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Predicting IM part toughness from melt viscosity.

Predicting IM Part Toughness From Melt Viscosity

Designers of structural plastics products frequently face a common but tough question - how to correctly utilize the resin supplier's data in their designs so that structural part properties can be predicted. Unfortunately, the resin supplier's data do not provide enough information to address this question. For example, melt flow data (a point melt viscosity data) are only a partial indicator of processability, although an excellent indicator for lot-to-lot process variation. Not given is how this parameter affects the resulting thermal and mechanical part properties, which are often substantially influenced by the processing parameters.

It is important to recognize that the mechanical properties given on the resin supplier's data sheet are valid only for the specific fabrication conditions employed and thus cannot be used directly for assessing the actual part performance. Injection molded part toughness, such as Izod impact, is a good example. If the designer uses the resin supplier's Izod impact strength data, generally obtained with highly oriented samples, he may commit a gross error. Figure 1 shows that the part toughness might not be too great even with a high Izod impact value.

This article investigates the effect of the melt viscosity of flame retardant impact polystyrene (PS) on the resulting toughness of the molded part. Easy-flow material will be shown to be preferred, not only for its better processability, but also for higher structural part toughness.

To avoid costly "surprises," product designers must understand that a resin's typical molding window, which defines the molding envelope to give acceptable exterior appearance to molded parts, is quite different from a true molding window, which gives both acceptable exterior appearance and the desired physical properties. Gardner impact data were obtained from specimens injection molded under different conditions from two flame-retarded PSs having widely different melt viscosities. From this data, a Gardner impact contour diagram was constructed as a function of melt viscosity and injection speed. The effect of melt viscosity on toughness and the "true molding window" were assessed from this diagram.

Experimental

The Table summarizes the key properties of the flame-retarded impact PSs used - a low flow rate, injection molding grade (LFPS), and a super high flow rate, structural foam molding grade (HFPS). The edge-gated, 2-in-diameter, 0.125-in-thick Gardner specimen discs were injection molded on a 28-ton reciprocating screw machine at three different injection speeds and four different melt temperatures (370 [degrees] F, 400 [degrees] F, 430 [degrees] F, and 450 [degrees] F). A typical injection molding condition is given below.
Barrel temp., [degrees] F Cycle time, sec
Nozzle: 350-450 Delayed inj: 1.0
Front: 400-450 Injection: 10.0
Center: 400-450 Holding: 10.0
Rear: 400-450 Cooling: 30.0
 Die opening: 1.0


Pressure (hydraulic), psi Initial injection: 700-1000 Holding: 500-800 Back: 150 or less

Mold temperature, 150 [degrees] F-155 [degrees] F Screw speed, 200 rpm Screw cushion, 4 in

After the specimens were conditioned at room temperature for 24 hrs, impact testing was conducted following ASTM method D3029.

True melt viscosities (Fig. 2) were measured at 200 [degrees] C, 230 [degrees] C, and 260 [degrees] C using an Instron Capillary Rheometer. To further assess moldability of the materials, spiral flow molding experiments (Fig. 3) were conducted at 400 [degrees] F and 450 [degrees] F.

Benefits of Highly

Processable Materials

The greater shear rate and melt temperature dependence shown by HFPS (Fig. 2) is translated to outstanding moldability, as shown by the spiral flow molding diagram (Fig. 3). The molding window constructed on the basis of the spiral flow data (Fig. 4) shows that LFPS requires about 2000 psi higher injection pressure than HFPS to achieve an equivalent mold filling capability. Such easy mold filling characteristics offer the following advantages:

* Large, thin wall (difficult to fill) part

molding is permitted. * Part weight is reduced and mold release

enhanced. * Less clamping force is required. * Molding operation energy cost is

reduced. * Key part physical properties are

improved, such as heat resistance,

dimensional stability, and toughness.

Melt Viscosity and

Toughness - True Molding

Window

The structural toughness of the molded part is significantly influenced by the processing characteristics of the materials as well as the processing conditions employed. Figure 5 shows that structural toughness, namely the Gardner impact strength, increases with increasing melt temperature but decreases with the increase in injection speed.

The effect of the processing characteristics of the material, such as melt viscosity, is demonstrated in a Gardner impact contour diagram (Fig. 6). Within the realistic molding conditions, where most flame retardant impact PSs are molded, the part toughness of HFPS material can increase more than seven times while LFPS material can increase only about four times. This diagram has an interesting, practical implication in product design. A designer examining the resin supplier's typical property data (the Table) for two different melt viscosity resins, such as LFPS and HFPS, will not find differences in impact toughness between the two resins. However, Fig. 6 indicates that only HFPS will do the job if the required molded part toughness is >50 in-lbf, while LFPS will never make it. The product designer must take into account in the initial material selection the key fabricated part property, such as Gardner impact strength, under the real fabrication condition to be used to avoid unexpected, often costly results.

Table : Key Properties of the Two FR Polystyrenes: Low Flow (LFPS) and High Flow (HFPS).
Property LFPS HFPS
Key features High heat/ High flow
 stiffness, structural
 V-O, inj. foam molding,
 molding V-O
Melt flow rate, cond. G,
 g/10 min 5.0 10.0


Notched Izod impact, 1/8 in,

ft-lb/in 2.0 2.0

Gardner impact, 1/8 in,

in-lbf 50 50

A true molding window - in which not only exterior appearance, but also the key part property, is taken into account - must be assessed prior to the design commitment. The true molding window is different, not only for different melt viscosity materials, but also for the product requirement constraints. For example, in Fig. 6, if the part toughness requirement is >50 in-lbf, then LFPS does not have any true molding window, and HFPS has a molding window covering the upper third of the total area where melt temperature is 425 [degrees] F. Note that this discussion has assumed no short shots or flashing within this area. If such molding limitations are considered, then the true molding window is even smaller than normally supposed.

Toughening Mechanism

The effect of fabrication parameters is most commonly explained in terms of polymer molecular "orientation" and the associated "frozen-in" stress (sometimes referred to as "anisotropy" and "molded-in" stress, respectively). The orientational effect can be beneficial or damaging to the product property, depending on its use. For example, this effect is maximized in many extruded sheet and film applications to achieve the maximum rupture (or tear) strength.

For most injection molding applications, where random or multidirectional flows are common, this effect is not necessarily beneficial. Figure 7 shows the directional flow lines during the mold filling process to be the same as the orientational directions on the molded part. The Izod impact point is thus 90 [degrees] against the orientation while the Gardner impact point is on top of the orientation and sees the orientational flow in both the parallel and perpendicular directions. Because the orientation reinforces the material strength against the impact direction, greater impact (rupture) strength is expected for the Izod specimens. Not only is the toughening effect from orientation lower for the Gardner specimens, but the impact point is also the crack initiation site.

The roles of molding (melt) temperature and injection speed on part toughness may be examined via the Gardner impact strength. Figure 6 shows that higher melt temperature or slower injection speed increases the Gardner impact strength. The higher melt temperature reduces the thickness of the frozen oriented layer. Since this minimizes the formation of significant crack initiation sites, the observed results can be easily understood.

However, the slower injection rate increased the frozen orientation layer while also increasing the Gardner impact strength. This suggests that the effect of injection speed on the molecular orientation is stronger than on the frozen layer thickness formation during the injection molding period. The contribution of melt temperature and injection speed to Gardner impact strength depends on the melt viscosity of the material. Melt temperature is significantly more important than injection speed with the low viscosity HFPS, but the effects are comparable with the moderate viscosity LFPS.

Conclusions

Gardner impact strength is shown to be substantially controlled by the fabricating conditions, but the extent of such dependence is highly sensitive to the resin viscosity. The low viscosity resin is preferred over the moderate viscosity resin, not only for its better processability, but also for higher part toughness.

PHOTO : FIGURE 1. Orientational dependence of impact strength - Izod impact vs. Gardner impact.

PHOTO : FIGURE 2. The HFPS melt viscosity shows greater shear rate and melt temperature dependence.

PHOTO : FIGURE 3. The HFPS melt viscosity behavior results in good moldability.

PHOTO : FIGURE 4. LFPS requires 2000 psi higher injection pressure to match HFPS's mold filling capability.

PHOTO : FIGURE 5. Gardner impact increases at higher melt temperatures and decreases at higher injection speeds.

PHOTO : FIGURE 6. Gardner impact contour diagram shows toughness achievable under realistic molding conditions.

PHOTO : FIGURE 7. Molecular orientation reinforces the material more against the Izod impact direction than the Gardner impact direction.
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Title Annotation:injection molded
Author:Song, James H.; Racanelli, Paolo
Publication:Plastics Engineering
Date:Aug 1, 1991
Words:1558
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