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PEEK Injection Molding: Material Features and Key Factors to Consider
Created on 05.18
As a semi-crystalline thermoplastic, PEEK is often used for injection molding in order to make parts that will withstand conditions when regular plastics are not capable of doing so. Nevertheless, working with the PEEK injection molding process demands comprehensive knowledge about how this type of plastic acts in both heated and cooled states.
In this article, we will take a closer look at PEEK's basic characteristics and other parameters that need to be taken into account during processing in order to proceed from designing a part to mass production of it in a high-quality manner.
Main Features of PEEK Material
PEEK is a member of the PAEK family, and its molecules' structure makes it very durable. In case one needs to consider using PEEK injection molding technology, he or she needs to focus on certain parameters of this material.
1. High Temperature Stability
Thermal stability at high temperatures is one of the most significant properties of the PEEK polymer material. The continuous service temperature of the material is about 250°C. Under short-term conditions, the material is able to withstand even greater temperatures without any loss of its characteristics. This allows PEEK to be used in places where high temperatures are constantly present, such as inside the engine block of an automobile or downhole oil and gas production.
2. Mechanical Strength and Wear Resistance
The material has good mechanical properties. It has very good tensile strength and stiffness, which makes PEEK capable of being used in place of aluminum and even titanium. In addition to mechanical properties, it has good resistance to cyclic loads and a low wear rate. Moreover, PEEK can operate in a state of dry friction without any wearout.
3. Chemical and Corrosion Resistance
Most chemicals do not affect PEEK. It is unaffected by oils, hydraulic fluids, acids, and bases at any temperature. However, the only common chemical substance that can dissolve PEEK is concentrated sulfuric acid. In the medical industry, chemical stability is highly significant since the product made from PEEK material can be subjected to many times of autoclave sterilizations without deteriorating.
4. Biocompatibility and Electrical Insulation
In the medical industry, one of the advantages of PEEK is its biological compatibility, implying the non-reactivity of PEEK with the tissues of human beings. Besides, it is worth mentioning that PEEK has good insulation capability and acts as an electrical insulator. Its dielectric properties are retained irrespective of changes in temperature or frequency.
5. Enhancing Properties with Filled Grades
Even though pure PEEK can offer great properties, it may be further enhanced by using various reinforcements that will enable certain properties to be improved as follows:
- Glass Fiber Reinforced Grade: Enhances stiffness and decreases the expansion coefficient of the component.
- Carbon Fiber Reinforced Grade: Improves tensile strength and increases thermal conductivity at reduced weight.
- PTFE/Graphite Grades: Specifically designed for "bearing" applications to provide the lowest possible friction and highest wear resistance.
Key Factors for Successful PEEK Injection Molding
Processing PEEK is more complexthan processing nylon or polypropylene. Because it has a high melting point and a specific molecular structure, the injection molding setup must be rigorous.
1. Material Preparation: The Role of Drying
PEEK is a hygroscopic polymer, which implies that it can absorb moisture from the environment. The presence of even small amounts of moisture can result in complete failures of molding. When the material is wet, it causes voids, silver lines, and even the loss of its mechanical properties due to hydrolysis at the elevated temperatures needed for the molding process.
Actionable Tip: Dry PEEK pellets using a desiccant dryer at 150°C for a minimum of 3 to 4 hours. Maintain the moisture level below 0.02%.
2. Temperature and Pressure Control
The thermal demands that PEEK has are much more demanding compared to typical plastics.
- Melt Temperature: The plastic has a melt temperature of 343°C; however, it is usually processed at a temperature between 360°C and 400°C.
- Mold Temperature: This is probably the most important of all factors. The mold needs to be heated to somewhere between 160°C and 200°C. A cold mold can cause the plastic to cool rapidly, producing an "amorphous" product that appears translucent and brown. On the other hand, heating the mold produces crystallinity.
- Crystallinity: It refers to the extent to which the polymer's molecules are arranged in an ordered structure. Having high crystallinity (which usually presents itself as opaque and beige-colored) gives the part its maximum hardness and chemical resistance.
3. Mold and Part Design Basics
When designing parts for PEEK, it is important to understand how the material behaves with respect to flow and shrinkage.
- Wall Thickness: A consistent wall thickness is preferred. Extreme differences can lead to internal stress buildup.
- Gate Sizing: PEEK is a highly viscous material ("thick" in its molten state). Large gates are critical in order to allow fast filling and to prevent freezing of the material until the mold is filled.
- Drafts: Since PEEK is a rigid material, it does not deform easily when being ejected from the mold. It is therefore important to incorporate large draft angles (1.5 to 2 degrees).
4. Equipment and Cleanliness
While standard injection molding machines can be used, they must be equipped with high-temperature heaters (often ceramic heater bands) and specialized insulation. It is also vital to keep the environment clean. At 400°C, any dust or leftover residue from a previous run of a different plastic will burn (carbonize). These burnt particles will appear as black specks in the PEEK part, which often leads to the part being rejected, especially in medical or aerospace sectors.
Common Challenges and Practical Tips
Even with the right equipment, PEEK injection molding presents specific hurdles that manufacturers must navigate.
1. Managing High Material Costs
PEEK is one of the most expensive polymers on the market. Consequently, waste must be kept to a minimum. Unlike cheaper plastics, where you can easily grind up and reuse "runners" (the plastic that fills the channels leading to the part), PEEK's properties can change slightly with each heat cycle.
Practical Tip: Minimize the runner system size or use a hot runner system if the production volume justifies the cost. This reduces the amount of material used per cycle.
2. Shrinkage and Warping
The shrinkage rate of PEEK is higher than that of most other engineering plastics. When the material cools, the molecular alignment leads to crystallization, resulting in shrinkage. The process is not uniform in all parts, causing warpage.
Advice: Make use of staged injection. In staged injection, you control the pressures of different stages, i.e., filling, packing, and holding. You will be able to counteract the effects of shrinkage during solidification.
3. Processing Demands and Maintenance
Because the machine runs at such high temperatures, the wear and tear on the screw and barrel are increased. Standard lubricants may break down, and the high pressures required to move the viscous PEEK can stress the mechanical components of the machine.
Advice: Perform regular maintenance and use specialized purging compounds specifically rated for high temperatures to clean the barrel. Never leave PEEK sitting in a hot barrel during a machine shutdown; it will degrade and harden, potentially damaging the screw upon restart.
4. Quality Control and Inspection
Because PEEK is often used for "mission-critical" parts, inspection is non-negotiable. Beyond checking dimensions, you should inspect for color consistency. A patchy or dark appearance usually indicates inconsistent mold temperatures or contamination. If the part is supposed to be semi-crystalline but comes out looking dark and glassy, it will likely fail in high-heat or high-stress environments.
Conclusion
PEEK injection molding is a sophisticated process that rewards precision. For businesses looking to leverage PEEK, the key is to prioritize material preparation and mold temperature management. Starting with a clear understanding of these factors reduces the risk of defects and ensures a reliable production cycle.
If you are planning a project that requires high-performance components, our team is ready to assist. We provide injection molding machines in different sizes, as well as injection molds. Contact us today to discuss your specifications, request a quote, or receive a technical processing checklist to get your project started.
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