Molder's guide to the new olefinic alloys.

Here's the first molding guide for a new family of in-reactor alloys of polypropylene and styrenic or acrylic polymers. These crystalline/amorphous combinations can be cost-effective alternatives to other engineering thermoplastics.

[TABULAR DATA FOR TABLE 1 OMITTED]

TABLE 2 - TYPICAL INJECTION MOLDING CONDITIONS(a)

                                Hivalloy W         Hivalloy G

TEMPERATURE, F

Nozzle                          425-500 F          425-500 F
Barrel                          425-500 F          425-500 F
Melt                            425-500 F          425-500 F
Mold                             70-180 F           70-150 F

DRYING CONDITIONS             3 hr (at) 195 F      Not required

PRESSURE, psi

Injection                      8000-13,000         8000-13,000
Hold/Pack                      5000-7500           5000-7500
Backpressure                     50-100              50-100

SCREW SPEED, rpm                 50-100              50-100

INJECTION SPEED, in./sec

First 10% of shot               0.7-1.5             0.7-1.5
Next 70% of shot                1.2-2.3             1.2-2.3
Remainder of shot               0.2-0.5             0.2-0.5

TIME, sec

Injection                         3-10                3-10

Hold

With packing stage                6-11                6-11
Without packing stage             9-15                9-15
Cooling                          12-20               12-20

a Conditions given are for parts of 3-4 lb. Adjustments may be
necessary for specific part geometry, weight, gating, flow length,
and wall thickness.

Made with a unique reactor granule technology, the Hivalloy family of olefinic engineering alloys combines the strength, ease of processing, and chemical and uv resistance of semi-crystalline polypropylene with the low shrinkage, high gloss, dimensional stability, and toughness of amorphous resins. Commercially introduced by Montell in late 1996, Hivalloy resins are an entirely new class of performance polymers.

Low-temperature impact resistance of these resins is truly exceptional. In addition, their significantly lower specific gravity results in lower part weight and up to 30% more parts per pound than with traditional engineering thermoplastics.

Presently, 23 Hivalloy grades are available in two product lines - the "W" or highly weatherable series of acrylic/PP alloys and the "G" or general-purpose series of polystyrene/PP alloys. Many of the resins are impact modified to varying degrees, and the Hivalloy G resins are available in unreinforced and glass-reinforced versions. Typical properties for four Hivalloy grades appear in Table 1.

The Hivalloy W series was developed for engineering parts requiring exceptional surface and weathering characteristics. In side-by-side testing, Hivalloy W resins have been proven to be up to 10 times more uv-resistant than ASA, the industry standard, in terms of retention of original gloss and color. Mechanical-property retention is also excellent. This series of resins can be colored on the press using standard color concentrates.

Hivalloy G styrene/olefinic alloys provide a slightly better balance of stiffness and impact, especially at lower temperatures. Their weatherability, while not equal to the W series, is also very good.

The versatility of Hivalloy resins has resulted in their use in a wide range of commercial and developmental applications in automotive and industrial/consumer markets. These end uses include truck fenders and automotive pillar posts, cowl screens, and fasteners. Non-automotive applications are marine deck hatches, geriatric walkers, sledge hammers, jet skis, worm gears, and dryer vents.

Process with ease

Due to their olefinic backbone, Hivalloy resins offer both outstanding processability and good surface appearance. Most significantly, Hivalloy resins have lower melt temperatures and improved flow properties compared with many conventional engineering thermoplastics - particularly amorphous ones such as polycarbonate. Even glass-reinforced Hivalloy grades can be processed easily.

Large and complex parts - like Bombardier's Sea-Doo storage cover, auto cowl screens, and Air Fender's truck fenders [ILLUSTRATION FOR PHOTOS OMITTED] can be injection molded with relatively few gates, which helps reduce the occurrence of weld lines. In addition, parts molded from Hivalloy resins tend to have lower levels of molded-in stress and less flash in difficult-to-fill molds. Representative melt viscosities of Hivalloy resins are shown in Fig. 1.

Warpage is dramatically improved over semi-crystalline polymers such as PET and acetal, due to the amorphous content of the alloys. After molding, no large dimensional changes are caused by water absorption, as can occur with nylon. In fact, even large parts like marine deck hatches made of Hivalloy resins retain their dimensional stability in hot, moist environments.

Typically, Hivalloy resins can be processed using lower-tonnage injection machines and lower pressures and temperatures than with competitive engineering resins. Thus processors benefit from reduced capital expenditure and energy consumption. Hivalloy resins usually require 1-3 ton/sq in. of clamp force - versus 3-5 ton/sq in. for polycarbonate and PC alloys and 2-4 ton/sq in. for nylon.

When starting up Hivalloy resins in new or existing tools, we recommend beginning with low melt temperatures and pressures, and then adjusting them as needed. Melt temperatures of about 450 F are typical. Table 2 suggests initial settings for standard screw injection machines.

Little or no drying needed

Since Hivalloy resins are primarily olefinic in character, they absorb less total moisture and take it up slower than other engineering thermoplastics such as polycarbonate and ABS (Table 3).

The general-purpose (G series) Hivalloy resins are non-hygroscopic and do not require predrying under normal conditions. In order to achieve optimum processability and maximize surface aesthetics, weatherable Hivalloy W resins should be dried 3-4 hr at 190 F in a hopper dryer. Table 3 shows that Hivalloy W resins absorb moisture at a rate 50% lower than that of PC or ABS.

Shrinkage is lower, too

Typically, Hivalloy resins exhibit lower and more uniform mold shrinkage than most semi-crystalline resins, owing to their amorphous character. However, mold shrinkage - as with many traditional engineering resins - is partly a function of part thickness, part geometry, and process conditions. Table 4 shows the effects of part thickness and holding pressure on a 4-in.-diam. disk.

TABLE 3 - MOISTURE UPTAKE OF HIVALLOY RESINS
COMPARED WITH PC AND ABS

Exposed Time, hr    Hivalloy W    Hivalloy G       PC       ABS

50% R.H., 73 F

24                    0.0620        0.0076       0.1293    0.2200
168                   0.1343        0.0070       0.1431    0.2140

98% R.H., 90 F

24                    0.2016        0.0278       0.2979    0.4534
168                   0.3925        0.0405       0.3425    0.4590

In general, shrinkage will increase with increasing wall thickness, decreasing hold pressure and/or time, and increasing melt and/or mold temperature. Unreinforced Hivalloy resins will exhibit isotropic shrinkage, while glass-reinforced versions will display anisotropic shrinkage.

Shrinkage measurements should always be taken 48 hr after the part has cooled to room temperature because earlier measurements may not accurately reflect final shrinkage of the part. For large parts and/or parts with tight tolerances, a [TABULAR DATA FOR TABLE 5 OMITTED] shrinkage study should be run as a function of process conditions on a similar tool and injection press prior to building a mold.

Healthy appetite for regrind

In studies of Hivalloy GXPA068 resin, the addition of five-pass regrind to virgin material at increasing levels does not affect physical properties, except for a trend toward slightly lower flexural modulus and flexural strength. Tensile strength remains unaffected, while elongation increases (Table 5). As would be expected, the melt flow increases with each successive regrind pass [ILLUSTRATION FOR FIGURE 2 OMITTED].

TABLE 4 - EFFECT OF THICKNESSES & HOLD PRESSURE ON SHRINKAGE

Material: Hivalloy WXPA39-02 resin
Mold: 4-in.-diam. disk with 0.250- in. tab gate

Thickness, in.             Shrinkage, in./in.

0.062                         0.008
0.125                         0.011
0.1875                        0.014
0.250                         0.018

Material: Hivalloy WXPA012 resin
Mold: 4-in.-diam. disk with 0.250- in. tab gate

Hold Press., psi           Shrinkage, in./in.

0                             0.0145
150                           0.0130
200                           0.01275
250                           0.0128
300                           0.0126
350                           0.0122
[greater than]350             Flashing

Increasing the percent of regrind has a minimal impact on notched Izod impact strength at 14 F. However, at sub-zero test temperatures, impact performance decreases slightly with 75-100% regrind. The total energy of failure for 5-mph instrumented impact remains constant, and ductile failure occurs down to -40 F.

Injection Molding Conditions For Hivalloy Resins

* Drying: The G series PS/PP alloys are non-hygroscopic and do not require drying unless surface moisture is apparent. W series acrylic/PP alloys require predrying to minimize surface flaws such as splay, drool, or voids. Dry for 4 hr at 190 F.

* Screw design: Use standard metering screws with L/D ratios from 20:1 to 24:1 and compression ratios between 3:1 and 3.5:1.

* Nozzles: Standard or reverse-taper nozzles with polished bores and streamlined flow are recommended. Heated nozzles (425-500 F) help avoid freeze-off.

* Melt and barrel temperatures: Residence time of 2 to 10 min is desirable and can be achieved by balancing barrel capacity with shot size. The maximum shot size should be between 45% and 65% of barrel capacity. Melt-temperature range for most grades is 425-500 F. Do not exceed 520 F. Long residence times at 475-500 F are not recommended. The feed zone should be slightly cooler to help remove volatiles.

* Backpressure: Machine hydraulic pressure of 50 psi will achieve uniform melt temperature and consistent shot size. Increase pressure to 100-150 psi when using color concentrate. If adequate mixing cannot be obtained, try using a static-mixing nozzle.

* Injection speed: Slow injection rates are recommended for high-gloss parts and when molding in color. For textured parts, medium to fast injection is allowable.

* Holding/packing time and pressure: A good starting point for hold/pack pressure is 50-75% of injection pressure. Optimal packing pressures do not cause flashing and can be identified when part weight is constant. A minimum cushion of 0.1-0.3 in. is usually adequate.

* Clamp force: One to three tons/sq in. is typical. Exact force will be dependent on cavity pressure, which is dependent on flow length, part thickness, and processing temperature.

* Mold temperature: Normal mold temperatures are 70-180 F. Run molds as cold as possible to minimize cycle time without contributing to molded-in stress. Higher mold temperatures (160-180 F) are necessary for higher gloss surfaces.

* Cooling time: This depends on the time needed to obtain proper "green" strength. The temperature of the part should be less than 150 F, depending on warpage and geometry constraints.

* Mold release: Release agents are typically not needed.

* Regrind: Regrind content should be held to 2540% of the total mix. Hivalloy resins retain a high degree of original properties after several passes.

* Purging: Use a low-flow PP homopolymer before and after molding with Hivalloy resins.

* Downtime: For downtimes up to 30 min, retract nozzle and take air shots to clear barrel before resuming molding. For downtimes of 30 min to 2 hr, decrease barrel temperatures to 320-350 F, shut off hopper feed, and purge barrel with screw in the forward position. When resuming, raise barrel temperatures to 425-500 F and take air shots prior to molding. For downtimes longer than 2 hr, purge barrel with a low-flow PP homopolymer.

Kenneth R. Dargis is the director of business development and Todd A. Glogovsky is the applications development manager for Hivalloy resins at Montell USA Inc., Wilmington, Del.