The text on this page is a sample from our full White Paper ‘Injection Moulding for Buyers’ -
This guide is intended for people who are looking to source plastic mouldings. It gives a much needed insight into all that is involved with creating plastic parts, from the mould tool required to the moulding process itself. It also explores what to look out for when obtaining quotes and comparing them. If you want to explore further, the guide covers types of mould tools, as well as special finishing processes such as colours & plating. Words that are underlined can be found in the glossary in the appendix...
Plastic injection moulding is a very precise process that offers several advantages over other plastic processing methods. Here are just 5 benefits:1. Precision Plastic injection moulding is perfect for very intricate parts. Compared to other techniques, moulding allows you to incorporate more features at very small tolerances. Have a look at the image to the right. You can hold this moulding in the palm of your hand and it has bosses, ribs, metal inserts, side cores and holes, made with a sliding shut off feature in the mould tool. That's an awful lot of features on a small part!2. Material choiceThere's a vast amount of materials available for plastic injection moulding. A range of standard materials, but also things like antistatic plastic, thermoplastic rubber, chemical resistant plastics, infrared, biocompostable...and with colour compounding or masterbatch colouring you have an endless choice of colours as well. The moulding above is just black, but it's made out of PPO which is an extremely rigid and flame-retardant material.....
The injection moulding process involves heating & injecting plastic material under pressure into a closed metal mould tool. The molten plastic cools & hardens into the shape inside the mould tool, which then opens to allow the mouldings to be ejected or removed for inspection, delivery or secondary operations.
Stage 1
Material granules from the hopper feed into the heated barrel & rotating screw.
Material melted by heat, friction & shear force is forced through a check valve to the front by the rotating screw....
The core & cavity design of the plastic injection mould tool is what gives the final product its shape, but there are several other functions of the tool that are crucial for the correct formation of the end product...
Manufacturing in China can save you a lot of cost, but it can come with problems such as delivery delays, miscommunications, inferior quality and the paperwork surrounding importation. Using a UK moulder with existing ties to China can remove the risk and still result in cost-saving....
Boss - On a moulded part, an upright column which can take a metal insert or a screw for example.
Cavity - The part of an injection mould tool that gives the plastic product its shape, that does the actual moulding of the plastic. Also see mould tool chapter on pages 5 and 6 for all terms associated with the mould tool
Cycle time - The time it takes for a mould cycle to be completed, i.e. from material feed & melting; material injection; cooling time and ejection to the re-closing of the mould tool ready for the next cycle.
Draft angles - The walls of a moulded part should be slightly tapered in the direction in which the part is ejected from the mould tool, to allow the part to be ejected easily. This angle at which they are tapered is called the draft angle.
Ejector stroke - The pushing out of ejector pins to eject the moulded part from the mould tool. Ejector stroke speed, length and timing needs to be carefully controlled to prevent damage to the ejectors and mould tool, but at the same time make the moulding cycle as short as possible.
High polish - A special finish on the cavity of the mould tool which ensures the plastic part is super smooth
Locked-in features - Features on a plastic part design that would make the plastic part impossible to remove from the mould tool, or that would cause the mould tool to need expensive mechanisms to be able to remove the part.
Overlocking - When a mould tool has been set into a moulding machine incorrectly, causing the tool to shut too hard and so damaging the mould tool
Part repeatability - The ability to create identical plastic parts time after time
Radii - Perfectly straight corners are impossible to eject from the mould tool. A slight radius should be added to any straight corners. Ribs - When a plastic part has thin walls, ribs are added to the design to make the thin walls stronger
Side cores - Side action which produces a feature on a moulded part, at an opposing angle to the normal opening direction of the mould tool. The side core needs to be able to retract as the plastic part cannot be ejected otherwise.
Sparked finish - A special finish on the cavity of the mould tool which ensures the plastic part has a slightly gritty texture – think about some car dashboards, keyboards, computer frames for example
Tolerance - The margin by which a moulded part is allowed to deviate from the sizes specified on the drawing
Tool bolster - A near complete standard mould tool which can take a core and cavity insert.
Walls - The sides of a moulded part
The text on this page is a sample from our full White Paper ‘Injection Moulding for Buyers’
Plastic components are used in many industries. From automotive to home appliances and medical devices, components in a variety of plastics are used to protect, enhance and build a huge range of products.
With its reliable, high-quality performance, injection molding is one of the most common processes used to produce plastic components. Indeed, the compound annual growth rate (CAGR) of the injection molded plastics market is expected to increase by 4.6% up to 2028.
Yet, despite its ability to produce high numbers of plastic components quickly, the injection molding process must be tightly controlled to maintain the quality of the final parts. This article will explain how injection molding works and how experienced manufacturers control the process to produce the best quality plastic components. We'll cover:
Injection molding is a complex manufacturing process. Using a specialized hydraulic or electric machine, the process melts, injects and sets plastic into the shape of a metal mold that’s fitted into the machine.
Plastic injection molding is the most widely used components manufacturing process for a variety of reasons, including:
This cost-effectiveness, efficient production time and component quality are just some of the reasons why many industries choose to use injection molded parts for their products.
As well as the fact that plastic injection molding process can be optimized to have a lower carbon impact. Find out more in our guide, Design for Sustainability: Optimizing Plastic Injection Molding Processes.
The injection molding cycle includes many parameters which need to be tightly controlled to ensure the overall quality of the plastic components produced. Understanding the process and parameters in some depth will help manufacturers to identify plastic components producers who can provide the quality and consistency they need.
Before the actual process begins, it’s key that the right thermoplastics and plastic injection molds are selected or created, as these are the essential elements that create and form the final components. Indeed, to make the right selection, manufacturers need to consider how the thermoplastic and mold interact together, as certain types of plastics might not be suitable for particular mold designs.
Each mold tool is made up of two parts: the mold cavity and the core. The mold cavity is a fixed part that the plastic is injected into, and the core is a moving part that fits into the cavity to help form the component’s final shape. Depending on requirements, mold tools can be designed to produce multiple or complex components. The repeated high pressures and temperatures that mold tools are put under mean they are typically made from steel or aluminum.
Due to the high level of design and quality of materials involved, developing mold tools is a long and expensive process. Hence, before creating a final bespoke mold, it’s recommended that tools are created, prototyped and tested using computer aided design (CAD) and 3D printing technology. These tools can be used to digitally develop or create a prototype mold that can then be tested in the machine with the chosen thermoplastic.
Learn more about how 3D printing can complement the injection molding process.
Testing the tool with the right thermoplastic is key to ensuring that the final component has the right properties. Each thermoplastic offers different characteristics, temperature and pressure resistances due to their molecular structure. Plastics with an ordered molecular structure are called semi-crystalline and those with a looser structure are known as amorphous plastics.
Each plastic’s mechanical properties will make them appropriate for use in certain molds and components. The most common thermoplastics used in injection molding and their characteristics include:
The final thermoplastic selection will depend on the characteristics that manufacturers need from their final component and the design of the mold tool. For example, if a manufacturer needs a lightweight part with electrical properties, then PC will be appropriate, but only if the mold doesn’t need to operate above 275°F (135°C) or at very high pressures, which the plastic won’t be able to resist.
Once the right thermoplastic and mold have been tested and selected, the injection molding process can begin.
Injection molding machines can be powered by either hydraulics or electricity. Increasingly, Essentra is replacing its hydraulic machines with electric-powered injection molding machines, showing significant cost and energy savings.
Injection molding machines consist of a feeder or ‘hopper’ at the top of the machine; a long, cylindrical heated barrel, which a large injection screw sits in; a gate, which sits at the end of the barrel; and the chosen mold tool, which the gate is connected to.
To start the process, raw plastic pellets of the thermoplastic material are fed into the hopper at the top of the machine. This could be virgin material, like plastic resin, or recycled plastic materials. As the screw turns, these plastic pellets are fed gradually into the barrel of the machine. The turning of the screw and the heat from the barrel gradually warm and melt the thermoplastic to melting point until it turns to a molten material.
Maintaining the right temperatures within this part of the process is key to ensuring the plastic can be injected efficiently and the final plastic part formed accurately for the injection molding project.
Once the melted plastic reaches the end of the barrel, the gate (which controls the injection of plastic) closes and the screw moves back. This draws through a set amount of plastic and builds up the pressure in the reciprocating screw ready for injection. At the same time, the mold halves close together and are held under high pressure, known as clamp pressure.
Injection pressure and clamp pressure must be balanced to ensure the part forms correctly and that no plastic escapes the tool during injection. Once the right pressure in the tool and screw is reached, the gate opens, the screw moves forward, and the molten plastic is injected into the mold.
Once most of the plastic is injected into the mold, it is held under pressure for a set period. This is known as ‘holding time’ and can range from milliseconds to minutes depending on the type of thermoplastic and complexity of the part. This holding time is key to ensuring that the plastic packs out the tool and is formed correctly.
After the holding phase, the screw draws back, releasing pressure and allowing the part to cool in the mold and the plastic solidifies. This is known as ‘cooling time’, it can also range from a few seconds to some minutes and ensures that the component sets correctly before being ejected and finished on the production line.
After the holding and cooling times have passed and the part is mostly formed, ejector pins or plates eject the parts from the tool. These drop into a compartment or onto a conveyor belt at the bottom of the machine. In some cases, finishing processes such as polishing, dying or removing excess plastic (known as spurs) may be required, which can be completed by other machinery or operators. Once these processes are complete, the components will be ready to be packed up and distributed to manufacturers.
At Essentra, injection molding is a key production process. As part of our ESG strategy, we are optimizing our plastic injection molding processes, including replacing old hydraulic machines with electric machinery.
We continue to invest in material innovation and have a new Centre Of Excellence at our UK headquarters, which will help us to test and develop new materials that will in turn enable us to offer even more sustainable product ranges globally for our customers.
Our team of experts are on hand to help you create consistent, high quality parts for your next project. With over 45,000 molds to choose from and custom solutions available, we can deliver the parts you need. As well as plastic parts, we also have a range of metal components manufactured using hot chamber die casting.
Free CADs are available for most solutions, which you can download. You can also request free samples (some exclusions apply) to make sure you’ve chosen the right product for what you need. Same day dispatch for sample requests received by 4pm.
If you’re not quite sure which solution will work best for your application, our experts are always happy to advise you.
Request your samples or download free CADs now.
Questions?
Email us at sales@essentracomponents.com or speak to one of our experts for further information on the ideal solution for your application 800-847-0486.