Injection moulding is one of the most commonly used methods of producing identical plastic products in high volumes. However, as with every process, it is important to understand the specific design restrictions that must be adhered to in order to facilitate the obvious benefits of successfully producing cost-effective, high-quality parts.
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To help, we have very simply laid out the advantages and disadvantages to consider.
Please be aware, however, that the following guidelines apply to what we would consider to be conventional injection moulding (quantities of 100,000+) and may appear at first glance to be fairly prohibitive if you are looking to develop a product with numbers out of this norm.
The good news is Plunkett Associates have spent many years developing strategies and techniques in order to take away some of the issues identified below, particularly for parts that would traditionally be considered too difficult or unusual to mould.
In addition, we are specialists in making injection moulding far more cost effective for much lower quantities of parts than is more commonly expected. Using a hybrid of techniques, our low volume production tooling allows us to customise to client/geometry/quantities. The result is a mould tool that reflects the requirement and opens up an exciting option to produce high quality, yet low volume components.
As an ISO:: Quality System certified supplier, we are confident we can help with your requirements, so please feel free to give us a call.
1) Fast production and highly efficient. Injection moulding can produce an incredible amount of parts per hour. Speed depends on the complexity and size of the mould, anywhere between 15-120 seconds per cycle time.
2) Low labour costs. Plastic injection moulding is an automated process whereby a majority of the process is performed by machines and robotics, which a sole operator can control and manage. Automation helps to reduce manufacturing costs, as the overheads are significantly reduced.
3) Design flexibility. The moulds themselves are subjected to extremely high pressure. As a result, the plastic within the moulds is pressed harder and allows for a large amount of detail to be imprinted onto the part and for complex or intricate shapes to be manufactured.
4) High-output production. Thousands of parts can be produced before the tooling needs to be maintained.
5) Large material choice. There is a large selection of polymer resins to choose from. Multiple plastic materials can also be used simultaneously; for example, TPE can be overmoulded onto PP parts.
6) Low scrap rates. Injection moulding produces very little post-production scrap relative to traditional manufacturing processes. Any waste plastic typically comes from the sprue and runners. Any unused or waste plastic, however, can be reground and recycled for future use.
7) Ability to include inserts. Metal or plastic inserts can be insert moulded.
8) Good colour control. Plastic parts can be manufactured in any required colours with the use of masterbatches or compounding.
9) Product consistency. Injection Moulding is a repeatable process; in other words, the second part you produce is going to be identical to the first one etc. This is a huge advantage when trying to produce high tolerances and part reliability in high volumes.
10) Reduced finishing requirements. There is often very little post-production work required as parts usually have a good finished look upon ejection.
11) Enhanced Strength. When plastic injection moulding, it is possible to use fillers in the moulding material. These fillers reduce the density of the plastic whilst it is being moulded, and can help add greater strength to the completed part.
1) High tooling costs and long set up lead times. Up-front costs are high due to the design, testing, and tooling required. There is the initial design and prototyping (probably via CNC or 3D printing), then the design of a prototype mould tool to produce replicas of the part in volume. Lastly, and only after extensive testing during both stages, you can finally injection mould a part.
2) Part design restrictions. Plastic parts must be designed with injection moulding consideration and must follow the basic rules of injection moulding, for example:
Don’t forget, because tools are typically made from steel or aluminium, it can be difficult to make design changes. If you need to add plastic to the part, you can make the tool cavity larger by cutting away steel or aluminium. But in order to take away plastic, you need to decrease the size of the tool cavity by adding aluminium or metal to it. This is extremely difficult and in many cases might mean scrapping the tool (or part of it) and starting over.
Also, the weight and size of the part will determine the tool size and necessary press size. The larger the part, the more difficult and expensive it will be.
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3) Small runs of parts can be costly. Due to the complexity of tooling, and the necessity to rid the machine of all previous material before the next product can be made, the setup time can be quite lengthy. Therefore small runs of parts have traditionally always been thought of as too expensive to injection mould.
Injection moulding is a great process! The range of materials and colours is huge, and there is no “abs like’ that we get in 3D Printing, it’s the real thing. Parts are repeatable and tight tolerance. The only potential downside is the tooling, but as we said at the beginning of this article, we’ve got that covered, with options to suit most requirements.
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.
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.
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.
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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