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How Injection Molding Machine Works Polymers Into Products: A Process Guide
Created on 09.05
Injection molding machines have reshaped the landscape of manufacturing plastics – increasing process speeds, and lowering material waste. This step-by-step guide takes you through how injection molding machine works polymers to create complex plastic shapes with dimensional precision in a standard manufacturing line today.
What Is Injection Molding?
Injection molding is a high-precision manufacturing process where raw materials like plastics and metal are melted before being injected into a cast or mold for shaping. While this can be applied to metal through die casting, injection molding is a choice method when working with thermoplastics – for these few specific reasons :
- Low Melting Point – Plastic polymers have lower melting points, typically between (180°C–320°C). This ability allows for efficient energy use and more precise control over the viscosity of the material for a more accurate, consistent mold filling.
- High Dimensional Precision – When melted, plastics have excellent flow characteristics and low shrinkage rates. This allows accurate reproduction of the mold, no matter how complex. Plastics can also capture micro-textures, thin wall sections, and layers of detailed geometric designs.
- Ease of Reworking – Owing to its lower melting point, faulty final products can be ground up, molten, and fed back into the injection molding machine without significant degradation. This greatly reduces waste, costs of raw materials, whilst forming an efficient, closed-loop production line.
- Simplified Finishing Process – Often, a molded plastic product comes with a glossy, matte, or textured finish. This reduces a step in the line, reducing costs and boosting production efficiency, without compromising the final product quality.
- Flexibility & Customizability – Fillers like glass, fiber, calcium carbonate, and silicate are added to plastics to engineer them to a specific mechanical strength or flexibility. Colorants, metallic additives, masterbatches, and in-mold decorations/labels let you customize the final look of the product with greater specificity.
Materials In Injection Molding
There are two main types of plastics used in mass production – thermoplastics and thermosetting plastics. The key difference between them is how they react to heat applications and keep shape post production :
Thermoplastics
Thermoplastics are organic polymers that melt and soften when heated, then re-solidify at cooler temperatures. This ability is why thermoplastics is used as the main raw material in plastics manufacture. Since the process is cyclic, thermoplastics can be melted and remolded, reducing waste.
Examples : Polypropylene (PP), Polyethylene (PE), Acrylonitrile Butadiene Styrene (ABS), Polystyrene (PS), and Nylon (PA).
Applications : Household items, electrical appliances, machinery/automotive parts, packaging, and medical disposables.
Thermosetting Plastic
Thermosetting plastics need to be cured once shaped, allowing them to have a more rigid crosslinking for better mechanical strength. The catch, however, is that it can no longer be remelted and remolded – heat will begin to degrade the plastic instead.
Examples : Epoxy resins, Phenolics (PF), Melamine-formaldehyde, and Urea-formaldehyde.
Applications : Electrical housings, insulator, construction/building materials, high-temperature components, circuit boards, and automotive/industrial parts.
Common Polymers In Injection Molding
Naturally, thermoplastics dominate the scene since they’re easier to melt, mold, and re-shape. The processability, cost-efficiency, and mechanical properties of these polymers is what makes them ideal for mass production of plastic articles :
- Polypropylene (PP) : This lightweight, chemically resistant, polymer offers excellent fatigue performance. Hence, you’ll see it widely used in mechanical/structural parts, packaging, caps, and automotive trim.
- Acrylonitrile Butadiene Styrene (ABS) : This polymer is rigid, robust, impact-resistant and has a smooth finish. Common uses of ABS plastics are in children’s toys (like LEGO), consumer electronics, household items, and automotive interiors.
- Polyethylene (PE) : PE plastics – both HDPE and LDPE grades – are tough, flexible, and moisture-resistant. As a result, they’re often the base material when making plastic bags, containers, medical tubing, and films.
- Polystyrene (PS) : This plastic type is rigid, easily moldable, and inexpensive. You can find it used to make disposable cutleries, CD casings, insulation materials, and housings for appliances.
To further fortify and beautify the final product, plastic makers add various additives and colorants to the hot resin before the injection process.
Additives & Colorants In Plastic Injection Molding
Plastic polymers have their tensile strength, heat resistance, flammability, and final appearance customized by combining them with other polymers and pigments. Depending on the intended final result, plastic makers often add a variety of functional additives, fillers, and various colorants to enhance the plastic polymers :
- Colorants & Pigments : For consistent coloration and aesthetics, colorants and pigments are often added to molten plastics as masterbatches for maximal color precision.
- UV Stabilizers : These additives help reduce plastic degradation when exposed to UV light sources – like sunlight. This ability is critical for plastics designed for outdoor or automotive use.
- Glass Fibers & Mineral Fillers : Fillers like these reinforce the links between plastic polymer atoms, reinforcing overall strength and durability. This is common for plastics made for structural, electrical, or automotive applications.
- Plasticizers : These fillers allow plastics to be more flexible, pliable, and less brittle. Often, this is essential when making piping and tubing.
- Flame Retardants : Flame retardants reduce chances of the final product catching on fire. This is good to have for plastics in electronics, automotive, and building applications.
- Antimicrobials : Often, these are added to plastic products used in medical applications – especially disposable surgical tools.
- Anti-static Agents : This reduces the static energy that plastics naturally have. It’s a common filler in manufacture of plastics used in electronics.
How Injection Molding Machine Works Plastic Polymers
The injection molding process starts with melting the raw plastic polymer pellets under precision-controlled temperatures. Once the right viscosity and consistency is achieved, the plastics are injected into the mold to form the product.
Here’s the detailed, step-by-step process of how an injection molding machine works with plastics in a common manufacturing setting :
Step 1 : Clamping
- The mold, usually composed of two steel halves (the negative and the positive) will be mounted onto the injection molding machine.
- A hydraulic or toggle clamping unit will apply an accurate amount of pressure – usually several tons – to keep the mold secure during the injection process.
Step 2 : Injection
- Plastic polymer pellets are melted in a long barrel at a specific temperature, usually between 180–300°C, depending on the polymer type and final application.
- The reciprocating screw within the barrel heats and melts the supply of plastic pellets by way of friction and electrical resistance heaters as a heat source.
- As the screw advances forward, molten polymer moves towards the mold, ready for injection.
- The molten polymer is injected into the mold under high pressures – usually between 500 to 2000 bar. These pressures help ensure that the mold is evenly and completely filled. The set speed of injection prevents defects in the product, as a result of cooling.
Step 3 : Cooling
- Upon contact with the mold, the polymer begins to cool and harden. For faster cooling, manufacturing plants have a water-powered cooling systems that pass through the mold, quickly cooling the product for ejection.
- Cooling temperatures are monitored and kept constant to avoid defects like shrinkage, warpage, and crystallinity of the polymer. Often, cooling takes up to 60% of production time.
Step 4 : Ejection
- Once cooled, the mold halves separate and the product is removed with air blasts, plates, or ejector pins. In some cases, this step would be done manually by human power.
- For more intricate designs or complex geometries, the machine would remove the product using lifters or sliders. This is to prevent damage, stress marks, or deformation as complex designs are fairly fragile.
Step 5 : Post-Processing (Optional)
- Depending on what is being produced, the plastic product may need finishing steps before it can head to packaging.
- Common finishing processes include trimming, finishing, dimensional checks, printing, assembly, and fusing.
Here’s a diagram of how the molten material travels during an active injection molding process :
Injection Molding : Revolutionizing Plastics Production
The efficient and accurate process of how injection molding machines work to mass produce plastic products has single handedly changes the game of manufacturing. For precision-engineered, energy-saving models, browse our catalog of injection molding machines and speak to our experts for a quote.
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