The plastic industry has grown over the years into a rather diverse and versatile industry. Plastics are one of the most ubiquitous groups of materials ever developed by present civilization.
From construction to biomedicals to several other industries, plastics find use in almost every aspect of our daily existence. As such the industry has developed several methods for processing plastics into countless product designs and specifications. Injection molding and thermoforming are both effective means of forming plastics into a variety of products. Some products can be manufactured using either injection molding or thermoforming. In some cases, the outcomes are even identical. Thus manufacturers and product designers often have to choose between these two. Several factors come to play when determining which method to go for when it comes down to these two. Here we see how they compare.
We begin by making a distinction between the two processes.
The Injection Molding Process
Injection molding is a plastic molding technique that involves the melting and mixing of plastics. The plastic when heated forms a viscous melt which makes it more processible. This is then injected into molds that are fabricated into the desired product form. The final product takes the shape of the mold cavity. It retains this shape after cooling. The process occurs in an injection molding machine. The machine comprises a hopper into which solid plastic pellets get fed in. These then pass into a barrel with a reciprocating screw. The barrel gets heated by heating plates. The plastic pellets get sheared by the screw rotating within the barrel. This converts them from a solid-state to a melt state. Heat is generated as the plastic gets sheared between the walls of the barrel and the flights of the screw. The melt gets held within the channel and is pushed into the melt chamber as the screw rotates. The amount of melt in the melt chamber is set by the limit switch. This determines how far back the screw goes. The back pressure controls how fast the screw moves backward as it rotates. Once the required shot size is reached, the screw is propelled forward. This action, aided by a hydraulic cylinder provides the injection pressure that pushes the melt into the mold. The melt fills the mold via the sprue, runners, and gates. The melt fills the mold and is held under pressure as the mold cools. The cooling of the mold occurs through cooling channels. Once the product cools and hardens the mold is opened and the part gets ejected. The ejection of the product is aided by ejector pins. The injection molding process is therefore a series of events that begin with plastic pellets getting fed into the hopper and ends with a finished product ejected from the mold. There are several variations to the conventional injection molding process. There are also variations to the different mechanisms within the process. These are determined by factors such as the type of plastic and the product design. Nonetheless, this gives a general overview of the injection molding process.
The Thermoforming Process
Thermoforming is another method that is well used in the industry for processing plastics into a range of products. It involves heating the plastic to its glass transition point where it is soft and pliable. The part then gets formed by pressure into the desired shape and geometry over a mold. For thermoforming, the starting material is a sheet. The process begins with the sheet placed over the mold. It is clamped in place and then heated to soften the plastic. The heating method varies with different process designs. The heating can be achieved in an oven or heating system attached to the unit. The next stage is product forming. This is where pressure is applied to the heated plastic to allow the sheet to fit into the desired shape using the mold. The pressure is exerted in different ways depending on the variation of thermoforming used. The formed product is then allowed to cool still attached to the mold with exerted pressure. Once cooled the product gets ejected.
Types and Subtypes
For the general process described above, there are several variations of thermoforming and injection molding. Within the different types the machinery, tooling, and circuitry also vary depending on manufacturer and operators. Thermoforming is mainly of three types: Vacuum, mechanical, and air blown thermoforming. These are depending on whether vacuum, a mechanical force from a male mold, or plunger gets used. You, therefore, have the choice of investing in a male mold, vacuum system, or air blower.
Injection molding offers a much more diverse option. The variation can be from the overall orientation of the whole machine. It can also be to do with the mold filling process. In injection molding, you have the option of a hot runner or cold runner system. You can also choose between a vertical or horizontal orientation. The mold can also be a single cavity or a multi-cavity system. Depending on the design and application of the material you can have gas-assisted injection molding or structural foam molding to have hollow or porous parts. Injection molding is not limited to thermoplastics. There is thermoset injection molding and metals powder injection molding or metal injection molding exist.
Here we see that injection molding offers more variety of choices than thermoforming. So where versatility is required one might be forced to opt for injection molding. Within the injection molding realm, the process can be modified in different ways to meet the required product specification. For example for the same machine, the mold can be changed or the runner system converted from a cold runner to a hot runner system.
Machinery, Molds, and Tooling Requirements
One key difference between injection molding and thermoforming is the mold type used. Injection molding uses two-part molds. There are also 3 to 4 part mold systems. The mold opening and closing mechanism can also vary from the simple split open to more complex mechanisms like the slide mold systems. This means that the injection mold requires more complex tooling and machinery. Furthermore, the plastic is injected in a more fluid form in injection molding. This requires more intricate designs to prevent the leakage of the melt from the mold. Injection molding requires the melt to flow through channels to get to the mold. This, therefore, involves the mechanics of heated fluid flow through nozzle, sprue, runners, and gates. Pressure and temperature control are also very important to ensure good part formation. Injection molding also requires excellent timing of events to minimize cycle time and increase efficiency.
Thermoforming on the other hand uses a more simplified molding system. It is generally used for large parts. Unlike injection molding that can be used for tiny parts like bolts, gears, and screws to moderately sized parts like plastic furniture. Thermoforming can use a single mold system where vacuum or air blown thermoforming is used. At most, a two part mold system is used where a male and female mold is required in the mechanical thermoforming type system. The opening and closing of the mold are fewer complexes in thermoforming as the plastic is used in a softened form rather than flowing fluid form. This means fewer chances of defects such as weld lines and flow lines.
Design Complexity and Precision
Both injection molding and thermoforming can achieve a good level of product design complexity and precision. However because injection molding brings the plastic to a more flowing fluid form, it can achieve better precision. This is particularly important for parts like gears, screws, and small fittings that require high precision for performance and efficiency. A more fluid form allows the melt to fill the intricate hollows of the mold cavity to replicate the design. Especially for small parts. The precision of thermoforming is largely aided by vacuum and pressure. Thermoforming is generally not used for small parts like bolts and gears. Also, the molding process limits it to parts with relatively thin walls. If the wall is too thick this affects temperature distribution. Also, a higher force is needed to form the shape required. Because the product forms within the mold cavity in injection molding, this gives more possibility of product design achievable. The range of possible designs achievable in injection molding is further extended with variations in mold opening and closing systems and also the options of using inserts. Today injection molding even extends to the production of parts with dimensions in the micrometer range.
Textures, Colors, and Blends
Often parts are required to have certain textures either for aesthetics or performance. Injection molds can be designed to achieve different textures more easily than thermoforming. The thermoforming process almost always limits you to a smooth surface finish. With injection molding textured, glossy or smooth finishes can be achieved by varying the mold material and design. The operating parameters can also be varied to control the type of finish achieved for certain plastics.
The injection molding machine mixes and melts the plastic before product formation. This in itself presents a whole lot of possibilities. Colors and other additives get mixed in with the feed-in injection molding. You can also have the pellets already containing these additives or mix pellets with different colors to achieve the desired color blend. However, with thermoforming you are limited to the color of the sheets used. Other than colors fibers, powders, and granules can be mixed in with the plastic pellets in injection molding. Different types of plastics also get blended into the injection molding machine. This allows the formation of composites with different properties, The properties of the plastic can be varied by mixing in different fillers. Examples are glass-filled nylon or a blend of PVC and ABS.
Design and Prototype Production
Injection molding has a whole industry dedicated to design and prototyping. The mold design and fabrication alone is a field of its own. Prototype production for injection molding can be done with processes like CNC machining and 3D printing. Molds for prototypes and short runs can also be designed from aluminum. The injection molding process can be simulated using 3D software from feed to mold. The fluid mechanics as well as the temperature profiles can be simulated to obtain optimum process parameters and mold design. Thermoforming is a simpler process and the phase change is narrower. Prototype production is often less demanding for thermoforming than injection molding.
Cost, energy, and material consumption
The cost of production depends on a lot of factors beyond the machinery alone. Typically injection molding is targeted at minimizing cycle times. This is achieved by filling the mold as quickly as possible and cooling it as fast as possible. Both were done without compromising on the quality of the part formed. The capital cost in injection molding is more than that on thermoforming. This is due to the complexity of the tooling and machinery required to achieve the level of efficiency required. Thermoforming is used for lower volume production. Typically only a few hundred parts get made per year using thermoforming. Whereas injection molding can achieve a million cycles or more per year.
Since the plastic is required to flow more in injection molding, it requires more energy. Energy is required in the form of heat and hydraulic pressure to fill the mold. Thermoforming only needs the plastic to be pliable while still in its rubbery state. The vacuum pressure required in thermoforming requires less energy than that in injection molding. Cheaper materials like aluminum can be used for molds in thermoforming. But for injection molding, in the typical case you are looking at high-grade tool steel. The molds used in injection molding can give you thousands and millions of cycles so you do get your money’s worth.
Depending on the type of injection molding scrap is minimal. You can avoid scrap completely by maintaining process efficiency and using hot runners. The use of one part mold in thermoforming means scrap is often unavoidable. Thus injection molding achieves more efficient material consumption.
Here we see that injection molding offers a lot more possibilities than thermoforming in terms of part complexity, size, and design. However, this comes with a need for more complex mold design, operation, and tooling. For large parts with relatively thinner walls that can get made at relatively lower pressure, thermoforming is a better choice. Although this limits you to smaller production volumes. Where you need parts to be made with high precision and large production volume from thousands to millions of parts annually, injection molding is a preferred option.