The most important criterion for the profitability of injection molding production is cycle time. For most molded parts (except thin-walled ones) the biggest component of cycle time is cooling time.
In the case of many molded parts with a complicated shape, it is not possible to easily dissipate the heat from narrow parts of the mold. Using advanced techniques such as conformal cooling requires building the die with additive manufacturing methods, which significantly increases the tool production cost. Simulation programs, equipped with modules for thermal analysis, are certainly helpful. However, when already fabricated mold does not achieve the “theoretical cooling time”, a possible remedy is to rebuild it from steel or alloy with high thermal conductivity. According to Fourier’s law, the heat flow from the molding surface to the cooling channel will be the greater the greater conductivity of the mold material. Let’s take a look at two interesting cases that have caused a problem with too long injection time.
Two-cavity form for flowerpot cover (Fig. 1). Injected material – PS. Cycle time achieved by the injection plant is 40 s. Dimensions of the molded part 125x125x200 mm, weight 210 g. Wall thickness 1.5 to 1.6 mm.
![](https://about-engineering.com/wp-content/uploads/2021/07/Injection-molding-cycle-time-reduction-1.png)
Using a simple practical rule to quickly estimate the cooling time:
tcool = a ∙ s2 [s]
where s is the wall thickness in mm, a = 2 to 3 (coefficient depends on the type of material; for crystalline a – closer to 2, for amorphous a – closer to 3) – let us estimate the “theoretical” cooling time:
tcool = 3 ∙ 1.6 ∙ 1.6 = 7.7 s.
Adding the remaining times, we come to the conclusion that the cycle time should be between 15 s and a maximum 20 s. Let’s look at the structure of this form (Fig. 2).
![](https://about-engineering.com/wp-content/uploads/2021/07/Injection-molding-cycle-time-reduction-2.png)
The form was very carefully designed. The matrices and stamps are made of NIMAX steel, which has a conductivity of 26 W/mK, typical for this type of steel and this type of application. Cooling channels surround the mold almost conformally. Only one area is not sufficiently cooled – it is the cavity in the bottom of the molded part, into which a hot runner is inserted. There are no channels here, as there is no way to introduce cooling by drilling holes. Hot runners with smaller outer diameter would not be up to the task either. So what we can do in such a situation, knowing that the mold could actually work in a cycle twice as short?
![injection molding simulation](https://about-engineering.com/wp-content/uploads/2021/07/Injection-molding-cycle-time-reduction-3-1024x501.png)
It has been proposed to collect heat from the cavity using high conductivity material. With a help from the simulation program, let us check the effect of the alloy and highly conductive steel. First, let us see what the situation is like in the existing mold made of NIMAX steel. The effect of the lack of cooling in the cavity is that the temperature of this section of the mold increases to 90 °C (Fig. 3). No wonder that the cooling time of the cavity is longer than 35 s (Fig. 4), which in turn leads to a cycle time of 40 s.
![](https://about-engineering.com/wp-content/uploads/2021/07/Injection-molding-cycle-time-reduction-4-1024x501.png)