The injection molding process refers to the process of producing semi-finished products of a certain shape by melting raw materials through operations such as pressure injection, filling, cooling, and demolding. It is typically used as a mass production process to manufacture thousands of identical items.
Injection molding is one of the indispensable processes in product engineering, and it has a wide range of uses, so it will be exposed to it when customizing products.
Process of Injection Molding
The process of injection molding mainly includes six stages: clamping, Injection, holding pressure, cooling, opening the mold, and demolding. These stages directly determine the molding quality of the product, and they are a complete continuous process.
Step 1 – Clamping:
The injection machine consists of three components – injection mold, fixture, and injection unit, where the clamping unit keeps the mold under certain pressure to ensure consistency of output.
Step 2 – Injection:
This step involves feeding plastic pellets into the hopper located at the top of the injection machine. These pellets are loaded into the main cylinder, where they are heated to a high temperature until they melt into liquid.
Then, inside the injection machine, the screw rotates and mixes the liquefied plastic. Once this liquid plastic reaches the desired state for the product, the injection process begins. The plastic liquid is forced through a running gate, and its speed and pressure are controlled by the screw or plunger depending on the machine type used.
Step 3 – Holding Pressure:
This step involves applying a certain pressure to ensure that each mold cavity has been completely filled. If the mold cavity is not correctly filled, it will result in scrap material. The role of the holding pressure stage is to continue applying pressure, compacting the melt, increasing the plastic density (densification) to compensate for plastic shrinkage behavior.
Step 4 – Cooling:
This process step allows the mold to cool for the required time. If this step is executed too hastily, the products may stick together or deform when taken out of the machine.
In the injection molding mold, the design of the cooling system is very important. This is because the molded plastic products can only avoid deformation caused by external forces after cooling and solidifying to a certain rigidity. Since cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can significantly shorten the molding time, increase injection molding productivity, and reduce costs.
Step 5 – Opening the mold:
The clamping device is opened to separate the mold. In the entire process, the mold is usually reused repeatedly, and their processing is very expensive.
Step 6 – Demolding:
Remove the finished product from the injection machine. Demolding is the last step in an injection molding cycle. Although the product has been cooled and solidified, demolding still has a significant impact on the quality of the product. Improper demolding methods may cause uneven stress on the product during demolding, resulting in defects such as deformation when ejected.
There are two main ways to demold: ejector pin demolding and stripper plate demolding. When designing the mold, suitable demolding methods should be chosen according to the structural characteristics of the product to ensure product quality.
This video explains the process of Injection molding process in detail. I hope it can help you.
Pros and Cons of Injection Molding Process
Pros of Injection Molding Process:
- The wall thickness distribution of the molded part can be precisely controlled using injection molds, which results in high accuracy for small-necked products.
- The final products do not require secondary processing, have no seams, and save time on product finishing.
- Injection molded blanks can be completely blown into shape in the blow molding mold without generating waste materials, which reduces the waste of molding materials.
- Smooth surfaced products can be obtained without scratches, with clear surface text and patterns.
- Economical for small-batch production.
- Wide range of plastic varieties applicable.
- High degree of automation, capable of multi-mold production, with high production efficiency.
Cons of Injection Molding Process:
- The molded product requires the use of two sets of molds for injection molding and blow molding, as well as a hollow core mold with a valve. The injection molded mold should be able to withstand high pressure, resulting in high equipment costs.
- The injection molded blanks have a high temperature and require a long cooling and setting time after being blown in the injection mold, which prolongs the entire molding cycle, affects production efficiency, and leads to long delivery cycles.
- The injection molded blanks have high internal stress, and due to uneven cooling during the mold transition, stress cracking is prone to occur when producing complex-shaped and large-sized products. Therefore, the shape and size of the product are limited, and it is only suitable for producing small-sized thin-walled products used in cosmetics, daily necessities, pharmaceutical packaging, food packaging, and other fields.
- High requirement for the operator’s skill level.
7 Common Injection Molding Processes
1. Plastic Injection Molding
Injection molding, also known as plastic injection molding, is a method of manufacturing plastic products.
The process involves melting granular or powdered materials in a hopper and injecting them into a mold cavity through a nozzle and mold casting system. The plastic then hardens and solidifies in the mold cavity. The quality of injection molding is affected by injection pressure, injection time, and injection temperature.
- Short molding cycle, high production efficiency, and easy automation
- Ability to manufacture plastic parts with complex shapes, precise dimensions, and metal or non-metal inserts
- Stable product quality
- Wide range of applications
- High cost of injection molding equipment
- Complex mold structure
- High production cost, long production cycle, and not suitable for small-batch production of single parts
Injection molding is used to manufacture a variety of industrial products, such as kitchenware (trash cans, bowls, water buckets, kettles, tableware, and various containers), electrical equipment casings (hair dryers, vacuum cleaners, food mixers, etc.), toys and games, various automotive products, and many other product parts.
2. Insert Molding
Insert molding refers to the process of inserting pre-prepared inserts of different materials into the mold and injecting resin to make an integrated product.
- Combining multiple pre-formed inserts to rationalize the subsequent production process.
- Combining the easy formability and flexibility of resin with the rigidity, strength, and heat resistance of metal to make complex and sophisticated metal-plastic integrated products.
- Particularly suitable for making products with electrical functionality by combining the insulation of resin and the conductivity of metal.
- For rigid molded products and curved elastic molded products on rubber sealing gaskets, the complex work of arranging sealing rings can be eliminated by molding integrated products on the base, making it easier to automate subsequent processes.
3. Two-Shot Injection Molding
Two-shot injection molding is a method of molding two different colored plastics in the same mold cavity. This process can create plastic parts with two different colors, presenting regular patterns or irregular cloud-like patterns, to improve the functionality and aesthetics of plastic parts.
- Low-viscosity materials can be used as the core material to reduce injection pressure.
- Recycled materials can be used as the core material for environmental considerations.
- Based on different usage characteristics, such as using soft materials for thick skin layer materials, hard materials for core materials, or foamed plastics for reducing weight.
- Lower-quality core materials can be used to reduce costs.
- Expensive materials with special surface properties, such as anti-electromagnetic interference and high conductivity, can be used to increase product performance.
- Proper coordination of skin layer materials and core materials can reduce residual stresses in molded products, increase mechanical strength, or improve product surface properties.
4. Micro-foam Injection Molding
Micro-foam injection molding is an innovative precision injection molding technology that fills products by expanding air holes and completes the molding of parts under lower and uniform pressure. Process Characteristics:
- Precision injection molding;
- Overcomes many limitations of traditional injection molding, significantly reducing the weight of parts and shortening the molding cycle;
- Greatly improves the warping deformation and dimensional stability of the parts. Applications: Automobile dashboards, door panels, air conditioning ducts, etc.
5. Nano Injection Molding (NMT)
NMT (Nano Molding Technology) is a process that combines metal and plastic using nanotechnology. After the metal surface undergoes nanoscale processing, plastic is directly injected and molded on the metal surface, allowing the metal and plastic to be formed as one. Nano molding technology is divided into two types of processes based on the plastic’s position:
- Integral molding of plastic as non-appearance surface
- Integral molding of plastic as appearance surface
- Products have a metallic appearance and texture.
- Simplifies product design and makes it lighter, thinner, shorter, and smaller, and more cost-effective than CNC machining.
- Reduces production costs, has high bonding strength, and significantly reduces the use of related consumables. Applicable metal and resin materials:
- Aluminum, magnesium, copper, stainless steel, titanium, iron, galvanized sheet, brass.
- Aluminum alloy is highly adaptable, including the 1000 to 7000 series.
- Resins include PPS, PBT, PA6, PA66, and PPA.
- PPS has a particularly strong bonding strength (3000N/c㎡). Applications: Mobile phone cases, laptop cases, etc.
6. Blow Molding
Blow molding is a method of using compressed air to inflate a molten thermoplastic material that has been extruded into a mold and adheres to the mold cavity surface, cooling and solidifying into the desired product shape. Blow molding is divided into two types: thin-film blow molding and hollow blow molding:
6.1 Thin-film blow molding
Thin-film blow molding extrudes molten plastic from the extruder head, forming a cylindrical tube in the annular gap of the mold, and simultaneously blowing compressed air into the inner cavity of the tube to blow it into a larger diameter tube-shaped film (commonly known as bubble tube), which is then cooled and rolled up.
6.2 Hollow blow molding
Hollow blow molding is a secondary molding technology that uses gas pressure to blow up the rubber-like blank closed in the mold cavity into a hollow product. There are three different methods for manufacturing the blank: extrusion blow molding, injection blow molding, and stretch blow molding.
6.2.1 Extrusion blow molding:
Extrusion blow molding uses an extruder to extrude a tubular blank, which is then clamped in the mold cavity while hot, and the compressed air is blown into the inner cavity of the tubular blank to inflate it into the desired shape.
6.2.2 Injection blow molding:
The blank used is obtained by injection molding. The blank is left on the core of the mold, and after the blow molding mold is closed, compressed air is blown into the core to inflate the blank into the desired shape. Blow the preform, cool it, and then obtain the finished product after demolding.
The finished product has uniform wall thickness, small weight tolerance, requires minimal post-processing, and generates minimal scrap material. This method is suitable for producing small refined products in large batches.
6.2.3 Stretch blow molding:
The blank is made by injection molding or extrusion blow molding. The blank is reheated and stretched before blowing to achieve the desired shape.
- Blown film is mainly used for manufacturing plastic film molds.
- Blow molding is mainly used for producing hollow plastic products such as bottles, packaging drums, spray bottles, fuel tanks, cans, toys, etc.
7. Extrusion molding (Profile):
Extrusion molding, also known as extrusion, is mainly suitable for the forming of thermoplastic materials, as well as some thermosetting and reinforced plastics with good fluidity. The molding process uses a rotating screw to extrude the heated and melted thermoplastic raw materials from the head with the required cross-sectional shape, then it is shaped by the shaping device and finally cooled and solidified by the cooler to become the product with the desired cross-section.
- Low equipment cost.
- Simple operation, easy to control the process, and easy to achieve continuous and automatic production.
- High production efficiency, uniform and dense product quality.
- Various cross-sectional shaped products or semi-finished products can be formed by changing the head mouth mold.
Extrusion molding has strong applicability in the field of product design. The products produced by extrusion molding include pipes, films, rods, monofilaments, flat belts, nets, hollow containers, window and door frames, sheets, cable sheaths, monofilaments, and other special-shaped materials.
Materials Commonly Used in Injection Molding Process
1. Polystyrene (PS)
PS is a colorless, transparent, and glossy granular material with the following properties:
- Good optical properties
- Excellent electrical properties
- Easy to process and mold
- Good coloring performance
- The biggest disadvantage is brittleness and low heat resistance (maximum use temperature of 60-80 degrees Celsius)
- Poor acid resistance
2. Polypropylene (PP)
PP is a colorless, transparent, or slightly glossy granular material, also known as “bendable plastic.” It is a crystalline plastic with the following properties:
- Good fluidity and molding performance
- Excellent heat resistance, can be boiled and sterilized at 100 degrees Celsius
- High yield strength
- Good electrical properties
- Poor fire safety and weather resistance, sensitive to oxygen and susceptible to aging from UV exposure
3. Nylon (PA)
PA is an engineering plastic made from polyamide resin and includes PA6, PA66, PA610, PA1010, etc. Nylon has the following properties:
- High crystallinity, high mechanical strength, good toughness, and high tensile and compressive strength
- Outstanding fatigue resistance, wear resistance, corrosion resistance, heat resistance, non-toxicity, and excellent electrical properties
- Poor light resistance, easy to absorb water, and not resistant to acid
4. Polyoxymethylene (POM)
POM is also known as “steel substitute,” POM is an engineering plastic with the following characteristics and uses:
- Highly crystalline structure with excellent mechanical properties, high elastic modulus, rigidity, and surface hardness, known as “metal’s competitor”
- Low friction coefficient, excellent wear resistance and self-lubrication, second only to nylon but cheaper
- Good solvent resistance, especially organic solvents, but not resistant to strong acids, alkalis, and oxidants
- Good dimensional stability, suitable for manufacturing precision parts
- Large molding shrinkage, poor heat stability, and easily decomposed when heated
5. ABS Plastic
ABS plastic is a high-strength modified polystyrene composed of acrylonitrile, butadiene, and styrene in a certain proportion. It has a light ivory color, is opaque, non-toxic, and odorless. Characteristics and uses:
- High mechanical strength, strong impact resistance, good creep resistance, and characteristics of hardness, toughness, and rigidity
- The surface of ABS parts can be electroplated
- ABS can be blended with other plastics and rubber to improve its performance, such as ABS+PC
6. Polycarbonate (PC)
PC is also known as “bulletproof glass,” PC is a transparent material that is non-toxic, odorless, and combustible but self-extinguishing after ignition. It has the following characteristics and uses:
- Special toughness and hardness, with the best impact resistance among all thermoplastic materials
- Excellent creep resistance, good dimensional stability, and high molding accuracy; good heat resistance (up to 120 degrees Celsius)
- The drawback is low fatigue strength, high internal stress, and prone to cracking, with lower wear resistance for plastic parts.
7. PC+ABS (PC+ABS alloy)
PC+ABS combines the advantages of PC (engineering plastic) and ABS (general-purpose plastic), and improves their performance. It contains the chemical components of both ABS and PC, providing ABS’s good flowability and molding processing performance, as well as PC’s impact resistance and resistance to thermal cycling. Its features are:
- Mold design with adhesive point/large water inlet.
- Surface can be oil sprayed, electroplated, or coated with metal film.
- Pay attention to adding surface exhaust.
The material is commonly used in hot runner molds and has been used in an increasing number of consumer communication products, such as phone and computer cases.
How to Effectively Reduce Injection Molding Cost?
Before understanding how to reduce injection molding costs, it is necessary to understand the composition of injection molding costs: The exact cost of injection molding can be calculated using the following formula:
Mold price = material cost + design + process and profit + value-added tax + trial cost + packaging and transportation cost.
Among these costs, materials and components account for 20%-30% of the total cost, while process and profit account for 30%-50%. Considering these factors, the cost of a small single-cavity injection mold is between $1,000 and $5,000. Larger or more complex molds may require $80,000 or more. However, on average, a typical mold cost is about $12,000.
Although mold costs are high, the actual production costs of injection molding are relatively low. Here are some common methods used to reduce costs in injection molding:
Method 1：Reduce risk and quality costs
Use scientific molding principles to develop repeatable and stable processes. After developing the process using sensors and molding technology, common injection molding defects can be monitored during injection molding to ensure 100% quality assurance for your customers.
Method 2：Increase efficiency through automation
When we start automating processes, we remove fluctuations caused by manual processes, improve efficiency, improve quality, and increase available space. Quality automation, through process monitoring, visual systems, or online dimension validation, can ensure that our customers never receive defective products.
Method 3：Reduce scrap
By quickly knowing when the process changes, you can solve problems faster. This means reducing the amount of waste, increasing the utilization rate of injection molding machines, and reducing waste costs.
Method 4：Purchase wide-specification resin
Resins with greater performance variation are cheaper, but it is difficult to ensure or maintain product dimensions for products with strict tolerances. This undoubtedly increases variability, leading to an increase in scrap rate.
Method 5：Shorten cycle time
Properly sized mold temperature control units or mold temperature machines also help shorten cycle times. 80% of molding cycle time is used to cool the product from melt temperature to demold temperature, making the product hard enough to withstand ejection force and maintain product size stability.
Method 6：Manufacture more efficient molds
The cost of manufacturing a product one by one is very high. If a higher number of mold cavities are used, more products can be manufactured in the same amount of time.
Method 7：Reduce mold transfer costs
Process development applications ensure that you can quickly and easily transfer molds from one injection molding machine to another without creating special conversion tools or recreating processes for each new injection molding machine, saving time, energy, and resources.
Q: Is Injection Molding Environmentally Friendly?
The impact of plastic forming equipment on the environment mainly manifests in several aspects:
Firstly, the production and use of equipment will generate a large amount of pollutants such as exhaust gas, wastewater, and noise, which will affect the surrounding environment and the health of residents.
Secondly, the plastic materials used in the processing process will also have an impact on the environment, including the chemical raw materials required for the production process of plastic materials and processing waste, etc.
Finally, discarded plastic products may be casually discarded and discharged, causing damage to the environment and ecosystems.
Therefore, it is recommended to strictly implement relevant environmental protection regulations to reduce pollutant emissions and reduce environmental pollution while using plastic forming equipment.
Q: What’s the Difference Between Injection Molding and Extrusion Molding?
Injection molding = injection molding machine + injection mold;
Extrusion molding = extrusion machine + die, core mold + cooling water tank.
The injection molding machine and the extrusion machine are similar in that they both melt the plastic. The difference lies in the mold: the injection mold has an upper and lower mold, which is closed with a shaped cavity inside, just like the shape of the product. After the plastic particles are melted, they are injected into the closed mold cavity, cooled, and the product is taken out by opening the mold. The extrusion mold is very simple, consisting of a die and a core mold, similar to two rings fitted together, which extrudes the shape of a pipe, and then cools and hardens through a water tank.
The biggest differences between injection molding and extrusion molding are as follows:
1. Injection molding can produce relatively complex products that are discontinuous and have one cavity each, such as phone cases. The characteristic is that it cannot be produced continuously and needs to cool one mold before producing another. And there is a sprue.
2. Extrusion molding can generally only produce products with the same cross-section and continuous production, such as pipes. The characteristic is that it can be produced continuously, and cooled while extruding through a water tank. As long as there is material, it can be extruded continuously, tens of thousands of meters or more, without a sprue.
Injection molding has a wide range of applications in manufacturing, especially in the production of large quantities of parts. Although tooling and moulding may be expensive, once completed, the production costs are low. If you are unsure which injection molding process is suitable for your product idea or need a high-quality injection molding manufacturer during the new product development, you can contact us.