Compression molding is a molding process that uses compressive force to squeeze a charge to conform to the shape of a mold that consists of a lower and upper half. Parts may consist of several components that work as one mechanism when compression is applied. When the halves of the mold meet, a cavity is left between them that defines the desired shape. Parts meet at the widest cross-section of the shape to facilitate the ejection of the product once it is cured.
Of the various types of molding, compression molding was the technique developed for synthetic material. It is the least expensive molding method for thermosetting materials. Compression molding are ideal processes for high consistency rubbers (HCRs).
High-consistency silicone rubber (HCR), also known as solid silicone rubber, is cured at high temperatures with either organic peroxides or a platinum catalyst. HCR contains long polymer chains with a high molecular weight, which provides the rubber with a solid consistency. Due to its outstanding mechanical properties, HCR is used in a wide range of static and dynamic applications, including:
Advantages of Compression Molding
Lower cost Tooling
Given that the process does not involve an Injection or Transfer cycle, the tooling has less infrastructure requirements than tools designed for other molding methods. There is little else needed in the way of additional features, other than the particular features of the cavity itself that will produce the molded parts, which clearly have to replicate the detail required of the product. Tools can be made of aluminum or lower cost grades of steel which can reduce costs, although any tool must be capable of withstanding the considerable molding pressures required.
Cost-effective for short production runs
Compared to injection molding, compression molding does not require special equipment and expensive complex tooling. As a result, the equipment and tools required for the operation are much simpler, which typically means less capital investment. Although it is a labor-intensive process, when handled correctly, the wear and tear sustained by the molds during operations is relatively small, which can mean less maintenance needed over time, resulting in lower cost quotes for customers.
Raw material wastage is minimum
Little material is lost during the molding process. For example, it does not use gates, sprues or runners, tooling features required by other production methods that result in greater waste.
Suitable for large parts
This form of molding is best suited to producing large parts that require a significant bulk of material to manufacture. Given that the material is directly loaded into the mold cavity, there is no limitation on the weight of part that can be made, other than the size of press and tonnage required, whereas Injection molding presses are limited in the weight of part they can produce by the volume of the injection barrel that fills the mold.
Disadvantages of Compression Molding
Higher labor cost
This production technique is fairly simple; however, it requires more man power than a semi-automatic injection molding process to run. The skill level is often proportionately higher, and for larger parts or tools, manual handling may become an issue.
Slower process times
Compared to injection molding, compression molding requires more time to bring materials to cured state. These slower processing times can affect production rates and costs in higher volume projects.
Not suitable for complex designs
While complex parts can be made using Compression molding, depending on the material required and the design of the product, it is generally better suited to the production of larger, simpler objects. This is because the limited flow of material within the cavity often makes the elimination of voids, air traps and knit lines difficult when trying to produce more intricate parts.
Higher chance of post molding costs
Flash is a necessary by-product of Compression molding due to the need to expel air during the molding process by overfilling the mold cavity. This flash can also vary in thickness depending on the exact fill of each particular shot, leading to issues with variable dimensions of the part over across the split line. The flash has to be mechanically cut off and, again, is a thermoset material that cannot be recycled easily. The resulting split line witness mark is often more evident than on an Injection molded part. This may create an aesthetic problem for the customer. Generally, Compression molded parts require more labor effort to trim and finish them, which can also increase the cost.
Overall, the process of compression molding is ideal when used appropriately and can help to keep overall project costs to a minimum, although individual part costs can be higher when compared to other techniques. It is critical to consider all the required aspects of a product, its performance and its life cycle when designing the most appropriate production solution, as each process technique has a different combination of costs, benefits and drawbacks. The proper solution may not require an expensive Injection mold tool and thus even if the part cost is higher, overall, Compression molded solutions remain as relevant as ever.
