Optimized extrusion techniques for ACM.

Over the past decide, the world wide automotive market has had to respond to governmental regulations and public demand for vehicles with reduced environmental emissions as well as improved fuel efficiency while maintaining vehicle performance. To meet these requirements the automotive industry has made great strides with such innovations as catalytic exhaust gas convertors, electronic fuel injection, front wheel drive-train systems, torque sensitive automatic transmissions and turbo charged engine designs. In addition, an overall trend has been the down-sizing of the vehicle itself. The sum result of these innovations is the production of increasingly smaller automobile designs with more and more compact engine compartments (ref. 1). This, in turn, has meant increased under-the-hood and service fluid operating temperatures.

Under these conditions, traditional elastomers used in these applications are being replaced by specialty elastomers with improved temperature and oil resistance (ref. 2). In the case of engine and transmission oil cooling hose, nitrile rubber (NBR) has been used as the polymer of choice for these applications, with good oil resistance (volume swell: ASTM #3 oil = 15 to 20%) but relatively poor high temperature resistance (110 [degrees]C) (ref. 3). This polymer is now being replaced by polyacrylic rubber (ACM), which has superior heat (165 [degrees]C), and oil (volume swell: ASTM #3 oil = 10 to 25%) resistance to meet new application requirements (refs. 4 and 5). ACM elastomers are polymers of acrylic acid esters, which are inherently resistant to hot air, oil and ozone due to a highly saturated polymer backbone (refs. 4 and 5)

Engine oil and transmission oil cooler hoses require the following properties (ref. 3):

* Low Compression set for good coupling retention ensuring leak free performance;

* good high temperature resistance for long term hose usage;

* good resistance to engine and transmission oil fluid;

* resistance to service fluid additive packages;

* low temperature flexibility.

Typical polyacrylic hose compounds are capable of providing what is shown in table 1. (ref. 6).

  Table 1 -typical polyacrylic hose compounds
           are capable of providing (ref. 6):

1. Compression set:            20/40%              ASTM D395
   (70 hr. @ 150 [degrees]C)                       Method B
2. Heat resistance             150 [degrees]C      Continuous
                               200 [degrees]C      Intermittent
3. Volume swell:               +10/25%             ASTM #3 oil
   (70 hr. @ 150 [degrees]C)   +2/10%              Engine oil
                                0/+3%              ATF
4. Low temperature             -30/-40 [degrees]C  ASTM D1053
    flexibility:                                   Gehman T100

To assist hose manufacturers in the processing of polyacrylic compounds, a design of experiments was developed to statistically model the extrusion process. These models can be useful in evaluating the complex interactions between ACM Compound, machine and tooling parameters, as they relate to productivity and extrudate appearance. Once refined, these models can yield extrusion settings required to achieve an optimum balance of output rate and surface appearance. No attempt was made to relate the findings to fundamental compound rheological characteristics.

Experimental

The polyacrylic compound formulation used in this experiment is shown in table 2. Mooney viscosity, rheometer and physical property data for this compound are listed in table 3. Extrusion evaluations of the ACM compound were performed on a cold feed extruder with dimensions listed in table 4.

     Table 2 -experimental

HyTemp ACM polymer:                    100
  (dual carboxyl/chlorine cure site)
Stearic acid                             1
Octylated diphenylamine                  2
Fatty acid ester                         2
N326 HAF, carbon black                  90
Ether/ester plasticizer                  5
Sodium stearate                          4
Diuron                                   2
                                        206

[TABULAR DATA OMITTED]

     Table 4

Screw type:          Double wave: constant double
                     and single lead feed screw
Screw diameter:      38 mm
L/D ratio:           20:1
Compression ratio:   2:1
Tube die:            11 mm internal diameter
                     15 mm outer diameter
Tube die:            Electrically heated
Die land length:     1x = 6mm
                     2x = 12mm

Machine parameters

Part one of this experiment statistically models the effect of the following machine parameters on the extrusion process (refs. 7-9).

  Screw speed (R):           RPM          10, 30,50
  Screw temperature (S)      ([degrees]C) 60, 70, 80

Barrel profile temps. (B) ([degrees]C) Constant-60; Constant-80;

Ramped (60, 70, 80)

A design of experiments worksheet was generated through the utilization of statistical software program RS/Discover (ref. 10). The worksheet was based upon the following statistical modeling criteria:

  Objective:           Response surface model
  Design type:         D-optimal
  Model type:          Quadratic
  Total candidates:    27 runs
  Design candidates:   14 runs
  Design model:        (9) terms
                       1          1
                       2          S
                       3          B
                       4          R
                       5          S*B
                       6          S*R
                       7          B*R
                       8          S**2
                       9          R**2

Machine parameter combinations according to the design worksheet were inputed into the extruder where the process was allowed to come to steady state conditions over a period of approximately 15 minutes prior to taking any response measurements on the resulting extrudate (ref. 7).

Tooling parameters

Part two of this experiment statistically models the effect of the following tooling parameters on the extrusion process (refs. 7 and 9):

Die temperature (D) [degrees]C 85, 95, 105

Land length (LL) 1X, 2X

A design of experiments worksheet was generated, as before. The worksheet was based upon the following statistical modeling criteria:

  Objective:           Response surface model
  Design type:         D-optimal
  Model type:          Quadratic
  Total candidates:    27 runs
  Design candidates:   14 runs
  Design model:        (5) terms
                       1        1
                       2        D
                       3        LL
                       4        D**2
                       5        D*LL

The extrusion settings listed below were used for this portion of the experiment:

  Screw speed (rpm):                  35
  Screw temperature ([degrees]C):     70

Barrel temp. profile ([degrees]C): ramped A list of measured design responses is in table 5.

[TABULAR DATA OMITTED]

Discussion of results

Response model analysis

Analyses of variance (ANOVA) (ref. 11) tables for each of the design response models were generated and are available from the authors. These tables enable us to look at the ratio of the output response signal to model variability in order to determine which machine/tooling parameters have a statistically significant effect on our output response. The accuracy or sensitivity of each model can be expressed as the percentage R-squared (R-SQ)(ref. 12). This number is defined as the percent variability in each response factor which can be explained by the correlation model. An R-SQ of 100% means all response variability can be explained by our design model.

In the upcoming discussion of results we will refer to the terms "predicted main effects" and "adjusted output responses." Predicted main effects can be defined as average effect or the slope of each response ignoring interactive and quadratic effects (ref. 10). This information is useful in determining which parameters have overall dominating effects on the output response being modeled. Adjusted output response curves are plots of selected machine/tooling parameters against the chosen response variable with all other machine/tooling parameters set at their average value (ref. 10). Though useful in seeing how a variation of a single setting might affect an output response, it should be noted that the response plots will differ with changes away from the average process settings. This is especially true where interactions between process parameters exist.

Extrusion response: based on machine parameter changes

Output temperature

This model shows that the only statistically significant machine parameter effect on output temperature is screw speed. Predicted main effects and adjusted response curve for extrudate output temperature are shown in figures 1 and 2. From these plots it is shown that screw speed has an increasing quadratic effect on extrudate output temperature.

Linear output

In this model we can see that not only are screw speed, barrel profile and screw temperature, to be statistically significant, but there also exists an interaction between screw and barrel temperature profiles with screw speed. Figure 3 shows the predicted main and interactive effects of screw temperature with screw speed. Figure 4 shows the predicted main and interactive effects of barrel temperature profile with screw speed. In each case it shows that we are capable of achieving a larger increase in linear output with higher screw and barrel profile temperatures. Figure 5 shows the adjusted linear output response as affected by screw temperature, barrel profile temperature and screw speed. From this figure we can conclude that increase in screw speed is the dominant factor in controlling linear output.

Mass output

Here our model indicates that not only screw temperature, barrel profile temperature and screw speed affect mass output, but there also exists an interaction between screw temperature and screw speed. Figures 6 and 7 show the predicted main and interactive effects of screw temperature and barrel profile temperature with screw speed. Here we see that screw speed is the overriding factor in controlling mass output. Adjusted weight output response for each of the machine parameters is illustrated in figure 8.

Head pressure

This model shows that the only machine parameter which has an effect on head pressure is screw speed. Figure 9 shows die predicted main effects of screw speed on head pressure. Figure 10 illustrates the quadratic effect of screw speed.

Appearance

In this model both barrel temperature profile and screw speed are shown to have statistically significant effects on surface appearance of the extrudate. Extrudate appearance is improved with decreases in screw speed and with a constant 60 [degrees]C barrel temperature profile.

Overall machine parameter effects

Screw speed

Screw speed has an overriding effect on all measured extrusion process responses. Here we can conclude that increased screw speed causes quadratic increases output temperature, mass output and bead pressure, as well as linear increases in linear output. Decreased screw speed causes improvements in surface appearance.

Screw temperature

Screw temperature has no statistically significant effect on output temperature, head pressure and surface appearance. Screw temperature does, however, have an interactive effect with screw speed for both linear and mass output. Increases in screw temperature cause significantly larger increases in linear and mass output at higher screw speed settings.

Barrel temperature profile

Barrel temperature profile has no statistically significant effect on output temperature and head pressure. A low, constant 60 [degrees]C barrel profile temperature will provide an extrudate with the best surface appearance, though, a higher constant 80 [degrees]C profile will yield the greatest mass output. There also exists an interaction between barrel temperature profile and screw speed on the linear output response. Here an increased barrel temperature profile of a constant 80 [degrees]C causes significantly greater linear output at higher screw speeds than al lower screw speeds.

Extrusion response: based upon tooling parameter changes

Output temperature

This model shows both die temperature and land length to have a statistically significant effect on output temperature. Increases in both die temperature and land length cause increases in output temperature.

Linear output

This model shows that both the temperature and land length have a statistically significant effect on linear output. Increases in both the temperature and land length cause increased linear output.

Mass output

This model indicates that both die temperature and land length have a statistically significant effect on mass output and an interaction exists between die temperature and land length. At low die temperatures, mass output actually decreases with an increase in land length. At higher die temperatures, this does not seem to be the case. Increases in both die temperature and land length cause increased mass output with die temperature being the dominating factor.

Head pressure

Here our model indicates both die temperature and land length have statistically significant effects on head pressure, and an interaction exists between die temperature and land length. It was found that with low die temperatures larger head pressure increases can be expected with increased land length. Increased die temperature causes decreased head pressure and increased land length causes increased head pressures.

Appearance

This model shows that both die temperature and land length have statistically significant effects on the surface appearance of the extrudate. Land length is the dominant factor in controlling surface appearance. Extrudate appearance is improved with increased die temperature and decreased land length.

Overall tooling parameter effects

Die temperature

Increases in die temperature cause significant quadratic increases in output temperature and linear increases in mass output when compared with the effect of longer land lengths. There is also an interactive effect between die temperature and land length on mass output. Increases in die temperatures cause a greater effect of increased mass output at longer land lengths than with shorter land lengths. It is also noted that increases in die temperatures have more significant effect on mass output than do increased land lengths. Of particular significance is the drastic improvement of extrudate surface appearance seen with increases in die temperature.

Land length

Increases in land length are found to have significant effects on increasing linear output but only marginal effects on increases in mass output. There also exists an interactive effect between land length and die temperature on head pressure. Head pressure increases associated with increased land length can be minimized with increased die temperatures. While increased land length is shown to have only marginal effects on increasing output temperature it has greatly negative effects on surface appearance.

Optimized extrusion settings

Table 6 shows optimized extrusion settings for linear output and appearance based upon the statistical models for machine parameters as generated by the RS-1 software program. It is noted that increased linear output is inversely related to surface appearance. Table 7 shows optimized extrusion settings for output and surface appearance based upon the statistical models for tooling parameters. It is noted here that high outputs and good surface appearance can be achieved through increased die temperatures.

[TABULAR DATA OMITTED]

  Table 7 -optimized tooling parameter settings

Factor/response       Range          Optimized settings
Factor:                                for: Appearance
 Die temp.          95 to 106             105
 Land length      Ramp/constant 80         1x
Response:
 Output temp.                             96.2
 Linear output                           121.5
 Mass output                             309.1
 Head pressure                            14.1
 Appearance                              (4.4)

Based upon the design models and optimization presented in this article we can make some general recommendations in the setting of machine and tooling parameters for the extrusion of polyacrylic tube profiles with the best surface appearance. Linear outputs would be maximized by increasing screw speeds to a level providing a stock temperature below that of its scorch temperature.

  Screw temperature:      80 [degrees]C
  Barrel temp. profile:   Constant 60 [degrees]C
  Land length:            Short (1x)
  Die temperature:        105 [degrees]

Conclusions

A statistical model was developed to evaluate the performance of a typical extrusion grade polyacrylic rubber compound. The model developed was capable of predicting key output responses of the extrusion process: extrudate output, temperature, surface appearance and head pressure. Complex interactive relationships between key extrusion parameters: screw speed, screw temperature, barrel temperature profile, die temperature and land length were identified. The statistical model, based upon relatively few experiments, was used to generate optimized extruder/tooling settings. The importance of optimizing extrusion parameter and tool settings to achieve high output rates and excellent surface appearance was determined.

References

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