Advantages and Disadvantages of Compression Moulding

Within this article, Martin’s Rubber  explains what Compression moulding is and demonstrates some of the main advantages and disadvantages of producing a Compression mould and the overall Compression moulding process.

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Developed in the ’s, Compression moulding was the original method for moulding rubber and is still ideal for manufacturing low to medium volumes of rubber products. It remains a useful moulding process for forming bulky parts. Martin’s Rubber has a wide range of Compression moulding machines, from 5 tonnes up to a 500 tonne press, with a maximum of a 1 square metre press area.

Compression Moulding is a traditional technique that involves taking a rubber compound and making a pre-form that is in the shape of the end product, but is larger than the final shape. A combination of heat and pressure is then used to mould thermoset rubber or plastic resin into the desired shape.

The rubber or resin material is shaped using a heated mould tool, and the clamping pressure of a press is used to force the flow of the material inside the tool. The thermoset characteristic of the material is instigated by this process and permanent chemical change referred to as Vulcanisation for rubber takes place, fixing the shape of the product.

Once this vulcanisation process is complete, the mould is then opened and the part extracted and allowed to cool, maintaining the shape of the mould that produced it. It is important to note that the part has a tendency to shrink during cooling to arrive at its final size. When the press is opened, the tool can be split and the part de-moulded.

This technique can be used to make a wide range of products, such as: wellington boots, machinery parts, seals, gaskets or even door stops and chair feet. This manufacturing process is capable of reproducing the same product a large number of times, but it is not necessarily suitable in all situations.

Here are some of the advantages and disadvantages that relate to Compression moulding as a process, to help you decide if it’s right for you and your product.

Advantages:

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 moulding 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 moulded parts, which clearly have to replicate the detail required of the product. Tools can be made of aluminium or lower cost grades of steel which can reduce costs, although any tool must be capable of withstanding the considerable moulding pressures required.

Good for small production runs
The lower capital cost of manufacturing a mould tool, setting up a press and beginning to run production parts means that Compression moulding is the most cost effective method of making smaller runs of parts. There is of course a breakeven point at which the higher capital cost of an injection mould tool becomes viable due to the lower cost of the parts that it makes. Correct assessment of the cost / benefit point is a key consideration when developing the best production solution for a new product.

No gates, sprues or runners
This form of moulding does not use gates, sprues or runners which are tooling features that materials have to pass through in other methods of production before entering the mould cavity. These can consume extra material and therefore cost and can also detract from the cosmetic requirements of a part.

Good for large parts
This form of moulding 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 mould 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 moulding presses are limited in the weight of part they can produce by the volume of the injection barrel that fills the mould.

Disadvantages:

Greater waste
Compression moulding is not as precise a method of making a product as injection moulding, due to the fact that the mould cavity has to be overfilled to some degree to achieve the correct pressure to cure the part. It is also often necessary to push air out of the cavity using excess material in order to create a void-free part, which again increases wastage, whereas an injection mould is more precisely filled and the material inherently drives out the air as the cavity is filled. Waste thermoset rubber or plastic generally cannot be melted down and reused so the cost of the part must reflect this.

Higher labour cost
This production technique is fairly simple, however it requires more man power than a semi-automatic injection moulding 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
Because the thermal conductivity of rubber and resin is relatively poor, bringing a blank of material up to curing temperature by contact with the hot surfaces of the mould takes time for larger parts, which limits the speed at which parts can be cured and de-moulded. On the other hand, Injection moulding shoots material into the mould that is already at a temperature just below curing, therefore the extra time required to begin cure once the press closes is drastically shorter. Depending on the size of a part, a typical cycle can take around ten minutes, whereas the equivalent cycle of injection moulding would take approximately five minutes to complete. This can make a significant difference when producing parts in higher volume, when production rate and part cost become the priority.

Not suitable for complex moulds
While complex parts can be made using Compression moulding, 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.

Contamination
It is much easier to produce clean, consistently coloured rubber parts by Injection or transfer moulding. However, the uncured blank of compound used in Compression moulding can pick up specks of dirt which then simply get pressed and cured into the surface of the part, which the results in poor cosmetic appearance. Despite the part being dimensionally and physically acceptable, this often leads to an increase in reject rates and waste.

Difficult to control flash
Flash is a necessary by-product of Compression moulding due to the need to expel air during the moulding process by overfilling the mould 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 moulded part. This may create an aesthetic problem for the customer. Generally, Compression moulded parts require more labour effort to trim and finish them, which can also increase the cost.

Moulds can be damaged
The repetitive nature of this process means that the moulds themselves can become victims of general wear and tear due to manually loaded blanks of material. Quite often, the mould is made to be run loose and is not bolted into a press. This can lead to deterioration of trim grooves and fine details, or in extreme cases, impact damage on mould faces caused by poor handling.

 

Overall, the process of compression moulding 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 mould tool and thus even if the part cost is higher, overall, Compression moulded solutions remain as relevant as ever.

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Plastic injection moulding is ideally suited to producing prototypes and even end-use parts. Once they’re constructed in the client’s desired shape, the steel or aluminium moulds and the rapid injection process offer precision, speed, scalability, and so many other advantages to expedite your production.

However, there are some use cases where injection moulding may not be the most suitable manufacturing method. Just as there are two sides to every coin, there are certain disadvantages inherent to the injection moulding process that may make it imperfect for your project.

That should come as no surprise since every project – including yours – is unique and requires a unique approach.

So, let’s discuss the advantages and disadvantages of injection moulding to help you determine if this is the right manufacturing process to help you achieve your objectives.

Main Advantages of Injection Moulding

1. Precision and Consistency

The moulds used during the rapid injection moulding process are shaped from steel or aluminum. This allows for a precise and detailed mould that exactly matches your vision. Furthermore, this mould is used over and over again, thus resulting in a consistent output.

2. Wide Material Choice

Plastic injection moulding offers a high degree of flexibility in terms of the material you want your part to be made out of. This is a huge advantage, as it allows for parts to be made to suit a variety of high-profile industries.

3. Fast and Efficient Production

Once the mould has been fashioned, the injection moulding production process is fast and efficient. Even the process to create the mould is short with a lead time of two weeks. This is much quicker than die casting moulds, which have lead times of three weeks.

4. Low Cost Per Part

The price per part produced using injection moulding is low and affordable. This means you could potentially gain a much better ROI on an order for 100,000+ parts using this manufacturing method versus, say, die casting, which has a higher cost per part.

5. Scalability

Injection moulding is highly scalable. Tens of thousands of units can be produced from one single mould, which greatly reduces the lead time and the cost per part in the long-term.

It’s no wonder, then, that this process is used to produce most consumer products, including LEGOs

Main Disadvantages of Injection Moulding

1. High Initial Cost

While it’s true that plastic injection moulding results in a low cost per part, the initial front-end cost is high. If you’re wanting a relatively small production, then you may need to go with a manufacturing method like vacuum casting, which has a much lower initial cost. That way, you’ll be able to achieve a higher overall ROI.

2. Initial Lead Time

The first part of the lead time for an injection moulding prototype project can, admittedly, take a long time compared to other methods. This is because the moulds must be built out of steel or aluminium first before production is even able to start. This can result in a lead time of at least two weeks or more.

If time is a concern, consider asking us about vacuum casting, cnc machining or 3D printing. Both of these methods have lead times of less than a week (depending on the size of the project).

3. Design Limitations

There are a number of part design restrictions that should be carefully considered when designing a prototype model for injection moulding, as there are several limitations you must work around.

For example, we must adhere to a certain range of thickness for the walls of the part and increase their draft angles by 1 degree for every 25 mm of height. These measures protect the parts from warping, sinking, and draft marks.

4. High Expertise Needed

Unlike 3D printing, where almost anyone with a design can pop a few prototypes out from a push of a button, building a mould tool for a prototype requires careful considerations. Not only is expertise required in designing the tool but also to properly setup the machine to reduce the risk of mould or machine damage.

Following these parameters can be a tedious disadvantage to those who are unfamiliar with designing for the injection moulding process. Fortunately, you can contact HLH Rapid at any time with any queries you may have, and we will aid you in making sure the final injection moulded prototype looks exactly as you envisioned.

5. Design Changes Are Expensive

Following our previous points, injection mould tools always require careful planning before production. This is because design changes can be very costly. Generally, it can cost from a few hundred to a few thousand pounds. If the changes you need can’t be done on the current tool, you may have to build a new mould which can cost between a few thousand to ten thousands of pounds.

If you want to learn more, please visit our website hardware mold.

Conclusion: Injection Moulding Isn’t Perfect for Every Project