Injection Molding
An injection molded plastic part is considered "safe" to remove from its mold once it is near or below the HDT. This means that part deformation will be held within acceptable limits after removal. The molding of plastics by necessity occurs at high temperatures (routinely 200 °C or higher) due to the high viscosity of plastics in fluid form (this issue can be addressed to some extent by the addition of plasticizers to the melt). Once plastic is in the mold, it must be cooled to a temperature to which little or no dimensional change will occur after removal.
A major drawback of this to industrial applications is that, in general, plastics do not conduct heat well and so will take quite a while to cool to room temperature. One way to mitigate this is to use a cold mold (thereby increasing heat loss from the part). Even so, the cooling of the part to room temperature can take too long for the mass production of parts.
As such, the heat deflection temperature plays an important role, as it allows for manufacturers to achieve a much faster molding process than they would otherwise. The HDT does not signify the part to be non-susceptible to dimension changes, but, as mentioned earlier, these dimension changes will be within certain acceptable limits.
Read more about this topic: Heat Deflection Temperature
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