Creating consistent plastic components: an in-depth technical guide
When you look at a small plastic cap, it’s hard to imagine the level of technical knowledge that’s gone into making it. Although plastic components may just be one small part of a much larger product or machine, it’s essential that they perform, both in terms of quality and function.
Just a small change in the tolerance or a slight fault in the production of a component can cause problems for your process or your customers, for example:
- A cap that doesn’t cover a valve properly can cause dirt and ingress to enter a system, or mean that materials leak from the process, threatening manufacturing downtime
- A motion component that doesn’t operate correctly in a washing machine can cause, at best, consumer inconvenience and at worst real threats to a customer’s safety and your company’s reputation tarnished
- An electrical part that malfunctions in a car can be a real threat to people’s safety and cause significant damage to the vehicle
This guide will explain the considerations that experienced manufacturers take into account to ensure they are creating accurate components. This means you can be aware of the variables involved in the process and can make an informed decision about the components manufacturer you choose.
What affects the consistency of injection moulded components?
To make sure that plastic components are of a consistent high quality, it is essential that the injection moulding process is as accurate and efficient as possible. As a highly complex process, there are several variables in injection moulding which will influence the final quality and consistency of an injection moulded component.
An established manufacturer will have an in-depth knowledge of injection moulding variables and will also:
- Understand the effects of changes in pressure and temperature
- Know the complexities of mould design and polymer selection
- Select the appropriate machinery for each type of component, experienced manufacturers provide consistent and high-quality component products.
As part of this guide, we’ll examine five different areas of the injection moulding process and the controls needed to ensure a consistent final product, namely:
- Pressure
- Temperature
- Mould design
- Polymer choice
- Machine specification
This will help you to have a greater understanding of the process so you can ask the right questions and guarantee you get the component quality you need.
The impact of pressure
Usually, the injection moulding process involved two different types of pressure, injection pressure and hold pressure. Injection pressure is the force with which molten plastic is pushed into the moulding tool. Hold pressure is the tension with which the cavity and core, the two halves of the tool, are pressed against each other to pack the final component into the correct form and size. Getting the balance of these two pressures is essential to ensuring that the final product has high material qualities and moulds into an accurate form.
With injection pressure, it’s important to consider the characteristics and volume of the polymer that is being used in the process. Particular polymers, such as nylon, are sensitive to intense pressure, so injecting it at a high speed will damage the plastic and mean that a final part isn’t formed correctly. Alternatively, injecting the polymer at too low a pressure will mean that the coverage of the polymer within the moulding tool will not be correct, causing faults in the aesthetics and strength of the final part.
The injection pressure also needs to be considered alongside the volume of polymer that’s being injected, as dependent on the size and design of the moulding tool, it will be filled to around 95% and 98% of its final form. The greater the volume of polymer, the more pressure that needs to be applied and vice versa. Getting this right also means that the polymer won’t get stuck in the gate or leak out of the moulding tool.
The hold pressure is equally challenging to get right as not only do manufacturers need to consider the capacity of the tool to handle the applied force (a smaller, more delicate tool won’t take as much pressure) but also consider the force that needs to be applied so there’s an even distribution of the polymer within the tool. Too little pressure and the component won’t form correctly, with polymer potentially leaking outside of the tool. Excessive pressure will affect the structural integrity of the part, potentially causing warpage or distortion in the part.
Considering the mould design
There are many important elements of the tool design manufacturers consider when creating a high-quality consistent part.
In combination with the design of the tool, manufacturers need to balance the polymer type and cycle time to ensure the plastic has adequate time to form and cool before being ejected. The more complex the design of the part or the more cavities it has, and the higher temperature the plastic can be heated to, the longer the cycle time needs to be to ensure the component is fully formed.
Aesthetic elements play a part in mould design too. The location of where the two halves of the tool, the cavity and core, come together is an important consideration, as it will leave a parting line at the join, affecting the final aesthetics if the mould tool isn’t positioned correctly.
Similarly, the position of where the ejector plates or pins push out the formed part needs to be considered and an experienced manufacturer will be able to advise how best to create the mould tool to avoid these aesthetic imperfections.
The importance of accurate temperature
Similarly to pressure, there are two types of temperature that have to be tightly controlled during the injection moulding process. Barrel cylinder temperature (which is the temperature of the heaters which surround the barrel feeding the injection moulding tool and shear heat temperature) is caused by the friction of the screw against the polymer during the screw back phase.
In addition, thermoplastics are the most common type of polymer used in injection moulding and are highly sensitive to temperature differences, meaning that even a decimal point change in temperature will have a huge impact on the final aesthetic and functional quality of a part.
By balancing the temperature of the barrel cylinder and that caused by screw back, manufacturers will prevent the plastic from becoming too liquid or too solid. As well as causing the polymer to become too molten and not fix into the correct shape in the tool, high polymer temperatures can also cause aesthetic damage to a component such as burn marks and flash, particularly if the tool becomes too hot to mould the part correctly. A lack of heat in the barrel however will cause inconsistent polymer flow into the tool, potentially causing blockages in the barrel or gate or leading to a lack of plastic distribution within the moulding tool.
It’s also important for manufacturers to take the cooling time of a polymer into account when designing the mould for the part because, as the polymer material cools, the part will start to shrink. By taking the cooling rate into account, manufacturers can ensure that the design of the tool will produce the correctly sized component and ensure that the hold pressure isn’t released before the component is fully packed and formed.
Selecting the correct polymer
Choosing a plastic that’s right for the aesthetic and functional needs of your component is a fundamental decision to make in the process.
Thermoplastics are the most common polymer used to create injection-moulded components. However, no two types of thermoplastic have the same materials characteristics. There are two main categories of thermoplastics, amorphous and semi-crystalline. The major difference between these two types of plastic is their molecular structure, causing them to behave differently under certain conditions.
The randomly ordered molecular structure of amorphous plastics means they gradually soften as temperature increases making them a more stable plastic to mould, with greater tensile strength and resistance when cooled. In contrast, semi-crystalline plastics have a highly ordered structure, meaning they’ll melt quickly at a specific temperature. This makes them tricky to mould with but tougher under stress, wear and loads.
The ultimate function, aesthetic and material qualities of your part will help you decide which type of plastic you need. Your manufacturer should be able to advise on this and adjust the injection moulding process and mould design to your selected polymer.
Using the right spec of machine
No single injection moulding machine is able to create all types of parts consistently and accurately. Different types, shapes and sizes of components require different machines, so ensuring that a manufacturer has the right spec of machine for the complexity of a part and the scale at which it needs to be produced.
If your business needs a range of different component types manufactured at scale, then it’s key you choose a company that has a variety of machine specs for different types of tooling and production scales. Also, making sure a manufacturer has a good maintenance and machine replacement strategy is essential to reducing the likelihood of production downtime and delays in delivering the parts you need. In short, a manufacturer with a variety of machine specs can deliver a wider range of accurate components.
How does Essentra Components consistently create high-quality components?
The key to making sure our components are of a consistent high quality is in the expertise we employ within our global workforce. As well as employing experienced technical experts who oversee the injection moulding process and develop both our standard and custom components for customers, we have distribution teams who regularly ship more than 17,000 orders each week across the world.
Our manufacturing and distribution expertise helps us to deliver thousands of consistent components efficiently from our 12 global sites. This means that customers can get the high-quality parts they need when they need them, reducing lead times and preventing disruptions on the production line, as well as ensuring that your final product or system functions and looks its best.