Annealing Info Sharepoint

For some plastics, the Glass Transition temperature is close to the melt temperature; for others, the temperature is MUCH lower, with amorphous structures having a degree of freedom well below 0 Celsius, resulting in retained flexibility. The melt deformation temperature or "soft melt point" may be relatively close to the lower temperature of crystallization, requiring careful temperature control for stress relief without overheating to deformation. The length of time held at the target stress relief temperature is known as the "soak", and directly corresponds to the extent of shrinkage- the longer the soak, the tighter the crystalline structures arrange, with corresponding increases in shrinkage and density. Recall, thermoplastics consist of 2 primary aspects: crystalline and amorphous regions. Plastic falls below the Glass Point, or Tg, when amorphous chains retain insufficient energy to slide past one another, "freezing", where as melting- Tm - occurs when temperatures promote movement of all chains, amorphous and crystalline. Both Tg and Tm are progressive regions, rather than abrupt cusps, with slowing and speeding up of chains in the vicinity of either. Distinctions may be made between freezing of the amorphous or crystalline regions, with Glass Points for either referenced. This is where things get tricky: within limits, the denser the plastic, the higher the temperature required for state changes. Take the example of 2 pressed parts, one annealed, the other not: the greater density of the annealed part will provide a degree of increased temperature stability. Moreover, the concept of "latent heat" applies, where the energy given up by crystallization results in crystals requiring a greater temperature change to melt than to form. Additionally, the pressure at which crystallization takes place also increases density and temperature stability. (edited)

Annealing schedules may be contrived to prioritize specific characteristics, such as tensile or yield strength, or uniformity, with uniformity typically achieved closer to the melt point of the plastic (such as 125C annealing temperature of HDPE with a soft-melt temperature of 130C). This should be kept in mind when interpreting proffered annealing schedules, such as this: https://www.boedeker.com/Technical-Resources/Technical-Library/Plastic-Annealing-Guidelines Notice the recommended annealing temperature for HDPE is only 93.33C/200F- to what end is not specified. The schedule is specific to the formulation and unknown priorities of the author, and utility to us dependent on what insight into motive and materials can be gleaned. Since we are employing recycled materials of unknown formulation, schedule testing and experimentation is essential. This burdened might be minimized by utilizing same-source scrap (ex: single color caps from one product), or working in large batches of well mixed shred.

Here is a proposed starting regimen for developing an annealing schedule for a part employing recycled HDPE, with priority put on minimizing strain, using the framework from the guide linked to in the preceding post. All temperature thresholds should be confirmed by experimentation and application of an optical heat sensor. For the purpose of this exercise, the part will be a 7.5" thick compression molded block, compressed at a temperature of 220C/428F. Rather than heating at the suggested rate to 200F/93.33C, after compression, the block will be cooled to the soak temperature. Since stress relief rather than strength is prioritized, the target annealing soak temperature will be closer to the soft melt point of 130C/266F; a survey of several research papers indicates stressed relief for HDPE may be maximized between 125-127C/257F-260.6F; to allow for fluctuations in oven temperature and avoid possible over heating past the melt temperature, the 125C/257F is selected for the soak temperature. The compressed part will be allowed to cool to 10F above the soft melt point, or 276F, then cooled as per the guide for approximately 2 hours, then held at the soak temperature for 18.75 hours (1 hour per 0.04" of thickness), followed by controlled cooling as prescribed for approx 16 hours to 100F. This provides a complete part cycle of approximately 2 days from the start of the initial melt. (edited)

Speaking to shrinkage and warpage of parts and eventual potential of stability, there are many process dependent factors; setting those aside, keep in mind that plastics, such as HDPE, when at room temperature are neither melted nor "frozen" (below the glass transition temperature), having both amorphous and crystalline constituents with some degree of molecular movement; eventually these constituents reach an equilibrium, where the amorphous and crystalline content percentage is stable. Injected, cast, pressed, or extruded parts- any that have been formed by heating above the melt point -on cooling will continue to shrink or warp to some degree until reaching that equilibrium point. How long does that take? In the case of HDPE, the last 2% of shrinkage may require up to 3 months to occur! HDPE may demonstrate 85% of shrinkage within 24 hours, 98% within a week, and that last 2% by 3 months. What does that practically amount to? Say you injected a meter long beam, with a gross shrinkage of 3%, or 3 cm. A day after de-molding, shrinkage might be measured at 2.55 cm; at a week, 2.94 cm, and at 3 months 3 cm. (Yes, that last bit is just 0.6 mm change!) This is just an abstract example- many factors will impact the actual shrinkage rate, such as pressure, mold temperature, and annealing. Other plastics shrink far less, or stabilize over a shorter period, or in the presence of moisture. While annealing time, temperature, and pressure contribute to total shrinkage, the increased density they may deliver also raise the glass transition temperature, shortening the distance to equilibrium. The take-away for me is 1) know your plastic, 2) know the purpose of your plastic, 3) build in plastic "set" time, as appropriate. For those formulating an annealing strategy for stabilization, it may behoove them to incorporate the time required to reach equilibrium in the plan.