Thermoplastic elastomer, referred to as
TPE , has the characteristics of energy
saving , resource saving and high production efficiency, and its annual growth rate is as high as 6%. In this context, the use of new thermoplastic elastomer materials by processing plants is gradually expanding, but the problems that arise (the surface roughness, deformation, etc. of molded articles) have also increased.
Problems related to materials and their countermeasures
The main problem of thermoplastic elastomers from materials
problem | phenomenon | the reason | Countermeasures |
Albino | Whitening of the surface of the molded product | ( a ) Excess additive ( b ) Polymers and stabilizers are not suitable or compatible | ( a ) reduce the amount, optimize ( b ) use a stabilizer with good compatibility or use a high relative molecular mass stabilizer |
Sticky | The surface of the product is sticky | ( a ) Low molecular ( b ) plasticizer due to thermal decomposition , migration of stabilizer | ( a ) Reduce the molding temperature, low shear ( b ) Do not place at high temperature, reduce the amount, replace the type |
Ageing | Mechanical strength decreases, appearance quality deteriorates | ( a ) heat ( b ) ultraviolet light | ( a ) adding a heat stabilizer ( b ) adding a weathering stabilizer or a filler having a shielding effect |
1.1 Whitening
Whitening refers to the migration of a compounding agent such as a stabilizer to the surface of a molded article, and the surface thereof exhibits a white phenomenon as if it were sprayed with powder. The reason is mainly due to excessive stabilizer or incompatibility with the polymer. Therefore, a stabilizer which is compatible with the polymer should be selected or the amount of the stabilizer should be controlled to an optimum range. Secondly, it is also very effective to replace it with a stabilizer having a relatively high molecular weight.
In addition, there are stabilizers that function by migrating to the surface of the molded article. For example, antistatic agents, lubricants, and the like. For such a stabilizer, it is necessary to select a stabilizer which is hard to be whitened even if it is migrated.
When the thermoplastic elastomer is used in a general environment, there is little problem of whitening. However, in the case of high temperature, humidity, and outdoor use for a long period of time, a heat-resistant stabilizer (anti-aging agent) and a weather-resistant stabilizer are added to improve the durability. It is very necessary. Especially at high temperatures, the choice of stabilizer is also very important because it is highly susceptible to migration.
1.2 sticky
The surface of the soft thermoplastic elastomer is more susceptible to stickiness than the resin. The reason is mainly caused by the aging to produce a low molecular weight polymer and a plasticizer, and the stabilizer migrates to the surface. In either case, it is relatively easy to determine the substance related to this by analyzing the sticky component by means of analytical methods such as infrared spectroscopy (IR). The stickiness is mainly due to the fact that the molding temperature is too high and the polymer is thermally decomposed to form a low molecular weight substance. Although the set temperature of the molding machine itself does not reach the thermal decomposition temperature, the shear heat generation in the molding process sometimes causes it to temporarily reach a high temperature. As a countermeasure against this, it is very effective to reduce the molding temperature and low shear, and purify the inside of the hydraulic cylinder of the molding machine with nitrogen. Further, when the molding machine is suspended, the rubber is retained in the molten cavity for a long time in the cavity of the molding machine, and sometimes it is sticky due to heat aging.
In addition, articles used at high temperatures are prone to stickiness. Therefore, the selection of the stabilizer and the type of softener and the determination of the amount are very important.
1.3 Aging
Compared with inorganic materials and metal materials, polymer materials have poor heat resistance and ultraviolet resistance (relatively aging). Most of the products have a markedly reduced mechanical properties due to aging, and the appearance quality is deteriorated. By blending a stabilizer such as heat resistance and weather resistance, the occurrence of aging can be suppressed to some extent.
Problems related to extrusion molding and countermeasures
problem | phenomenon | the reason | Countermeasures |
Rough | rough surface | ( a ) Poor plasticization and mixing
( b ) Poor liquidity
( c ) the material is damp | ( a ) Using an extruder with good mixing effect to increase the set temperature, using a fine filter
( b ) increase the set temperature
( c ) sufficiently dry before molding |
Scaling | Retaining sediment at the edge of the mouth | ( a ) the material is damp
( b ) characteristics of the material itself | ( a ) sufficiently dry before molding
( b ) using a lubricant and coating the material with Teflon to modify the material |
Unstable extrusion | Unstable extrusion | ( a ) The melting point in the extruder is unstable
( b ) The temperature of the extruder fluctuates greatly
( c ) Screw and pellet slippage | ( a ) Change extrusion conditions and screw shape
( b ) Improve temperature control accuracy
( c ) preheating the pellets to increase the temperature of the barrel under the filling bucket |
Deformation | Product deformation | ( a ) uneven cooling
( b ) Double extrusion with different shrinkage rates | ( a ) uniform cooling
( b ) Adjust the shrinkage rate |
stripe | Stripes appear in the same area | ( a ) Oral damage, scaling
( b ) the appearance of dissimilar materials
( c ) does not eliminate the welded joint | ( a ) cleaning mouth type
( b ) cleaning the inside of the barrel, the screw and the head
( c ) Increase the length from the meeting (welding) to the mouth shape, and increase the pressure at the end of the mouth and the temperature of the resin. |
In particular, in recent years, in the context of environmental issues, the pace of replacing olefinic thermoplastic elastomer materials with soft polyvinyl chloride resins has been accelerated. However, there are many examples in which the extrusion molding of an olefin-based thermoplastic elastomer is carried out using an extruder for a flexible polyvinyl chloride. For the plasticization of the olefin-based thermoplastic elastomer containing a crystalline resin, it is necessary to use an extruder having an aspect ratio L/D of 22 or more and a screw compression ratio of 3 or more. However, since the extruder for soft polyvinyl chloride generally has an L/D of 20 or less and a compression ratio of 2.5 or less, the olefin-based thermoplastic elastomer cannot be sufficiently plasticized, so that the extrusion process of the molded article is performed. In the middle, there is often a problem that the surface is rough and the unmelted resin is foreign matter. Therefore, it is also necessary to select a molding machine suitable for the material.
2.1 The surface of the molded product is rough
The surface of the molded article is not smooth and rough, and most of it is caused by poor plasticization. As a countermeasure, it is needless to say that an appropriate molding machine is selected, but optimization of molding conditions such as setting temperature and type of screw rotation speed filter is also very important. In addition, the use of high temperature, fine mesh (filter) method can also improve the plasticizing efficiency.
Among the thermoplastic elastomers, some grades themselves contain a large amount of moisture-absorbing components or fillers, so it is necessary to dry them before molding. When it is not dried and molded, problems such as foaming and surface roughness occur.
2.2 scaling
Fouling is a deposit that accumulates on the edge of the mouth during extrusion, either from extruded materials or from special additives. As a countermeasure, the characteristics of the material can be effectively improved by reducing the amount of the lubricant, and the lubricant itself is one of the factors causing fouling when the lubricant is excessively added. The use of a Teflon-coated mouth pattern also reduces fouling. In addition, the use of moisture-containing materials should also be avoided.
2.3 Unstable extrusion
Fluctuations in the amount of extrusion are unstable phenomena. The fluctuation of the melting point at the screw feeding portion or the compression portion is the main cause of instability, and the magnitude and period of the extrusion fluctuation are irregular. These phenomena can be considered as the screw shape and the extrusion conditions are not suitable for the material, so it is necessary to adjust the extrusion conditions and replace the screws of different shapes.
In the case of large fluctuations in the temperature of the extruder, the fluidity of the material in the extruder is also one of the causes of this problem. In addition, the screwing of the screw and the pellet at the feeding site can also cause the phenomenon of unstable extrusion. While preheating the pellets with a Dodger dryer, it can be improved by increasing the temperature of the barrel below the bucket.
2.4 deformation
Extrusion of thermoplastic elastomers is usually carried out from a die and cooled through a water bath. At this time, if the molded article cannot be uniformly cooled, deformation is easily caused. In particular, for a molded article having a complicated shape and high dimensional accuracy, it is necessary to use a cooling calibration or detecting device.
In addition, during the multilayer extrusion process, deformation is easily caused by the shrinkage of the composite material.
2.5 stripes
Extrusion molded articles sometimes have streaks or marks on the same part. At this time, it is necessary to check or confirm whether there are signs of damage and dirt, scale, etc. on the mouth. Secondly, cleaning or eliminating uncleanness when changing materials is one of the reasons for this problem. In addition, it is easy to cause streaking when the material of the mouth-shaped meeting portion is poorly melted.
Problems related to injection molding and their countermeasures
problem | phenomenon | the reason | Countermeasures |
Stomata | Molded product Down hole | ( a ) Insufficient filling and insufficient pressure holding
( b ) slow cooling
( c ) in the thick part of the product | ( a ) increase the filling amount and increase the pressure holding
( b ) reduce mold and molding temperature
( c ) set a glue port near the thick part |
Burr | Resin Spill from the cavity | ( a ) Filling amount, overpressure
( b ) Low mold accuracy
( c ) insufficient clamping force | ( a ) reduce the filling amount and hold pressure
( b ) repairing the parting surface
( c ) use a larger molding machine |
Flow marks | ( a ) narrow interval recording stripe ( b ) Wide-interval isophase stripes ( c ) Wide spaced out-of-phase stripes | ( a ) The flow rate at the end is slower than the growth of the epidermis
( b ) low temperature resin intermittently clogs the injection port
( c ) End flow disorder | ( a ) increase injection speed, mold temperature and molding temperature
( b ) increase the flow port to increase the molding temperature and injection speed
( c ) increase the mold, molding temperature, and reduce the injection speed |
Poor release | The product is not easy to demould | ( a ) insufficient cooling
( b ) Adhesion of the material
( c ) The mold structure design is unreasonable | ( a ) Extend the cooling time and reduce the mold temperature
( b ) Replace materials and use release agents
( c ) Improved mold design |
Silver stripe | Radial streaks appear from the flow port | ( a ) Gasification of volatile and volatile components
( b ) Thermal decomposition gas of the material | ( a ) should be fully dried before molding
( b ) lower molding temperature |
Missing glue | Not filling the end of the cavity | ( a ) insufficient filling, pressure and fluidity
( b ) Insufficient exhaust
( c ) The injection port is not balanced | ( a ) Formed under optimal conditions
( b ) Improved exhaust method
( c ) Improved mold design |
Uneven color | Color such as paint | Poor dispersion of pigment | Improve screw back pressure or back pressure |
Charred | The end becomes like Burnt state | Insufficient exhaust | Improve the exhaust method and reduce the injection speed |
3.1 stomata
A pit phenomenon occurs in the molded article, mainly due to shrinkage of the molded article during cooling in the mold. In addition to requiring the material to fill the cavity, rapid cooling is also necessary. Specifically, it is to increase the holding pressure (secondary pressure) and lower the set temperature of the resin and the mold.
In addition, there is a great dependence on the shape of the molded article. Since the pores are highly prone to occur in thick areas, such a product should be provided with a model design such as a glue injection port or a flow rubber lane around it.
3.2 Burr
This is caused by the resin overflowing from the mold cavity. For the injection molding of rubber, it is normal to have a burr, but it is not normal for a resin or a thermoplastic elastomer. The reason is as follows: (1) Compared with the rubber compound, the molten resin or the thermoplastic elastomer has higher fluidity and lower injection pressure; 2 the mold is contacted and cooled, and the liquid can be solidified and terminated in an instant. Therefore, a resin or a thermoplastic elastomer is generally a phenomenon in which burrs are less likely to occur.
As a countermeasure, it is first necessary to reduce the amount of filling, reduce the holding pressure, and shorten the holding time. In addition, for molds with poor dimensional accuracy and gaps in the parting surface, repairs are necessary. In the case where the projected area of ​​the molded article is large and the mold clamping force is relatively lower than the injection pressure, burrs sometimes occur, so a larger molding machine must be used.
3.3 Flow marks
A streak phenomenon in which gloss is different on the surface of the molded article. Generally speaking, in the injection molding of resin:
(1) a narrow strip of recording stripe;
(2) a relatively wide stripe pattern of the same phase appearing above and below the surface of the molded article;
(3) Three types of relatively narrow spaced stripes appearing on the surface of the molded article.
Solving this problem can be solved by these methods, such as increasing the injection speed, mold temperature, increasing the glue injection port, increasing the resin temperature and injection rate, increasing the molding temperature, mold temperature or reducing the injection speed. It is effective by increasing the injection speed and the mold temperature.
3.4 Poor release
The poor mold release property means that the molded article is difficult to remove from the mold or is completely deformed during the removal process. Adhesive materials can easily cause this problem, but the method of applying a release agent to the material or applying a release agent to the mold before molding can be improved. Insufficient cooling of the molded article (under-curing) is also likely to cause such a problem, and therefore it is necessary to sufficiently cool the molded article. In addition, the irrational design of the mold will become a cause of difficulty in demoulding, especially in the parts that are easy to stick to the glue injection port, the rubber feed path, etc., it is very important to increase the extraction angle of the injection port and widen the glue path. Effective.
3.5 silver stripes
The phenomenon of radial streaks around the injection port is caused by the vaporization of moisture or volatile components in the material. Among them, the air trapped in the mold or in the mold during the plasticization process will also cause this phenomenon. Therefore, it is necessary to sufficiently dry the moisture absorbing material before molding and to reduce the molding temperature at which the decomposable gas material is easily generated.
3.6 lack of glue
The phenomenon of not filling the end of the cavity is called lack of glue. This is mainly caused by the discomfort of the molding conditions such as insufficient filling amount, but the exhaustion of the molding or the unevenness of the flow passage (multi-cavity mold) may also cause this phenomenon.
3.7 charred
It means that the part that is not filled to the end and that is not filled with the cavity has an aging phenomenon like scorch. This is mainly due to insufficient exhaust gas, heat or compression caused by air or generated gas, and the result of a sudden increase in temperature (ie, heat aging on the surface of the molded article). Improving the exhaust method is a better solution. In the case of a lighter degree, reducing the injection speed can also be solved.
3.8 uneven color
When the precursor mixture using the thermoplastic elastomer particles and the dry blend as a pigment is colored, the color unevenness of the molded article is likely to occur. As a countermeasure, it is effective to increase the back pressure of the screw and to knead the filler.
Ways to solve problems quickly
The problems that arise in actual production are often transitioned to complex levels by an extremely simple problem (inappropriate molding temperature). Especially for the problems caused by the composite factors, in order to be resolved as soon as possible, we must first correctly collect the relevant information. As shown in the above table, although it is a phenomenon of rough surface, in order to find out its specific cause, it is necessary to understand the situation related to this.
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