The Complete Guide to Injection Molding Machines

Injection molding is one of the most common and important manufacturing processes used today. It allows for the efficient, high-volume production of plastic parts with good dimensional accuracy and repeatability. At its core, injection molding uses heat and pressure to mold plastic resin into a desired shape via a mold. But there is much more to it than that simple explanation. This comprehensive guide will provide you with everything you need to know about injection molding machines and the injection molding process.

Table of Contents

What is an Injection Molding Machine?

An injection molding machine, sometimes known as an injection molder or injection press, is a large industrial machine used to produce plastic products at high volumes via the injection molding process. It consists of two main parts – an injection unit and a clamping unit.

The injection unit is responsible for heating, melting, and injecting the plastic material into the mold. It contains a hopper, barrel, screw, and nozzle. The raw plastic pellets are fed into the hopper. Then the screw, powered by a motor, pushes and heats the plastic inside the barrel until it melts. Once molten, the material is injected through the nozzle and into the mold.

The clamping unit provides the force necessary to keep the two halves of the mold closed and withstand the pressure from the injection process. This is done through a hydraulic or mechanical system that clamps the mold together. The clamping force is measured in tons – for example a 300 ton injection molding machine produces 300 tons of clamp force.

Once the molten plastic is injected and the mold is clamped shut, it is allowed to cool and harden into the desired shape before the mold opens and the part is ejected. The mold is then closed and the cycle repeats for continuous production of parts.

Types of Injection Molding Machines

There are several different types of injection molding machines that provide various levels of automation, clamping force, and production capabilities:

Plasticizing Machines – The oldest and simplest injection molders that require an operator to manually feed plastic into the injection barrel between cycles. Used for short runs.
Reciprocating Screw Machines – Uses a screw that reciprocates to melt the plastic and inject it into the mold. Provides more automation and efficiency than plasticizing machines.
Hydraulic Injection Molding Machines – Hydraulic fluid and cylinders are used to generate the clamping force as well as inject the plastic. Allows for higher clamping forces.
Hybrid Injection Molding Machines – Combines hydraulic and electric technology, providing energy efficiency for clamping and precision control over injection.
All-Electric Injection Molding Machines – Rely solely on electric servo motors to drive the screw, clamping, and other motions. Provide the most energy efficiency and precision control.
Two Platen Machines – Have a stationary platen and a moving platen for opening/closing the mold. Common for smaller presses.
Three Platen Machines – Have two stationary platens with a movable platen in between to allow larger molds. Used for making larger parts.

Main Parts of an Injection Molding Machine

While injection molding machines vary in complexity and configuration, most contain the following main components:

Hopper – This funnel-shaped container holds the raw plastic pellets right before they get fed into the barrel. It’s mounted on top of the barrel.
Barrel – The heated metal cylinder that the plastic pellets get fed into from the hopper. It contains the reciprocating screw which pushes, melts, and mixes the plastic.
Reciprocating Screw – This rotating screw inside the barrel is what mixes, melts, and injects the plastic into the mold. It can move axially to inject and retract the material.
Clamping Unit – Provides the force necessary to keep the mold closed during injection phase. Usually driven by hydraulic or electric motors. Measured in tons.
Platens – The large metal plates that hold the halves of the mold and connect it to the clamping unit. There are typically two or three platens.
Tie Bars – Long steel rods that connect the stationary and moving platens to guide them and withstand clamping forces.
Mold – Made from high strength steel or aluminum, the mold has the cavities that form the desired plastic parts. It may be machined in two or more pieces.
Nozzle – At the front end of the barrel, the nozzle ensures the melted plastic gets injected directly into the mold through the sprue.

The Injection Molding Process Step-By-Step

Now that the main components have been covered, let’s look at what exactly happens during the injection molding process:

Clamping – The two halves of the injection mold are securely clamped together by the clamping unit. This will keep the mold closed during injection.
Injection – Raw plastic pellets are fed into the hopper then through the barrel via the rotating screw. Friction and external heating melt the pellets. Once molten, the screw moves forward and injects the plastic into the mold through the nozzle and sprue.
Cooling – Cooling channels in the mold draw heat away from the molten plastic, allowing it to solidify into the final shape. This typically takes 10-60 seconds depending on the part size and plastic type.
Clamping Release – Once cooled and solidified enough, the clamping force is released from the mold. The two halves open.
Ejection – Ejector pins push the finished plastic part(s) out of the opened mold. The parts are then trimmed and collected.
Clamping – The mold closes again and the clamping force is reapplied, preparing for the next injection cycle to start.

This sequence is typically fully automated and can produce parts at a rate of seconds per cycle. The most efficient injection molding processes are those that minimize cooling time to allow faster cycling.

Key Process Parameters

There are several important injection molding process parameters that technicians and engineers adjust to optimize production efficiency, part quality, and material properties:

Clamping Force – The amount of force applied by the clamp to keep the mold closed. Measured in tons. Too little can cause flashing.
Shot Size – The volume of plastic injected into the mold during each cycle, usually 1.1-1.5x the part volume. Too much causes flash, too little causes short shots.
Injection Pressure – The pressure at which the molten plastic is injected into the mold, usually around 10,000-30,000 PSI. Effects filling of mold.
Fill Time – The injection phase time for the plastic to fill the mold cavity, normally 1-3 seconds. Faster fill can increase production speed.
Packing Pressure – Pressure applied after initial injection to pack more material into the mold and prevent shrinkage, around 5,000-20,000 PSI.
Cooling Time – Time allowed for the plastic part to solidify in the mold before opening it. Typically 10-60 seconds. Longer prevents defects.
Back Pressure – Pressure maintained in the barrel during cooling phase to counteract shrinkage of the cooling plastic. Usually 20-100 PSI.
Cycle Time – The total time taken for one complete injection molding cycle. Key metric to minimize.

Plastic Resins Used in Injection Molding

Not all plastics can be molded through injection. The most common resin types used are:

Polypropylene (PP) – Versatile, low cost polymer used for containers, appliances, automotive parts, medical devices, and consumer goods. Easy to mold with good strength.
Acrylonitrile Butadiene Styrene (ABS) – Amorphous thermoplastic with good dimensional stability, impact resistance, and temperature tolerance. Used for automotive body panels, appliance housings, keyboards.
Polyamide (Nylon) – Engineering plastic with high strength, wear resistance, and temperature resistance. Common in automotive components. Difficult to process.
Polycarbonate (PC) – Optically clear, high impact resistant, dimensionally stable resin. Used for lenses, glazing, protective gear, electronics housing.
Polyethylene (PE) – Most common plastic with low processing temperature and cost but also low strength. Used for bottles, containers, tubes, sheets. Comes in LDPE and HDPE grades.
Polyvinyl Chloride (PVC) – Versatile and durable but can be brittle. Used for pipes, siding, flooring, bottles, packaging, cards.
Polystyrene (PS) – Low cost resin for low strength applications like packaging, cups, cutlery, boxes, foam, models.
Polyethylene Terephthalate (PET) – Clear but tough and shatter resistant. Most commonly used for beverage bottles and food packaging.
Acrylic (PMMA) – Optically transparent thermoplastic with good weather/UV resistance. Used for lenses, glazing, taillights, displays.

Benefits of Injection Molding

There are many benefits that make injection molding one of the most popular manufacturing processes:

High Volume Production – Once the initial fixed costs are paid, injection molding provides very low per-unit costs for high volume production. Millions of parts can be made.
Design Flexibility – The injection molding process allows complex parts with fine details and thin walls to be created, unlike other methods. Very complex geometries are possible.
Material Options – Many different plastic resin types, including engineered plastics, can be used to achieve the desired material properties like strength, aesthetics, thermal or electrical properties.
Accuracy & Repeatability – Injection molded parts can be produced to tight tolerances with very little variability between parts and production cycles. Automation increases consistency.
Low Labor – Automated injection molding machines have very low labor requirements to keep production running, reducing overall costs. Little manual work is required.
Fast Production – Once molds are designed and fabricated, injection molded parts can be produced very rapidly, some in just seconds per part.
Minimal Secondary Processing – Parts can be ejected from the molds with very fine details already molded in, requiring little to no additional finishing or machining.

Drawbacks of Injection Molding

Despite its many benefits, there are some downsides to injection molding that should be considered:

High Initial Costs – The injection molds themselves are very expensive to design and machine. New molds can cost thousands to tens-of-thousands of dollars. Makes short runs costly.
Part Size Limitations – Extremely small or extremely large injection molded parts can be difficult to produce. Best for smaller parts up to 4 feet long.
Low Strength Materials – While some engineered plastics have good strength, the most commonly molded thermoplastics like PP and PE have fairly low strength compared to other materials.
Limited Resin Types – Only thermoplastic resins can be used since the plastic must be melted and recast. Thermosets and elastomers like rubber cannot be injection molded.
Dimensional Stability Concerns – Flow of the molten plastic during injection can cause warpage in parts as they cool. Keeping walls thin and uniform can help.
Sink Marks – Thicker sections of plastic material can shrink and cause visible dents or marks in finished parts as they cool inside the mold.
Flash – Excess material escaping due to too much pressure or clamp force can leave thin mold lines or “flashing”. This must be trimmed from parts later.

Applications of Injection Molding

Because it offers so many advantages, injection molding is used to make millions of different plastic parts and products across virtually every industry:

Packaging – For plastic bottles, containers, caps, packaging, tubes, cases, trays, etc.
Automotive – Interior and exterior components, lighting, trims, panels, knobs, dials, bumpers, grills.
Consumer/Electronics – Laptop and electronics housings, cases, phone and camera components, power tool housing.
Medical – Diagnostic products, sample collection ware, IV, catheters, syringes, trays.
Appliances – Washer/dryer components, refrigerator parts, dishwasher baskets.
Construction – Light switches, electrical boxes, fixtures, power tools, vinyl siding.
Aerospace – Ducting, seating, galley components, panels, covers, cowlings.
Toys & Sporting Goods – Action figures, balls, frisbees, slides, play equipment, protective gear.
Housewares – Kitchenware, furniture, storage containers, office accessories.

Nearly every plastic product today is manufactured economically through injection molding. It has allowed plastic to become such a ubiquitous material in consumer and industrial applications.

Size and Tonnage of Injection Molding Machines

Injection molding machines are typically referred to by the clamping force they provide, which is measured in tons. As mentioned earlier, this clamping force is what keeps the mold closed during injection. The required clamp force is determined by the area of the mold and the injection pressure.

Some examples of common injection molding machine sizes:

Micro Injection Molding – 2 to 10 tons
Small Machines – 25 to 100 tons
Medium Sized – 70 to 500 tons
Large or Advanced – 200 to 5,000+ tons

For extremely tiny, high precision plastic parts like medical components, micro injection molding with machines under 10 tons is used. At the other end, giant machines with over 4,000 tons can mold large automotive parts. The most common sizes for consumer plastic products are in the 100 to 700 ton range.

Larger parts typically require larger presses, but part geometry is also a factor. Complex parts with small thicknesses may need higher clamp force to prevent flash while simple large part shapes can be molded in smaller machines.

Size and Tonnage of Injection Molding Machines

Factors That Determine Injection Mold Costs

Creating and manufacturing a new injection mold can be very expensive due to the complexity of machining, finishing, and polishing required. Typical factors that determine the total mold fabrication costs include:

Part Size – Larger molds and molding surfaces cost more due to increased material and machining time.
Complexity – Intricate molds with small details, multiple sides, and precision features have higher machining costs. Simple molds can be made faster.
Material – Most molds are made from tool steel or aluminum. Steel is stronger but slower to machine. Aluminum is cheaper but less durable.
Surface Finish Requirements – Molds that require mirror finishes or special coatings for quality finishes on parts have higher polishing costs.
Production Volumes – Molds expected to produce millions of cycles will utilize harder tool steels and higher precision to last longer, increasing costs.
Side Actions – Molds that require moving slides, lifters, collapsible cores, or other side actions to release parts will be more complex and costly.
Custom Features – Specialized features like conformal cooling channels, integrated sensors, or quick change components add engineering and production costs.

For a small plastic part to be produced in low-mid volumes, the injection mold may cost $5,000 to $10,000. Very high production molds for large components with complex engineering may cost over $100,000.

Mold Manufacturing Process

There are several key steps involved in the complete manufacturing process when making a new injection mold:

Mold Design – The mold is carefully designed based on the final plastic part geometry and production requirements. Usually done with advanced CAD software. Critical features are defined.
CNC Machining of Mold Cores and Cavities – The individual cores and cavities that make up the mold halves are precision CNC machined from steel or aluminum blocks. This can take days to weeks depending on mold size and complexity.
Polishing – Mold surfaces are manually polished and finished to remove any rough edges and provide a smooth finish to molded parts. Multiple fine grit polishing steps are used.
Component Insertion – Any ejector pins, cooling lines, sliders, guide pins, sensors, or other parts are added to the mold.
Testing and Validation – Test injections are done to verify part quality and tweak mold geometry or process parameters as needed.
Assembly and Tuning – The two mold halves are assembled together and clamped onto an injection molding machine for final tweaking and machine setup tuning to get the optimal production process.

The machining and polishing steps take up the bulk of the total manufacturing lead time. Very simple molds may be produced in 1-2 weeks, but 8-12 weeks is typical for most molds, and complex precision molds can take over 20 weeks.

Key Suppliers of Injection Molding Machines

There are many equipment manufacturers around the globe that design and produce injection molding machines and technology:

Arburg – Leading German injection press manufacturer with an emphasis on ALL-electric machines. Makes machines from 170 to 5000 kN of clamp force.
Engel – Another large European manufacturer of injection molding machines, offering hydraulic, hybrid, and electric options from 200 to 6000 kN.
Haitian – Major Chinese machinery producer known for more affordable hydraulic and electric injection molding equipment.
Sumitomo (SHI) – Japanese conglomerate that manufactures a diverse range of injection molding machines.
Husky Injection Molding – Leading supplier of injection equipment and turnkey solutions for rigid plastic packaging applications.
KraussMaffei – Renowned German machinery company with large hydraulic and hybrid injection molding machines.
Netstal – Swiss manufacturer specializing in premium all-electric injection presses. Part of Milacron group.
Wittmann Battenfeld – Offers a wide range of injection molding machines.

Toshiba – Diversified electronics conglomerate from Japan that also manufactures smaller injection molding equipment.
Chen Hsong – Chinese company focusing on energy efficient hydraulic and all-electric injection molding solutions.
UBE Machinery – Another well-known machinery producer from Japan offering a full range of injection molding systems.
JSW – Large supplier of hydraulic and electric injection presses based in Japan with precision molding focus.
LS Mtron – South Korean company specialized in all-electric multi-component injection molding technology.
Mitsubishi – Japanese industrial giant that manufactures hydraulic, hybrid, and electric injection molding equipment through the Mitsubishi Heavy Industries group.
Nissei – Major Japanese manufacturer of injection molding machines that pioneered the 2-platen molding process.

This list covers some of the most prominent global manufacturers of new injection molding machines that support the production of millions of plastic parts every year. Many also offer refurbishing services for used equipment.

Buying Used Injection Molding Machines

In addition to buying brand new injection molding machines, purchasing quality used and refurbished injection molding equipment can be a very smart financial decision:

Lower Capital Investment – Used injection presses can cost 50-70% less than brand new equipment, significantly reducing startup and investment costs for molding operations.
Faster Delivery Lead Times – While new custom presses can take months for delivery, used machines are often readily available much quicker.
Proven Reliability – Pre-owned equipment from leading OEMs like Arburg and Engel already have years of reliable service with known maintenance history.
Support Available – Many used machine dealers offer equipment warranties, maintenance contracts, spare parts inventories, and technical support.
Flexible Financing Options – Used machinery can be financed over longer terms with lower monthly payments, preserving working capital.
Ability to Upgrade Later – Start with an affordable used press now, then upgrade to higher tonnage or newer technology models later as needs grow.

Used injection molding machines should be purchased through reputable dealers that fully inspect, repair, and refurbish the equipment before resale. This ensures reliable operation.

Conclusion

Injection molding is a ubiquitous manufacturing process used to produce millions of identical plastic parts quickly and affordably. The technology leverages heat and pressure to mold thermoplastic resins into final parts matching a desired design. Injection molding machines provide the high clamping forces and injection capabilities required, enabling mass production of complex geometries not easily achievable through other fabrication methods. While the initial mold creation is expensive, the per-unit cost of each additional injection molded part is very low. When produced through automated injection molding, plastic components can offer strength, precision, and economical production at scale across nearly every industry.