If it’s the same plastic, why the two different densities?
The first thing we need to understand: What is density? Density can be thought of as how many grams of a plastic will fit into a 1cm x 1cm x 1cm cube as illustrated in Figure 1. When working with plastics, there are two types of densities that need to be considered: solid density and melt density. But why the two different densities? After all, it’s the same plastic we are talking about, right? True, but we need to understand what happens to the molecular chains when we apply heat to plastic. The chains expand and separate, thus increasing the volume that will be occupied.
To put this into example, think of a five sided container whose dimensions are unaffected by temperature changes. The container is open at the top with inside dimensions as show in Figure 1. We then machine a solid cube of a given plastic to have the same outside dimensions at room temperature as the inside dimensions of the container (Figure 2). We place the solid 1cm³ plastic cube on a scale and find out that it now weighs 1.054 grams. We then place the solid plastic cube inside of the container such that the plastic cube occupies 100% of the volume of the inside of the container. Now we apply heat to the plastic cube until it is molten. What will happen to the plastic? The molecular chains of the plastic will expand and separate. Given a container that does not expand nor break, some of the plastic will flow over the container thus effectively reducing the weight of the plastic inside the container. When the plastic that expanded beyond the container is removed we now find that the remaining cube of heated plastic inside the container weighs only 0.933 grams. Therefore, for the same 1cm³ volume, the melted plastic weighs less than the solid plastic (0.933 g/cm³ vs. 1.054 g/cm³).
The material in the example above was an ABS, which is an amorphous material. The percent difference between the two densities for this material is approximately 11.5% when the plastic is raised from room temperature to 260°C. For semi-crystalline materials, a similar change in temperature can create a difference that can easily be up to 20-30%.Finding the melt density data for your materials can be problematic since this data is rarely, if ever, published on the material data sheet. You will need to turn to the material database websites in hopes of finding it, look at the material specifications inside your mold filling simulation material database, or you can calculate it using a melt flow index machine or your molding machine. As a general guideline, you can estimate the melt density for an amorphous material by using 90% of the solid density, and for a semi-crystalline material by using 80% of the solid density. Those approximations will get you in the neighborhood, but actual values will vary by specific material types, grades, temperature, and pressure applied to the plastic. The relationship between pressure, volume, and temperature for a given plastic is described within PVT graphs.
So why discuss this topic? It is important because the fundamental principle of solid density vs. melt density needs to be understood so that you can then understand two other very important topics related to plastics:
- Making sure your molding machine has enough capacity to mold your part (Figure 3)
- Material shrinkage and related warp (Figure 4)
These topics, along with PVT relationships, will be covered in more detail in future Tech Tips from Beaumont, so be sure to watch for them in upcoming newsletters. For more tech tips, visit beaumontinc.com.
