PROCESSING | COOLING
Injection moulding cooling time: a breakdown
In this guide by injection moulding training group RJG, Jeremy Williams outlines how to approach part cooling
If you have attended one of RJG’s training events, there is a value that probably sticks out in your mind: 80%. That is how much of the moulding cycle is spent cooling the plastic part to a temperature that it is rigid enough to withstand the forces of ejection.
As engineers (part, mould, or process), we need to understand which factors influence cooling and how that ultimately determines the cycle time. So where does that 80% number come from? In Figure 1 is an equation used to estimate cooling time. In this article, we will review four areas and their impact on cooling: part design, material selection, mould design, and processing. The basis for cycle time is rooted in the decision made by the product design engineer. The thicker the product must be to meet its working conditions, the longer the cycle will be to produce the product. In the formula in Figure 1, h² represents part thickness. Since the thickness is squared in the equation, it has the most influence over cooling time. For this analysis, we utilised a American Society of Testing and Material (ASTM) Tensile Test Bar. The dimensions are an overall length of 63.25mm, a width of 10.41mm, and a thickness of 3.3mm. Considering material selection, it should be noted that by nature, plastic is an insulator. In a melted or molten state, plastic transfers heat slightly better. As it gives up heat, however, its insulation properties increase. The material
Figure 1: Cooling equation
properties that are used in the equation are: 1. Melt Temperature — Temperature at which material transitions from solid to liquid
2. Mould Temperature — Temperature range to best achieve surface finish replication of the cavity surface
3. Heat Deflection/Distortion Temperature (HDT) — Temperature at which a material will deflect under load. Typically, the eject temperature in the equation uses the HDT, or a temperature slightly below HDT. The ASTM test for HDT closely represents what a part endures during ejection with an ejector pin pushing on a single side, while the opposite side is unsupported. The alpha symbol in the equation in Figure 1 is an important factor in determining cooling time. But what does it mean? Figure 2 shows how to find alpha. The variables in the thermal diffusivity equation
include: 1. Thermal Diffusivity — Rate at which a thermal disturbance (a rise in temperature) will be transmitted through a substance
2. Density — The quantity of a substance per volume (g/cm³ for plastics)
3. Specific Heat — Heat in calories required to raise the temperature of one gram of substance one degree Celsius For this test, we utilised a Toyolac 100 ABS
Figure 2: Thermal diffusivity equation
material from Toray Plastics with a melt tempera- ture range of 230 to 250°C, mould temperature of 40 to 80°C, and HDT of 83°C. Density can typically be found on a material data sheet, but for thermal conductivity or specific heat, it’s best to contact the material supplier directly or utilise the data within the simulation software. Based on the part geometry and material selection, the estimated cooling time is 18.00 seconds in simulation. Given all the energy required to melt a material,
it’s impractical to remove all of it while the part is still in the mould. Only 40% of the energy must be removed so the part is rigid enough for ejection.
36 INJECTION WORLD | March 2019
www.injectionworld.com
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