How to injection mold metallocene polyolefins.
New families of metallocene-based polyolefin plastomers and elastomers are offering injection molders broad new opportunities. With these polymers, molders can target high-value applications and can obtain higher productivity from their existing equipment. Good low-temperature brittleness properties, excellent optics, good flexibility over a broad range of densities, and the capability to run on either hot- or cold-runner systems are among the benefits these new resins deliver. Because these are new types of polymers, understanding their processing nuances will allow molders to use these products to their fullest advantage.
Note that there is more than one family of metallocene-based polyolefins on the market and there may be differences in how they process. Therefore, the following processing guidelines are tailored specifically to Affinity polyolefin plastomers (POPs) and Engage polyolefin elastomers (POEs) made by Dow Plastics. These are low-density linear ethylene-octene copolymers made with Insite metallocene catalyst technology, which imparts very narrow molecular-weight distribution and highly uniform comonomer distribution.
In general, Affinity and Engage resins process similarly to other linear ethylene copolymers such as LLDPE and ULDPE. Since much of the initial molding work with the new polyolefins has been targeted toward applications served by flexible PVC (f-PVC), this article discusses processing conditions using f-PVC molding as its primary point of reference. Some comparisons will also be made with styrene block copolymers (SBCs), EVA, and EPDM-modified PP, since they also compete with POEs and POPs.
LOWER DENSITY = COST SAVINGS
A significant cost advantage of POPs and POEs derives from their low density. Flexible PVC, for example, has a density of 1.23-1.30 g/cc. POPs and POEs designed for the same applications have densities of 0.870 to 0.885 g/cc. That 40% difference in density translates into 40% more parts per pound of resin with POEs and POPs.
MOLDS: THE COLDER THE BETTER
When running POPs and POEs, the lower the mold temperature the better. The target is 50 F for both core and cavity, whereas f-PVC calls for running a "hot side" and a "cold side." High water circulation rates are recommended. Those are the key differences between these new resins and other materials aimed at similar injection molding applications (see Table 1).
For comparison, standard mold temperatures for f-PVC are 90 F (cold side) and 120 F (hot side). SBCs and EPDM-modified PP are often run at 80 F with minimal water circulation.
Cold molds are recommended for POPs and POEs because of their low densities and hence low melting and solidification points. If molds are not kept sufficiently cold, parts may stick.
Cold molds together with fast injection rates can reduce cycle times by as much as 25-30% relative to f-PVC, as shown in Table 1. This can help molders squeeze more value from existing capital equipment.
FAST INJECTION, HOT RUNNERS
Because POPs and POEs exhibit more shear thinning than does f-PVC, molders are encouraged to adjust their injection speeds downward to 0.4 sec at a pressure of 1500-2000 psi. If you can't inject that fast, go as fast as you can. Injection speeds in this range are a goal but not a requirement. Faster injection speeds reduce viscosity and allow for more efficient filling.
TABLE 1 - INJECTION MOLDING METALLOCENE RESINS VS. FLEXIBLE PVC(a)
Conditions POP/POE f-PVC
Barrel Temp., F 300-360 340-360 Feed-Throat Temp., F 290 340 Mold Temp., F(b) 48/48 125/100 Mold Water Circulation Maximum Minimum (Turbulent) Part Weight, g 48.3 62.8 Injection Time, sec 1.3 8.1 Cooling Time, sec 21.0 33.0 Total Cycle Time, sec 52.0 70.0
a Actual run conditions for a vacuum cleaner part. b Both mold halves should be at same temperature for POP/POE, as compared to using a "hot side" and "cold side" for PVC.
Hot-runner systems offer significant labor and material savings over standard cold runners. More thermally stable than f-PVC, POPs and POEs are easily processed with hot-runner systems. Flexible PVC can be run on hot runners, but this requires close monitoring of the molding equipment to prevent thermal-instability problems (e.g., black specks). Generally speaking, POPs/POEs can be hot-runner molded similarly to other polyolefins using similar manifold design and temperature ranges. Good results can be achieved with hot tips run 50 [degrees] F cooler than the melt temperature.
Hot-runner molding increases the cycle-time savings possible with POPs/POEs relative to f-PVC. Thus, although switching to hot runners requires investment in new molds, this cost can be offset by labor and material savings, by higher molding productivity, and by lower maintenance expenses over the lifetime of the tool. Tools last longer because POPs/POEs are not as corrosive as PVC. These factors were enough to yield a projected 26.5% cost saving per part in the case of the medical face mask pictured above. What makes this comparison all the more noteworthy is that the POE part cost included amortization of a brand-new four-cavity hot-runner tool. But the f-PVC cold-runner parts were assumed to come from an existing four-cavity tool and so bore no tool amortization cost at all. (Assumed production volume was 5.1 million parts/yr.)
OTHER MOLDING GUIDELINES
Besides the differences in mold temperature and injection speed, and the added factors of lower density and hot-runner availability, several other processing characteristics of POPs/POEs should be taken into account:
* Part weights: Because these new polymers have very low melting and solidification points, aim for maximum part weight in order to prevent "back-flow." Increase your hold time to make certain the part is completely filled. The latter is important to avoid a phenomenon that can resemble flow lines. The problem can be avoided by adding a little bit of extra hold time to allow for complete solidification of the part and to achieve a uniform surface. Hold time can be extended without lengthening the overall cycle by subtracting a comparable amount from the cooling phase that follows hold time at the end of the cycle.
* Shrinkage: POPs and POEs have been run in f-PVC molds without shrinkage problems. As with traditional polyolefins, the amount of mold shrinkage depends on density - The higher the density, the greater the shrinkage. Average mold shrinkage with POPs/POEs is between 0.5% and 1.5%, when processing guidelines are followed.
* Barrel temperatures: Because POPs/POEs have low melting points, it's important to run cold (225-300 F) in the feed-throat area. Running too hot may cause bridging or clogging problems in the hopper. So keep it cold in the back.
POPs and POEs are thermally stable, so their processing window is fairly wide - from 300 to 550 F - as compared with those of f-PVC or EVA. If you need to increase barrel temperatures, you can do so without risk of thermal degradation. You can run at the same temperatures you currently use for f-PVC or you can run hotter if you need slightly lower viscosity to fill the mold. You need not be overly concerned if the melt temperature at the nozzle slightly exceeds the barrel set-point temperature because of shear heating. Unlike more sensitive materials, such as PVC and EVA, POPs, and POEs can take a bit of extra heat.
* Melt temperatures: If you're having any flow difficulties, perhaps due to tight restrictions in the mold, increase the temperatures to enhance the flow rate. Note that POPs and POEs will shear heat, so the melt temperature will exceed the set temperature. Keep in mind that too high a temperature will lengthen the cooling time. So seek a balance: Target a melt temperature that's high enough to flow easily and cool enough to set up quickly.
* Ejection: When designing a new mold for POPs/POEs, use air ejection instead of ejector pins in order to avoid nicks or cuts on the parts. Blow off parts, don't push them off. If you're using an existing mold that has ejector pins, be sure to allow the part to cool sufficiently before ejection.
* Mold design: As noted earlier, hot runners are recommended. And since the mold may be running colder than you're used to with other materials, make sure the gate is big enough to allow adequate filling and packing of the part without premature gate freeze-off. If there are wall-thickness variations in the part, the gates should be placed in the thickest area to ensure a steady melt-front advancement through the mold ("hesitation" of the melt front as it passes from a thin section to a thicker one can leave surface marks) and to avoid over-packing and high residual stresses.
Since fast injection is recommended, remember that improper and inadequate venting can cause voids, burns, sticking, and stresses in molded parts. However, don't overdo the venting or else flashing may result. A flat groove recessed below the mold's parting surface at the point in the mold farthest from the gate is usually sufficient. Optimum vent depth can be determined by trial and error. Start out cautiously - you can always relieve the vent a little more if you need to, but it's difficult to put steel back once it's removed.
* Screw design: Plasticating-screw design has not been a concern in processing POPs/POEs. They have been run successfully in machines set up to process polyolefins, f-PVC, EVA, and SBCs. However, equipment optimization can be expected as POPs and POEs are commercialized further.
RELATED ARTICLE: More Advice on Plastomer Molding
Plastics technology asked Exxon Chemical Co., the only other commercial producer of plastomer resins in North America, whether its recommended molding conditions differ from those of Dow Plastics. The following response was submitted by George Wagner, Exact Plastomers market development manager, and Dr. Joe Domine, senior staff scientist at Exxon's Baytown, Texas, Polymers Center.
Exxon Chemical believes its Exact plastomers (made with Exxpol metallocene catalyst technology) will process in injection molding essentially the same as Dow's Insite-technology polymers. Exxon's Exact plastomer molding grades are strictly linear polymers. Dow's molding grades appear to contain a very small amount of long-chain branching, as indicated by Dow's DRI (Dow Rheology Index) of 0.4. Exxon Chemical believes this small amount of long-chain branching will have essentially no effect on injection moldability.
Metallocene-catalyzed ethylene polymers, especially those with densities below 0.900, replicate mold surfaces very well. This attribute, combined with their relatively low modulus and low shrinkage, can lead to part ejection problems, especially in highly polished molds with low draft angles. Internal mold releases, such as Axel's Mold Wiz INT-33P/A or INT-38H, have been found to be very effective in improving part ejection of Exact plastomer parts without adversely affecting clarity. (Mold Wiz products come from Axel Plastics Research Laboratories, Inc., Woodside, N.Y.)
Although Dow reports that metallocene-based PEs are more pseudoplastic (i.e., shear thinning) than some f-PVC formulations, molders may find that increasing gate sizes may be beneficial in facilitating very fast mold filling without inducing "gate blush." For some gate geometries, this may marginally increase the time for gate freeze-off, but frequently this will be offset by reduced fill time and improved part appearance.
WENDY HARRIS & DAN MOLDOVAN are project leaders at Dow Plastics Technical Service & Development, Freeport, Texas.
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|Title Annotation:||includes related article|
|Date:||Apr 1, 1995|
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