Plastic moulding is a manufacturing process for producing plastic precise injection molding parts by injecting molten plastic material into a mould cavity and allowing it to solidify. This process is also known as plastic injection moulding.
Introduction to Plastic Moulding
Plastic moulding or injection moulding is a commonly used manufacturing technique for producing high volumes of plastic parts. It is most commonly used to process thermoplastics, but thermosetting polymers like epoxy and silicon can also be moulded.
The plastic moulding process involves melting plastic pellets and injecting the molten plastic into a mould cavity under high pressure. The plastic cools and solidifies into the shape of the mould cavity. This results in the creation of a plastic part which can be ejected from the mould.
Advantages of Plastic Moulding
There are several advantages of using plastic moulding over other manufacturing processes:
- High volume production – injection moulding is ideal for producing large quantities of plastic parts efficiently and fast. It has high production rates compared to other methods.
- Consistent quality – moulded parts maintain excellent mechanical properties and consistency as the process can precisely controllable.
- Complex geometries – complex 3D geometries with tight tolerances.
- Low labour costs – labour costs are much lower.
- Minimal waste – injection moulding generates very little waste material. Any scrap plastic can be recycled.
Due to these benefits, injection moulding is used to manufacture a variety of plastic products like containers, bottles, packaging, cases, caps and closures, fittings and valves, toys, consumer electronics, medical devices and automotive parts.
The Plastic Injection Moulding Process
Plastic injection moulding transforms plastic pellets into finished products through a series of steps. Let’s delve into the active voice overview of the complete injection moulding process:
In the initial step, machine firmly clamp the two halves of the injection mould together to prevent any potential leakage of the molten plastic. This phase involves applying high clamping forces to ensure the mould remains securely closed while maintaining precise alignment.
Following the clamping phase, machine inject the molten plastic into the mould cavity through the sprue. This action requires the use of plungers or screws, which exert high pressure to force the polymer melt into the mould.
Dwell refers to the critical phase during which the molten plastic cools inside the mould cavity, solidifying into the desired shape. machine meticulously control the cooling time to prevent any defects from occurring.
Once the cooling process reaches its completion, it proceed to eject the moulded part from the mould. For this, we employ ejector pins to push the part out of the opened mould. It’s important to note that the part may still retain some residual heat and may remain relatively soft.
The final step, trimming, involves the removal of excess material such as sprues, runners, and flash from the moulded component. This is accomplished using cutting tools once the part has been ejected and has sufficiently cooled.
This concludes a single moulding cycle, and the process cyclically repeats as the mould closes again for the next injection.
Key Components of an Injection Moulding Machine
A plastic injection moulding machine consists of two main parts – an injection unit and a clamping unit. It also contains moulds, temperature control systems, safety interlocks etc.
1. Injection Unit
This unit plasticizes and injects the polymer into the mould. It consists of a hopper, barrel, screw and a nozzle. Granules are fed from the hopper into a heated barrel. The reciprocating screw rotates to melt, mix and inject the plastic through the nozzle into the mould.
2. Clamping Unit
It opens and closes the mould halves, and exerts clamping force during moulding to prevent leakage. This unit contains the platens and a hydraulic system to apply clamping force.
3. Injection Mould
The mould has two halves and contains cavities for forming the desired plastic parts. Moulds are often made of tool steel or aluminum. The mould is mounted on the platens and clamped.
4. Mould’s Cooling System
Cooling channels circulate water or oil to extract heat from the mould. This cooling solidifies the molten plastic.
5. Ejection System
It ejects the solidified part from the mould after cooling. Ejector pins are used to push the part out when the mould opens.
The Injection Moulding Process Cycle
The injection moulding process can be broken down into four stages within the cycle:
- The mould halves are closed and clamped together with sufficient pressure to prevent leakage
- Molten plastic is injected into the mould cavity through sprue/runners
- It fills the empty space taking shape of the mould
- Cooling channels allow the plastic to solidify while in contact with the mould walls
- Once solidified it can maintain shape after removal
- After adequate cooling time, the mould opens
- Ejector pins push the part out of the opened cavity
- The component is then trimmed of excess material
- The mould closes and the next cycle starts
This sequence of clamping, injecting, cooling and ejecting continues in a cyclic manner. The total cycle time ranges from 10 seconds to few minutes depending on part size, plastic material etc.
Factors Affecting the Injection Moulding Process
Several important factors influence the injection moulding process including:
- Melt and mould temperatures – Affects flow properties of molten plastic and its solidification rate.
- Injection pressure – Determines how well the material fills the mould cavity.
- Cooling time – Longer cooling produces stronger parts with better uniformity.
- Clamping force – Prevents leakage at high injection pressures.
- Injection speed – Faster speeds improve filling but may cause jetting.
Careful selection of these process parameters is required to produce good quality moulded components. Maintaining process control is also critical.
Different Types of Injection Moulding Processes
There are several variants of the basic injection moulding process:
- Thermoplastic injection – Most common type using thermoplastics like Nylon, PP, ABS etc.
- Thermoset injection – Uses thermosetting plastics like epoxy, silicone etc. Needs heat curing.
- Gas-assisted injection – create hollow interior volumes.
- Water-assisted injection – Water flow into the cavity along with plastic to produce hollow parts.
- Injection blow moulding – Used for producing hollow bottles and containers using stretch blow moulding.
- Multi-component moulding – Combines two or more materials into a single plastic part.
- In-mould decoration – Method for decorating plastic parts during moulding through insertion of films, fabrics etc.
Advantages of Plastic Injection Moulding
Some of the many advantages of injection moulding include:
- High production rates and low cycle times
- Ability to produce complex shapes with good tolerances
- Parts have excellent mechanical properties and consistency
- Low labour costs due to automation
- Minimal scrap losses and plastic waste
- Variety of thermoplastic and thermoset materials
- Coloring, texturing, finishing of parts is possible
- Allows high-volume production at low per unit cost
- Well suited for manufacturing large plastic components and parts
These benefits make injection moulding ideal for mass-manufacturing plastic products across many industries.
Disadvantages of Plastic Injection Moulding
Despite many advantages, injection moulding also has certain limitations:
- High initial costs for mould design and fabrication
- Only suitable for high volume production to justify mould costs
- Limited to thermoplastics, thermosets and elastomers only
- Not ideal for prototyping or short production runs
- Cycle time restrictions on very thick and large parts
- Poor aesthetic surface finish may require secondary operations
- Residual stresses may cause dimensional inaccuracies
- Limited strength capabilities compared to metal machining
- Colouring pigments can be difficult to mix evenly
Applications of Plastic Injection Moulding
Injection moulding manufactures different types of plastic products across various industries:
- Bottles, containers, caps, closures
- Food packaging like clamshells
- Cosmetics packaging
- Household items
- Interior and exterior trim parts
- Lighting components like headlamp lenses
- Under-the-hood components
- Dashboard instrumentation
- IV fluid bags, blood oxygenators
- Syringes, vials
- Medical trays, containers
- Diagnostic devices, disposables
- Furniture, storage boxes
- Housewares like kettles, cutlery
- Toys, recreational products
- Appliance housings, components
- Mobile phone and electronics housings
- Electrical fittings, plugs, sockets
- TV cabinets, frames, stands
- Pipe fittings, valves, connectors
- Electrical conduits & trunking
- Wall tiles, roofing sheets
- Door/window panels, frames
This concludes a detailed overview about plastic moulding and the injection moulding process. In summary, injection moulding is a versatile high-volume manufacturing process for producing plastic parts using reusable moulds. Across industries make different products we use everyday. With continuous innovations in materials and processing, injection moulding will continue to be an indispensable manufacturing technique for mass-production in the future.