Plastic Injection Molding: Application and Practice in Typical Products

Plastic Injection Molding (PIM) is a core plastic molding process widely used in mass production, which realizes the rapid shaping of plastic products by injecting molten plastic into a precision mold cavity under high pressure and cooling and solidifying. With the advantages of high production efficiency, stable product quality, and strong adaptability to complex structures, it has become the preferred process for manufacturing plastic products in consumer electronics, home appliances, new energy, and other fields. Unlike general process introductions, this article will focus on three typical products closely related to daily life and industrial production—mobile phone middle frames, washing machine control panels, and new energy vehicle charging pile housings—to deeply analyze how plastic injection molding technology fits product needs and solves actual production pain points.

I. Mobile Phone Middle Frame: High-Precision Injection Molding Empowers Lightweight and Durability

As a core structural component of mobile phones, the middle frame undertakes the functions of fixing internal components (motherboard, battery, screen), bearing external impact, and ensuring the overall flatness of the fuselage. With the trend of mobile phones being thinner, lighter, and more durable, the requirements for the middle frame are extremely stringent: thickness is usually controlled within 1.5-3mm, dimensional tolerance is less than ±0.02mm, and it must have excellent impact resistance, scratch resistance, and heat dissipation. Plastic injection molding has become the mainstream process for manufacturing mobile phone middle frames due to its ability to achieve high-precision and integrated molding.

1. Material Selection for Injection Molding

The middle frame needs to balance rigidity and toughness, so engineering plastic alloys are the first choice. At present, PC/ABS alloy is the most widely used material—PC provides high strength and heat resistance (heat distortion temperature up to 120℃), which can withstand the heat generated by the mobile phone during operation; ABS ensures good impact resistance and processability, making the molten material easy to fill the narrow mold cavity. For high-end flagship phones, glass fiber-reinforced PC/ABS (with 10%-20% glass fiber content) is often used, which can increase the tensile strength of the middle frame by 30%-50% while reducing the weight by 20%-30% compared with metal middle frames (aluminum alloy, stainless steel).

2. Key Injection Molding Process Points

The biggest challenge in injection molding of mobile phone middle frames is solving the problems of uneven wall thickness, shrinkage deformation, and surface defects (such as weld lines, flow marks). To this end, manufacturers usually adopt the following process optimization measures: first, use a hot runner system to ensure uniform temperature of the molten material, avoid cold material marks at the gate, and improve the surface finish of the product; second, control the injection parameters precisely—injection pressure is set at 80-120MPa, injection speed is divided into 3-5 stages (slow filling at the gate, fast filling in the middle section, slow pressure maintaining at the end) to prevent the molten material from scouring the mold cavity and causing deformation; third, adopt a precision cooling system—design a serpentine cooling water channel in the mold that fits the shape of the middle frame, control the cooling water temperature at 25-35℃, and ensure uniform cooling of each part of the product to reduce shrinkage deformation.

3. Advantages of Injection Molding for Middle Frames

Compared with traditional metal processing (CNC machining), injection molding of mobile phone middle frames has obvious advantages: first, high production efficiency—a single mold can have 8-16 cavities, and the cycle time of each cavity is only 25-40 seconds, which can achieve mass production of 100,000+ pieces per day; second, low production cost—the mold cost can be amortized quickly in mass production, and the unit cost of plastic middle frames is only 1/3-1/2 of that of metal middle frames; third, strong design flexibility—integrated molding of structures such as screw holes, positioning columns, and buckle grooves can be realized in the mold, reducing subsequent assembly processes and improving production efficiency.

II. Washing Machine Control Panel: Injection Molding Realizes Functional Integration and Aesthetic Design

The washing machine control panel is the "command center" of the washing machine, integrating buttons, display screens, circuit boards, and sealing structures. It not only needs to have good waterproof performance (IPX4 level or above) and chemical resistance (resistant to detergent corrosion) but also needs to meet the aesthetic requirements of home appliances (smooth surface, uniform color, no obvious defects). Plastic injection molding can perfectly match these needs by virtue of its ability to realize multi-functional integration and surface customization.

1. Material Selection for Injection Molding

The control panel is divided into two parts: the outer shell (visible part) and the inner bracket (used to fix the circuit board). The outer shell usually uses HIPS (high-impact polystyrene) or PP (polypropylene)—HIPS has good surface gloss and can be easily colored (spraying or in-mold coloring), which is suitable for control panels of mid-to-low-end washing machines; PP has better chemical resistance and waterproof performance, and its toughness is not reduced at low temperatures (below 0℃), which is suitable for washing machines used in northern regions. The inner bracket needs higher rigidity, so ABS is usually used, which can bear the weight of the circuit board and ensure the stability of the internal structure.

2. Key Injection Molding Process Points

The core requirements of the washing machine control panel for injection molding are surface quality and sealing performance. For surface quality control: first, the mold cavity is polished to a mirror finish (Ra≤0.02μm) to ensure that the surface of the product is smooth and free of burrs; second, in-mold decoration (IMD) technology is adopted for high-end products—put the printed decorative film into the mold in advance, and the molten plastic is combined with the film during injection molding, which can realize patterns such as wood grain, metal texture on the surface of the control panel, and avoid the problem of peeling of the surface coating. For sealing performance control: design a sealing groove with a width of 2-3mm and a depth of 1-1.5mm on the edge of the control panel through the mold, and ensure that the dimensional accuracy of the sealing groove is within ±0.03mm during injection molding, so that the control panel can be closely fitted with the washing machine body to prevent water from entering the internal circuit board.

3. Advantages of Injection Molding for Control Panels

Plastic injection molding can realize the integrated molding of the control panel's outer shell, inner bracket, and functional structures (button holes, display windows), reducing the number of parts from 5-8 (traditional assembly) to 1-2, which not only simplifies the production process but also improves the overall sealing performance of the product. At the same time, injection molding can achieve batch coloring of products—by adding masterbatch to the plastic raw materials, the control panel can be made into white, gray, black, and other colors to match different home decoration styles. In addition, the mold change cycle of injection molding is short (usually 2-3 hours), which can quickly respond to the personalized needs of different brands of washing machines.

III. New Energy Vehicle Charging Pile Housing: Injection Molding Meets High-Strength and Weather-Resistant Requirements

The charging pile housing is a key protective component of new energy vehicles, which needs to withstand harsh outdoor environments (high temperature, heavy rain, ultraviolet radiation) and ensure the safety of internal electrical components (charging module, circuit breaker, connector). It has strict requirements for strength (can withstand 50kg impact without deformation), weather resistance (no fading or cracking after 2000 hours of ultraviolet radiation), and flame retardancy (reaching UL94 V-0 level). Plastic injection molding has become the core process for manufacturing charging pile housings due to its ability to produce large-scale and high-performance plastic products.

1. Material Selection for Injection Molding

The charging pile housing is usually made of glass fiber-reinforced PP (GFRPP) with 20%-30% glass fiber content. PP has good chemical resistance and low temperature resistance, and the addition of glass fiber can significantly improve its tensile strength (up to 50MPa) and dimensional stability, avoiding deformation of the housing due to temperature changes. For the top cover and front door of the charging pile (parts with higher aesthetic requirements), PC/ABS alloy is used, which has better surface gloss and impact resistance, and can be sprayed with anti-ultraviolet coating to extend the service life of the product. In addition, all materials must meet the flame retardant requirement of UL94 V-0, which can prevent the spread of fire in case of internal short circuit.

2. Key Injection Molding Process Points

The charging pile housing has a large size (length 1000-1500mm, width 300-500mm) and a complex structure (with reinforcing ribs, mounting seats, and sealing grooves), which brings great challenges to injection molding. The key process measures are as follows: first, use a large-scale injection molding machine (clamping force ≥ 1600 tons) to ensure that the molten material can fill the entire mold cavity; second, design a multi-point gate system (usually 4-6 gates) to make the molten material fill the mold cavity evenly and avoid weld lines (which will reduce the strength of the product); third, strengthen the cooling system—arrange dense cooling water channels in the mold's reinforcing ribs and thick-walled parts, control the cooling time at 60-90 seconds, and ensure that the product is fully cooled before demolding to prevent deformation; fourth, adopt post-molding treatment—use ultrasonic welding to connect the housing and the bottom plate, which can ensure the sealing performance of the product (IPX5 level) and avoid water leakage.

3. Advantages of Injection Molding for Charging Pile Housings

Compared with traditional metal housings (steel, aluminum alloy), injection-molded plastic housings have obvious advantages: first, lightweight—weight is reduced by 40%-60%, which is convenient for transportation and installation of charging piles; second, corrosion resistance—can resist acid rain, salt spray, and detergent corrosion, and the service life in outdoor environments is more than 10 years (twice that of metal housings); third, low production cost—for mass production (10,000+ pieces), the unit cost of plastic housings is 1/2-2/3 of that of metal housings, which can effectively reduce the overall cost of charging piles and promote the popularization of new energy vehicles.

IV. Conclusion: Injection Molding Adapts to Product Demand and Drives Industrial Upgrade

From the small and precise mobile phone middle frame to the large and durable charging pile housing, plastic injection molding technology has shown strong adaptability and scalability. Its core value lies in not only realizing the mass production of products but also matching the personalized needs of different products (precision, functionality, aesthetics, performance) through optimized material selection, process design, and mold manufacturing. With the development of industries such as consumer electronics, home appliances, and new energy, the requirements for plastic products will become more stringent, and plastic injection molding technology will continue to upgrade—such as the combination of 3D printing (rapid mold making), intelligent injection molding (real-time monitoring of process parameters), and multi-material co-injection (integrating different performance materials), to provide more efficient and high-quality solutions for product manufacturing. For enterprises, mastering the application of plastic injection molding in specific products is the key to improving product competitiveness and realizing industrial upgrading.