Injection moulding vs compression moulding: when should each process be used?

Injection molding and compression molding offer different manufacturing possibilities. Part geometry, expected production volume, and required tolerances all influence the choice of process.

Injection Molding and Compression Molding: Two Different Processes

Injection molding and compression molding can both be used to manufacture rubber and silicone components, but the way the material is introduced and formed inside the mold differs between the two processes.

How Does Each Process Work?

In compression molding, the compound is placed inside the mold. When the mold closes, pressure and heat cause the material to fill the cavity, take the shape of the part, and cure.

In injection molding, the compound is plasticized or conditioned in the injection unit, metered, and injected under pressure into the mold cavity, where vulcanization or curing takes place. The way the material is introduced affects tooling design, cycle times, and the possibilities for automation.

Materials and Applications

Injection molding can be used with different rubber compounds and silicone formulations specifically developed for this process.

Compression molding is also suitable for a wide range of elastomers and can be an appropriate alternative for larger parts, less complex geometries, or production runs where tooling cost is an important consideration.

The specific application influences the decision. A small seal with numerous details and a high production volume may benefit from injection molding, while a larger and simpler part may be better suited to compression molding. Material formulation, geometry, tolerances, and expected production volume should all be evaluated together.

Tooling Costs and Production Volume

Tooling cost is one of the factors that should be considered before choosing between injection molding and compression molding. The initial investment should be assessed in relation to the expected production volume and the manufacturing cost per part.

Initial Investment and Cost per Part

Molds used for injection molding generally incorporate runners, gates, and other elements required to introduce the material into the cavity. Their development can therefore be more complex than that of a compression mold.

In compression molding, the material is prepared before the mold closes, which can allow simpler tooling to be used for certain parts.

However, the comparison should not be limited to mold cost. The investment difference between the two processes can vary considerably depending on the number of cavities, the feeding system, part geometry, and the level of automation planned. Material utilization and post-molding operations should also be taken into account. Flash, the feeding system, and deflashing requirements can all affect both unit cost and total manufacturing time.

What Happens as Production Volume Increases?

For long and repetitive production runs, injection molding can offer advantages due to its automation capabilities and cycle stability once the process parameters have been optimized.

For shorter production runs, compression molding can be suitable when the part geometry is compatible with the process and the aim is to limit the initial tooling investment.

There is no universal minimum production volume that determines which process should be selected. The break-even point depends on the part, the mold, and the manufacturing conditions. To compare both processes, it is advisable to evaluate the total program cost, considering not only the initial tooling investment but also the unit manufacturing cost in relation to the expected production volume:

Tooling investment + (unit cost × production volume)

Cycle Times and Repeatability

Cycle times and repeatability become particularly important as production volumes increase. The characteristics of each process can have a significant impact on manufacturing efficiency.

Long Production Runs and Repetitive Manufacturing

Injection molding is particularly attractive when large quantities of the same part are required. The metering system and number of cavities make it possible to organize stable and repetitive production.

In compression molding, material preparation and placement can have a greater impact on cycle time, particularly when some of these operations are performed manually. The level of automation available and the characteristics of the part also influence process efficiency.

Injection molding can reduce the time associated with material preparation and loading while enabling more automated production cycles. However, total cycle time still depends largely on part thickness, elastomer formulation, and vulcanization conditions. For this reason, high-volume production can justify a higher initial investment if the reduction in cycle times and increased automation offset this cost throughout production.

Tolerances and Part Geometry

Tolerances, part size, and complexity also influence the choice of process. Each manufacturing method offers different dimensional capabilities that should be assessed together with the component design.

Complex Parts and Tight Tolerances

Injection molding allows the amount of material introduced during each cycle to be controlled and can fill cavities containing small details or complex geometries. This can facilitate the manufacture of components with demanding dimensional requirements.

Compression molding can perform effectively when the part geometry is suited to mold closure and the required tolerances are compatible with the process.

In both cases, achievable tolerances do not depend solely on the molding process. They are also influenced by the elastomer, shrinkage, part geometry, mold design, and vulcanization conditions.

Injection Molding or Compression Molding?

The choice between injection molding and compression molding depends on the relationship between the part, material, production volume, and manufacturing requirements. As a general guideline, the following are some of the situations in which each process may be worth considering.

When Should Injection Molding Be Considered?

It may be particularly suitable when:

• the expected production volume is high or recurring;
• high repeatability between cycles is required;
• the geometry includes details or areas that are difficult to fill;
• automated material feeding and manufacturing are desirable;
• the tooling cost can be amortized over a sufficient production volume.

When Should Compression Molding Be Considered?

It may be particularly suitable when:

• the production volume does not justify more complex injection tooling;
• large or relatively simple parts are being manufactured;
• the geometry is compatible with mold loading and closure;
• limiting the initial investment is a priority;
• the resulting unit cost remains competitive for the expected production volume.

These conditions are only general guidelines. Two apparently similar parts may require different manufacturing processes due to the material, tolerances, number of cavities, or operating conditions. The decision should therefore be based on the specific part and production requirements rather than on the process normally used.

From Part Design to Manufacturing

At Moelsi, we assess the characteristics of each project to determine which manufacturing process best suits the part and the expected production volume.

A Decision Based on the Requirements of Each Project

Geometry, material, tolerances, and expected production volume help define manufacturing conditions from the early stages of a project. Based on this information, the required tooling and the most suitable production alternative can be assessed.

Tell us about your project and we will recommend the most suitable manufacturing process.