the lowest density, and the greatest absorption capability, both from water and from humid air. Not only the density but also the absorption capability was influenced by the porosity level.

Extensional viscosity of microfibrillated cellulose suspensions.

The extensional properties of micro fibrillated cellulose (MFC)-suspensions at different fibril concentrations and with different amounts of added sodium chloride were evaluated. The MFC-suspensions were obtained by diluting a stock solution consisting of 0.95 wt.% cellulose with either deionized water or sodium chloride solution, giving a series of different concentrations and sodium chloride contents. The extensional viscosities of the suspensions were measured utilizing contraction flow geometry. Here the specimens were forced through a hyperbolic nozzle and the required pressure drop over the nozzle was measured. The extensional viscosity exhibited an extensional-thinning behaviour over the extensional strain rates used. Furthermore the extensional viscosity decreased with decreasing concentration of the suspensions, in similarities with the shear properties of the specimens. For the suspensions containing sodium chloride, the extensional viscosity appeared to increase when the concentration of sodium chloride was increased. But excessive amounts of added sodium chloride promoted an agglomeration of the suspensions.

Influence of strain rate on the stress relaxation behaviour of polyethylene and cadmium

Abstract Stress relaxation measurements have been carried out on high density polyethylene (HDPE) and polycrystalline cadmium (Cd) after constant strain-rate loading at strain rates varying from 10 −6 9 o 10 −2 s −1. When testing the applicability of a spectrum of relaxation times, increasing the straining time during the initial loading was found to cut off the initial part of the spectrum for HDPE; for Cd the spectrum remained largely unchanged, shifting its position towards longer times. The relaxation process was also analyzed using the exponential law of flow [rate ≈ exp(} − σ i )] for the initial-, and the power law [rate ∼ ( σ − σ i ) n ] for the final part of the curves. For Cd, the exponential region comprised the main part of the process, while it was less well developed for HDPE. In agreement with earlier results, the maximum slope, F , of the stres vs . In time curves, was found to be related to the initial stress, σ o , as F ≈ 0.1( σ o − σ i ), where σ i is the internal stress. At the longest straining times used, a slight increase of this value was found. The value of σ i was independent of strain rate; it increased with the strain.

Internal stresses in cold-drawn and irradiated polyethylene

The influence of high-energy radiation on the internal stress level in cold-drawn, high- and low-density polyethylene, with and without subsequent annealing, has been determined using a stress relaxation method described earlier. The internal stress level (σi) is found to increase substantially with the radiation dose for doses below 40 Mrad, while the increase at higher doses is only moderate. It is suggested that the initial increase is due to a preferred cross-linking of the amorphous phase or the fold surface of the lamellae. An exception to this behaviour was observed for cold-drawn low-density polyethylene, where the internal stress decreased as the radiation dose was raised.

Analysis of cellulose networks by the finite element method

Paper can be regarded as a network of cellulosic fibres, especially at lower basis weights. When the elastic behaviour of paper sheets is modelled, it is normally essential to know or to assume how the stresses (and strains) are distributed at the fibre level. This article presents an attempt to estimate how the stresses are transferred throughout a simple fibre network using the finite element method (FEM). Attention is mainly focused on the axial fibre stress distribution when the network is uniaxially deformed. The presence of fibre ends is found to induce local stress increases (“stress concentrations”) in the deformed network, which presumably have a bearing on the ultimate properties of the sheet. The influence of the properties of the bonds between crossing fibres on the mechanical properties is also investigated. It is noted that the bond stiffness has no significant effect on the stress transfer between fibres provided that the stiffness is above a critical value. Below this value the stress transfer deteriorates rapidly.

Foaming behavior of water-soluble cellulose derivatives: hydroxypropyl methylcellulose and ethyl hydroxyethyl cellulose

Hydroxypropyl methylcellulose and ethyl hydroxyethyl cellulose could be interesting candidates for production of lightweight, foamed packaging material originating from non-fossil, renewable resources. The foaming ability of nine different grades of the two cellulose derivatives, using water as the blowing agent, was investigated using a hot-mold process. The foaming process was studied by evaluating the water loss during the heating, both in a real-time experiment and by thermal gravimetric analysis. Further, the development of the rheological properties of the derivative-water mixtures during a simulated foaming process was assessed using dynamical mechanical thermal analysis and viscosity measurements. Five of the studied derivatives showed promising properties for hot-mold foaming and the final foams were characterized with regard to their apparent density. It was concluded that the foamability of these systems seems to require a rather careful tailoring of the viscoelastic properties in relation to the water content in order to ensure that a network structure is built up and expanded during the water evaporation.