Precision Injection Molding 101: Process, Advantages And Applications
- Gavin Leo
Precision injection molding is required in the manufacture of some precision electronic component housings or medical device parts, because the dimensional tolerances of these products are usually required in a very small range, which may only be a few microns to tens of microns.
What is Precision Injection Molding?
Precision injection molding is an injection molding process that is mainly used to manufacture high-precision and high-quality plastic products. It requires precise control of various parameters in the injection molding process to ensure that the dimensional accuracy, shape accuracy and surface quality of the product can reach a high standard.
Precision Injection Molding Process
Plastic raw material preparation:
Raw material inspection: Quality inspection of plastic raw materials, including appearance, particle size, melt flow rate and other indicators. For example, check whether the raw material has impurities, whether the color is uniform, and so on.
Drying treatment: Drying according to the characteristics of the material, such as ABS plastic is generally dried at 70-80 ° C for 2-4 hours.
Mold installation and commissioning:
Mold installation: Install the mold on the injection molding machine to ensure that the installation position is accurate and fixed firmly.
Mold debugging: including opening and closing mold debugging, ejection system debugging, etc., to check whether the action of the mold is smooth.
Plastic Injection molding process:
Injection stage: The melted plastic is injected into the mold cavity at a precise injection pressure and speed. For example, injection pressure can range from 50-150MPa depending on the size and complexity of the product.
Pressure holding stage: After the injection is completed, a certain pressure is maintained for a period of time to compensate for the volume change caused by the cooling shrinkage of the plastic. The holding pressure is generally 50%-80% of the injection pressure.
Cooling stage: The plastic is cooled and solidified through the cooling system of the mold, and the cooling time is determined according to the thickness of the product and the material characteristics, generally between 10-60 seconds.
Product release and post-processing:
Demoulding operation: The product is pushed out of the mold cavity through the ejector mechanism of the mold. For some easily damaged precision products, the demoulding speed should be moderate.
Post-treatment: including removal of flash, surface grinding, painting and other operations to improve the appearance quality and performance of the product.
Key elements of precise injection molding
Mold design and manufacturing:
Mold structure design: It is necessary to consider the demoulding method of the product and the layout of the cooling system. For example, for some sophisticated injection molded products with complex shapes, special ejection mechanisms such as sliders and inclined tops may be needed to ensure that the products can be successfully removed without damage.
Mold material selection: Usually choose high-quality mold steel, such as P20 steel, H13 steel, etc. These steels have good wear resistance, corrosion resistance and thermal stability, which can maintain the accuracy of the mold cavity during the long injection process.
Machining precision control: The processing of molds requires the use of high-precision processing equipment, such as CNC machining centers, EDM machine tools, etc. In the machining process, the machining size and surface roughness should be strictly controlled through accurate programming and tool path planning.
Injection molding equipment and parameter control:
Injection system: The injection system of the precision injection molding machine needs to be able to accurately control the injection pressure, injection speed and injection volume. For example, the control accuracy of the injection pressure may reach ±0.1MPa, and the injection speed can be finely adjusted over a wide range.
Temperature control system: including cylinder temperature, mold temperature control. Different plastic materials have different processing temperature ranges, and precision injection molding requires temperature control accuracy between ±1 and ±3 ° C to ensure that plastic materials can be uniformly melted and formed.
Clamping system: The clamping system of precision injection molding machine should be able to provide sufficient clamping force, and the clamping accuracy is high. The control accuracy of the clamping force can reach ± 1%-3% to prevent defects such as flash in the injection process of the plastic melt.
Plastic material selection:
Material performance requirements: Plastic materials with good fluidity, dimensional stability and mechanical properties need to be selected. For example, polycarbonate (PC) materials have high transparency, high strength and good dimensional stability, and are often used in the manufacture of precision optical products and electronic products.
Material drying treatment: Many plastic materials need to be dried before injection molding to remove moisture. For precision injection molding, the degree of drying is higher. For example, nylon (PA) material needs to be dried for 4-8 hours at a temperature of 80-100 ° C before injection molding to ensure the quality of the injection product.
Advantages and Disadvantages of Precision Plastic Molding
Pros
High Precision
It can produce plastic products with extremely high dimensional accuracy, and its dimensional tolerance can be controlled between ±0.01mm-±0.001mm, or even smaller, which can meet the needs of products with demanding precision.
Good product performance
Precision injection molding products usually have better mechanical properties, mechanical properties and dimensional stability, which can maintain stable performance under different environmental conditions, thereby improving product reliability and service life.
High production efficiency
Once the mold commissioning is completed and put into production, precision injection molding can achieve efficient and stable mass production, and can produce a large number of products with consistent dimensional accuracy in a short time, which is suitable for large-scale manufacturing.
High design freedom
Product molding of complex shapes and structures can be achieved, providing greater freedom for product design, and designers can design more innovative and unique products according to functional and appearance needs, without having to think too much about the limitations of the manufacturing process.
Cons
High cost
Precision injection molding demands high-precision molds with strict design, manufacturing, and processing requirements and more selective material choices, thus sharply increasing mold costs. For instance, a set of precision injection molds can cost several to dozens of times more than ordinary ones.
Long production cycle
Due to complex mold manufacturing and debugging and strict production process requirements, the production cycle of precision injection molding is long. From design to mass production, it may take much time, which is disadvantageous for urgent product projects.
The mold life is relatively short
Precision injection molds endure high pressure and wear during production. To ensure product accuracy, they need more frequent maintenance, and their service life is shorter than ordinary ones, typically ranging from tens of thousands to hundreds of thousands of injection cycles, thus raising mold replacement costs.
Applications Of Precision Molding Services
Electronics Industry
Mobile phone parts: such as mobile phone buttons, camera brackets, SIM cards, etc. Taking mobile phone keys as an example, their small size and high precision requirements, precision injection molding can ensure that the size of the keys is accurate, the surface is smooth and the touch is good.
Computer hardware: such as the case of the hard drive, the plastic parts of the CPU heat sink, etc. The hard drive housing requires precision injection molding to ensure its tightness and dimensional accuracy to protect the internal disk and read/write head.
Automotive industry
Interior parts: car dashboard buttons, interior lighting shell, etc. These interior parts should not only meet the requirements of aesthetics, but also ensure the accuracy of installation and the reliability of use.
Engine peripheral parts: some sensor housing, small plastic pipe joints, etc. The environment around the engine is complex and the temperature is high, and these plastic parts need to be molded with high precision to prevent leakage or damage.
Medical device field
Disposable medical devices: plastic parts of syringes, infusion sets, etc. These parts are in direct contact with the human body, requiring very high dimensional accuracy and quality, and precision injection molding can ensure their safety and accuracy.
Medical device housing and internal components: For example, the housing of small ultrasound devices and the plastic housing of some implantable medical devices, precision injection molding can meet stringent medical standards.
What are the common defect of precision injection molding?
The common quality problems in precision injection molding are mainly as follows:
Dimensional Accuracy Problems
Excessive Dimensional Deviation
Reasons: In mold making, poor equipment precision or improper techniques may create deviations between the actual and designed cavity sizes.
Unstable injection process parameters also lead to dimensional issues. Pressure fluctuations change melt filling, impacting product dimensions.
Temperature control matters too, as barrel and mold temperature changes vary shrinkage rates and cause dimensional differences.
Examples: When producing the housings of precision electronic components, if the cooling system of the mold is not designed properly and the mold temperature is uneven, the product will have inconsistent shrinkage during the cooling process, resulting in dimensional deviations and making it impossible for the housing to match the internal components precisely.
Poor Dimensional Stability
Reasons: The unstable quality of the plastic material itself is a key factor. Different batches of plastic raw materials may vary in composition, molecular weight distribution, etc., which in turn affects the physical properties and dimensional stability of the molded products. Inappropriate settings of injection molding process parameters can also lead to this situation.
For example, if the holding time is too short, the plastic melt will not receive sufficient pressure compensation during the cooling process, and volume shrinkage is likely to occur, resulting in unstable product dimensions.
Examples: When manufacturing high-precision plastic parts for medical devices, if the process parameters are not readjusted after changing the batch of plastic raw materials used, the product dimensions may be unstable among different batches, affecting the assembly and performance of the medical devices.
Appearance Quality Problems
Flash (Burrs)
Reasons: Insufficient clamping force is a common cause of flash. When the clamping force of the injection molding machine cannot effectively counteract the mold-expanding force of the plastic melt during the injection process, the melt will overflow from the parting surface of the mold or the gaps of the moving parts, forming flash. Mold wear can also lead to the appearance of flash.
For example, the parting surface of a mold that has been used for a long time will gradually wear out, increasing the gap and the possibility of flash formation.
Examples: When injecting automotive interior parts, if the mold clamping force is insufficient or the mold has been used for a long time, flash is likely to appear on the edges of the product, which not only affects the appearance of the product but may also affect the assembly and use safety of the interior parts.
Surface Sink Marks
Reasons: Uneven wall thickness of the product is the main cause of surface sink marks. During the injection molding process, the thick-walled parts cool slowly, and the plastic melt cannot obtain sufficient compensation when shrinking, so sink marks will form on the surface.
In addition, improper settings of holding pressure and holding time can also cause sink marks. Insufficient holding pressure cannot compensate for the melt shrinkage, resulting in surface defects.
Examples: When injecting plastic products with ribs, since the wall thickness at the ribs is relatively large, if the holding parameters are not adjusted reasonably, surface sink marks are likely to appear around the ribs, affecting the appearance quality of the product.
Flow Marks and Weld Lines
Reasons: Unsmooth flow of the plastic melt in the mold cavity will produce flow marks. This may be due to the unreasonable design of the gate position, size, and number, resulting in a disordered flow state of the melt when filling the cavity. Weld lines are formed when the plastic melt fails to fully fuse when converging in the cavity.
For example, when injecting with multiple gates, weld lines are likely to appear at the meeting points of the melt fronts from different gates, or poor mold venting can also lead to the formation of weld lines.
Examples: When injecting large plastic housings, due to the unreasonable design of the gate, flow marks may appear on the product surface during the flow of the melt, affecting the smoothness of the surface; in products with complex shapes and multiple gates, weld lines may be more obvious, reducing the appearance quality of the product.
Internal Quality Problems
Excessive Internal Stress
Reasons: During the injection molding process, excessive injection speed, too high injection pressure, or uneven cooling can all lead to excessive internal stress in the product. For example, when the injection speed is too fast, the plastic melt is forcibly injected into the mold cavity in a short time, and the molecular chains are stretched and compressed during the rapid flow process, and large internal stress will form after cooling.
Examples: When producing precision optical lenses, excessive internal stress may cause birefringence in the lenses, affecting their optical performance.
Porosity and Bubbles
Reasons: Plastic raw materials containing moisture or volatile substances are common causes of porosity and bubbles. During the injection molding process, these moisture or volatile substances turn into gases and form pores in the melt. In addition, excessive injection speed or poor mold venting can also prevent the gases from being discharged in time, resulting in bubbles inside the product.
Examples: When injecting transparent plastic products, porosity and bubbles will seriously affect the transparency and appearance quality of the product and reduce its performance.