Material Selection for Mass Production Injection Molding

November 15, 2021

Cooling mass production injection molding is about waiting, but with the right part, mold and process design, the time important for cooling is easy to reduce.

If you’ve attended any of the training events, the figure of 80% was probably on your mind. This number refers to how much of the cycle time is needed for cooling the plastic part. It is for temperature that makes it rigid enough to withstand the ejection forces.

What are the areas that affect the cooling stage?

Through the following exercise, the following four areas that impact the cooling stage is good to review:

  • Mass production design
  • Moreover, selection of material
  • Mold design

Mass production injection molding design

The basis for the cycle time lies in the decisions by the mass production injection molding design engineer. 

The thicker the mass production to meet usability conditions, the longer the cycle time in mass production will be. By having a squared variable, this factor becomes the one with the greatest influence on the cooling time.

How material is selected?

Plastic is an insulator par excellence. In a melted or molten state, plastic transfers heat slightly better. By releasing heat, its insulation properties increase. Moreover, the properties of the material that were common in the equation are:

  • Melting temperature : Temperature at which there is the transition of the material from solid to liquid
  • Mold temperature: The temperature range to replicate the surface finish that machines in a cavity on the plastic mass production
  • Thermal Deflection / Distortion Temperature (HDT): Temperature at which the material deflects under load.

Typically, the ejection temperature of mass production injection molding in the equation uses the HDT or a temperature very close to below the HDT. The ASTM test for HDT very accurately represents which zone is hard. Similarly, it comes during ejection by the action of an ejector pin on only one side, while the opposite side is has no support.

What are the variables in the thermal diffusivity equation?

The variables in the thermal diffusivity equation include:

  • Thermal diffusivity: Rate at which a thermal disturbance (in an increase in temperature) is going to transmit through the substance.
  • Density: Moreover, the amount of substance per unit of volume (g/cm3 for plastics).
  • Specific heat: Calorie heat requires raising the temperature of a gram of a substance by one degree Celsius.

For this test, Toyolac 100 resin was common with a melt temperature in the range between 230 and 250 ° C, mass production injection molding temperature of 40 to 80 ° C, and HDT of 83 ° C. Density can usually in the technical sheet of the material, but for thermal conductivity or specific heat.

Moreover, it is better to contact the supplier directly or use the information from the data bank of simulation software. Based on the geometry of the mass production and the selection of the material, the estimated cooling time was 18.00 seconds, from the mass production injection simulation.

Is it possible to remove all the heat?

Given the energy important to melt the material, it is not practical to remove all the heat while the part is still in the mold. Only 40% of the energy must remove to achieve the necessary rigidity and thus expel mass production.

Generally, it is not important to choose a cooling timer for mass production injection molding that only meets the HDT, rather it is good to choose a maximum value as a safety factor.

Moreover, a rule of thumb is to add 20% to the cool-down timer to account for incoming material variation and to lighten machine performance changes. For mass productions with close tolerances, the factor of safety should increase.

Which mold design is suggested?

An eight cavity mold is common for this mass production, with an H feed channel pattern and overlapping edge mass production ion injection points. 

Further, the cooling lines are located in the cavity, core and block of the feed channel following the established guidelines for diameter/depth/pitch. Using proven refrigeration line design methods, warpage and cycle time will be less.

Method of mass production injection molding prosecution

So where does the 80% figure come from? Let’s look at the data that comes for mass production injection molding. The stages of the process were taken as:

  • Filling
  • Packing / post-pressure
  • Cooling
  • Mold opening / ejection / closing

What is a robust decoupled II process?

For this experiment, a robust Decoupled II process was developed, resulting in the following mass production injection molding process parameters:

  • Actual melt temperature:235 ° C.
  • Actual mold temperature: 4 ° C with a 3.0 GPM flow rate for the cooling loop.
  • Filling time: 26 seconds with a transfer pressure of 576 bar
  • Packing / post-pressure:0 seconds at 286 bar
  • Cooldown time: 0 seconds

If the process times add together and divide by the total cycle time, we reach a value of 0.85. This means that the proportion of the cycle that comes cooling the part is 85% to withstand the ejection forces.

Moreover, this graph also indicates that, at some point, leaving the part in the mold does not represent an added value. Based on the concepts of geometry, material, mold and processing, leaving the mass production injection molding closed for 24.73 seconds the mass production does not cool much more than what is important.

Conclusion mass production injection molding

The cooling of the part is always a function of the wall thickness and the material that the design engineer chooses. It is up to the mold designer engineer to locate the cooling channels in the proper zone to allow minimal cooling time. As a process engineer, the old adage to “fill as fast and consistently as possible with quality” still holds true.

For this geometry and material, much of the heat is removed during the filling and packing stages. However, to remove the last 3.2 ° C and reach a lower temperature than HDT, it takes more time than to fill and post-pressure the plastic mass production. Cooling is a waiting game, but with better engineering, the time important to cool the material can be less.


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