Polymers are a very diverse group of compounds. They are also ubiquitous. They exist in nature and are also synthesized by man. Polymers exist in food, drugs, textiles, electronics, and medicine. Examples are proteins, cellulose, polyethylene, nylon, and polystyrene. All polymers have a common characteristic of a repeating unit within their structure. Let’s take for example polyethylene. Plastic used in products like shopping bags and bottle caps. It is a polymer made up of repeating units of ethylene. Ethylene reacts with thousands of other ethylene molecules to form one polyethylene molecule. These ethylene molecules that form the repeating units are the monomers. Another example is polystyrene, the polymer this article focuses on. Polystyrene has styrene as its monomer.
Every polymer distinguishes itself from other polymers by its chemical structure. The chemical structure of the monomer forms the main identity of the polymer. Interaction within and with other polymer chains form the polymer’s identity. Polystyrene for example is a vinyl polymer with a benzene ring as a pendant group. The pendant group refers to the group attached to the main chain. It does not form part of the backbone of the polymer. Think of it as the pendant of a chain. It attaches to the chain but it is not part of the rings that link the chain. Such features are very important determinants of how the polymer behaves. This includes its mechanical, thermal, electrical, and even optical properties.
This is why manufacturers prefer some plastics for certain applications over others. Polystyrene is one of the 6 most common plastics today. It takes plastic number 6 on the recycle number scale. You would find the recycle number written within the chasing arrows. Look for this number of products around you and identify which use polystyrene. Some you might notice are party cups, plastic spoons, CDs, and appliance casings. The properties of polystyrene make it suitable for these applications.
Plastic Properties for Injection Molding
In injection molding, thermal and mechanical properties are very important. This is because the process involves heating and deforming the plastic. Once the desired shape forms, the plastic is then cooled and solidified. After cooling, the plastic retains this new shape. This form of heating and molding applies only to thermoplastics.
The injection molding process can produce a variety of products. From small parts made in large quantities to large parts of equipment or vehicles. Thermoplastic, thermosets, metals, and glass are injection molded. This article focuses on the injection molding of thermoplastics. The particular focus is on polystyrene. Thermoplastics used in injection molding must melt at a reasonable temperature. This reduces the energy needed to soften the material into a moldable state. The plastic must also be able to withstand high shear. Injection molding exerts high shear to mix and melt the plastic. The plastic must also have a good flow in its molten state to allow it to fill into the mold. The plastic must maintain a uniform consistency in the molten state. This ensures that the product formed is uniform and to required structural detail.
Methods for measuring the properties of plastics are well established in the industry. The properties relevant to injection molding are; melt temperature, melt flow index. Other properties such as the degradation temperature are also important for quality. The measurement of some of these properties is offline while others are online. Measurement of melt viscosity for example is offline. This property is then monitored or controlled online using the related property, temperature. Temperature sensors are placed at different points on the injection molding machine. Some manufacturers work with the same plastic using the same settings. For some, the parameters change for different types of plastics. For example, working with high-density polyethylene and then switching to polystyrene. This would need changes to settings. The temperature at which both melts is different. The manner in which the melts flow is also different. Differential scanning calorimetry or thermogravimetric analysis determines the thermal properties. Also known as DSC and TGA. These give thermal properties like the melting point and temperature profiles.
The screw-in injection molding pretty much drives the whole process. You can think of the screw is like a rolling pin kneading a dough. Except the pin is within a heated barrel. Also rather than rolling back and forth on the dough it rotates in the middle of the barrel. Other articles in this blog cover injection molding. One titled Introduction to Injection molding is available. Using the dough analogy, let’s talk about the power of the screw and plastic melting and mixing. Plastics have a gradual transition temperature. Between their solid-state and liquid state, they have a soft rubbery state. This makes it possible to process them below the actual melting point where they turn to liquid. This is where there is a balance between the heat applied to the barrel and the torque applied to the screw. The softer the melt, the less work the screw needs to do. In order words, This requires less force for the screw to make one rotation if the plastic is less viscous. The type of plastic used determines the mixing and melting regime used. That is whether to apply more heat or more power to the screw. The viscosity of the plastic at different temperatures determines this. Some plastics have a wide softening point while others have a sharp softening point. With wider softening temperature, the plastic molding can occur at different temperatures. Plastics with a sharp melting point leave little room for temperature variation.
Shrinkage is another factor to consider when injection molding plastics. Plastics tend to reduce in volume when they change phases. This can be between 0.1 and 3%. This might seem like a low value but it becomes important when considering precision. For example, you might have noticed some plastic products with the label as part of the design. For example, the company logo or the recycle number. These are sometimes part of the mold design. Such detailed needs are in the millimeter range. For a product measuring centimeters in dimensions, even 0.1% is relevant shrinkage. If a plastic shrinks so much the inscription is not transferred to the product. The part could flatten out as the product shrinks. For products like screws, this is important to the functioning of the product. A screw wouldn’t work well if the groves aren’t quite right.
The mechanical properties of the plastic are also important in injection molding. This might not seem obvious since mechanical property affects functioning. It becomes important when considering the removal of the product from the mold. Depending on the design of the mold, the removal of the product from the mold might exert some force on the product. If the product is for example to frail or brittle then it might break in the process. So the design of the ejector pins or manual removal must consider this.
Thus far we’ve discussed the properties of plastics relevant to injection molding. This helps us understand the properties we need to consider for specific plastics. The focus of this article is polystyrene. The next section looks at polystyrene injection molding. This covers how the properties affect the injection molding process. We look at the pros and cons of polystyrene in injection molding
Injection molding of polystyrene
Polystyrene is the top choice for products such as cases, disposable spoons, and cups. Injection molding is the preferred method for making many of these products. So polystyrene must be well adapted to the injection molding process. Here we will look at some of the properties of polystyrene that make it great for injection molding. We will also look at those properties that make it not so great for the process. The picture below gives an example of some injection-molded polystyrene products.
Examples of injection-molded polystyrene products
Thermoplastics are generally preferred over thermosets for injection molding. The injection molding of thermoset plastics is quite cumbersome. The process has the challenge of preventing cross-linking within the barrel. So as thermoplastic polystyrene is injection moldable. Polystyrene has a high melting point compared to the other common plastics. Although its melting point is around 270oC, it begins to soften at around 205oC. Injection molding of polystyrene occurs at or above its softening temperature. This temperature can get the plastic soft enough to melt in the barrel. This temperature is high compared to plastics like polyethylene. This means the injection molding of polystyrene requires higher energy input.
A lot of packaging foams are polystyrene. These are common in the packaging of fragile products and appliances. You would notice the white foam that comes with your fridge, laptop, or a new tablet. A variation of injection molding is structural foam molding. This process produces foams using plastics. Polystyrene works for this because it can maintain good rigidity in the porous form. If the material is too flexible, the foam would be too soft. Such soft foam would not have the desired cushioning effect to protect a product from damage. Polystyrene foam is rigid enough to withstand impact yet soft enough to absorb force.
Polystyrene is quite brittle. Plastics need enough mechanical strength and flexibility for effective ejection from the mold. For thick structures, mold release should not be much of a problem. Thinner structures are more prone to breakage. Designing the ejector pins in such a way that the force applied is even, prevents this. This way the product does not experience too much flexural stress.
Polystyrene has low shrinkage compared to plastics like polyethylene. The shrinkage of polystyrene is around 0.5%. The shrinkage % can be as high as 3% in other plastics so this is quite low. This is an advantage when considering product design. The low shrinkage means that polystyrene will replicate the design details better. Shrinkage makes release from the mold easier. This need not be high shrinkage, a little gap between mold and product is all that’s needed.
The transition temperature of polystyrene is sharp compared to other common plastics. Polystyrene has a benzene ring as a pendant group. This bulky pendant group limits the mobility of the polymer chain. A less mobile chain means more energy needed to make it flow. So polystyrene needs to absorb energy close to its melting point to flow. Polystyrene melt flow index is between 12.0 to 16.0 g/10min (200oC/5kg). This is low compared to that of polyethylene which is 180g/10min at 145oC. It tends to go from a hard solid-state to a melt state within a shorter temperature range. This is unlike plastics with more mobile structures. So the temperature of plastics must rise fast enough to flow or the gap between the screw and barrel is wider. There are different measures taken to handle plastics with different melt behaviors. One way is using screw designs that only compress the melt towards the end of the barrel.
Conclusion
Polystyrene is well suited for the injection molding process. It can be injection molded into either solid rigid products or foam products. It’s production demands higher energy input as it has a higher melting point. It compensates for this with its low shrinkage and this allows for good design detail.
