Defects and solutions of injection molding surface of elastomer material

Time:2020-01-10 10:32:50 / Popularity: / Source:

One of reasons for surface problems of TPE injection parts is poor quality of raw materials, which is also related to uneven dispersion of formula, and it is also related to setting of some parameters of injection moulding process.
Thermoplastic elastomer 

Whitening phenomenon: refers to compounding agent such as stabilizer migrates to surface of molded product, and its surface appears as a white phenomenon like spraying powder.

Reason: It is mainly caused by excessive blending of stabilizer or incompatibility with polymer. Incompatibility can be solved by adding related compatibilizers. A stabilizer with good compatibility for polymer should be selected or amount of stabilizer should be controlled in optimal range. Secondly, it is very effective to replace it with a stabilizer with high molecular weight. In addition, there are stabilizers that perform their functions by migrating to surface of molded product. For example, antistatic agents, lubricants, and so on. For such a stabilizer, it is necessary to select a stabilizer that is unlikely to cause whitening even when it migrates. Thermoplastic elastomer rarely cause whitening when used in general environments, but in the case of high temperature, humidity, and long-term outdoor use, it is necessary to add heat-resistant stabilizers (anti-aging agents) and weather-resistant stabilizer to improve durability. Especially at high temperatures, it is very important to choose a stabilizer because it can cause migration easily.

Tackiness: Compared with resin, surface of soft thermoplastic elastomer is more prone to stickiness.

Cause: It is mainly caused by aging to generate low molecular weight polymers, migration of plasticizers and stabilizers to surface. However, in any case, by using infrared spectroscopy (IR) and other analytical methods, related substances can be easily identified by analyzing sticky components. Tackiness is mainly due to excessively high molding temperature and formation of low molecular weight substances due to thermal decomposition of polymers. Although set temperature of molding machine itself does not reach thermal decomposition temperature, shear heat generated during molding process sometimes causes it to temporarily reach high temperature.
Solution: Reduce molding temperature and reduce shear. Purifying inside of hydraulic cylinder of molding machine with nitrogen is very effective. In addition, when molding machine is suspended, rubber compound is left in cavity of molding machine in a molten state for a long time, and sometimes it is sticky due to thermal aging. In addition, products used at high temperatures are prone to stickiness. Therefore, it is very important to choose type, amount of stabilizers and softeners.

Aging phenomenon: Mechanical properties of product are significantly reduced, and appearance quality is deteriorated.

Reason: Compared with inorganic materials and metal materials, polymer materials have poor heat resistance and UV resistance, which causes product aging. Due to aging, mechanical properties of most products are significantly reduced, appearance quality is deteriorated.
Solution: By adding stabilizers such as heat resistance and weather resistance, adding ultraviolet absorbers and light stabilizers, aging phenomenon can be suppressed to a certain extent.

Air holes: phenomenon of concave holes in molded products, mainly caused by shrinkage of molded product during cooling process in mold.

In addition to requiring material to fill mold cavity, rapid cooling is also necessary. Specifically, it is to increase holding pressure (secondary pressure), reduce set temperature of resin and mold. In addition, there is a large dependence on shape of molded parts. Since air holes are easy to appear in thick parts, for such products, a model design such as a glue injection port or a flow channel should be used around it.

Burr: This is caused by resin overflow from cavity. Bursting is normal for injection molding of rubber, but abnormal for resin or thermoplastic elastomer.

Reason is that compared with rubber compounds, molten resins or thermoplastic elastomer have higher fluidity and lower injection pressure; contact with mold, cool, can solidify and stop flow in an instant. Therefore, resins or thermoplastic elastomers are generally less prone to burrs. As a countermeasure, first of all, it is necessary to reduce filling amount, holding pressure and holding time. In addition, for molds with poor dimensional accuracy and gaps on parting surface, repairs are necessary. When projected area of molded products is large and clamping force is relatively lower than injection pressure, burrs may also occur. Therefore, a larger molding machine must be used.

Flow marks: Streaks with different gloss appear on the surface of molded products.

Generally speaking, in resin injection molding, there are: narrowly spaced stripes of recording; relatively wide spaced stripes of same phase appear on the surface of molded products; narrowly spaced stripes of heterogeneous phase appear on the surface of molded products. This problem can be solved by these methods, such as adding pure monomer resin, increasing injection speed, mold temperature, increasing injection port, increasing resin temperature and injection rate, increasing molding temperature, mold temperature, or reducing injection speed.

Poor demoldability: Poor demoldability means that molded products is difficult to take out of mold or is completely deformed in the process of demoulding.

Adhesive materials can easily cause this problem, but it can be improved by adding a release agent to material or applying a release agent to mold before molding. Such problems are likely to occur due to insufficient cooling (inadequate solidification) of molded product, and therefore it is necessary to sufficiently cool molded products. In addition, irrational design of mold will also become reason for difficulty in demolding, especially in parts that are easy to stick, such as injection port and injection channel. Increasing extraction angle of injection port and widening injection channel are very effective.

Silver streaks: Phenomenon of radial streaks around injection port is caused by vaporization of moisture or volatile components in material.

Among them, air entrained in plasticizing process or air remaining in mold can also cause this phenomenon. Therefore, it is very necessary to sufficiently dry moisture-absorbing material before molding and to reduce molding temperature of material that easily generates decomposable gas.

Short mold: Phenomenon of not filling end of cavity is called short mold.

This is mainly caused by inadequate molding conditions such as insufficient filling amount, but insufficient exhaust or uneven flow path(multi-cavity mold) during molding can also cause this phenomenon.

Scorching: refers to phenomenon of aging like scorching in part that is not filled to end and not filled in mold cavity.

This is mainly due to inadequate exhaust gas, thermal insulation compression caused by air or generated gas, and instantaneous rise in temperature (ie: thermal aging on the surface of molded products). Improving exhaust method is a better solution, and in light cases, reducing injection speed can also be solved.

Uneven color

When using thermoplastic elastomer particles and dry blends as pigment's parent mixture for coloring, it is easy to have uneven color of molded products, uneven mixing or poor combination. As a countermeasure, an appropriate coupling agent and compatibilizer are used. Increasing screw back pressure and strengthening filler are effective for mixing.
Thermoplastic elastomer 

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