Taking the real sample for PEEK plastic injection
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Hello dears,
Today we want to have a simply talking of PEEK material.
PEEK polyetheretherketone is a special engineering plastic with excellent properties such as high temperature resistance, self-lubrication, easy processing and high mechanical strength. It can be manufactured and processed into various mechanical parts, such as automobile gears, oil screens, shift start disk ; Aircraft engine parts, automatic washing machine runners, medical equipment parts, etc.
PEEK polyetheretherketone is a special engineering plastic with excellent properties such as high temperature resistance, self-lubrication, easy processing and high mechanical strength. It can be manufactured and processed into various mechanical parts, such as automobile gears, oil screens, shift start disk ; Aircraft engine parts, automatic washing machine runners, medical equipment parts, etc. Due to the relatively high price of PEEK material, it is relatively difficult to form, and it has become one of the areas that many injection molding companies focus on.
Polyetheretherketone (PEEK) is a high polymer composed of repeating units containing one ketone bond and two ether bonds in the main chain structure, and is a special engineering plastic. PEEK has the advantages of high mechanical strength, high temperature resistance, impact resistance, flame retardant, acid and alkali resistance, hard texture, long service life, etc., and has a large number of applications in the automotive industry, aerospace, medical equipment and other fields.
PEEK resin was first used in the aerospace field, replacing aluminum and other metal materials to manufacture various aircraft parts. In the automotive industry, PEEK resin has good friction resistance and mechanical properties. As a raw material for the manufacture of engine inner covers, various parts such as bearings, gaskets, seals, clutch gear rings, etc. made of PEEK resin are used in the transmission and braking of automobiles. and air conditioning systems are widely used.
PEEK resins can also withstand up to 3000 cycles of autoclaving at 134°C, a feature that makes them useful in the production of sterilization-critical, reusable surgical and dental equipment. The molding temperature of PEEK is 320 degrees ~ 390 degrees, the baking temperature is 160~1855H~8H, and the mold temperature is 140~180. The molding temperature of this material is too high, and the damage to the screw is serious. When setting the screw speed, the speed should not be too fast, and the injection pressure should be 100 ~130MPa injection speed 40~80. After molding, the screw should be quickly cleaned with PE wax, and the PEEK material should not be allowed to stay in the screw.
PEEK material has extremely high performance in many aspects, and its injection molding parts also have great development prospects. However, PEEK injection molded parts also have various defects, including cold spots, silver streaks, pores, weld lines, warpage, etc. The author takes an aviation cabin adjustment and environmental control valve part as an example, and discusses the defects and process optimization of PEEK injection molding process.
Below is an example of analysis of forming defects of PEEK product.
Pictured below is a black PEEK with insert valve part, injection molded using Victrex PEEK 450G. Product diameter 110mm, thickness 15mm, with M6×1 steel inserts.
In the molding process of this product, the main difficulty is that the product has appearance defects, including cold material, silver streaks, pores and weld marks, etc., and there are gate marks in the front and middle feed. Appearance defects were analyzed from the aspects of man, machine, material, method and environment, and corresponding measures were taken, and good results were obtained. The gate marks are removed by machining after molding to ensure that the front surface is flat and the thickness size meets the design requirements.
Cold stock problem
There are two main reasons for the cold material problem:
Mold cold wells were not specified or removed during production. The polymer in the molten state is brought into contact with the mold with a lower temperature during production, and the temperature of the front part in the molten state will decrease rapidly due to heat transfer. When the melting point of PEEK is reduced to 343 ° C, the PEEK that should be in the molten stage will be It will rapidly solidify and become cold material. In the production process, if the cold slug well is not added at the end of the runner, the product will easily flow into the cold slug, causing defects in the injection molded parts.
PEEK material has a special production process. When the thickness of the product is required to be large, it needs to be cooled for a long time. At this time, the temperature of the nozzle is as high as 400 ℃, but the temperature around the material does not reach this value, and the great temperature difference causes the heat to dissipate rapidly. If the sprue sleeve at low temperature is in contact with the injection molding machine nozzle for a long time, after the temperature drops to the melting point of PEEK, the molten material is very easy to solidify, thus interfering with the progress of the injection molding process. Therefore, after the material injection and pressure holding are completed, the barrel should be removed immediately to keep the mold and the nozzle at a distance (pre-feeding). In order to prevent the nozzle from casting, it should be set to properly pump glue. The melt directly contacts the air and is prone to solidification due to the large temperature difference. If the treatment is lacking, the solidified PEEK will enter the mold, and at the same time, it will appear on the surface of the product by means of springs, resulting in cold material defects.
Silver streak problem
For some PEEK injection molded parts, silver streaks near the gate are usually caused by moisture or gas. The plasticization process of the product will be accompanied by gas generation, which can easily cause silver streaks in the product. When the screw speed of the injection molding machine is large, the gas will take the opportunity to enter the plasticizing process, and flow to the surface of the injection molded part by means of the fountain flow. Because it flows through the gate under high pressure, silver streaks will be generated on the surface of the injection molded part. Setting back pressure during molding can effectively avoid gas entrapped by molten material during plasticization.
Moisture produced by raw materials that have not been fully dried is also prone to silver streaks. PEEK materials will continuously absorb moisture in the air during placement, and the water absorption rate can reach 0.4%. The moisture absorbed in the production enters the gate through the melt. Due to the large temperature difference between the mold cavity wall and the molten material, the encounter between the two leads to silver streaks. In order to avoid the occurrence of silver streaks, attention should be paid to controlling the moisture within 0.2% in actual production. When storing materials, pay attention to placing them evenly and not stacking them, and control the height of the material layer within 20-30 mm. Put PEEK in a drying oven at 150-160°C for 6-8 hours before use.
Warping problem
When the shrinkage and cooling are uneven, the product and the mold cavity will deviate, and warpage will occur at this time. In fact, the contraction itself will not have much impact, mainly the difference in the contraction process. Normally, PEEK injection molded parts will only change in volume when the shrinkage is uniform. When the shrinkage is uneven, the warpage of the injection molded part will occur.
There are three main factors:
(1) Warpage caused by uneven wall thickness of injection molded parts.
(2) Warpage caused by sharp bending angles of injection molded parts.
(3) Warping deformation cracking caused by stress.
Suggestions for optimization of PEEK injection molding process
Mold temperature and cooling rate recommendations
In injection molding of PEEK, controlling the mold temperature and cooling rate can change the mechanical properties of the product. In most cases, the mold temperature for PEEK molding should be controlled at around 175°C. When the mold temperature is higher than 140 ℃, the PEEK crystal grows better, more complete, and the structure is relatively stable.
Appropriate cooling rate is beneficial to improve the mechanical properties of the product; slow cooling and rapid cooling will reduce the mechanical properties of the product. The crystalline shape of PEEK at a certain temperature is uniform nucleation, and if a slow cooling method is used, it is easy to form larger non-uniform crystals. However, when the surface of the product is rapidly cooled, the internal cooling rate is not fast enough, and internal stress is easily generated, resulting in cracks, resulting in a decrease in the mechanical properties of the material.
Injection pressure selection
When high-pressure injection is used, the material is filled tightly, and the molding effect will be good, which is beneficial to improve the mechanical properties of PEEK. However, if the pressure is too high, the product will be difficult to demold, resulting in scratches on the surface of the product, so injection
The pressure should be moderate. When the injection pressure is 9.5 MPa, the pressure holding cylinder is 7.5 MPa, and the back pressure is 0.5-1.5 MPa, the product can be formed smoothly.
temperature advice
PEEK is a special thermoplastic crystalline plastic. Its glass transition temperature is about 143°C, and its melting point is about 343°C. The optimum range of barrel temperature is 360～400℃, at this time, the surface of the product is smooth and shiny. The thermal decomposition temperature of PEEK is 550°C. However, when molding at high temperature (above 410°C), due to mechanical action and the presence of a small amount of air, it is easy to degrade, and cross-linking reactions occur. These reactions directly lead to changes in its melt viscosity, resulting in uneven product gloss.
time suggestion
The injection time needs to ensure that the entire mold cavity is filled with enough molten material. Due to the large thickness of the processed product, the injection time is selected to be 6 s. The holding pressure time needs to ensure that the volume of the material is replenished due to the reduction of the melt temperature in the mold cavity. At the same time, in order to avoid the generation of pores after the removal of the sprue condensate, the selected time is 8 s. The optimal value of cooling time should be to open the mold cavity when the cavity pressure is close to atmospheric pressure, so that there will be no demoulding scratches on the appearance of the product. Mold cavity pressure greater than atmospheric pressure or less than atmospheric pressure to open the mold will have an impact on the appearance of the product. According to the appearance change of the product, after debugging, it is determined that the cooling time is 35s.
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