The amount of time it takes for plastic to travel from the feed throat of the molding machine to the mold cavity is known as the residence time. For custom molders, knowing this value is critical to part quality and consistency. I want to cover why knowing residence time is important and how to calculate it.
One “shot” of plastic is the amount of plastic used to fill a mold. This may or may not include a runner system, depending on if the mold is a hot or cold runner. Using the shot size to match a mold to a molding machine is often more important the physical limitations of tonnage and mold size. However, some molds you may have to run in a larger press simply because the mold would not fit into a smaller one. Depending on the molding material, having too large or too small of a total shot capacity compared to the shot size of the mold can cause several molding issues.
Usually, we want the shot size to account for about 20-65% of the barrel capacity. Having too small of a shot size (less than 20%) causes longer residence times. Meaning the material is being heated/processed in the barrel for a long period of time. This can cause material degradation which leads to quality issues and poor process control. Having too large of a shot size (more than 65%) causes short residence times. The material is being processed through the barrel quickly and may have melt quality issues and poor process control.
For custom molders, working with this can be difficult. We are running several different parts in a particular molding machine, leading to varying residence times. In more ideal circumstances, we would run one job in one press with the perfect barrel size and screw design, but that’s not the nature of custom molding. We try to have an array of machines and shot capacities so we can fit a mold into a machine based on shot size. So how do we figure out residence time for our molding machines?
We could divide the shot capacity of the machine by the shot size of the mold. This yields the number of shots in the barrel. Multiplying that by the cycle time gives residence time but an inaccurate one. The example below is from one of our 250 Ton Toyo machines with a mold shot capacity of 4 ounces and a cycle time of 20 seconds using the simple calculation.
(Shot Capacity) / (Shot size of the mold) = Number of shots in the barrel
(Number of shots in the barrel) X (Cycle Time) = Residence Time
(Shot size of the mold) / (Shot Capacity) = % of Barrel Usage
17.161 / 4 = 4.3
4.3 X 20 seconds = 86 Seconds
4 / 17.161 = 23%
The problem with this method is the shot capacities of the barrel (by weight) can change depending on the material. This is because different polymers vary in density, and their densities also change as the material reaches it’s melting point. So as plastic goes from room temperature in the hopper to being molten in the barrel, it’s density is changing. As it turns out, there’s only one additional step to make the calculation significantly more accurate.
Typically the capacity of a barrel will be given based on the density of general purpose polystyrene (GPPS). So we need to offset either the shot size or the barrel capacity by comparing the density of the material being processed to GPPS. For this calculation, I will be offsetting the shot size. Note that I am using the melt density of the plastics here. If you were looking at a datasheet for given plastic, it would show the room temperature density of the plastic. It may take some searching to find accurate melt densities.
Let’s take the same scenario from above and assume that we are processing High Density Polyethylene (HDPE). First, let’s calculate the shot size of the part (4 oz.) as if it were GPPS. We will divide the melt density of GPPS by that of HDPE.
(Density of GPPS) / (Density of HDPE)
(.945 G/cm³) / (.764 G/cm³) = 1.237
This offset means that if we were to have the same weight of each material, HDPE would take up 1.237 times the volume of GPPS. Once we have the offset, we can modify the original part weight by that ratio.
4 X 1.237 = 4.96
Since HDPE takes up more volume per unit of weight, it makes sense then that offsetting to a higher density material would make the theoretical part weight increase. Using the same equations from above gives the following residence time and barrel usage.
17.161 / 4.96 = 3.46
2.31 X 20 seconds = 46.2 seconds
4.96 / 17.161 = 29%
Note that you can calculate this in a few different ways, like offsetting the barrel capacity instead of part weight or working only in volume and not part weight. Either way, the results should be the same in time and barrel capacity usage. Just keep in mind that less dense materials take up more volume of the barrel. This will decrease residence time and increase barrel usage.
You can see the difference in the values by offsetting the shot weight by the material being processed. The difference between the calculations may not seem like much but could be the difference between making good and bad parts or fighting to get a good molding process. We can also take this a bit further by considering some other factors.
If we wanted to be more precise, we could even detail out the different zones of the screw. Because the plastic changes density as it transitions to its melt temperature, we can break the barrel down by zones and modify the densities. We can also look at different types of screws and how they handle the plastic.
These more detailed calculations are important when purchasing a new barrel and screw that is job-specific. For most applications, understanding the calculations from above will be a good starting point.
As you can see, the residence time may have a large influence on part quality and process consistency. Allowing too high or too low residence time can cause several issues for the processor as well as the molded part. Fitting a mold to a press by checking the residence time is the critical first step. Using these calculations makes it quick and easy to do so.
