Injection molding under the microscope. Materials and production process

One of the most efficient technologies used for plastic component production is injection molding. In this article, you will learn how this process works, the properties of materials used for molding, and the advantages and disadvantages of its application.

The injection molding process is used for mass production. Thanks to the high repeatability of the entire cycle, it is very easy to produce thousands of units of a product with a very consistent level of quality. This is a technology that offers a wide range of possibilities and is used to manufacture various parts – from the smallest components to automobile body panels. In the later part of the article, we will provide a more detailed overview of the production process.

Materials used in injection molding

Injection molding of plastics allows for the use of most polymers. These can be polymer resins, all thermoplastic materials, some thermosetting materials (such as epoxy and phenolic), and elastomers. Most thermoplastics are characterized by biodegradable properties, wide-ranging applications, and the ability to become pliable when heated. When choosing materials, it’s important to consider criteria such as cost, strength, elasticity (the ability to return to its original shape and volume after the removal of external forces causing deformation), chemical resistance, and water absorption.

The most critical aspect when selecting the appropriate parameters is the shrinkage of the plastic material. Shrinkage refers to the change in volume or dimension of the molded part compared to the volume and dimensions of the molding cavity. Shrinkage varies for each material, ranging from 0.2% to as much as 4%.

The properties of each material make it suitable for specific conditions. Some of these properties are presented below:

• Acrylonitrile-butadiene-styrene (ABS) – Due to its smoothness, rigidity, and strength, ABS is excellent for parts that require impact resistance and dimensional stability maintenance.
• Polyamide (PA) – available in various variants, different nylons offer different properties. They are resistant to high temperatures and chemicals, have high hardness, good sliding properties, and shape stability.
• Polycarbonate (PC) – lightweight, has high strength and shape stability, as well as good electrical properties, a wide temperature range of operation, and is flame-resistant.
• Polypropylene (PP) – with good fatigue resistance, PP is physiologically neutral, making it suitable for food contact.
• Polyethylene (PE) – relatively easy to process, suitable for food contact, chemical resistance, and the most commonly used material in rotomoulding technology.
• Polyethylene terephthalate (PET) – high mechanical strength, dimensional stability, high rigidity.
• Polymethyl methacrylate (PMMA) – high hardness, excellent optical properties, making it commonly used in the lighting industry, gloss.
• Polystyrene (PS) – dimensional stability, high rigidity.

The process of injection molding of plastics

In this process, plastic granules are heated within the barrel of the injection molding machine, and then a rotating screw inside the barrel moves the material towards the mold. Under the influence of heat, the material transforms into a plasticized mass. The combination of high pressure and temperature causes the material to become pliable, followed by ejection in the form of a finished molded part.

The injection temperature is typically around 200 degrees Celsius, which allows the plastic material to become molten and form a pliable mass. The introduction into the screw causes the injection of the molten plastic into the cavities located within the closed injection mold. The injection pressure depends on the type of material used, its geometry, and its mass. The hardening of the injected plastic enables the removal of the finished molded part from the injection mold. The cycle is repeated to produce the desired number of parts or for a specified period.

One of the techniques in plastic processing is a method called insert molding. Thanks to this method, there is no need to create a large, expensive mold. Instead, inserts are placed in the mold where the mold cavities are located. The use of this method significantly reduces the production time of an injection mold (usually less than 30 working days) and reduces costs compared to a mold used in traditional injection molding – the mold cost can be up to 50% lower. This solution is dedicated to the production of small, uncomplicated parts with lower volume (10,000-100,000 pieces). The manufacturing process remains unchanged when using this technology.

Injection molding of plastics – advantages and disadvantages

Advantages:

• Very short cycle time.
  The typically lasting several, sometimes dozens of seconds, allows for the production of hundreds or even thousands of product units in a short period.
• Versatility of shapes.
  The ability to produce components with highly complex shapes and tight dimensional tolerances.
• Low waste generation
  . Plastic injection molding is characterized by a low amount of waste compared to other manufacturing methods (e.g., CNC machining). Additionally, the used material can often be recycled. Environmental advantages also include low emissions of harmful substances.
• Flexibility in mold design and material selection.
  The manufacturer has significant flexibility in creating shapes and choosing the thermoplastic used for the production of individual components.
• Energy efficiency.
  Modern electric injection molding machines can consume relatively little energy.
• Production repeatability and dimensional stability of produced molded parts.
  Proper control of process parameters ensures the quality and geometry of components, which remains consistent even during rapid production of a large number of molded parts.
• No need for finishing machining.
  The use of injection molding technology does not require additional post-production machining.
• The possibility of full process automation.
  An integrated production line, supported by computer systems, allows for a reduction in the human resources required to operate it.
• High quality of produced molded parts.
  Customers will be looking for durable, flexible, and complex components. Injection molding of plastics proves itself in their production, maintaining a high level of quality.

Disadvantages:

• High initial mold cost.
  The cost of an injection mold can be very high. Therefore, it is important to carefully choose the manufacturing technology to optimize costs.
• Long mold production time.
  Typically, this process takes around 12-16 weeks.
• Highly skilled personnel are necessary.
  The project-execution team must, among other things, plan the process, design the mold, and select the appropriate processing parameters.
• This technology is not cost-effective for small production runs.
  For production to be cost-effective, the order volume should start from a minimum of 10,000 pieces.
• Control of wall thickness.
  The walls cannot be too thick because this can lead to the outer surface of the part cooling and solidifying prematurely. Meanwhile, the interior material may still be in a plastic state, resulting in reduced part strength and potential defects (it is recommended to keep the thickness below 4mm). On the other hand, being too thin (below 1mm) for most materials can also cause issues as they may not be strong enough and could lead to defects.
• The need to maintain tight tolerances of production parameters.
  Continuous monitoring of processes is required, with a particular focus on quality control.

Examples of injection molding applications

• RTV (Radio, Television, and Video) and household appliances components
• Molded shapes or fittings
• Nuts
• Cassettes for testing in the medical industry
• Spools
• Sockets
• Furniture components (e.g., legs)
• Rubber components
• Plugs
• Cable connectors
• Building blocks for children
• Electronic components
• everyday use items
• Medical industry components
• Boxes and furniture components
• Door stops

Summary

Injection molding has become one of the most important technologies in the plastics processing industry. Market forecasts predict that the interest in this technology will continue to grow year by year due to its increasing applications in new fields such as optics, electronics, aviation, biomedicine, and communication.

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