Injection molding: Types, process, & design guide

Injection molding is additive manufacturing in the contemporary market, for producing plastic parts in bulk quantities. In the process, the polymer granules are melted under high pressure, and injected into the molds. They are typically a type of thermoplastic polymers that harden after molding. Almost everything from car parts to electronics is made this way. The method is well-suited for high production volumes since unit costs are relatively low.

Parts of an injection molding machine

Injection Molding Machine Parts

The molding machine has vital elements to promote the correct formation of parts from the plastic. These parts include hopper, barrel, screw, heaters, movable platen, nozzle, mold, and mold cavity.

• Hopper: Where the raw material, the plastic(polymers) granules are fed to the device for processing.
• Barrel: The main housing of the machine; the barrel holds the reciprocating screw and the plastic material. It has heating structures that melt the granules before they are going to be injected through the nozzle.
• Reciprocating Screw: The spiral mechanism is used to transport and place pressure on the plastic as it melts within the barrel, and consequently, provide a constant flow toward the mold.
• Heaters: Heaters are necessary to generate enough heat to melt solid plastic pellets into pellets of PVC. These are fixed at different positions around the barrel.
• Movable Platen: Attached to the mold core, the movable platen exerts pressure that maintains the mold’s shut during injection. Further, they release the mold core easily should an attempt be made to form a part through the mold core.
• Nozzle: The nozzle’s temperature and pressure must be at a particular level because it helps dispense the molten plastic through which it enters the mold cavity.
• Mold: Mold is typically made of two halves to give the injected plastic its form. Moreover, injection mold parts feature parts like ejector pins, cooling channels, and vents to assist in coming up with the final shape of the product.
• Mold Cavity: The mold cavity is a critical part of the molding machine. Through cavities, the molten plastic is poured to have the desired part. They also serve as gates, runners, and sprues which somewhat are crucial because they assist in the flow channeling of the plastic during the injection practice.

Injection molding process

The injection process emphasizes a well-designed mold and sophisticated machinery to form the material.

1. Mold Creation

Molds are generally made of aluminum or steel based on the nature of the project in hand. These are shaped and sized in a manner that tends to be similar to the certain part that you want to create.

2. Material Preparation

Polymer or metal granules are placed into a barrel and then heated. A helical screw blends the material, while heating bands cook it to the right texture.

3. Material Injection

In the molten form, the material is then introduced into the mold cavity at a high pressure.

4. Part Ejection

After the alloy has solidified the mold opens and some rods push the final part out of the mold. This automated step is useful for keeping up the right production rate and quality.

5. Two-Shot Molding

In two-shot molding, it is possible to connect various materials in one part of the product. It is applied to provide shade, and feel, or to develop an item, which has several characteristics of performance.

6. Cavity Design

Molds can be single or multiple parting. They can hold more than one cavity to create many pieces at once. Cavities can be made to mirror one another to make similar components or the shape of the cavity may be different to make different designs.

7. Material Selection

Aluminum molds are cheaper but are not as strong as the other molds. Usually, they are preferred for low production volume. The steel molds are employed for high-volume runs due to the toughness of steel.

8. Accuracy and Design Elements

The shape, the material, and the features of the mold part must be well thought out. These considerations ensure the part is within the required dimensional tolerance and performs the intended application. However, initial mold costs are high, and production lead times can be over four weeks. Therefore, plastic molding may not be optimal for small runs.

Types of injection molding technologies

Let’s discuss common types of injection molding employed in industries;

1. Gas-assisted injection molding

Gas assisted injection molding

In the gas-assisted process, after molten the plastic, highly pressurized inert gas like nitrogen is used in the injection. This makes hollow sections as the material is forced into the mold walls. They found the layering of carbon fiber helped in reducing weight, improving cooling, and lessening the chances of warping because of the uniform thickness of the walls. However, it is more effective for single-cavity molds.

Materials: ABS, Polycarbonate, Nylon, PET, PP.

2. LSI – Liquid silicone injection molding

LSI – Process

Liquid silicone molding employs use the of liquid silicone to inject cold-cured silicone rubber into heated tooling for practicing silicone rubbers in high volumes. The method applied here is vulcanization. It makes the material right and elastic as well. LSI is usually automated, yet silicone cannot be recycled once it has been set, which makes any mistakes made in the manufacturing process irreversible.

Materials: General silicone, Health care silicone, Industrial silicone, Optical silicone.

3. Structural foam molding

Structural foam molding

This process involves the incorporation of a polymer with an inert gas to form a structure with a skin-like outer surface. It is used in production to produce large, thin, and rigid structures like automobile roofs or medical equipment shells. When comparing the foam to the conventional design, weight is decreased but there may be additional surface finishing needed.

Materials: Polyurethane, Polycarbonate, ABS, PBT.

4. Metal injection molding (MIM)

Metal Injection Molding

Metal injection molding(MIM) is a process of producing a part from metal powder and a binder is injected during which it is sintered to become a solid metal part. It is used to produce tiny metallic components such as a tool head or an aerospace part. While it is accurate, the process is costly, and the technique is suitable for small components only.

Materials: Austenitic and martensitic stainless steel, Titanium, Cobalt-chrome, Nickel superalloys.

How much does injection molding costs?

On the whole injection molding is quite economical. However, the start-up costs are high. Normally, production runs vary from 1000 to over 100000 pieces, cost per part is low, generally between $1 to $5. This makes it suitable for use in industries that require the frequent production of a single product.

These high costs are chiefly attributable to the requirement for unique molds. The cost of a mold will depend on its design and the material used, and normally it will be between $3000 and over $100,000. Aluminum molds are cheaper and suitable for low volume while steel molds are expensive, accurate, and economical for large quantities.

Types of material used in injection molding

Injection molding operates with almost all types of thermoplastic materials, which increases its pliability. Some of the silicone and thermoset resins are also compatible with the process.

In addition, polypropylene is considered as most preferred material for use in the production of CDs and DVDs, with a global production capacity of approximately 38%. Second to polypropylene is ABS followed by PE and PS with global production capacities of 27%, 15%, and 8% respectively. These four materials account for more than 40% of the total number of plastic parts produced in the world. These analytics underlines the significance of plastic molding in contemporary production.

Considerations for optimal injection molding

To achieve efficiency and low costs before engaging in injection molding, several factors require consideration.

1. Financial Investment

However, one must appreciate the fact that IM involves much capital investment. The required technology is expensive and creating particular molds also raises the cost. So, the initial investment has to be offset by the production level. Generally speaking, needs to be put in place to recoup the investment costs. It’s better to seek out a reliable Injection Molding Service provider to expediently lower the costs.

2. Production Volume

The expected number of parts must be estimated to determine if injection molding is the right choice. Since the process is normally applied where large quantities are required.

3. Design Simplicity

The simplicity of the product design also has a significant effect of preventing the complexity of the manufacture. Parts count reduction and continuous geometries contribute to the reduction of the likelihood of forming problems as well. Hence, the mold tool design must be done effectively to minimize the happening of an incident to slow down the process of production.

Products made by using the injection molding process

Many products in various industries are made using injection molding. Here’s a list of common products made using this streamlined manufacturing process:

• Toys
• Packaging
• Miniatures
• Automotive Components
• Electrical Enclosures
• Healthcare Products

Comparing 3D printing and plastic injection molding

3D printing and rapid injection molding are two main processes in rapid prototyping. They are different as described below:

3D printing forms parts from layers of a digital model. The inside of the formed object is often complicated. This process takes longer, sometimes several hours to days based on the design of the web page. Such materials as thermoplastic, thermosetting plastic, metals, as well as ceramics are used.

Meanwhile, plastic injection molding involves mold use. It forces molten plastic to take all the desired forms at once under pressure. So, this makes it possible to produce quality products in minimal turnaround, at high rates of production, ideal for assembly line production. However, setting up for a mold will set you back a great deal of money. But making the actual piece relatively becomes cheaper in the long run. Additionally, plastic molding mostly works with thermoplastic material but extends to other classes of material.

Key comparisons

Conclusion

Injection molding is one of the most promising techniques for the production of many identical parts. Although the initial expense for tooling and mold creation may be comparatively steep. But the cost per unit after is comparatively low. The process allows to production of parts, similar to a prototype to a very great accuracy. This adaptability, when it comes to the types of materials used also adds to its allure, and guarantees that IM will continue to be the go-to process for mass production with minimal compromise on quality. However, the process is a bit challenging. As it requires enriched engineered designs to effectively bring out a unique prototype. This way, it’s economical to seek out professional Rapid Prototyping Services, to minimize potential issues, and achieve desired outcomes on a constrained budget.