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The Izod-orientation concept for impact characterization.

The Izod-Orientation Concept for Impact Characterization

Plots of Izod impact strength vs. orientation shrinkage can be used to show the effect of molded-in orientation on impact values, thereby sharpening comparisons of material toughness.

Notched Izod impact strengths are often quoted to describe the toughness of polymers; they are used in the plastics industry primarily to assist in material selection. Values are typically based on testing of injection molded specimens, and product bulletins generally state a single value of Izod impact for a given polymer product. However, a range of values can exist, which depend upon the manner in which the test specimens were injection molded.

Studies on ABS show that the injection molding process influences Izod impact through two mechanisms--thermal degradation and orientation. Thermal degradation does not usually come into play unless the polymer is abused by processing outside of the recommended melt temperature or heat history limits. Most often, it is orientation that creates a range of values, and this range can be quite wide for a given polymer. For example, a general-purpose ABS can be molded to yield Izod impact values from 214 to 427 J/m. Published Izod impact values are generally developed by breaking the specimen "across flow," that is, at right angles to the orientation. The low end of the range can be extended even further, to 107 J/m, if the broken "with-flow" data is included. In moderately or highly oriented test specimens, the with-flow values may be only a fraction of the across-flow values. This anisotropy is not always appreciated when published Izod values are related to the impact strengths at various points in finished parts.

Because injection molding variables affect orientation, which in turn can significantly modulate Izod impact, product characterization can also become significantly complicated, especially when two or more polymers are compared. Molding parameters can vary from machine to machine and even day to day on a given machine. Thus, unless one has explicitly defined, measured, and controlled all the key processing variables so that they are identical, the comparison may lack validity. It is possible for two nominally identical molding machines, operating at the same machine settings to turn out test specimens that vary in Izod impact by 20% or more.

This article summarizes a study of the relationship between molding parameters, orientation, and Izod impact in ABS polymers. A goal is to show that a particular relationship can be helpful in removing some of the uncertainty and confusion caused by the influence of orientation on Izod impact. This relationship is called the "Izor" plot, a contraction of the words Izod and orientation.

Method and Equipment

End-gated ASTM D638 tensile bars (inset, Fig. 1) were injection molded with a 12.7-mm-wide and 3.18-mm-thick gage area. In most cases, the 63.5-mm-long impact specimen (ASTM D256 method A) was taken from the middle (M) of the gage area. For reasons that will be covered later, some impact specimens were also cut to the same width from the wider "grip" areas at the gated (G) and non-gated (E) ends of the bars. Impacting was done at room temperature on a TMI model 43-10 impact apparatus with a 0.909-kg pendulum weight. Each data point is the average of three breaks.

Orientation in the flow direction was measured by a simple oven shrinkage test on duplicate 38.1-mm segments of the tensile bar. After the segments were carefully prepared with clean-cut flush ends, they were measured to the nearest 0.03 mm and then put in an oven at 152 [degrees] C on a talc pan and allowed to shrink for two hours. They were then re-measured, and the orientation was expressed as the shrinkage divided by the original length. Although there is an orientation gradient across the thickness, this simple "bulk-average" orientation value will be seen to correlate quite well with Izod impact.

Test specimens were molded on several molding machines, but always with equivalent mold design. Most of the data quoted here came from molding done on three presses: a 142-g, 136-metric-ton unit, and two different 85-g, 68-metric-ton units. All had closed-loop microprocessor control of the injection side. Ram speeds and injection pressures were charted on a high-speed recorder. Melt temperatures were carefully taken from air shots with a calibrated fast-response needle pyrometer. Molding conditions of melt temperature, fill rate, and velocity-to-pressure transfer (VPT) point were manipulated to create various levels of molecular orientation in the flow direction of the test specimens.

Typical Izor Curve

A Typical Izor curve for a medium-impact ABS, shown in Fig. 1, presents an obvious relationship of increasing impact with increasing core orientation. (The difference between skin and core orientation has been described. Izod impact is not sensitive to the degree of orientation on the very skin of the part.) The shape of the curve results from a number of mold filling factors that affect the degree of orientation in the flow direction. For this plot, test specimens were taken from the gate, middle, and end of the tensile bar, as shown in the inset. There is a natural orientation gradient from one end of the bar to the other because of flow time past a given point in the cavity and axial packing pressure gradients. The highest orientation is at the gate end, and advantage of that gradient was taken to generate points along the curve just at one set of molding conditions. In addition, specimens were molded at several fill rates and VPT points. Molding variables influence the locus of points on an Izor curve by the following general rules:

* Increasing the fill rate moves points "down" the curve. Fast fill reduces core orientation because of more shear thining and less influence of mold cooling.

* Increasing the melt temperature (but not above the degradation point) shifts points "down" the curve. Lower viscosities equate to lower shear stresses, which means less flow-induced orientation. There is also more relaxation of orientation after fill ceases and before freeze-up. Melt temperatures high enough to cause degradation shift points onto a different lower curve, as will be discussed later.

* For a given packing pressure, moving the VPT towards the gate moves points "up" the curve. For a given VPT point, increasing the packing pressure first moves points "up" the curve, then "down" in a complicated relationship.

The lower portion of the curve in Fig. 1 flattens out to an "intrinsic" impact level at around 187 J/m. This is an estimate of the "zero-orientation" impact, a parameter of interest in product development. Impact strengths of annealed, compression-molded specimens will fall on this line at zero orientation. Figure 2, an Izor curve for a specialty grade of ABS, shows this intrinsic point very well at about 96 J/m. It also has a curiously flat and extended plateau up to about 39% orientation shrinkage, after which there is a sharp ascent up to 320 J/m over the next 6% additional orientation. This increase in impact above the intrinsic plateau is called the impact "boost." A plot like this easily shows how a slight drift in the molding conditions could significantly change the impact if one were operating on the steep part of the curve. For example, just changing the VPT point from 16-mm to 6.4-mm ram travel dropped the Izod impact from 310 to 123 J/m.

Izor Fingerprints

A study of numerous commercial types of ABS has revealed that each material has its own Izor curve (Fig. 3) with a corresponding intrinsic impact, plateau, and impact boost. Studies are under way with model materials to gain an understanding of the relationship between polymer structure and the nature of the Izor curve, that is, rubber level, grafting, and rigid molecular weight. The relationship between Izor behavior and falling dart impact will also be studied.

It is possible for two polymers to have the same Izod impact when molded under the same conditions, leading one to believe that the materials are altogether equivalent. However, the Izor analysis can reveal hidden differences in the orientation behavior that can become important to other properties. In Fig. 4, data points for two types of ABS (squares and triangles, respectively) that correspond to the same molding conditions, in terms of melt temperature and fill rate, are connected. For each set of molding conditions, the two polymers have essentially the same Izod impact. However, one material (triangular data points) has considerably more orientation at a given impact level. Orientation differences like this can affect other properties such as dart impact and the tendency to warp at elevated temperatures.

Separating Effects

The two ways that increasing melt temperature affects impact were previously mentioned. If a processor raises the melt temperature and then notes a lower Izod impact, he cannot tell without an Izor plot if it has been lowered because of reduced orientation or because of degradation. An example of how this confusion can be resolved is shown in Fig. 5, an Izor plot for a high-impact, general-purpose ABS. The data points from specimens molded at 218 [degrees] C, 229 [degrees] C, and 246 [degrees] C all fit on the upper curve. However, when the polymer was abused by being molded at 274 [degrees] C, a new lower curve was created, showing a significant reduction in Izod impact at a given orientation level. The Izor concept thus allows an easy determination of significant polymer degradation and can be a handy diagnostic tool.

Equating Molding Machines

Despite the pronounced effect of molding conditions on impact, the Izor tool allows two polymers to be compared even when molded on different machines under different molding conditions. This is because the molding parameters that induce variation in orientation simply move points up and down the Izor curve; they do not affect the shape or position of the curve itself. (This, of course, assumes no polymer degradation and the same mold geometry.)

To illustrate this idea, a high-impact general-purpose ABS was molded on eight different machines in five different laboratories. The molds were identical. Several different molding conditions were used on each machine. Izod impacts ranged from 267 to 422 J/m. The Izor plot, Fig. 6, shows that all these data points reasonably fit one line. There is some scatter; the standard deviation of the regression is 22.4 J/m. However, other work has shown that single-machine/single-condition molding produces scatter on the order of 10.7 J/m just because of testing variance.

Applications

Consider the problem of two different laboratories that wish to verify the impact strength of a given material. They could be two QC labs of a material supplier located on opposite sides of the continent or the material supplier's lab and a customer's lab located in different countries. The molding machines are of different brand, size, and state of maintenance. It would be most improbable that the molding conditions in the cavity would be exactly the same. Consequently, specimen orientation levels will be different, and, as expected, the two labs will not get the same impact value. The Izod impact values could differ by 20% to 30% or more, creating obvious difficulties about the perception of the quality of the material in question. However, if each lab would also measure the orientation level and plot their data on an Izor master plot for that material, they would find that both sets of results fit the same curve.

Another application concerns QC labs that use injection molded Izod impact as a product/process control tool. Things can happen to a molding machine that change the true in-mold filling parameters without the operator being aware of it. This can involve drift over months due to machine wear, or it can happen quickly as a result of electronic calibration failures or a non-return valve cracking. Anything that changes the true in-mold filling rate, even for only the last centimeter of the test bar, will change the orientation level in the Izod impact specimen. Product can apparently drift out of specification when in fact it is the molding conditions that are changing. If an Izor curve is first developed for on-specification material, then subsequent molding can be done at any single non-degrading condition. By simply measuring both the impact and the orientation level of a new lot of material, seeing if the data fits on the reference Izor curve, and determining that other properties are in line, one can have confidence that the product is good. If the data point does not fall on the curve, further investigation into product quality is warranted. But suspicions regarding molding condition equivalence or drift are removed from the picture. [Figures 1 to 6 Omitted]
COPYRIGHT 1989 Society of Plastics Engineers, Inc.
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Author:Fritch, L.W.
Publication:Plastics Engineering
Date:Sep 1, 1989
Words:2106
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