Part design, material selection, mold design and processing all play a role when it comes to injection molding cooling time.

Jeremy Williams, RJG

Here’s a value that probably sticks out in your mind if you’ve been to an RJG training event: 80%. That’s how much of the molding cycle is spent cooling the plastic part to a temperature that’s rigid enough to withstand the forces of ejection.

But where does the 80% come from? Here’s an equation used to estimate cooling time:

Let’s review four areas that go into this equation:

1. Part design

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 previously mentioned formula, h² represents part thickness. Since the thickness is squared in the equation, it has the most influence over cooling time.

For this analysis, we utilized an American Society of Testing and Material (ASTM) Tensile Test Bar. The dimensions are 2.49 in. for length, 0.41 in. for width, and 0.13 in. for a thickness.

2. Material selection 

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.

Material properties that are used in the equation are:

• Melt temperature: temperature at which material transitions from solid to liquid;
• Mold temperature: temperature range to best achieve surface finish replication of the mold;
• Heat deflection/distortion temperature (HDT): The temperature at which a material will deflect under load.

Typically, the eject temperature in the equation uses a value slightly below HDT. The following equation shows how to calculate thermal diffusivity (alpha symbol or α).

The variables in the thermal diffusivity equation include:

• Thermal diffusivity: rate at which a thermal disturbance (a rise in temperature) will be transmitted through a substance;
• Density: quantity of a substance per volume (g/cm³ for plastics);
• Specific heat: heat (in calories) required to raise the temperature of one gram of substance by 1 °

For this test, we utilized a Toyolac 100 with temperature ranges for melt of 446 °F to 482 °F, mold 104 °F to 176 °F and HDT of 181 °F. Density can typically be found on a material data sheet, but for thermal conductivity or specific heat contact the material supplier directly or utilize the data within the simulation software.

Based on the part geometry and material selection, the estimated cooling time is 18 seconds in simulation.

Given all the energy required to melt a material, only 40% must be removed so the part is rigid enough for ejection.

Generally, we do not recommend selecting a cooling timer that only meets the HDT. A good rule of thumb is adding 20% to the cooling timer to account for variation. Tighter tolerance parts will require a larger safety factor.

3. Mold design

The mold design shown with this article is an 8-cavity with an “H” pattern runner with a lapped edge gate. Cooling lines have been placed following established guidelines for diameter/depth/pitch. By using proven methods for cooling line design, warp and cooling time are minimized. The mold is also fully instrumented with cavity pressure sensors at post gate and end of fill in conjunction with in-cavity temperature sensors.

4. Processing

So where does that 80% number come from? Let’s look at the data we collected in each of the following process segments:

1. Fill
2. Pack/hold
3. Cooling
4. Mold open/eject/mold close

For this experiment, we developed a robust Decoupled II process, resulting in the following process parameters:

1. Actual melt temperature –  456 °F
   Actual mold temperature – 121 °F with a flow rate of 3.0 gpm per cooling circuit
2. Fill time –  0.26 seconds with a transfer pressure of 8,356 PSIp
3. Pack/hold time – 8.0 seconds at 4,150 PSIp
4. Cooling time – 10.0 seconds
5. Overall cycle time – 21.43 seconds

If we add the process times together and divide by the overall cycle time, we reach a value of 0.852, or 85.2%.

In the graph below are the actual part temperatures measured with a surface probe at different time increments during process development. In our experiment, the tensile test bar cooled from 456°F to 203°F in 8.26 seconds (fill/pack/hold time). For the part to get below the HDT, it took an additional 8.47 seconds.

A mold closed time past 24.73 seconds is non-value add because the part temperature is no longer decreasing at a high rate.

Cooling is a complex subject with many variables. However, with better engineering from part designers, mold designers and processing we can achieve results close to theoretical.

Jeremy Williams is a consultant/trainer at RJG (Traverse City, Mich.) with more than 17 years of experience in the plastics industry serving the medical, automotive, furniture and appliance industries.

The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.