Everything You Need to Know About Polycarbonate Injection Molding
Learn more about the polycarbonate injection molding process and uses.
Polycarbonate Injection Molding: The Basics
Polycarbonate injection molding refers to the process of injecting melted polycarbonate into a mold at high pressure and temperature to create complex plastic parts. Polycarbonate is an amorphous thermoplastic polymer with functional groups linked with carbonate groups that can be processed using injection molding. Injection molding is a high-volume, high-speed production technique that can create millions of components per year with limited human intervention. Very few manufacturing technologies can achieve the same scale at the low costs offered by injection molding.
For more information, see our guide on Plastic Injection Molding.
Polycarbonate is just one of many thermoplastics that can be injection molded. It is the material of choice when optical clarity and toughness are required. Polycarbonate can also be formed by extrusion, blow molding, and thermoforming. Injection molding, however, provides the most flexibility in terms of shape, production quantity, and mechanical and optical properties for high-volume production at a low cost per part. It provides improved performance when compared to acrylic, another common transparent plastic used in injection molding.
For more information, see our guide on All About Polycarbonate.
The Applications of Polycarbonate Injection Molding
Polycarbonate is used in a number of industries due to its strength, optical clarity, and thermal resistance. Some key application examples of polycarbonate injection molding are listed below:
- Eyeglasses and Lenses: Polycarbonate is often used for eyeglass lenses due to its clarity and resistance to UV (Ultraviolet) light when UV stabilizers are incorporated into the base material.
- Medical Devices: Polycarbonate has excellent corrosion and thermal resistance and can be sterilized using irradiation, ethylene oxide, or steam autoclaving. Coupled with its optical clarity, polycarbonate is an ideal material for medical devices that require sterilization as these processes often require elevated temperatures. Applications include catheters, syringes, and incubators.
- Consumer Products: Polycarbonate has excellent toughness and optical clarity, and is food safe, making it ideal for kitchenware like food processor mixing bowls, for example. Polycarbonate can withstand the temperatures required to keep equipment that contacts food clean and hygienic. It must be noted that polycarbonate can also be made to be opaque.
- Automotive Products: Polycarbonate is widely used in car headlights due to its toughness, resistance to UV light, high and low temperature, and transparency. Its resistance to impact also makes it excellent at resisting impacts from road debris.
Understanding the Polycarbonate Injection Molding Process
The polycarbonate injection molding process is generally the same as the injection molding process used for most other thermoplastics. The raw material, in the form of pellets, is fed into the heated barrel from a hopper where high levels of friction and pressure between the screw and the heated barrel melt the plastic. Once the correct volume of plastic has been melted, the plastic is forced into the mold at high pressure and speed and the screw then retracts. Polycarbonate has a high viscosity which makes it necessary to heat the plastic to a high temperature and inject it quickly into the mold to prevent the polycarbonate from suffering degradation due to prolonged exposure to high temperatures.
Polycarbonate Injection Molding Techniques
Listed below are some common techniques used when injection molding polycarbonate components.
- Rapid Heating & Cooling (RH&C): Polycarbonate is often used for the manufacture of transparent parts and as such surface blemishes are unacceptable. When molten material is injected into a mold, a thin layer is formed on the mold surface due to the temperature differential between the plastic and the mold. This layer can form weld lines and other surface imperfections. In RH&C, the mold is heated just before injection, then rapidly cooled to prevent the formation of this layer.
- Water-Assisted Injection Molding: Injection molding large thin-walled or hollow parts from polycarbonate is challenging. In cases such as these, it may be beneficial to make use of water-assisted injection molding. This technique uses high-pressure water inside the mold to press the plastic against the sides of the mold to produce uniform thickness and smooth internal volumes.
- Compression Injection Molding: Manufacturing thick-walled polycarbonate parts with standard injection molding techniques can produce sink marks on the part. One method of getting around this is to make use of compression injection molding, in which a specific amount of molten plastic is injected at low pressures into an open mold that is then closed to force the plastic into the required shape. This technique also reduces internal stresses typical with injection molding.
- Overmolding: If multi-material components are required then techniques such as insert or over-molding can be used. This process refers to the act of injection molding plastic over a pre-existing part that has been placed in the mold.
The Advantages and Disadvantages of Polycarbonate Injection Molding
injection molding offers many advantages compared to other polycarbonate processing technologies. Some of the key benefits are listed below:
1. Quicker Cycle Time
Polycarbonate injection molding is significantly faster than other processing techniques like vacuum forming and machining for example. Quicker cycle times mean more parts can be manufactured in a given time frame, thereby reducing the cost per part.
2. Aids in Producing Parts with Precise Tolerances
Injection molding is an inherently accurate process provided the process conditions are precisely controlled and the molds are machined to the required tolerances and properly designed to compensate for shrinkage during cooling. Molds are designed to be slightly oversized to account for the shrinkage as the polycarbonate cools. Injection molded parts can be made to have consistent tolerances of ± 0.1 mm.
3. Makes it Possible to Accommodate a Range of Part Sizes
Injection molding allows for the creation of parts with a wide variety of sizes. Since injection molds can be made in almost any size, the lower end of the size range is limited by the ability to force the viscous polycarbonate to melt into small spaces. The upper end of the size range is limited by the space available to accommodate the mold halves in the injection molding machine.
4. Makes for Excellent Part-to-Part Repeatability
Injection molding makes use of a precision-machined mold that can produce parts with very high levels of consistency. There will always be small variations in part dimensions, within required tolerances. Statistical process control can prevent these small variations from becoming larger over time due to machine and mold wear, inconsistent process parameters, or material variation. This repeatability is especially beneficial for parts like lenses that must be manufactured with precision.
5. Aids in the Mass Production of Parts
When it comes to the mass manufacture of complex plastic parts, no technology beats the production rate of injection molding. Injection molding is often used to produce millions of parts per year with limited human intervention. This helps maintain a low cost per part.
6. Aids in Avoiding Surface Flaws
The surface finish of polycarbonate is extremely important in applications that benefit from its optical clarity like car headlights. Injection molded parts will take on the surface finish of the mold used. Injection molds can be polished to a mirror finish which means that this highly smooth surface will be transferred to the injection molded parts.
7. Uses Raw Materials Effectively
Injection molding makes efficient use of raw materials with minimal wastage. Subtractive techniques like CNC (Computer Numerical Control) machining will result in a lot of waste, whereas injection molding fills the mold with only the precise amount of material required. The only waste present during injection molding is the small amount of material found in the mold runners. Any wastage that does exist can be recycled.
8. Low Labor Costs
Injection molding is one of the fastest production technologies when it comes to the manufacture of plastic parts and is a highly automated process and requires very limited human intervention when compared to other processes like machining. Reduced human intervention results in reduced labor costs and ultimately results in a low cost per part.
9. Makes Products With Various Designs and Shapes
Provided the correct DFM (Design for Manufacturing) principles are followed, injection molding can be used to produce parts with a wide range of designs. These can include parts that use over molding and insert molding to create multi-material parts.
Despite its benefits, there are some challenges in successfully injection molding polycarbonate.
Some disadvantages of the process are listed below:
1. Expensive for Low-Volume Polycarbonate Parts
Injection molding requires a high upfront investment to manufacture the complex injection mold that must first be designed, machined, heat treated, surface hardened, and polished before it can be used. For that reason, injection molding is not typically feasible for low-volume production with any material. The high cost and long lead time for injection molds make the cost per part too high for low volumes.
2. High-Cost Raw Materials
Polycarbonate is one of the more costly injection molding materials. As such, this material should only be used if other, cheaper thermoplastics will not be able to meet the design requirements.
3. Difficult to Injection Mold
Polycarbonate is one of the more complicated materials to use for injection molding. This is due to its high viscosity, which makes it difficult to injection mold thin-walled or fine-featured parts without increasing the injection temperature to the point where the material faces an increased risk of chemical degradation.
Common Problems in Polycarbonate Injection Molding
Some of the challenges that must be overcome when injection molding with polycarbonate are listed below:
- Moisture Control: Polycarbonate easily absorbs moisture prior to injection andcan cause splaying in the final part and undesirable streaking on the surface. The mechanical properties of polycarbonate can also be negatively affected by the inclusion of moisture. For this reason, polycarbonate pellets must be dried with a suitable desiccant or with a dehumidifier prior to being used for molding. The optimal moisture content is less than 0.02 %.
- Metal Adherence: Polycarbonate has a tendency to adhere to injection mold materials and machine equipment made from high iron-content alloys. If molds or machine parts like screws have high iron content, it may be necessary to coat them with chrome to prevent adherence.
- High Viscosity: Polycarbonate has a high viscosity which makes it difficult to mold thin walls or parts with complex flow requirements. This problem can be managed with high injection pressure and high injection temperature. However, higher temperatures require reduced injection speeds.
Other Materials Used for Injection Molding
Injection molding can be carried out using almost any thermoplastic (such as polycarbonate). Thermoplastics can also be filled with reinforcing additives like glass or carbon fiber fillers. Metal can also be injected if combined with a plastic filler material to allow the metal powder to flow through the mold. However, additional sintering is needed for metal injection molding.
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