Compression Molding vs Injection Molding: Pros, Cons, and Cost Comparison

When it comes to creating plastic parts. Oftentimes two traditional methods come into mind: injection molding services and compression molding services. While both molding techniques rely on pressure and heat to shape materials, they operate differently and serve different applications.

So how is compression molding vs injection molding different? In simple terms. Injection molding melts plastic pellets and injects them into a mold at a high pressure. In contrast, compression molding places a pre-measured material charge into a heated mold and uses heat and pressure to compress the material into shape.

But which method is best for your project? And what factors should you consider? We’ll break down the key differences, and applications of injection molding vs. compression molding to help you make the right decision.

What Is Injection Molding?

Injection molding involves melting plastic material, such as resin pellets, in a heated barrel. The plastic material is first injected into a mold under high pressure. Once the plastic cools and solidifies, the mold opens, and the finished part is ejected. Although the process sounds relatively simple, it does have a complex process. Let's go through it in greater detail below.

The Injection Moulding Process

The whole process revolves around three main components of an injection molding machine:

• The Injection Unit
• The Mold
• The Clamp

Let’s break down the injection molding process.

Step 1: Feeding the Material

The process begins with plastic pellets or resin which are made of thermoplastics. These materials are fed into a hopper, and then funnelled into the injection unit’s barrel.

Step 2: Melting the Plastic

Inside the barrel there’s a rotating screw that transports the pellets forward. The rotating screw is designed with a specific geometry to provide consistent movement and melting of the material. In addition, there are heater bands wrapped around the barrel that provide the necessary heat to warm the pellets.

The heat and mechanical friction from the screw gradually soften and melt the pellets, as they move forward. By the time they reach the front of the barrel, they form a fully molten, homogeneous liquid. It’s now ready for injection.

Step 3: Injecting the Molten Plastic

The injection phase begins once enough molten plastic accumulates at the front. The screw will at this stage act as a plunger. It pushes forward the molten plastic with high pressure to force it into the mold, which is shaped like the desired part.

Step 4: Cooling and Solidification

After the mold cavity is filled, it begins to cool and solidify. The actual cooling time will depend on factors such as the part’s thickness, material properties, and mold design. For example, thin-walled parts can cool within seconds, while thicker components require more time to solidify.

Step 5: Ejecting the Part

The mold will open once the plastic has fully solidified, and then ejected using so called "ejector pins". The mold then closes, and the process repeats for the next cycle.

 

What Is Injection Molding Used For?

What’s the best use case for injection molding? Overall it’s suitable to produce high volumes with precision, consistency and cost effectiveness. You’ll find injection-molded parts everywhere - from everyday consumer products like chairs, toys, and electronic casings to dashboard panels to interior trims in the automotive industry.

What Is Compression Molding?

As its name implies. Compression molding uses pressure to form the part with the help of a hydraulic press with a split mold. The process starts with placing a measured amount of polymer (material charge) between two halves of a heated mold. Once the mold closes, heat and pressure soften the material to take the shape of the mold cavity. After cooling or curing, the mold opens, and the finished part is ejected using an ejector pin.

There are three main types of compression mold designs:

• Open flash type
• Fully positive type
• Semi-positive type

 

What Is Compression Molding Used For?

Compression molding is one of the widely used molding processes. It’s in particular effective for thermosetting and thermoplastic materials that contain a high amount of filler. This is because it reduces their viscosity which makes them less suitable for injection molding.

You’ll often see it in applications where parts need to be strong, durable, and have a simple geometry. For example:

• Automotive sector - battery trays, dashboards, engine covers, brake pads.
• Construction - roof tiles, electrical boxes, wall panels
• Electronics industry - circuit breaker housings, switch plates, electrical insulation parts.
• Sporting goods - helmets, protective padding, rubber shoe soles.

 

What Is the Difference Between Injection Molding and Compression Molding?

The key difference lies in how the material is shaped within the mold.

Compression molding machine uses heat and high pressure to press a (often) solid/semi-solid material charge into the mold cavity until it takes its final shape. Whereas injection molding injects the molten material into the mold under high pressure, where it then cools and solidifies.

• Compression molding: Suited for large, durable parts with simple designs. Often used for thermosetting plastics and high-filler materials.
• Injection molding: Ideal for high-volume production of finer detail and complex parts.

So which method should you choose? At the end of the day. It depends on your design, material, production volume, and cost considerations of the final product. Let's compare them in greater detail.

What Are the Compression Molding vs. Injection Molding Pros and Cons?

Each molding process has its strengths and limitations. They’re suited for very different applications.

When you’re looking for speed, precision and efficiency. Particularly with thin-walled intricate parts, then injection molding is the go-to choice. The main drawback is that the initial investment is higher than compression molding and it’s less efficient for thick and bulky components.

Meanwhile, compression molding is better suited for thicker, durable parts with simpler shapes. That’s because the heat and pressure applied during the process creates a stronger and impact-resistant component. What's the drawback? It can’t produce complex, finer details and is slower than injection molding.

Compression Molding vs Injection Molding Cost

The cost difference between compression molding and injection molding depends on factors such as:

• Tooling costs
• Production volume
• Material efficiency
• Cycle time

Generally, injection molding is a good choice for mass production and compression is more cost-effective for low- to medium-volume production or specialised applications.

Frequently Asked Questions

What Is the Difference Between Molding and Injection Molding?

Molding is a broader term. It refers to shaping materials using a mold. On the other hand, injection molding is a type of molding where molten plastic is injected into a mold under high pressure.

What Is the Difference Between Transfer Moulding and Injection Moulding?

Both methods use molds, but transfer molding involves preheating the material in a chamber before forcing it into the mold cavity, while injection molding directly melts plastic pellets and injects them at high pressure.

Is Injection Molding More Expensive Than Compression Molding?

Yes, injection molding is more expensive. This is because of the higher mold costs and the need for precision machinery. However, it becomes cost-effective for high-volume production since the cycle times are faster and it produces less waste.

Compression molding has lower initial tooling costs but may be slower and less efficient for finer designs.

Conclusion: Compression Molding vs Injection Molding

A common way to create plastic parts is by using molding methods. In this article we’ve looked at injection molding vs compression moldings. Injection molding (which forces plastic into a mold under high pressure) and compression molding (uses heat and pressure to shape pre-measures material into the desired form) are both popular methods.

Both of these molding methods have their pros and cons. And at the end of the day, it comes down to factors such as your design, material, production volume, and cost considerations of the final product.

If you’re looking for quotations on molding costs for your product. Submit a free RFQ to Asia’s largest marketplace for molding services at Haizol and receive same-day quotes from multiple suppliers.