Injection mold: the new polyketones.

Just now available, these challengers to nylons, acetals, and polyesters set up fast and are not moisture sensitive.

The world's first aliphatic polyketones became available in commercial quantities on October 30, 1996. Carilon Thermoplastic Polymers from Shell Chemical Co. are semi-crystalline terpolymers of ethylene, carbon monoxide (CO), and small amounts of propylene. The perfectly alternating structure of CO and olefin results in a material with a unique set of properties that is easy to process in most conventional injection molding equipment.

These engineering resins offer a combination of strength, stiffness, chemical and impact resistance, and good wear and friction characteristics. Also, polyketone resins retain their performance properties over a wide temperature range. Carilon Polymers should be particularly attractive for automotive, electrical, and industrial applications, as well as business machines and consumer appliances.

Currently, Shell offers three developmental grades of Carilon Polymers in commercial quantities: DP P1000 (medium-flow, unreinforced), DP R1000 (high-flow, unreinforced), and DP R1130 (30% chopped-glass fiber). Typical properties appear in Table 1. Flame retardant and lubricated tribological grades of polyketone are also available.

MOLDING BASICS

Carilon Polymers generally have a wide processing window. The most important processing characteristic that molders need to be aware of is the fast set-up time. This often can result in cycle-time reductions of 15% to 40% compared with acetal or nylon 66, assuming use of properly sized sprues, runners, and gates.

These polyketones are also hydrolytically stable. Thus, the presence of a small amount of water has little effect on processing or physical properties. All the same, you should take precautions to keep the polymer dry (no more than 0.2% water). Excess moisture can cause bubbles in the melt, resulting in drool, voids in parts, or surface problems such as splay.

Other thermoplastics may evolve hazardous by-products during processing upsets. But the primary by-product of Carilon Polymers, if processing degradation occurs, is water.

Table 2 outlines suggested starting-point settings for reciprocating-screw injection machines. The accompanying chart of processing tips should help you mold Carilon Polymers with minimal start-up time.

DESIGN MOLDS FOR FAST FILL

When you are designing gates, runners, and vents, it is important for you to keep in mind that medium to high injection speeds and pressures will be required because of the rapid set-up of Carilon Polymers. Also remember that the melt-flow index for unfilled polyketone (grade DP P1000) is slightly lower than that of many acetals and is significantly lower than that of nylon 66.

Both factors point to using runner and gate systems with minimal flow restrictions in order to minimize pressure drop between the nozzle and cavities. To take full advantage of polyketones' potential to reduce cycle times, pay careful attention to mold-temperature control. Ensure the proper location, sizing, and spacing of hot-water or oil channels. Separate temperature control of each mold half is best.

[TABULAR DATA FOR TABLE 1 OMITTED]

TABLE 2 - TYPICAL INJECTION MOLDING CONDITIONS

                                                         DP R1000
&
                                      DP P1000           DP R1130

TEMPERATURE, F

Barrel

Rear                                  470 450
Middle                                480 460
Front                                 480 460
Nozzle                                490-500            480
Melt                                  480-500            460-480
Mold                                  175                175-300
Hot Runners                           490-520            480-510

PRESSURE(a), psig

Injection                             650-1200           400-800
Hold/Pack                             400-900            300-600
Backpressure                          50                 25

SCREW SPEED, rpm                      50-100             40-75

TIME, sec (for 25-g to 180-g parts)

Injection                             1-3                1-4
                                      (Medium            (Slow to
                                      Rate)              Medium)

Hold                                  2.5-6              2-5
Cool/Clamp                            6-10               4-8

a Approximate machine hydraulic pressure, assuming a force
multiplier of 10:0.

Figure 1 shows how mold temperature can be used to fine-tune mold shrinkage. The graph also shows the close similarity between flow-direction and transverse-direction shrinkage of Carilon in a fiat, narrow test bar. Note also the similarity of shrinkage for acetal and polyketone, indicating that both can be molded in the same tools.

An advantage of unreinforced Carilon Polymers is that very little dimensional change occurs due to shrinkage after parts cool down, unlike some other semi-crystalline plastics such as acetals. Another advantage is that no large dimensional changes occur after molding as a result of water absorption, unlike nylon 66.

For example, when parts are conditioned in air at 73 F and 50% relative humidity, the dimensional growth resulting from water absorption will stabilize at the equilibrium moisture content. For plaques of nylon 66, dimensional growth stabilizes at approximately 0.7%. For acetal plaques, that growth stabilizes at about 0.2%, and for unreinforced Carilon Polymers, the growth stabilizes at around 0.1%.

For most polyketone parts, [TABULAR DATA FOR TABLE 3 OMITTED] dimensional changes due to water absorption will not be a problem. However, for parts requiring extreme accuracy in dimensions after molding - such as high-precision gears - adjustment for water absorption can be made by changing molding conditions such as mold temperature [ILLUSTRATION FOR FIGURE 1 OMITTED] without affecting part performance.

INJECTION MOLDING CARILON POLYKETONES

Drying: Maximum recommended moisture content: 0.2%. Dry 4 hr at 140 F. Resin is supplied predried. Although not moisture sensitive, Carilon Polymers should be kept dry to prevent splay, drool, or voids.

Screw design: Use standard metering screws with L/D ratios from 18:1 to 20:1 and compression ratios between 2.5:1 and 3:1. Free-flowing check rings minimize possibility of resin overheating.

Nozzles: Use conventional or reverse-taper nozzles with polished bores and streamlined flow. Positive shut-off is not usually required. Heated nozzles are recommended to prevent freeze-off problems due to polyketone's rapid crystallization.

Melt and barrel temperatures: Balance shot size with barrel capacity to give residence times of 2-10 min. Suggested melt-temperature range is 465-520 F (see Table 2). Do not exceed 575 F. Long residence times at high end of the temperature range can cause thermal degradation and loss of physical properties. In extreme cases, crosslinking and large increases in melt viscosity can result.

Start with a flat barrel-temperature profile (Table 2). Feed zone should be slightly cooler to aid removal of volatiles. A reverse profile is usually better with screw L/D's above 20:1.

Backpressure: In most cases, machine hydraulic pressure of 50 psi will achieve uniform melt temperature and consistent shot size. (This assumes a force multiplier of 10 between hydraulic and melt pressure.) When using color concentrate, 100-psi backpressure may be necessary for proper mixing. Avoid backpressures above 100 psi since they can lead to hot spots along the screw.

Screw speed: Use slow to moderate speeds as shown in Table 2.

Injection speed: Start with medium speed. Keep in mind polyketone's rapid set-up times. In tools designed for slow injection, you may need to add venting and raise mold temperature to prevent air entrapment and localized burning. See Table 2 for recommended injection pressures. Figure 2 shows shear-thinning behavior of polyketones.

Holding/packing time, pressure: Polyketone's fast set-up allows short holding times. Start with packing pressure at 50-75% of injection pressure. A minimum cushion (0.25-0.50 in.) is usually sufficient

Clamp force: Use low to moderate pressure of 3-6 tons/sq in.

Mold temperature: Start at 175 F (Table 2). Parts have been molded successfully at 85 to 320 F. Rapid crystallization at elevated temperatures allows use of hot molds to improve filling, weld lines, and surface finish with little effect on cycle times. As shown in Fig. 1, mold temperature can be used to help control part shrinkage.

Cycle time: Typically 10-15 sec for thin-wall moldings (0.025 to 0.060 in.), 30 sec or less for thicker sections up to 0.125 in.

Mold release: Sticking is rare in well-designed tools. If specific part geometry causes problems, light application of standard release agents may be used.

Regrind: Limit regrind content to a maximum of 25-40%. Lab tests show that Carilon Polymers can be reground several times and exhibit little loss in properties at suggested processing conditions (Table 3). However, regrind tends to pick up moisture and irregular particles can impair good flow in the hopper.

Purging: Use a fractional-melt-flow HDPE or polypropylene to purge Carilon Polymers before molding other resins or vice versa.

Downtime procedures: When interruptions occur in the molding cycle, do one of the following:

1. For periods up to 30 min, retract the nozzle and take a sufficient number of air shots to clear the barrel before resuming.

2. For interruptions between 30 min and 2 hr, decrease barrel temperatures by 120-212 [greater than] F, shut off hopper feed, and purge the barrel, leaving the screw in the forward position. At start-up, raise temperatures to proper settings and take several air shots prior to molding.

3. For interruptions greater than 2 hr, purge the barrel as recommended above.

TIPS ON MOLD DESIGN

Materials: Use standard tool steels such as P-20, H-13, and S-7. Polyketones do not emit corrosive compounds during normal processing.

Gating: All common gate types - edge, fan, flash, submarine, and pinpoint/hot-tip - are suitable. Make them as large as possible - 25-50% larger than for acetals and 40-75% larger than for nylon 66. Keep land length to a minimum due to polyketone's rapid set-up.

Runners, sprue bushings: Full-round runners minimize melt chilling during passage through the runner, though trapezoidal and half-round runners also can be used. Keep as short as possible, with a minimum diam. of 1/4 in. Minimize secondary runner lengths, too, and use 3/16-in. minimum diam.

Use standard sprue bushings with 2-4 [degrees] draft angles. Cold wells and sprue pins used with other engineering thermoplastics also work with Carilon Polymers.

Hot runners: Externally heated manifolds and nozzles reduce the chance of material degradation due to flow stagnation. Both manifolds and nozzles should have streamlined flow channels. Nozzle tips can be used to keep the gate area from freezing. Tips should not use resistive heating but rely upon heat conduction from the nozzle to the gate. Nozzle-tip geometry should not allow flow stagnation or blockage.

Vents: Vent generously to accommodate rapid fill rates. Use vent depths of 0.0003-0.0005 in. at final fill points and areas where air traps are likely.

Shrinkage: Transverse shrinkage of unreinforced grades is usually within 10-15% of shrinkage in flow direction (see Table 1). Mold temperature can help fine-tune part dimensions [ILLUSTRATION FOR FIGURE 1 OMITTED]. Tools designed for acetal are often suitable for polyketone.

Robert Farris is senior research engineer and John W. Kelley is staff research engineer at Shell Chemical Co. in Houston.

If you have any questions or encounter problems when using Carilon Polymers, technical assistance is available by phone at 1-888-Carilon (1-888-227-4566) or at Shell Chemical's World Wide Web site at www.shellchemical. com.