Molding method for PTFE by mould pressing

The crystallization melting point of PTFE is 327℃, but the resin cannot be in the melting state until it is above 380℃, and the melt viscosity is as high as 1 010 Pa*S. In addition, PTFE has strong solvent resistance. Therefore, it can neither melt processing method, can not be dissolved processing method, usually the production of its products can only be like metal and ceramic processing - sample, first powder compaction, then sintering and mechanical processing, or through extrusion molding, isotactic pressing molding, coating molding and calendering molding and other ways of processing.

1.  Molding

Molding is currently the most widely used molding method for PTFE. Molding technology is a certain molding materials (powder, granule, fibrous material, etc.) into the metal mold, in - - fixed temperature, pressure - - a method of forming. Molded polymers are not limited by their molecular weight, and almost all plastics can be molded. The main features of molding are; Low cost, simple equipment, low investment, not limited by the molecular weight of the processed plastics; The disadvantages are low production efficiency, high labor intensity and unstable product quality. PTFE is of high molecular weight and extremely poor fluidity. When other processing methods are not mature, PTFE products are mainly processed by molding all over the world.

In the molding can be subdivided into five methods according to the difference of specific process: (1) pressing - sintering a pressing method (also known as free sintering method);(2) sintering - pressing method;(3) rapid heating a pressing method;(5) simultaneous pressing and sintering method.

2.  Hydroform method

Hydraulic pressure method, also known as equalization method, isobaric pressure method or rubber molding method, is to add PTFE resin evenly between the bag and the mold wall, and then to the bag into the liquid (commonly used water), the pressure of the rubber bag to the mold wall expansion, compacting the resin and become a preformed product - a method. This method can be used to manufacture large-volume sleeve, bottom storage tank, hemispheric shell, tower column, large slab, etc., as well as complex products with PTFE composite structure, such as tee, elbow and profile. The main advantages of hydraulic molding are the simple structure of the equipment and mould - a common water pump replaces the high-tonnage press, and the products are compressed evenly and densely - resulting in the manufacture of large components, complex shapes and simple lining structure.

3.       Push molding

Push pressure is also known as paste extrusion molding, 20-30 mesh sieve of disperse resin and organic additives (toluene, petroleum ether, solvent oil, ratio of resin weight of 1/5) mixed into a paste, prepress into thick wall cylindrical blank, then put into the push press machine barrel, under heating with the plunker push molding. After drying and sintering at temperature of 360~380°C, strong and tough push and pressure tube and bar products are obtained after cooling. The pushing and pressing products are limited to the rod with diameter of 16mm or less and the pipe with wall thickness of 3mm or less.

4.       Spiral extrusion molding

The screw extruder of PTFE powder is different from the extruder used by other thermoplastic plastics. The extrusion molding of ordinary thermoplastic plastics is to push the material forward with the help of screw rotation, and meanwhile to compress, shear and mix the material. The material is also melted by the heat generated by shear force and the external heating of the material cylinder. However, the screw of PTFE extruder only plays the role of conveying and pushing, so that the material passes through the head of a single-screw extruder with double-head thread and the same pitch and depth, and then enters into the mouth mold for sintering and cooling, and forms with the pressure provided by the counter-pressure device to achieve the continuous purpose. It is often difficult to process PTFE by single screw extruder. The low friction coefficient of PTFE powder causes skidding during the feeding process, which greatly reduces the conveying capacity of the screw. And because of friction heat, may also make powder adhesion to the screw or barrel, making the feeding more difficult and unstable.

In recent years, twin-screw has also been applied in processing materials of this special nature. Its feeding principle is different from that of single-screw extruder, and it has a positive transport function, which can overcome the sliding problem of UHMWPE powder in the screw and greatly improve the feeding capacity of the screw. The counter-rotating twin-screw extruder has better mixing and homogenization effect than the same-direction twin-screw extruder, but due to its larger separation force, the shearing action at the gap of the screw is larger, which causes the material to overheat, and the molecular weight of the extruder can drop by about 40%.If the gap is large and the screw is not engaged, the material will stick to the hot metal. However, the use of the same rotation of the twin screw extruder, there is no such problem. Material in the extruder by the shearing action of smaller, plasticizing the required quantity of heat, all from plus heat source, and thus can be precise control, which can make the material in the extrusion process to minimize heat degradation, at the same time in order to maintain the material flow of normal and stable in the nose, the nose section size of design should be compatible with the volume of a screw conveying material. Screw speed is not fast, generally about 10 revolutions per minute. In order to avoid material suddenly sticking to the metal surface, the extrusion temperature must be strictly controlled.

5.        Plunger extrusion molding

Plunger extrusion processing plastic, plastic processing is a relatively ancient method, since the emergence of this material, people began to use this method to process plastic. PTFE is processed with a plunger extruder by pressing the quantitative resin into the inlet mold, making the plunger reciprocate and pressing it into a preformed product. So back and forth, in the mouth mold to form a multi - stage pre - molding products. Due to the friction between PTFE resin and the friction between PTFE resin and the mold wall, and the volume expansion of the preformed product during sintering in the mold, the preformed product is sintered and cooled into a continuous whole under pressure. The advantages of this method are as follows: no shearing occurs in the molding process, the relative molecular weight decreases less, the product quality is good, and is not limited by the relative molecular weight. However, due to the small contact area between raw materials and heating parts in the extrusion process, the heating efficiency is low, which limits the extrusion speed.

6.        Other processing methods;

PTFE can also be processed by injection molding, calendering molding, coating molding or secondary molding.

Polymer micro-mechanical mold assembly molding

In-mold assembly molding of polymer micromachines is a new technology in which technological bottlenecks of micromachines micro-assembly manufacturing process can be effectively solved,  however, the thermal-fluid-solid coupling effect between polymer  high temperature viscoelastic melt flow and preformed micro solid part can be caused which induces preformed micro solid part producing frequently the thermal-fluid-solid coupling deformation and  necking fusing fracture phenomenon, how accurate control and predict thermal-fluid-solid coupling deformation and necking fusing fracture phenomenon is key scientific issues of in mold micro assembly molding technology industrialized application.

Therefore, based on the developed variable combination  mold and Auto model FB-110C polymer co-injection molding machine, the polymer micromachines In-mold assembly molding experimental research platform was established, by which the influencing rule and influencing mechanism of process parameters and properties of polymeric materials on preformed micro shaft thermal-fluid-solid coupling deformation and necking fusing fracture phenomenon were  systematically studied, key regulatory parameters of thermal-fluid-solid coupling deformation and necking fusing fracture phenomenon in in-mold micro assembly molding process was clarified; the scientific foundation of the research and development of thermal-fluid-solid coupling deformation precision-shaped control technology  and the  prevention technology of necking fusing fracture phenomenon was laid. The main innovations and achievements as follows.

The variable combination mold of  typical micro mobile motion pair in-mold micro assembly molding was developed, and Based on the developed variable combination  mold and Auto model FB-110C polymer co-injection molding machine, the polymer micromachines In-mold assembly molding experimental research platform was established.

The influencing rule and influencing mechanism of secondary molding melt injection temperature, secondary molding melt injection pressure  and  screw stroke on necking fusing fracture phenomenon of preformed micro shaft, the research results show  key regulatory parameter of preformed micro shaft necking fusing fracture phenomenon is secondary molding melt injection volume,  when the secondary molding melt injection volume exceeds its critical melt injection volume, the critical melt injection volume of induced preformed micro shaft necking fusing fracture phenomenon depends on secondary molding melt injection temperature, which is positively proportional to secondary molding melt injection temperature. Improving of secondary molding melt injection temperature will help improve the critical melt injection volume of induced preformed micro shaft necking fusing fracture phenomenon, which increase the secondary molded part density and strength in In-mold assembly molding process, at the same time, but also to avoid preformed micro shaft necking fusing fracture phenomenon.

Experimental research results show that the micro shaft unidirectional necking fusing fracture phenomenon possible ways  have two main factors, firstly, the axial tensile force is too large; secondly, the preformed micro shaft near-surface temperature in a secondary molding process  is too large, which led to partial melting  and a sharp decline of elastic modulus. The heat transforms time contact between secondary molding high temperature and preformed micro shaft will prolong with increasing of secondary molding melt injection volume, which make the preformed micro shaft near-surface temperature improve with increasing of screw stroke.

 When the preform micro shaft near-surface temperature exceeds the glass transition temperature, local micro assembly interface of preformed micro shaft will experience continuous phase transition evolution from elastoplastic solid state to viscoelastic plasticity glassy state ,final to viscoelastic high-elastic state, which led to a sharp decline of elastic modulus and make polymer material in local phase transition evolution region lost the ability to resist deformation, preformed micro shaft in local phase transition evolution region of micro assembly interface must produce necking phenomenon under tensile force effect.

When the tension force reaches a certain level, preformed micro shaft will produce the necking fusing fracture phenomenon. In order to preform micro shaft near-surface temperature does not exceed the glass transition temperature, which requires its secondary molding melt injection volume does not exceed the critical injection volume. Based on above necking fusing fracture mechanism, the preparatory proposed necking fusing fracture mechanism theory and prevention technical method of preformed micro shaft necking fusing fracture provide technical support for the industrialized application of In-mold micro assembly molding technology.

Experimental research results show that the preformed micro shaft thermal-fluid-solid coupling deformation depends on secondary molding melt injection temperature，secondary molding melt injection pressure ，secondary molding melt injection volume and preformed micro shaft material properties，and the process parameters affect its thermal-fluid-solid coupling deformation  by adjusting pre-formed micro shaft micro assembly interface to withstand the impact of the thermal-fluid-solid coupling pressure, viscoelastic supporting normal stress, viscous friction drag shear stress and micro shaft stiffness against deformation，the preformed micro shaft thermal-fluid-solid coupling deformation increase with increasing of secondary molding melt injection temperature，secondary molding melt injection pressure, secondary molding melt injection volume, and based on key regulatory parameters of viscoelastic supporting normal stress, viscous friction drag shear stress and micro shaft stiffness against deformation, the formation mechanism of preformed micro shaft thermal-fluid-solid coupling deformation was revealed.

[---This article is from the network, if there is infringement, please contact delete, thank you!--- ]

Properties of molded products

Molded products have excellent electrical properties (especially anti-leakage properties), mechanical properties, heat resistance, fire resistance, chemical corrosion resistance and dimensional stability, and can be adjusted according to the needs of each component type and dosage to obtain products with special performance requirements. The main performance is as follows：

1.Electrical property

Molded plastics are widely used in high voltage electrical applications due to their arc resistance, acer performance, flame resistance, dimensional stability, moulding and low cost. The ability of arc suppression is mainly due to the presence of inert inorganic fillers such as hydrated alumina, silica and ceramic. Add a small amount of polyethylene powder (5% by weight) and use nylon fiber to improve arc resistance. In order to obtain better arc resistance, the resin and glass fiber content should be reduced to the minimum, however, such results reduce the mechanical properties.

2. Mechanical property

The mechanical properties of molding materials vary with the types and proportions of reinforced fibers and resin substrates used. In DMC, when the fiber length exceeds 6.35mm, the modification of product properties is very small.

Most SMC products are made from short-cut felt, and there is no published data on the effect of fiber length changes. Although the length of the general fiber is 50mm, this length is not necessarily suitable for all applications.

Increasing the content of reinforcing materials can improve the mechanical properties, but too much fiber content will bring inconvenience to molding. For example, when the fiber content in DMC exceeds 20%, it has little influence on its mechanical properties.

3. Heat resistance or fire resistance

Heat resistance refers to the ability of products to withstand thermal decomposition for long periods of time below the flammable temperature. "Short-term thermal strength" or "thermal strength" is related to the thermal deformation temperature of the resin. Although some fillers can improve the heat resistance of products, the heat resistance and thermal strength mainly depend on the properties of the resin. Flammability is a measure of apparent combustion, divided into "non-ignition", "self-extinguishing" and "flame-retardant" according to the ability and speed with which a material is able to extinguish easily or when the ignition source is removed. Polyester resins achieve flame resistance by adding halogen and phosphorous compounds and by using alumina hydrate as the main filler in the components.

Phenolic resins are inherently fire-resistant. Halogen compounds and phosphorous compounds, as well as HET anhydride used as hardener, can make epoxy resins flame resistant.

4.Dimensional stability

General molding materials have good dimensional stability, water absorption rate: small, thermal expansion coefficient and aluminum is very similar, when continuous exposure to high temperature, size is almost no change.

5. Corrosion resistance

The chemical resistance or corrosion resistance of molding material mainly depends on the selected resin matrix. The suitable resin and filler can be used to prepare the moulded material which can meet the requirement of resisting special bristle dew

DMC can be made of acid-resistant and alkali-resistant ligulates and acid-resistant epoxy resins. The choice of a resin for SMC is currently limited because the resin must also have a chemical structure that thickens easily. In common fillers, clay and silica have better corrosion resistance and other properties.

6.Contractility

The shrinkage of moulded material is very low after release, the typical maximum shrinkage rate is 0.004, and the shrinkage rate of many moulded materials is close to 0.001, which is mainly due to the small thermal shrinkage of glass fiber and inorganic filler. However, the combination of low-shrinkage, high-strength fibers with the resin systems commonly used for rapid curing and high thermal shrinkage results in greater stress on the resin matrix between the fibers. The collateral effects of this stress cause surface ripples, cracks, warping, and internal voids. These defects can be reduced by the use of organic fiber reinforced materials compatible with resin shrinkage or by the use of short or filamentous glass fibers.

Raw material for molding

The commonly used molding materials are mainly composed of resin matrix, reinforcing materials, fillers, pigments, etc., and according to the process and performance requirements, the resin matrix is also added with curing agent, thickener, internal release agent, solvent and other additives. Reinforcement material is the skeleton material of die pressing material, which mainly endows die pressing material with excellent mechanical properties and prevents the propagation of micro-cracks. The main varieties of reinforcement materials are various glass fiber felt, no twist roving, no twist coarse gauze, chopped fiber, ground fiber, glass yarn and fabric and other varieties of fiber. The main function of resin matrix in molding material is to bond the reinforcing material and filler together, which not only protects the reinforcing material, but also makes it uniform force under external load. Theoretically, most types of thermoplastic and thermosetting resins can be used as resin matrix materials. There are polyester resin, epoxy resin, amino resin and phenolic resin. A large number of various types of fillers are used in the molding material, the main function of which is to reduce the material cost, improve the process, improve the molding material some physical properties, appearance and give some characteristics. Fillers are mostly in powder form, such as clay, calcium carbonate, talcum powder and some fillers with special performance requirements.

The basic requirements of resin in molding are as follows: good infiltration performance of reinforcing materials and fillers to improve the bonding strength between resin and glass fiber; Resin to have the appropriate: viscosity, good fluidity, so that in the molding process of resin and glass fiber at the same time full of all corners of the cavity, to obtain a balanced strength of the molding products; Resin curing temperature is low, less volatile in the curing process, good technology (such as viscosity easy to adjust, good solubility with various solvents, easy to release mold, etc.), and can meet the mold products specific performance requirements. In addition, from the point of view of application or other point of view, the resin should also meet some other special performance requirements, such as corrosion resistance, heat resistance and so on. From the perspective of production efficiency, fast curing speed of resin is required, but for some large products with complex structure and higher requirements, the curing speed should be properly controlled. Resin curing after high mechanical strength, good toughness, avoid from the mold when taken out, in the case of bending cracking or breaking. Therefore, the selection of resin, will become a very important factor affecting the whole process.

Molding products are commonly used in thermosetting resin, thermoplastic resin in the molding process of the application of less. The main thermosetting resins are phenolic resin, epoxy resin, epoxy phenolic resin, polyvinyl butanal resin, unsaturated polyester resin, etc.