Density is a basic property to understand and compare materials. The measurement of how much mass fits into a unit of volume is called density. This property is an important part of materials in their physical changes, their usage, and other related fields. In the manufacturing sector, processes like injection molding, CNC machining, and 3D printing, the need a suitable density for a material can have a ripple effect on product performance, the entire process, and production costs.
If you are dealing with metals or plastics, familiarizing yourself with the concept of different densities in different substances will help you choose the right material, predict the behavior of materials under temperature variations, and make sure that parts are functional and structurally stable.
What is Density?
Density is a physical property that explains how much mass is crammed into a set volume of a substance. It is expressed by the following formula:
Density (ρ) = Mass (m) ÷ Volume (v)
This formula helps us calculate density and tells us how dense a material is. For instance, if two materials have the same volume but one is heavier, it is denser.
In production, density gives a reliable way to tell between substances that seem similar but act quite differently. For example, stainless steel and aluminum might seem similar at first sight, but aluminum is much less dense, something that has direct bearing on its weight, strength, and application.
Density is also a useful quality-control indicator that allows machinists and engineers to check whether a material satisfies the specifications for a product or part. It is very important in uses where weight is a key factor, like aerospace, automotive, and consumer electronics.
Units of Density
Density may be measured in a number of units, depending on the industry, the region, or the material. Among the most commonly used units are:
Kilograms per cubic meter (kg/m³): the SI (International System) unit
Grams per cubic centimeter (g/cm³): used in plastics and laboratory environments
Pounds per cubic foot (lb/ft³): frequently used in U.S. industries, particularly construction and aerospace
For comparison: 1 g/cm³ = 1000 kg/m³ = 62.43 lb/ft³
Unit selection is not just a formality. Using the incorrect or other units like cubic meter instead of cubic centimetre in engineering calculations has the potential to result in expensive design flaws or even component failure. That is why accuracy in conversion matters when dealing with global teams, suppliers, or multi-unit systems.
Factors Influencing Density of Different Materials
Density may appear like an unchanging value, but actually, it will change according to a number of material and environmental parameters. All these changes are important in manufacturing, especially when dealing with plastics or composites. Let’s look at the most important ones:
Crystallinity
Crystallinity in polymers is a term used to describe the orderliness of molecular chain arrangement. Highly crystalline materials, such as HDPE (High-Density Polyethylene), show molecules closely packed, leading to increased density.
Amorphous plastics with different elements like polystyrene (PS) or PMMA (acrylic) show a more disorganized molecular structure and lower density overall.
Crystallinity can also influence stiffness, heat resistance, and shrinkage and is an important processing consideration.
Temperature
The majority of materials expand on heating and contract on cooling. As temperature increases, a material’s volume tends to increase at a faster rate than its mass, reducing the density.
This sensitivity to temperature is especially significant under:
Injection molding (where part size is influenced by cooling rate)
Thermal testing
Dimensional stability tests
Additives and Fillers
Plastics usually contain additives such as glass fibers, carbon black, talc, or flame retardants. These materials usually have a greater density than the base polymer, increasing the compound’s overall density.
On the other hand, blowing agents or hollow glass spheres decrease density for lightweight applications such as automotive interior panels or packaging foams. Their presence and content specifically influence not just the density but also the mechanical properties of the final product.
How to Determine the Density of Plastics?
Density of plastics is a basic property that indicates how tightly the polymer chains are packed inside the material. The packing can be very different among various plastics, mainly due to differences in the molecular structure and the degree of crystallinity.
In the crystalline regions, the molecules are packed in a highly ordered, dense materials, resulting in higher density, while in the amorphous regions, the molecular structure is less organized, providing lower density. Also, additives such as fillers, plasticizers, or reinforcements can affect the overall density either by decreasing or increasing the mass per unit volume.
Common Methods to Measure Plastic Density
Plastic density measurement plays a big role in material selection, product design, and quality control. The main methods widely used are:
Water Displacement Method:
This is one of the simplest and most convenient methods, where a plastic sample is immersed in water, and the volume of water displaced is recorded. Density can be identified by taking the weight of the sample and dividing it by this volume. This method is suitable for solids with irregular shapes.
Pycnometer Method:
A pycnometer is an instrument that measures with high accuracy the constant volume of a solid or liquid from fluid displacement in a sealed vessel. This method provides more precision and is commonly used in laboratory settings for plastics with complicated shapes.
Calculation Method:
If the exact dimensions of the sample are known (for example, a molded shape of the same form), its volume can be calculated geometrically. The density is found by dividing the mass of the sample by this volume. This method requires using precise measuring instruments such as calipers and balances.
Factors Affecting the Accuracy of Density Measurements
There are many factors that can change the accuracy of density measurements. One is temperature, as plastics change in size when heated or cooled, so their volume and density change. To prevent mistakes, the measurements are usually made at standard conditions, often 23°C (73°F). How pure the sample is also has an influence on it. Dirt or air bubbles in the plastic will change the numbers. The way the machine is set and the skills of the person who is doing the work also have an effect on the results.
Test Methods to Determine Plastics Density
Density, by its nature, is mass divided by volume, which is written as:
Density (ρ) = m ÷ V
For plastics, accurate measurement of density is important since minor differences can mean varying grades or additives in the material and impact performance directly.
Direct Calculation Method
To find the plastic density, the simplest way is to measure the volume and mass of a sample directly. Mass is measured on a precision balance. Volume measurement depends on the shape:
For regularly shaped objects (cubes, cylinders), volume is found by multiplying the dimensions (length × width × height).
For irregularly shaped objects, water displacement is used from Archimedes’ principle.
Water displacement method implies placing the plastic sample in a liquid (most likely water) and measuring the volume of the liquid that was displaced, which is equal to the volume of the sample. The formula that is used is:
Volume (V) = (Mass in air (mₐᵢᵣ) minus Mass in water (m₍𝓌ₐₜₑᵣ₎)) divided by Density of water (ρ₍𝓌ₐₜₑᵣ₎)
Where:
mair = mass of sample in air
mwater = apparent mass of sample submerged in water
ρwater= density of water (approximately 1 g/cm³ at 20°C)
This method requires accurate temperature control as the density of water changes with temperature, and this affects the accuracy of the measurement.
Density Gradient Column Method
The method of using a density column to identify the type of plastic or to detect contamination is quite common in quality control. Different liquids of known densities are layered in a column. The sample of plastic will find its place where it is neither sinking nor floating i.e. at the point where the density of the liquid is equal to that of the plastic, effectively “floating” at that position.
This technique is non-destructive and quick, suitable for comparative analysis of pellets or granules.
Pycnometer Method
Pycnometers are sensitive instruments that determine the volume of a solid or liquid by displacement. For solid samples, the pycnometer is filled with a known liquid (usually water or alcohol) and the sample is deposited in it. The differences in weight before and after the addition of the sample are determined and volume is calculated to allow the determination of density.
This technique provides high accuracy, commonly applied in laboratories where precise density values are important.
Other Methods: Gas Pycnometry and Helium Displacement
Advanced techniques like gas pycnometry use gas (usually helium) displacement rather than liquid to assess volume. The sample is inserted into a sealed chamber, and the gas volume displaced by the sample is determined. This technique is beneficial for porous or extremely minute samples where liquid displacement is not suitable.
Considerations for Accurate Density Measurement
emperature Control: Density is temperature dependent. Measurements are always to be taken at a controlled temperature (at 20°C or 23°C).
Sample Preparation: Samples should be dry, free of air bubbles or voids, and clean to prevent measurement errors.
Repeatability: Several measurements are advisable for accuracy and to cover probable variations.
Density of Common Plastics
Plastics come in a variety of densities, corresponding to their varied structures and uses. The density of ordinary plastics is useful for manufacturers and designers when choosing the appropriate material according to weight, strength, and performance requirements.
Why Density Matters in Plastic Material Selection
Density is more than a mere datasheet number. It has a direct role in how a plastic acts, how it is manufactured, and where it is used. Knowing this property is what allows manufacturers to make the most of everything from cost to performance.
Weight vs. Strength
In applications where reducing weight is important, such as in automotive, aerospace, and consumer electronics applications, lower-density plastics are used. For instance, polypropylene (PP) and LDPE offer lower weight, which can improve the fuel efficiency of a vehicle and portability in products.
However, it also may imply lower stiffness or strength. Engineers have to find the best balance between reducing weight and mechanical performance requirements. That is why materials such as ABS or polycarbonate are selected for parts that will sustain repeated impact and a series of falls even if they are heavier.
Cost Efficiency
Density also has an influence on the cost in two ways:
Material cost per volume: Heavier plastics might be more expensive to transport and to process.
Yield per kilogram: Lower-density plastics provide more volume per kilogram, meaning manufacturers can produce more parts from the same amount of material, improving cost-efficiency.
For example, a kilogram of PP will yield more parts than a kilogram of PVC, due to its lower density.
Processing and Tooling
The density of a plastic can influence how it flows in molds, how it cools, and how long it takes to solidify. Lower-density materials generally fill molds faster and cool more quickly, potentially improving production speed. However, this also depends on other properties like viscosity and thermal conductivity.
Conclusion
Density is one of the major properties that directly influences the selection, processing, and application of plastics in various industries. The density of plastics changes the materials’ properties from weight and strength to cost-efficiency and even performance.
Through the understanding of density and how to gauge it correctly, producers and technical staff can be more efficient in choosing the correct material and also, save money through the elimination of design change or unplanned production.