Injection molding is an efficient and widely used plastic molding process. It uses an injection machine to inject molten plastic material into a mold, and after cooling and solidification, the desired plastic product is obtained. This process is particularly suitable for mass production of products with complex shapes and occupies an important position in the field of plastic processing. The main steps of injection molding include clamping, injection, pressure holding, cooling, mold opening and product removal. These steps are achieved through the precise control of the injection machine to ensure the high quality and precision of the product. By repeating these steps, the injection molding process can continuously and efficiently produce a large number of plastic products to meet the needs of various industries.
What is injection molding ?
A common plastic molding technique is injection molding, which involves using an injection machine, also known as an injection molding machine, to inject molten plastic material-typically thermoplastic-into a mold. After the plastic solidifies and cools down, the desired result is extracted.
The injection molding process consists of six key phases.
- Clamping: To guarantee that the molten plastic does not leak during injection, the mold is firmly sealed during this phase.
- The front and rear of the product are often represented by two or more components that make up the mold.
- The dimensions of the mold and the characteristics of the plastic substance determine the clamping force.
- Injection: Using a heating element, the injection machine melts the plastic particles.
- High pressure is used to inject the molten plastic into the locked mold.
- Two important factors that influence the product's look and quality are injection pressure and pace.
- Holding pressure: The injection machine maintains a certain pressure after the plastic material is injected into the mold. This pressure is known as holding pressure. Maintaining pressure helps guarantee that the plastic fills the mold completely and makes up for any potential cooling-related shrinkage.
- The plastic type, mold design, and product specifications all affect the holding time and pressure.
- Cooling: To create the appropriate product shape, the plastic is cooled and hardened in the mold.
- Cooling water channels in the mold are often used to do this, although fans and other external cooling devices may also help.
- The product's performance and dimensional correctness are significantly impacted by the cooling rate and homogeneity.
- Opening the mold: The mold is opened after the plastic has completely set. To prevent harm to the mold and the product, the mold opening procedure must guarantee that the mold's components move smoothly. The product stays on one side of the mold-typically the moveable mold side-after the mold is opened, ready to be taken out.
- Product removal: A robot or a human is often used to remove products. In order to prevent damage, make sure the product is entirely out of the mold before removing it. Post-processing such as deburring, trimming, etc., could be necessary for the removed product.
The injection molding technique may continually create a huge number of plastic goods with intricate forms and exact measurements by repeating the aforementioned stages. Injection molding has a significant place in the plastic processing industry because of its high efficiency, high level of automation, and excellent flexibility.
What plastic is used in injection moulding?
The plastic materials used in the injection molding process are extensive and cover a variety of types. The following are some common injection molding plastic materials and their characteristics:
Thermoplastics:
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Polyethylene (PE): has good heat resistance, chemical resistance and mechanical properties, and is often used to make plastic packaging films, plastic bags, bottles, etc.
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Polypropylene (PP): has good mechanical properties, wear resistance and chemical resistance, and is widely used in automotive parts, plastic containers, home appliances and other fields.
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Polyvinyl chloride (PVC): has good electrical insulation, weather resistance and solvent resistance, and is often used to make building materials, wires and cables, water pipes, etc.
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Polystyrene (PS): has good transparency, mechanical strength and electrical insulation properties, and is often used to make plastic cups, toys, insulation materials, etc.
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Polycarbonate (PC): has high strength, high rigidity and excellent heat resistance, and is often used to make glasses, automotive parts, etc.
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Polyamide (PA): also known as nylon, has good mechanical properties, wear resistance and solvent resistance, and is often used to make gears, bearings and other parts.
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Acrylic (PMMA): A strong, transparent thermoplastic that is a good alternative to glass and is often used for transparent components in architectural and lighting applications.
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Acrylonitrile butadiene styrene (ABS): An engineering-grade thermoplastic known for its strength and impact resistance, commonly used in automotive, electronics and other fields.
Thermosetting plastics:
Polyurethane (PU): Has good wear resistance, oil resistance and solvent resistance, and is often used to make vehicle suspension systems, seals, etc.
In addition, there are other injection molding materials such as polyethersulfone (PES), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), each of which has its own specific physical properties and application areas.
When choosing an injection molding material, it needs to be determined based on the processing requirements and use environment of the product. For example, for products that require high transparency, PS or PMMA may be selected; for products that require high strength and heat resistance, PC or PA may be selected. At the same time, factors such as material cost, processing performance and environmental requirements also need to be considered.
Importance of molds
An essential part of the injection molding process are molds. A well-designed hollow metal block with an internal form that matches the intended plastic product's outside shape is called a mold. Molten plastic is pumped at high pressure into the mold cavity during the injection molding process. The plastic substance eventually creates a plastic product that precisely matches the shape of the mold cavity via the cooling and solidification of the mold.
To prevent plastic from leaking during the injection molding process, molds often consist of many components, such as fixed and movable molds, that are precisely closed by a mechanism. To guarantee that the product can be smoothly removed from the mold and to optimize the adhesion and demolding qualities of the plastic, the mold surface is often specifically treated.
The intricate runner and gate mechanism of the mold directs the molten plastic to swiftly and evenly fill the hole. The product's quality and manufacturing efficiency are directly impacted by the runner design, whether it is appropriate or not. In order to regulate the pace at which the plastic solidifies and the end product's quality, the mold is further outfitted with a temperature control system that allows it to be heated or cooled using hot water, oil, or a heater.
The injection molding machine automatically controls the mold's opening and closure during the injection molding process. The injection molding machine's ejector pin pushes the mold's ejector plate to release the molded object from the mold after the plastic has completely set. To guarantee that the product can be demolded easily and without harm, this procedure has to be precisely controlled.
What is an injection molded part?
The runner, gate, and cavity in the mold make up the molded object during the injection molding process. Melted plastic resin is injected into the mold using the mold runner, which is connected to the injection molding machine nozzle by a channel called a gate. The molten plastic is guided from the gate to each cavity by a channel system called a runner. The plastic is cooled and hardened in the cavity to produce the finished product, with each cavity representing one or more items.
Molds having many cavities linked by runners are often intended to increase production efficiency and save costs. The variation in the time it takes for the molten plastic to fill each hole, however, might lead to some issues if the runner lengths to each cavity change. For instance, there's a chance that the closer cavity develops flaws from early plastic solidification, while the distant hole could not have enough plastic filled in time. This might lead to variations in the items' dimensions, looks, or functionality when molded into various holes.
The so-called balanced runner design is often used to address this issue by making the runner the same length from the gate to each cavity. This design guarantees the uniformity and caliber of every molded product by allowing the molten plastic to enter each cavity simultaneously and fill it equally. Injection mold design requires careful consideration of balanced flow channel design, which is crucial for raising product quality and manufacturing efficiency.
Injection molding examples
We will take the production of a refrigerator door switch box part as an example to describe the steps and parameters.
Material selection
Material: ABS plastic
ABS plastic has good strength, toughness, wear resistance and heat resistance, and is suitable for making parts with high appearance requirements.
Mold design
Design points:
One mold four cavity design to improve production efficiency.
The parting surface is selected at D-D to ensure smooth opening and closing of the mold.
The mold adopts point gate feeding and double parting surface structure to ensure uniform feeding and product quality.
Core pulling mechanism: The large rectangular groove on one side uses the outer core pulling of the inclined guide column, and the two small square grooves on the other side use the inner core pulling of the inclined slider to simplify the mold structure.
Process parameter setting
- Injection temperature: According to the characteristics of ABS plastic, set the appropriate injection temperature range.
- Injection pressure: According to the structure of the plastic part and the mold design, select the appropriate injection pressure to ensure that the plastic can fully fill the mold cavity.
- Mold temperature: Control the mold temperature to obtain the best molding effect and product quality.
- Compression pressure (if injection compression molding is involved): Set appropriate compression pressure when necessary to ensure that the plastic is fully compacted.
Production process
- Plastic heating: Add ABS plastic particles into the hopper of the injection molding machine and heat them to a molten state through a heating device.
- Injection: After the mold is closed, the injection molding machine injects the molten ABS plastic into the cavity of the mold through a nozzle.
- Pressure holding: After the injection is completed, continue to apply a certain pressure holding pressure to ensure that the plastic is fully filled in the mold and compensate for possible shrinkage.
- Cooling and curing: The plastic is cooled and cured in the mold through the cooling water channel or other cooling equipment in the mold.
- Mold opening and demolding: When the plastic is fully solidified, the mold is opened and the molded product is removed from the mold through a push rod or other demolding mechanism.
Quality control
Throughout the production process, strict quality control is carried out, including checking parameters such as the melt temperature of the plastic, injection pressure and speed, mold temperature and cooling time, to ensure that each link meets the process requirements.
The molded products are visually inspected, dimensional measured and performance tested to ensure that the product quality meets the requirements.
Summary
Through the above injection molding examples, we can see the application of injection molding process in the manufacture of plastic products. In actual production, it is necessary to select appropriate molds and process parameters according to specific plastic materials and product requirements, and conduct strict quality control to ensure the quality and stability of the final product.