PEEK belongs to high-performance special engineering plastic, with excellent high temperature resistance, high strength, corrosion resistance, wear resistance and insulation properties. However, it has poor melt fluidity, high molding temperature range, and strict requirements for equipment and technology. Every link from raw material drying, equipment selection, mold design, process parameters to post-processing needs to be strictly controlled to avoid molding defects such as cracking, insufficient filling, silver streaks, scorching and excessive internal stress.
1. Raw Material Storage and Drying ControlAlthough PEEK is not extremely hygroscopic, trace moisture is easy to decompose to produce bubbles and silver streaks under high-temperature melting conditions. The raw materials must be sealed and moisture-proof stored after opening to avoid long-term exposure to humid air. New raw materials generally do not need long-term deep drying. For raw materials that have been placed for a long time in humid environment or glass fiber modified PEEK, constant temperature drying treatment must be carried out. The drying temperature is controlled at 150 to 160 degrees Celsius with constant temperature drying time of 3 to 4 hours to reduce raw material moisture to an ultra-low standard. The dried raw materials shall be put into production immediately. Raw materials shall be kept sealed and insulated during machine shutdown interval to prevent secondary moisture absorption, and it is strictly prohibited to directly feed into injection molding without thorough drying.
2. Adaptation Requirements of Injection Molding EquipmentProcessing PEEK must adopt special high-temperature injection molding machine. The barrel, screw and nozzle need to be made of high-temperature alloy materials, with wear resistance, corrosion resistance and high-temperature creep resistance. The screw material of ordinary general plastic injection molding machine cannot withstand the high-temperature melting working condition of PEEK for a long time, which is prone to wear, carbonization and material sticking. The equipment temperature control system needs to be precise and stable with small temperature control error, and has multi-stage sectional temperature control function to accurately match the temperature gradient of each section of the barrel. The rated injection pressure and clamping force of the injection molding machine shall have sufficient margin. PEEK melt has high viscosity and high filling resistance. Too small tonnage equipment is prone to insufficient filling and unstable clamping force resulting in flash. The screw length-diameter ratio and compression ratio are reasonably selected to adapt to the plasticizing demand of high-viscosity melt.
3. Mold Design and Production Key PointsPEEK has small molding shrinkage but high rigidity, easy to produce internal stress cracking. The mold design needs to reasonably control the shrinkage allowance and reserve sufficient dimensional compensation. The mold steel is selected from high temperature resistant, high polishing and high hardness materials such as S136 and H13. The cavity surface is finely polished or textured according to product demand to avoid melt retention and carbon deposition. The mold temperature shall be equipped with an independent precision temperature control mold temperature machine. The conventional molding mold temperature is controlled at 160 to 200 degrees Celsius. High mold temperature can improve melt fluidity, reduce internal stress, and optimize product surface gloss and dimensional stability. The gating system preferentially adopts large-diameter main runner, thick runner and large fillet transition design. The gate size is appropriately enlarged, and tiny point gates are not recommended to prevent excessive melt shear from scorching and degradation. Multi-cavity layout must ensure runner balance to realize synchronous and uniform filling of each cavity. The exhaust groove shall be reasonably opened with depth adapted to PEEK material characteristics to timely discharge gas inside the mold cavity and avoid trapped gas scorching, bubble and pitting defects.
4. Control of Injection Molding Process ParametersThe overall molding temperature of PEEK barrel is much higher than that of ordinary plastics. The nozzle and each section of barrel are set with graded high temperature parameters. Too low temperature will cause poor plasticization and difficult filling, while too high temperature will lead to thermal degradation, discoloration and performance decline of materials. The injection speed is controlled in sections at medium and low speed, with slow filling in the front section to avoid overheating of high-speed shear, and uniform filling in the middle and rear sections. Full high-speed injection is not recommended. The injection pressure and holding pressure need to be appropriately increased to overcome the high viscosity resistance of melt. The holding time should not be too short or too long. Too short will cause shrinkage marks and dimensional deviation, and too long will increase internal stress of the product. The cooling time is appropriately extended compared with ordinary plastics. Slow and uniform cooling under high mold temperature can reduce the risk of product warping, deformation and later cracking.
5. Standard of Molding Operation, Material Change and Barrel Cleaning
PEEK high-temperature melting is easy to retain carbonized materials. Material change and barrel cleaning must be carried out independently. Empty the original plastic in the machine first, and then repeatedly clean the barrel, screw and nozzle with special high-temperature cleaning material or pure PEEK raw materials until the melt has no black spots, no burnt materials and uniform color before formal production. Reduce the number of shutdowns as much as possible during production. Maintain the barrel holding temperature during temporary shutdown instead of randomly lowering the temperature. Empty the residual materials in the barrel if the shutdown time is slightly longer to prevent materials from carbonizing due to long-term high-temperature retention. Do not adjust temperature, speed and pressure parameters randomly and greatly during the molding process. The adjustment range each time should not be too large to slowly adapt and stabilize the production state.
6. Product Demolding and Post-ProcessingPEEK material has high hardness and strong toughness with large demolding resistance. The mold ejection system needs to design uniform multiple thimbles or push plate integral ejection structure to avoid product whitening, deformation and cracking caused by single-point ejection. There is generally certain internal stress after PEEK products are demolded. Precision structural parts, thin-walled parts and stressed parts must be annealed for post-processing. Keep constant temperature for 2 to 4 hours and then cool slowly with the furnace to effectively release internal stress, improve product dimensional stability, crack resistance and service life. The processed PEEK products shall avoid sudden cooling and heating, strong bending and collision, and be placed in a dry and normal temperature environment for storage to prevent hidden cracks caused by temperature difference deformation.
7. Prevention of Common DefectsSilver streaks and bubbles on molding products are mostly caused by raw material moisture, incomplete drying or material degradation due to excessive barrel temperature. Insufficient product filling is related to low temperature, insufficient injection pressure and speed, and small gate runner size. Black spots and scorching on the surface come from incomplete barrel cleaning, excessive temperature and poor exhaust. Warping and cracking are mainly caused by uneven mold temperature, too fast cooling, unreleased internal stress and unreasonable mold gate layout. Adjustment from multiple dimensions of drying, temperature, mold and annealing in production can stabilize PEEK injection molding quality.
8. Process SummaryPEEK special engineering plastic injection molding has high technical threshold, involving systematic control of raw material drying, equipment matching, mold design, process parameters and post-processing. Only by strictly implementing standard operation specifications and precise parameter control can we avoid various molding defects, give full play to the high-temperature resistance, high strength and corrosion resistance advantages of PEEK materials, and meet the application demands of high-precision and high-standard industrial parts.
