Injection molding involves melt processing. The raw material must be converted from a solid to a fluid in the injection cylinder and then back to a solid in the mold. The time required to cool the material back to the solid is the primary factor determining cycle time, a parameter that has a substantial impact on the cost of the part and the amount of press time that must be allocated in the production schedule. The point at which a part can be ejected from the mold is a function of part geometry and the temperature at which the material becomes stiff enough to withstand the forces of ejection without deforming. The time that the cooling process takes is therefore dependent in large measure on the temperature of the material entering the mold cavity from the barrel.
Melt temperature selection is not an exact science in the world of injection molding today. The range of allowable melt temperatures for a given material can be quite narrow for a thermally sensitive material like PVC or it can be extremely wide for a thermally stable material like polyethylene. Exceeding the recommended upper limit has the obvious consequence of degrading the polymer. This can manifest as changes in color or a loss in properties, particularly impact resistance. But within the recommended melt temperature range there are also hazards and hidden costs associated with running the melt at a higher temperature than necessary, even in a forgiving material.
Consider polypropylene. Polypropylene materials have a crystalline melting point somewhere between 300-335°F depending upon whether they are homopolymers or copolymers. The published process temperature range for most grades of polypropylene is 375-550°F, which presents the molder with a very broad selection when establishing a new process. The hidden cost of selecting a melt temperature that is higher than necessary is difficult to capture unless a detailed study is performed relating melt temperature to cycle time. When this work is done, the results can be surprising.
Productivity, regrind problems
A common polypropylene appliance part chosen for such a study was run at melt temperatures of 400°F and 480°F. The higher melt temperature was being used for no particular reason. When the melt temperature was reduced to the lower value, the minimum achievable cycle time dropped from 46 seconds to 39 seconds. This is a 15% reduction in cycle time, which translates to an 18% increase in output. This increased productivity is the result of achieving the stiffness required to eject the part more quickly because the extra energy required to raise the melt temperature up to 480°F does not have to be removed during the cooling portion of the cycle. Plus, there is an additional cost in running the higher melt temperature that is not related directly to cycle time; this is the energy cost required to raise the melt temperature by 80°F.
But the problems go well beyond the cost factors associated with cycle time. A melt-flow-rate (MFR) test on the raw material and the molded parts shows that the MFR for the material processed at 400°F increases by only 4% while the MFR for the part produced at 480°F increases by 35%. The increases are associated with the reduced molecular weight of the polymer. Increases of up to 40% are considered to represent an acceptable preservation of the molecular weight of the polymer, so both parts are good.
However, the runners and any rejected parts run at the higher temperature that have to be reground and reclaimed will almost certainly exceed the upper limit after one more heat history. In fact, an orchestrated regrind study using both melt temperatures shows that the cumulative change in MFR from five passes through the process at 400°F is less than the 35% that occurs from one pass through the molding machine at 480°F.
This regrind study revealed another effect of the higher melt temperature. The color of the product started to drift off target after two passes through the molding process at 480°F while the product run five times at 400°F still provided a suitable color match. This color drift is an indication of another process that is not necessarily captured by the MFR test: the chemical reaction known as oxidation.
The threat to antioxidants
Polypropylene materials rely on small amounts of additives known as antioxidants to survive the molding process and the application environment. A review of UL relative thermal index data for a variety of polypropylenes shows that some materials can withstand long-term exposure to temperatures as high as 115-120°C (239-248°F) while others are only usable over the same time frame at temperatures up to 60°C (140°F). These materials are not distinguishable by their molecular weight. Instead, they differ according to their antioxidant content.
Elevated melt temperatures can consume antioxidants that are intended for use when the part is in the field. This will shorten the life of the product. The amount and effectiveness of the antioxidant can be measured in a relative manner by conducting a test designed to promote rapid oxidation in a controlled environment. When this test is performed on the parts molded at 400°F and 480°F, the time required to achieve oxidation is five times longer for the product molded at the lower temperature.
So the use of high melt temperatures has a number of consequences for the molder as well as the end user, and they are all negative. Energy costs increase. Cycle times increase, reducing throughput and extending the length of the production runs. And the parts that do come out of the mold at these elevated temperatures are of lower quality and less suited for their intended uses, particularly if the application calls for long-term field service.
Part 1 of this series on improved mold cooling appeared in June 2006 IMM, www.immnet.com/articles/2006/June/2901. Watch for Part 3 in October, authored by processing expert John Bozzelli.
| Cycle time reduction = output Example: PP appliance part PP process temperature range: 375-550°F Cycle time at 400°F: 39 seconds Cycle time at 480°F: 46 seconds Cycle time reduction: 15% Output increase: 18% Results: improved part quality, energy savings, more productivity |