Authored By: SDI Plastics

Plastic injection moulding is one of the most common and important manufacturing processes used today. It involves injecting molten material, usually plastic or metal, into a mould to form a part or product. But how exactly does injection moulding work and what are its key benefits over other manufacturing techniques?

This guide will provide an in-depth look at plastic injection moulding, its variations, and how it compares to other popular manufacturing processes like blow moulding, compression moulding, 3D printing, thermoforming, and more.

Overview of plastic injection moulding

Plastic injection moulding is a versatile, cost-effective manufacturing process ideal for producing high volumes of plastic parts with complex geometries to tight tolerances. The basic process involves melting plastic resin pellets in the Plastic injection moulding machine and injecting this molten plastic under high pressure into a steel mould tool. The mould tool contains a cavity that is machined into the desired shape of the final plastic part.

Once the molten plastic has been injected and fills this cavity space, it is held under pressure until it cools and solidifies into the final part shape. The mould tool then opens and the finished plastic part is ejected. This cycle then repeats over and over, with most injection moulding machines capable of cycling every few seconds.

Some key benefits of Plastic injection moulding include:

• High production rates and low per unit costs, ideal for large volume manufacturing
• Ability to produce complex geometries not possible with other processes
• Excellent dimensional accuracy and repeatability
• Various material options, including common plastics like ABS, nylon, and PET
• Variety of finishing options like textures, colours, and coatings
• Faster and cheaper than other manufacturing solutions for medium to high production volumes

Plastic injection moulding is used across vast industries including automotive, medical, aerospace, consumer products, packaging, and many more. Common injection moulded parts include plastic components, consumer packaging, automobile dashboards, medical devices, plastic containers, toys, pocket combs, and millions of other plastic products we use daily.

Popular plastics used in plastic injection moulding

There are thousands of different plastic materials available for plastic injection moulding. Some of the most widely used include:

Polypropylene (PP) – Versatile, low cost polymer used in packaging, medical, appliances, automotive parts, toys, and consumer products. Offers good chemical resistance.

Acrylonitrile butadiene styrene (ABS) – Common in automotive trim components, toys, computer keyboards, power tools. Strong and durable.

Polyethylene (PE) – General purpose plastic with excellent chemical resistance often used in packaging films, bottles, pipes, and flexible containers.

Polycarbonate (PC) – Extremely strong and impact resistant. Used for motorcycle helmets, eyeglass lenses, automotive lenses, medical devices, and transparent plastic panels.

Nylon (PA) – Excellent strength and wear resistance ideal for engineering and automotive components like gears, bushings, bearings. Also used in consumer goods.

Polyvinyl chloride (PVC) – Versatile and inexpensive, used for plumbing pipes and fittings, flooring, medical products, credit cards, wire insulation.

Polyethylene terephthalate (PET) – Most common plastic used for beverage bottles but also packaging films, food containers, clothing, carpet, automotive parts.

The type of plastic chosen depends on factors like tensile strength, flexural modulus, operating temperature range, chemical resistance, cost, aesthetics and more. Engineering-grade plastics like PC, ABS, nylon offer great mechanical properties while commodity plastics like PP and PE are inexpensive and offer good chemical resistance.

Plastic injection moulding process step-by-step

While plastic injection moulding may seem like a simple process of melting plastic and injecting plastic into a mould, there are actually several complex steps and machine components that enable it to produce intricate, high precision plastic parts in vast quantities.

Here is a simplified step-by-step overview of the complete plastic injection moulding process:

1. Plastic pellets are fed into the plastic injection moulding machine’s hopper from gaylords. These small bead-like pellets are gravity fed into the feed throat and then forwarded into the heating barrel using a rotating screw.
2. The screw turns and reciprocates to melt the plastic. As the pellets move along the screw towards the front of the barrel they are sheared apart, mixed, and exposed to conductive band heaters causing the pellets to melt.
3. When enough molten plastic accumulates, the screw moves forwards injecting the plastic into the mould. This happens under extremely high pressure, usually between 5,000 – 30,000 PSI to completely fill every crevice of the mould tool.
4. The plastic takes shape as it cools and solidifies in the tool. Cooling channels circulate water and air to carry away heat and solidify the thick sections first. Ejector pins are driven forward to pop the finished part out of the open mould.
5. The part is then removed from the tooling either by hand or using automation. Robots with suction cups are often used to pick finished parts from the mould area. The machine then resets itself and starts another cycle.

Modern plastic injection moulding machines are precisely engineered, computer-controlled systems designed to inject plastic with repeatable accuracy at high speeds. Machines can apply clamping forces from 50 to over 6,000 tons to keep moulds sealed against leaking during injection.

Other plastic moulding processes

In addition to injection moulding, there are other notable plastic injection moulding processes used in manufacturing:

Blow moulding – This method uses air pressure to inflate hot plastic in a mould cavity to form hollow parts like bottles, tanks, and containers. Common for high volume packaging and liquid containers.

Compression moulding – Heated plastic material is placed in an open mould cavity and compressed under high pressure. Used for moulding thermoset plastics, rubber, and composites in applications like automotive and aerospace.

Key types of plastic injection moulding processes

While the basics of plastic injection moulding are largely the same, over the years different processes have been developed to allow specialised injection moulding capabilities and improve part quality. Some key types include:

Multi-Shot moulding – Produces parts using multiple plastic resins or material compounds moulded in succession into one mould tool. Allows different properties like colours, textures, and materials to exist in one part.

Overmoulding – A multi-shot process where one material is moulded over another to create multi-material or soft touch parts like toothbrushes, grips, buttons, and more.

In-Mould decorating – Method to injection mould plastic parts with decorative films, logos, graphics already in the mould so they permanently fuse to the surface. Replaces post-mould painting or pad printing.

Gas-Assist injection moulding – Injects high pressure gas (nitrogen or argon) into molten plastic in the mould to create hollow interior channels and shapes not possible with solid plastic flow. Used to make tubular plastic parts.

Foam injection moulding – Uses chemical blowing agents or inert gases to expand plastic into a foam structure, reducing material usage and weight while increasing dimensional stability. Used for lightweight furniture, cushions and packaging.

Thermoplastic injection moulding – More environmentally friendly process that uses thermoplastic resins that can be reheated and remoulded again after the initial moulding. Recycles and reuses scrap plastic.

Liquid injection moulding – Uses liquid silicone or liquid rubber instead of molten plastic to accurately produce medical grade silicone or rubber components at high volumes. Used for seals, stoppers, valves, and O-rings.

Key advantages of plastic injection moulding

Now that we’ve covered the basics of the plastic injection moulding process and its variations, let’s examine some of its key advantages that make it such a ubiquitous manufacturing process, especially for medium to high production volumes.

High production volumes at low cost – 

Once mould tooling is fabricated, injection moulding offers very low per unit costs ideal for large production quantities. Mould tools often produce hundreds of thousands if not millions of identical plastic parts over their lifespan.

Complex geometries and detail – 

Molten plastic can flow into and fill incredibly complex part geometries with fine details that machining cannot easily achieve. Undercuts, threads, holes, slots, logos, and many shapes unique to injection moulding are possible.

Accuracy and consistency – 

Modern machining precision of mould tools combined with advanced injection moulding machines allow plastic parts to be replicated within microns of variation. Automotive grade moulds can often hold ±0.005” dimensional tolerances.

Material options – 

Thousands of plastic grades with diverse mechanical, thermal, electrical, and aesthetic properties to choose from including advanced engineered plastics suitable for auto, medical, aerospace use.

Automation – 

Plastic Injection moulding is highly automated for maximum efficiency. Parts can be produced around the clock with minimal operator intervention using robots, conveyors, sorters and other automation.

Low labour  – 

One plastic injection moulding machine and mould tool can replace dozens of workers producing plastic parts manually. It consolidates multiple manufacturing processes into one automated production cell.

Fast production – 

Cycle times can be as fast as a few seconds, allowing individual machines to produce hundreds of parts per hour. This speed enables massive quantities in short periods.

Clearly, injection moulding offers many benefits, but how exactly does it compare head to head against other popular manufacturing methods? Let’s take a look.

Injection moulding vs. 3D printing

3D printing builds up parts layer-by-layer in plastic, resin or metal powders whereas injection moulding produces parts in a mould cavity using molten plastic. Let’s compare both processes.

Speed – 

Consumer desktop 3D printers print slowly, often taking hours to produce one part. Industrial 3D printers are faster but low volume production. Injection moulding cycles in seconds, ideal for mass manufacturing. Vastly faster throughput.

Cost – 

Printed prototypes initially cost less in material but many hours of 3D print time. As volumes increase, injection moulding costs significantly less per part than 3D printing production which remains slow. Injection moulding also has higher upfront tooling costs.

Part size – 

Consumer 3D printers have small build volumes under 1 cubic foot. Industrial printers can print metre-sized parts. Injection moulding, only limited by mould tool size, routinely moulds parts over 3 feet long. Larger max part sizes.

Accuracy – 

Laser sintered and fused deposition modelling 3D prints have lower accuracy of ±0.005-0.02”. Parts often require sanding and machining to achieve tighter tolerances. Injection moulding routinely moulds plastic parts within ±0.003” due to precision tooling.

Appearance – 

Layer lines are faintly visible on 3D printed parts. Smoothing and painting required for fine appearance. Injection moulded parts directly produce flawless Class A finishes right out of the mould. Superior cosmetics.

Materials – 

Hundreds of injection-moulded plastic grades including advanced engineered plastics available. 3D printing is restricted to amorphous plastics like ABS with less strength. Wider material selection with injection moulding.

For prototypes, 3D printing excels due to no tooling. But for end use parts and mass production, injection moulding produces higher quality plastic components at far lower costs.

Injection moulding vs thermoforming

Injection moulding thermoplastics into metal cavity moulds differs from thermoforming which heats plastic sheet material against one-sided moulds. Let’s compare them:

Part wall thickness – 

Thermoforming relies on gravity and pressure to form typically thinner gauge material from 0.010” to 0.080”. Injection moulding injects material allowing thicker, stronger cross-sections from 0.080” to over 1”.

Complexity – 

Thermoforming is limited to relatively shallow, simpler part geometries without complex features due to the sheet forming process. Injection moulding can produce highly complex shapes with ribs, bosses, holes and threads.

Strength – 

Thin thermoformed parts lack rigidity and structural strength for load bearing applications. Injection moulded parts can be produced in glass filled plastics like nylon for immense rigidity and strength.

Finishes – 

Single sided moulds used in thermoforming generally produce one Class A finished side with B side appearance. Injection moulding easily makes flawless, glossy Class A finishes on all sides and faces.

Cost – 

Thermoforming generally has lower upfront tooling investment than injection moulding steel tools. However injection moulding has lower per unit cost at higher production volumes. Break even depends on annual quantities.

Volumes – 

Thermoforming suited for low to moderate production quantities up to approximately 50,000 units per year. Plastic injection moulding is ideal for large production volumes over 50,000 per year.

For shallow trays, blisters, and other basic components not requiring high strength or complexity, thermoforming competes well with injection moulding based on lower initial tooling costs. But for robust, complex components needed in high volumes, injection moulding is superior.

Making plastic injection moulding even more cost effective

While plastic injection moulding is already highly efficient, here are some tips manufacturing engineers use to make the process even more cost effective:

• Optimise part design – Eliminate excessive thick sections, undercuts, side pulls to reduce cycle times, clamp tonnage, and material usage. Design for mouldability.
  
• Use resins with lower material costs – For commodity parts, use PP or ABS instead of nylons and engineering plastics to lower raw plastic prices.
  
• Standardise on a few plastic grades – Reduces changeover scrap and allows purchasing in bulk for volume discounts.
  
• Multi-cavity moulds – Multi-cavity tools producing 4, 8 or 16 parts per cycle drastically increase output per injection press.
  
• Prevent process defects – Adjustments and monitoring to minimise scrap from common defects like short shots, flash, warp, sink marks.
  
• Automate finishing steps – Use robots or conveyors to automate labour-intensive finishing steps like degating, sanding, painting, assembly.
  
• Reuse plastic runners and sprues – Grind runners, sprues and reject parts into regrind material that can be reintroduced at 10-30% levels.
  

With wise production planning and optimisation, injection moulding costs can be lowered further while maintaining high quality standards.

Applications of plastic injection moulding across diverse industries

Now that we’ve covered the plastic injection moulding process from start to finish and compared it to other manufacturing methods, let’s take a look at some of the countless products and applications across various industries that utilise injection moulding to meet exacting standards:

Medical industry – 

Disposable syringes, IV connectors and taps, pill bottles, inhalers, clamps, medical trays, DNA analysis chips, orthodontics, prosthetics.

Automotive industry – 

Taillights, headlights, signal lights, door locks and handles, gear shift knobs, steering wheels, bumpers, trim, console parts, fluid reservoirs.

Aerospace – 

Aircraft interior trim, seats, camera housings, oxygen masks, overhead compartments, flight control surfaces.

Appliances – 

Refrigerator liners, washer tops and drums, air conditioner panels, range knobs and handles, dishwasher racks, microwave plates.

Consumer goods – 

Toys, musical instruments, sporting goods, remote controls, furniture, garden supplies, electronics, housewares.

Packaging – 

Bottle caps, food containers, vials, clamshells, trays, bulk bins, pallets, labels, tape cores.

Nearly every industry utilises injection-moulded plastic parts due to their precision, strength, durability, corrosion resistance, electrical insulation, low cost, and ease of complex designs. Injection moulding produces specialised components meeting stringent specifications across diverse applications.

In conclusion

We’ve explored the plastic injection moulding process, the various types of manufacturing, its advantages over competing manufacturing methods, and the vast array of applications benefiting from this high volume process. Plastic Moulding produces net shaped plastic components to tight tolerances with excellent repeatability. For medium to high production quantities, it offers rapid manufacturing of detailed and robust plastic parts at very low unit costs.

Plastic injection moulding is a constantly evolving manufacturing method, with newer technology enhancing product quality and expanding material options. Work with experienced injection Moulding partners like SDI Plastics, leveraging the benefits of modern plastic injection moulding and cutting-edge manufacturing processes to achieve your production goals. Their expertise in manufacturing can meet your programme needs from initial prototypes through to full production.

Contact SDI Plastics today to discuss your upcoming project requirements and discover how injection moulding provides a fast, cost-effective solution for producing flawless plastic components at production volumes from 1,000 to over 1,000,000 units annually.