For injection molding applications, Eastman Tritan™ copolyester can replace polycarbonate, typically without requiring new tools. Molding cycle times can be shorter with inherently lower residual stress often doing away with the need for a separate annealing stage. Lower processing temperatures and the potential for reduced cycle times combine to lower energy use during processing. The exceptional formability of Tritan allows molders the creative freedom to fashion more intricate shapes than ever before.
To achieve superior results with Tritan, there are several parameters to consider in choosing the right molding machine, including machine capacity, clamping force capacity, and ability to profile injection speed. With regard to machine capacity and essential preparation to run Tritan, this section contains information on drying, airflow rates, and moisture measurements. You will also find important factors to consider and recommendations for barrel and melt temperatures, injection speed, screw speed, cushion size, back pressure, and purge materials. For more detail, a recommended list of Eastman literature and links is provided.
Some of the parameters to consider in choosing a machine for molding Eastman polymers are:
Drying is an absolute necessity to prepare polymers for molding.
All polymers readily absorb moisture. Desiccant dryers must be used to dry the pellets prior to processing in the injection molding machine. If pellets are not dried, the moisture will react with the molten polymer at processing temperatures, resulting in a loss of molecular weight. This loss leads to lowered physical properties, such as reduced tensile and impact strengths. Molded parts may not show any noticeable defects, such as splay, but may still exhibit lower physical properties.
Pellets to be dried need to be in the hopper at the conditions shown on the data sheets for each specific polymer. If the dryer is turned on from a cold start, it must warm up to the proper temperature and the dew point of the air must be reduced to –30°C (–20°F) or below before drying time can be counted.
Dryness of air
Dry air comes from the desiccant beds in the closed-air circulation loop of the dryer/hopper system. Desiccant beds must be heated and regenerated before they can dry incoming process air. After regeneration, it is beneficial to cool down the regenerated bed with closed-loop (previously dried) air as opposed to ambient air.
Returning process air from the top of the pellet hopper is filtered before it is blown through the desiccant bed and onto the heater and hopper. Dryers used for polyesters should be equipped with aftercoolers to cool the returning process air. Return-air temperature should be below 65°C (150°F) to increase the desiccant’s affinity for moisture, thus improving efficiency.
The usual airflow rate requirement for drying is 0.06 cubic meter of hot, dry air per minute for each kilogram of material processed per hour (0.06 m³/min per kg/h) or 1 cubic foot of hot, dry air per minute for each pound of material processed per hour (1 cfm per lb/h). For example, if 109 kg (240 lb) of material is used per hour, airflow should be at least 6.7 m³/min (240 cfm). Minimum airflow to ensure good air distribution is usually about 2.8 m³/min (100 cfm) for smaller dryers.
Dew point meters measure only the dryness of the air, not the dryness of the plastic pellets in the hopper. Use of the dew point meter along with measurements of temperature, airflow, and time can give an accurate indication of whether the plastic pellets are being dried properly.
Weight-loss-type moisture meters are instruments that measure the moisture inside pellets. These meters can give a general indication of the effectiveness of the drying system in reducing the moisture level in the plastic pellets. However, most are usually not accurate enough to use as a method of quality control to ensure adequate dryness of polyesters, which prevents degradation during processing. A moisture level in the range of 0.020%–0.030% is desired, and this is determined using analytical means other than the preceding.
Common dryer problems
Barrel and melt temperatures
The first consideration in setting barrel temperatures is how much shot capacity will be used. Typically, if about half the machine’s shot capacity is used in each shot, barrel temperatures are set almost the same from back to front or slightly cooler at the feed end. If the shot is small relative to machine capacity, temperatures are set significantly cooler at the feed end to minimize degradation due to long residence times at high temperatures. If the shot size is most of the machine’s capacity, flat or higher temperatures at the feed end are typically used. These polymers often require a descending profile with higher rear-zone setpoints to achieve proper screw recovery.
Another important factor is expected cycle time. For example, if the expected cycle time is long because of limited mold cooling, barrel temperatures should be lower. Different screws add different amounts of shear heat, but it is common to see melt temperatures 10°–20°C (20°–40°F) above the barrel settings.
Actual melt temperature should be checked with a needle pyrometer. Melt temperature is best taken when the cycle is established, and an on-cycle shot is caught in an insulated container.
To minimize gate blush, splay, or both, the fill speed used for copolyesters is slower than for some other plastics. Machines with fill speed programming capability are recommended. Start the fill at a very slow speed, such as 10%–20% of available capacity for the first 3%–5% of the shot; then increase to 40%–60% to complete the shot. An average fill rate of 50–250 g/s (1.76–8.8 oz/s) is typical.
The screw should be run at the minimum rpm that will allow it to recover 2–5 seconds before the mold opens. This minimizes viscous heat generation, tends to make the melt more uniform, and minimizes dead time.
Pack and hold
A common problem with direct sprue-gated parts is a shrinkage void at the base of the sprue. Long hold times of 8–12 seconds and lower hold pressures of 275–550 bar (4,000–8,000 psi) (nozzle plastic pressure) will feed material to the sprue at a rate that will eliminate voids but not overpack the sprue. Overall cycle time does not have to be extended if the cooling timer is decreased by the amount the hold timer is raised. A shrinkage void can also form with a conventional runner at the junction of the runner and sucker pin; this can be eliminated by using the preceding methodology.
Cushion size should only be large enough to ensure the screw does not hit bottom and the pack-and-hold pressures are transmitted to the part. The cushion left at the end of the pack-and-hold phase of the cycle is typically 3–13 mm (0.125–0.5 in.), depending on machine size and injection speed. Larger cushions can increase holdup time in the barrel and contribute to degradation. Continued forward movement of the screw at the end of the shot indicates a leaking check valve. A leaking check valve will prevent a cushion from being maintained and can cause random short shots and shot-to-shot variability.
Typical back pressure is 7–10 bar (100–150 psi), though it may be as low as 3.5 bar (50 psi). To improve melt uniformity, increase melt temperature, or eliminate air entrapment (air splay), back pressure can be increased to as much as 28 bar (400 psi). Excessively high back pressures can aggravate drooling into the mold since decompression is usually kept to a minimum.
In general, minimal decompression is used. Decompression tends to pull air back into the nozzle, causing splay in the next shot. Small amounts of decompression can be used to reduce drool.
The material most effective in purging is a polymer similar to the material to be run. Polyethylene and polypropylene should be avoided because they can mix with the new material and cause streaks for extended periods of time. For difficult-to-remove materials, nozzle and front-barrel-zone setpoints are sometimes increased up to 300°C (570°F) to soak and purge, then cooled back to running temperatures. Use caution and refer to the manufacturer’s recommendations for the material used in the previous run.
After any cycle interruption of more than approximately 5 minutes, purging 3–5 shots is good practice.
In general, the feed can be shut off and molding continued on cycle until the screw is run dry. If you are changing to another material, purge with polycarbonate, acrylic, styrene, or commercial purge compound; run the screw dry; and turn off the power.
ALWAYS LEAVE THE SCREW FORWARD; otherwise, a large slug of material must be remelted. If the slug does not fully melt before the screw is injected forward, damage to the check ring may result.
My production manager has just told me we have been awarded a project using Eastman Tritan™ copolyester. As a production technical lead, where would I get processing information on this product?
An excellent source for Tritan processing information is available at Eastman.com/products/all polymers. Select the appropriate Tritan formulation from the listing. These product pages will provide access to physical data sheets, SDS, and processing guides as well as secondary operation guides.
Does Tritan have to be dried?
Yes. Tritan is hydroscopic and needs to be dried before processing. All drying information for Tritan is available on page 3 of our brochure Drying and processing guidelines for injection molding Eastman Tritan™ copolyester. This is Eastman publication TRS-237 and is available on Eastman.com.
When selecting an injection molding manufacturing cell to produce parts from Tritan, what would be a reasonable melt residence time?
An appropriate melt residence time should be between 3 to 5 minutes when the polymer is being processed at recommended temperatures. Screw and barrel information is available on page 6 of Drying and processing guidelines for injection molding Eastman Tritan™ copolyester. This is Eastman publication TRS-237 and is available on Eastman.com.
When producing parts from Tritan, what should be my process melt temperature?
An ideal actual melt temperature for Tritan would be 282°C (540°F) or lower. Note that lower polymer-processing melt temperatures are better, but care must be taken to not lower polymer melt temperatures to a point that excessive process injection pressures are experienced. Your actual polymer melt temperature may be significantly different than barrel temperature setpoints on the molding machine. Always confirm your actual polymer melt temperatures with a pyrometer.
After completing a production run with Tritan, do I have to idle the molding machine's barrel temperatures?
No. Barrel temperatures do not have to be idled between Tritan production runs. A suggested shutdown procedure would be to purge the molding machine barrel until it is empty; then turn barrel heaters off with the screw in its home position.