PEEK injection molding process

PEEK polyether ether ketone is high-temperature resistance, self-lubricating, easy processing, and high mechanical strength and other excellent performance of special engineering plastics, can be manufactured and processed into a variety of mechanical parts, such as automotive gears, oil screens, shift start disc; aircraft engine parts, automatic washing machine rotor, medical device parts, etc.

PEEK material due to the general price is relatively high, and relatively difficult to mold, and become many injections molding, one of the key areas of concern for companies.

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 devices, and other fields.

PEEK resin was first used in the aerospace industry, replacing aluminum and other metal materials in the manufacture of various aircraft parts.

In the automotive industry, due to PEEK resin has good friction resistance and mechanical properties, as the raw material for manufacturing engine covers, with its manufacturing of bearings, gaskets, seals, clutch rings, and other parts in the transmission, brakes, and air conditioning system in the car is widely used.

PEEK resin is the ideal electrical insulator, in high temperature, high pressure and high humidity and other harsh working conditions, but still maintain good electrical insulation properties, so the electronic information field gradually become the second largest application area of PEEK resin.

The manufacture of ultra-pure water transport pipes, valves, and pumps, in the semiconductor industry, is commonly used to manufacture wafer bearers, electronic insulation diaphragms, and a variety of connected devices.

As a semi-crystalline engineering plastic, PEEK is insoluble in almost all solvents other than concentrated sulfuric acid, and is therefore commonly used to make compressor valves, piston rings, seals, and various chemical pump bodies and valve components.

PEEK resin can also withstand up to 3,000 cycles of autoclaving at 134 degrees C. This feature allows it to be used in the production of surgical and dental equipment with high sterilization requirements and repeated use. PEEK molding temperature 320 degrees ~ 390 degrees C.

Baking temperature 160 ~ 1855H ~ 8H mold temperature 140 ~ 180 This material injection molding temperature is too high, the damage to the screw is more serious, in setting the Screw speed can not be too fast, injection pressure in 100 ~ 130MPa injection speed 40 ~ 80. molding should be completed promptly with PE wax to quickly clean the screw, can not let the material of PEEK stay in the screw.

PEEK material has extremely high performance in many areas, and its injection molded parts have great prospects for development. However, PEEK injection molded parts can also have a variety of defects, including cold material points, silver lines, porosity, fusion marks, warpage, etc.

The author takes an aircraft cabin control ring door as an example and discusses defects in the injection molding process and process optimization of PEEK injection molded parts.

PEEK products molding defects analysis

The figure below shows a black PEEK product with inserts for a valve, which was injection molded using Victrex PEEK 450G. The product is 110mm in diameter and 15mm thick with M6 x 1 steel inserts.

The main difficulty of this product in the molding process is that the product has appearance defects, including cold material, silver pattern, air holes and fusion marks, etc. There are traces of gating in the middle of the front feed.

The appearance defects were analyzed from the aspects of human, machine, material, method and environment, and corresponding measures were taken, and good results were received. The gate marks are removed by machining after molding to ensure that the front surface is flat and the thickness meets the design requirements.

Cold material problem

The cold material problem has two main reasons as follows：

During the production process, the cold material well of the injection mold is not designed according to the specifications or the cold material well is removed.

So that the molten state of the polymer in the production and the lower temperature mold contact, the front part of the molten state will be in heat transfer and the temperature will rapidly decrease, and so lower to the melting point of PEEK 343 ℃, should be in the molten stage of PEEK will rapidly solidify into cold material.

In the production process, if there is no cold material well at the end of the runner, then the product will easily flow into the cold material, causing defects in the injection molded parts.

PEEK materials are made in a special process. When the thickness of the product is large, the product needs to be cooled for a long time. The temperature of the nozzle can be as high as 400°C, but the temperature around the material does not reach that value, and the great temperature difference causes the heat to dissipate quickly.

If the low temperature of the gate sleeve and injection molding machine nozzle contact time is long, after the temperature drops to the PEEK melting point, the molten material is very easy to solidify, thus interfering with the progress of the injection molding process. Therefore, when the material is injected and pressure-holding is completed, the barrel should be removed immediately to keep the mold and nozzle at a distance (before filling).

In order to prevent the nozzle from the phenomenon of flow delay, proper rubber extraction should be set. Direct contact of the melt with the air because of the temperature difference is prone to solidification, if the lack of treatment, has solidified PEEK will enter the mold, while appearing on the surface of the product with the help of springs, resulting in cold material defects.

Silver pattern 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 the generation of gas, which is very likely to cause the product silver streak.

When the screw speed of the injection molding machine is high, the gas will enter the plasticizing process and flow to the surface of the injection molded part with the help of the fountain flow, which will produce silver lines on the surface of the injection molded part because it flows through the gate under high pressure. Setting the back pressure in the molding process can effectively avoid the molten material from entrapping gas during the plasticizing process.

Moisture from raw materials that have not been adequately dried is also susceptible to silver streaks, and PEEK materials can absorb up to 0.4% of the moisture in the air while sitting.

During production, the absorbed moisture enters the gate through the melt, and the temperature difference between the mold cavity wall and the molten material is large, resulting in silver streaks. In order to avoid the silver pattern, we should pay attention to control the moisture within 0.2% in the actual production.

The material should be stored evenly and not stacked, and the height of the material layer should be controlled at 20-30 mm. Before use, PEEK should be dried in a drying oven at 150-160°C for 6-8 hours.

Warpage problem

When shrinkage, cooling uneven products and mold cavity will be deviated, then will produce warpage phenomenon. In fact, shrinkage itself does not have too much impact, mainly in the shrinkage process generated by the difference.

Under normal circumstances when the shrinkage is uniform, PEEK injection molded parts will only change in volume. When the shrinkage is not uniform, the injection molded part will be warped and deformed.

There are three main factors, which are:

(1) Warpage caused by uneven wall thickness of the injection molded part.

(2) Warpage caused by sharp bending angle of the injection molded part.

(3) Warpage and cracking caused by stress.

PEEK injection molding process optimization recommendations

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 about 175°C.

When the mold temperature is higher than 140°C, the mold temperature should be controlled at about 175°C. When the mold temperature is higher than 140°C, PEEK crystals grow better, are more complete, and have a relatively stable structure.

The appropriate cooling rate is conducive to improving 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 a uniform nucleus, if you use the slow cooling method, it will be easy to form large inhomogeneous crystals.

However, when the surface of the product is cooled rapidly, the internal cooling rate is not fast enough and is prone to internal stresses and 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 conducive to improving the mechanical properties of PEEK. But the pressure is too high, the product will be difficult to release, resulting in product surface abrasion, so the injection pressure should be moderate. When the injection pressure is 9.5 MPa, the holding cylinder pressure is 7.5 MPa, and the back pressure is 0.5 to 1.5 MPa, the product will be molded smoothly.

Temperature recommendations

PEEK is a special type of thermoplastic crystalline plastic. Its glass transition temperature is about 143°C and melting point is about 343°C.

The best range of barrel temperature is 360 ~ 400 ℃, when the product surface is smooth and glossy. 550 ℃ PEEK thermal decomposition temperature. But at high temperatures (above 410 ℃) molding, due to mechanical action and the presence of a small amount of air, prone to degradation, and cross-linking reactions, these reactions directly lead to changes in its melt viscosity, so that the product luster varies.

Time recommendation

The injection time should be 6 s to ensure that the whole cavity is filled with enough molten material, because the thickness of the processed product is large, and the holding time should be 8 s to ensure the replenishment of the volume reduction caused by the decrease of the melt temperature in the cavity and to avoid the generation of air holes after the removal of the condensed material from the sprue.

The appearance of the product will not be scratched by the release. The cavity pressure is greater than atmospheric pressure or less than atmospheric pressure to open the mold, both have an impact on the appearance of the products. According to the change of product appearance, the cooling time was determined to be 35 s after debugging.