Injection Molding of Polyethylene
Thermoplastics have been one of the most significant materials in the industrialized world. These materials have had a pronounced impact on modern industry and modern living. Polyethylene is a very important type of thermoplastic. Plastics are polymers that are rigid at their service temperature. They have limited elasticity. They will continue to stretch beyond their yield point until breakage. This stretching is irreversible beyond the yield point. They are unlike rubbers which have very high flexibility at their service temperature. Plastics can be either thermoset or thermoplastic. Thermoset materials need a chemical reaction to go from a liquid state to a solid-state. These cannot be by heating because they do not melt. Rather they degrade when heated above their temperature resistance. The change from viscous liquid to a solid thermoset plastic is irreversible. At or above their glass transition temperature, thermoplastics soften. In this state, they are molded into different shapes. When cooled the thermoplastics return to their solid-state.
The formability of thermoplastics is desirable in injection molding. The versatility of the injection molding process complements the versatility of polymers. Polyethylene plastics are in particular quite versatile. They have very diverse chemistry that results in a wide range of properties. Such properties include the softening point and tensile stress. These properties depend on the chemistry of the polymers. Molecular weight and degree of branching are two important chemical properties. These can distinguish one type of polyethylene from the other.
Polyethylenes
Polyethylenes (PE) are of different types according to their chemistry. The most common types are High-density polyethylene (HDPE) and low-density polyethylene (LDPE). HDPE is plastic number 2 in the recycle number scale. LDPE is plastic number 4. They are both polyethylenes and have some properties in common. They also differ in some ways. The differences are equally important in their processing and application. Other types are linear low density and ultra-high molecular weight polyethylene. This article focuses on HDPE and LDPE as these are most common.
The main distinction between HDPE and LDPE lies in the chemical structure. Both plastics have the same basic monomer structure, ethylene (-C2H4-). LDPE is a more branched polymer while HDPE is a more linear polymer. The degree of branching varies in LDPE and it is possible to have some branching in HDPE. The following analogy explains the effect of the branching of the polyethylene. Imagine packing together straight rods. These can pack together close as there is a little hindrance between them. Now imagine these rods have protrusions on the sides. These then become more difficult to pack close together. The protrusions on the sides will cause hindrances. Thus LDPE has lower density due to more branching. HDPE has a higher density due to less branching which allows closer packing. These differences in their chemistry manifest in their physical properties. At service temperature, HDPE is generally more rigid while LDPE is softer.
The differences in these properties define their applications and processing methods. Examples of LDPE applications are in shopping bags, shrink films, and squeeze bottles. One of its desirable properties is that it can form thin films and flexible sheets. This makes it useful in applications that need moderate moisture barrier and strength. LDPE is also a low-cost material compared to other options such as paper and other polymers. This contributes to its widespread use as shopping bags and packaging film.
Examples of applications of HDPE are bottle caps in water bottles and beverages. Its low water absorbance and chemical resistance make it applicable in such products. It is also used in other products such as toys, toolboxes, and garbage bins. As a low-cost material that can form diverse shapes, it is suitable for such applications. Also, its higher rigidity compared to LDPE makes it better suited for such uses. HDPE is also used in pipes, fuel tanks, and cables. These applications are also attributed to its water and chemical resistance. Its high flexibility makes it useful in applications such as ropes and fishing nets. In such applications, it gives good flexibility and tensile strength required for manipulation. LDPE and HDPE are FDA approved as packaging for many food products.
Injection Molding Process
This section of the article gives a brief overview of injection molding. This is to ensure a prior basic understanding of the process. The injection molding process begins with the plastics in the pellet form. These pellets are from the polymerization of ethylene, converted to pellets using extrusion. The pellets pass through the hopper into a reciprocating screw within a barrel. The plastic pellets melt as they move through the gap between the screw and the heated barrel. The uniform melt formed goes into the injection chamber. The screw moves backward as he rotates hence the term reciprocating. Once at the required shot size the backward motion of the screw stops at a limit switch. This then triggers a hydraulic cylinder which pushes the screw forward. This forward motion of the screw forces the molten plastic into the mold cavity. A check valve prevents the backward flow of the melt. The molds shut and the product cools by running cooling fluid through the cooling lines. While the product cools the screw moves backward as another cycle begins. After the set cooling time, the mold opens and the cooled product comes out of the mold. Ejector pins aid the removal of the product from the mold. The manufacturing of many products is by injection molding. Examples are phone cases, shoes, buckets, furniture, toys, medical tools, and vehicle parts.
Polyethylene in Injection Molding
A material might be suitable for an application but not for a process to form the product. The process and the material must be a good match. Manufacturers consider the suitability of the material for the product and the process. This applies when discussing the injection molding of polyethylene. The focus is on the properties which are relevant to their injection molding.
For a material to suit injection molding it must meet certain requirements. One of such is that the material must withstand the high shear in injection molding. The high shear occurs during the mixing and melting of the plastics in the injection chamber. Hear the screw rotates within a heated barrel. As the screw rotates the plastic moves between the screw and the walls of the heated barrel. This shearing also referred to as compounding ensures even heat distribution. This prevents parts of the plastics from getting hotter parts leading to an uneven melt. If used, colors and additives are well mixed to form an even product.
Shearing also occurs when the mold is being filled with the melt. The process involves forcing the melt through the sprue, runner, and past the gate into the mold. This occurs at high pressure and velocity. This is necessary to ensure the mold fills to achieve the product design precision. So the plastic must withstand the different stages of shearing.
Pseudoplastic behavior is most suited for injection molding. Such materials will show less resistance to flow as the shear rate increases. Most molten polymers show pseudoplastic behavior. This allows for better processing in injection molding. When using additives such as pigments and fillers, these alter the behavior. The flow tends more toward dilatant behavior. Minimizing the concentration of the additives, including plasticizers can address this.
Flow is another very important factor in injection molding. Any flow problems can result in disruption of production or even damage to the equipment. In some cases, these damages and disruptions result in safety issues. When heated above their glass transition temperature thermoplastics flow. Thermoplastics have different flow properties. There are different ways to measure these. These include the melt flow index, melt viscosity, and the melt flow rate. The viscosity of a polymer melt relates to temperature. in the William Landel Flory Equation. The equation relates viscosity at the glass transition temperature with viscosity. This allows online control of viscosity through temperature. This is important because the temperature is easier to measure.
The injection molding machine has temperature indicators at different points. This is for different reasons. One reason is to ensure flow and another is to prevent overheating. It is also important that the plastic solidifies to form the product. Thus the plastic used must flow well enough to allow effective injection and filling of the mold. It must also cool fast enough for product formation. LDPE softens at about 50oC and HDPE softens at about 80oC. Hardening occurs within 20oC below these temperatures. So LDPE and HDPE have good heating and cooling profiles for injection molding.
Thermoplastics differ in the temperature at which they melt and also in how they melt. Unlike other materials such as metals and water. Polymers do not have a gaseous state. Beyond the liquid state, they degrade rather than transition to a gaseous state. The degradation products may pose health hazards. Proper temperature control prevents this.
Polyethylenes have lower melting point compared to other plastics. Polycarbonates for example soften at over 195oC. This means they need less energy input to melt. LDPE softens at around 50oC while HDPE softens at around 80oC. This low softening temperature also means they are only used at low temperatures. Now for how they melt. Some plastics absorb a lot of heat and have a broad softening temperature. This means they can keep their soft rubbery state over a wide temperature range. Others transition from a rigid state to a soft melt state at a very short temperature range. Once at their melting temperature, there is little room to go above or below that temperature. Heating and cooling for such plastics must be well controlled.
The melt properties of polyethylene make them suitable for injection molding. They melt at low temperatures and flow well in their molten state. Polyethylenes also tend to withstand repeated heating cycles better than other plastics. This makes them suitable for recycling. All polymers degrade when put through repeated heating and cooling. This is due to altered chain conformations and shortening of polymer chain lengths. Recently the company ESE world reported that HDPE can withstand up to 10 heating cycles. This thermal stability of HDPE is likely attributed to its simple chemical structure. HDPE is unlike polymers with pendant groups and more complex structures. There is less conformation change likely to occur in HDPE when heated. It is a linear repeating unit of ethylene. This simple structure could explain why it withstands many heating cycles.
Polyethylenes have higher shrinkage compared to other plastics. This has both its pros and cons. Shrinkage eases the release of a product from the mold cavity. The shrinking upon cooling will mean space between the cavity walls and the product. This way the product is not fitted to the mold hence easier ejection. This is desirable. As a con, shrinkage also affects the product conforming to the intended design. The product loses design precision if this is not considered in the design of the mold. Manufacturers and mold designers account for shrinkage in the injection molding of polyethylene.
Elastomers such as polyisoprenes have superior mechanical properties. These are desirable in many products. In the conventional form, elastomers are thermoset. The injection molding of thermosets is more challenging than that of thermosets. Thermoplastic elastomers combine the recyclability of HDPE with the mechanical properties of elastomers. These are plastics suited to injection molding with the mechanical properties of elastomers. HDPE based thermoplastic elastomers can be injection molded.
Conclusion
In conclusion, Polyethylenes are a good choice for injection molding. While LDPE is more suited to film and sheets, HDPE is well suited to injection molded products. Their thermomechanical properties, chemistry, and rheology make them suitable for injection molding. Recent developments present the potential for HDPE which can withstand many molding cycles.
