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, then 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, then 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 actual melt temperatures 10°–20°C (20°–40°F) above the barrel settings due to shear effects.
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 preceeding 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.