There are several factors that play a major role in the efficiency and part quality of any injection molding process, and cooling time is at or near the top of any such list. Cooling plays a huge part in the overall cycle time of a given mold, taking up more individual time than any other part of the process, and knowing how it works and the proper timing here is often what separates great molders from lower-quality ones.
At EnviroTech Custom Injection Molders, we’re here to help with a wide range of custom injection molding services and products, plus services like finishing, plastic bonding and more. The cooling process is one we take very seriously for any of our products — why is it important, how is it estimated during the mold process, and what are the risks of insufficient cooling? Here’s a primer on everything you need to know.
Why Cooling Time Matters During Injection Molding
When it comes to the overall injection molding cycle, cooling takes up 80-85% of this entire cycle. To help illustrate this, let’s go through the entire molding cycle and highlight the role cooling plays:
- Resin heating: Base resin is heated to a certain temperature, usually between 125 and 150 degrees Fahrenheit, which liquefies it.
- Pouring/Injection Phase: The heated resin is inserted into the mold and injected under pressure.
- Packing and holding: As some material that’s already been injected begins to cool and shrink, more material is injected to completely fill the cavity and hold the material, preventing backflow.
- Cooling: At this point, once all packing and holding is complete, the cooling stage has begun officially. The liquefied resin begins to cool and shrink, which often takes a significant period of time.
- Hold: After cooling, the empty cavity is held for a period of time to allow everything inside to cool and set. This step typically lasts between 5-10 minutes.
- Opening or ejection: The mold opens, usually at the end of a press cycle, allowing whatever product has been created inside to be ejected from the mold.
- Quality assurance: The mold is shut and inspected to ensure it has been created properly, and any issues are noted for the future.
Now, it’s vital for cooling time to be found using several important variables — which we’ll go over in our next section.
Estimating Cooling Time for Injection Molding
In fact, there’s a specific equation to determine the required cooling time for a given plastic part, and this equation utilizes both the part thickness and a number called “effective thermal diffusivity.” This latter number refers to a measure of how heat travels into and out of a given material, and it can be calculated by dividing the thermal conductivity of a part by the product of its density multiplied by the specific heat required to raise its temperature by one degree (all this information can be found on the spec sheet for the material, or through contact with the manufacturer, both of which we utilize regularly to ensure ideal cooling).
And once you know the thermal diffusivity, it can be plugged into the rest of the equation, which is a bit more complex than we’d prefer for this blog — but if you have specific questions about how it’s applied, our experts will be happy to tell you more.
Also important in the cooling time for any injection mold is the actual design of the mold in question. Cooling channels, for instance, are one of the biggest factors here. These are the passages on either side of a given mold cavity, which are designed to allow air in from outside and circulate it through the part, directly adjacent to it. They’re included in some molds, but not in all of them — and those that have these channels will typically require shorter cooling times, which in turn will allow for a speedier mold process that can create a greater volume of quality parts.
Problems With Insufficient Mold Cooling
There are a few issues that may arise with improper cooling for any injection mold, and the most common risk is defective or rejected parts. In the case of a given mold that has inadequate or insufficient cooling, an incomplete cycle will likely occur where the material in the cavity fails to set properly before it is ejected from the mold.
This can result in poor quality parts, which won’t last as long and are more likely to break when put under any pressure. These problems can be solved by utilizing a cooling test, which will ensure that any of the parts being produced are actually created to the highest standards possible.
It’s important to realize here that proper cooling temperature is not room temperature, nor is it a point where the part has cooled to the human touch — rather, proper cooling temperature refers to the point at which the part will retain its shape and can be safely ejected from the mold. Any cooling after this point is actually doing very little for the mold, and will simply lengthen the cycle time. For this reason, while ensuring adequate cooling is maintained for all parts, it’s just as important to ensure cooling isn’t overdone — as this will add no major benefit while also adding to production time, which results in lost money.
For more on the cooling process during any plastic injection molding service, or to learn about our bulk molding, slab stock molding or any of our other services, speak to the staff at EnviroTech Custom Injection Molders today.