Common problems and solutions on the surface of TPE injection molded parts

Time:2021-01-12 16:00:55 / Popularity: / Source:

TPE injection molded parts have surface problems. One of reasons is poor quality of raw materials, which is also related to uneven dispersion of formula, and secondly, it is also related to parameter settings of injection molding process.
Thermoplastic elastomers 

1. Whitening phenomenon:

Refers to migration of stabilizer and other compounding agents to surface of molded product, and surface shows a white phenomenon like powder.
Reason: It is mainly caused by excessive blending of stabilizer or incompatibility with polymer. Incompatibility can be solved by adding compatibilizer related to Nanjing Sutai. Stabilizer with good compatibility with polymer should be selected or amount of stabilizer should be controlled in optimal range.
In addition, there are stabilizers that perform their functions by migrating to surface of molded article. For example, antistatic agents, additives, etc. For this stabilizer component, it is very necessary to select a stabilizer that is difficult to whiten even if it migrates.
Thermoplastic elastomers rarely have whitening problems when used in general environments, but in high temperature, vertical, and outdoor long-term use occasions, in order to improve combination, a heat-resistant stabilizer (anti-aging agent) is added. Weather-resistant stabilizers are very necessary. Especially at high temperature, it is very easy to cause migration, so choice of stabilizer is also very important.

2. Stickiness:

In the name of resin, surface of soft thermoplastic elastomer is more prone to stickiness.
Reason: It is mainly caused by formation of low molecular weight polymers and plasticizers due to aggravation, migration of stabilizers to surface. However, no matter what situation is, using infrared spectroscopy (IR) and other analytical methods, relevant substances can be easily identified by analyzing sticky components.
Stickiness is mainly due to high molding temperature and formation of low molecular weight substances due to thermal decomposition of polymer. Although set temperature of molding machine itself does not reach thermal decomposition temperature, shear heat generated in molding process sometimes causes a temporary high temperature.
Solution: As a countermeasure, it is very effective to reduce molding temperature, reduce shear, and purify inside of hydraulic cylinder of molding machine with nitrogen. When molding machine is suspended, rubber material is continuously placed in molding machine in a molten state, mold cavity sometimes becomes sticky due to heat. In addition, products used at high temperatures are prone to stickiness. Therefore, selection of stabilizers and softeners, determination of amount are very important.

3. Aging phenomenon:

Mechanical properties of product are significantly reduced, appearance quality is deteriorated.
Reason: Compared with inorganic materials and metal materials, polymer materials have poor heat resistance and UV resistance, which can cause product wear. Mechanical properties of a large number of products are significantly reduced due to aging, appearance quality is deteriorated.
Solution: By adding stabilizers such as heat resistance and weather resistance, by adding ultraviolet absorbers and light stabilizers, aggravation phenomenon can be suppressed to a certain extent.

4. Stoma:

Phenomenon of concave holes in molded product is 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), lower set temperature of resin and mold.
In addition, there is a great substitute for shape of molded product. Since pores are prone to appear in thick parts, for such products, a model design with glue injection ports or glue channels around them should be adopted.
TPE injection molded parts 

5. Raw edges:

For rubber injection molding complications, appearance of flash is normal, but it is abnormal for resin or thermoplastic elastomer.
Reason is:
Compared with rubber compound, molten resin or thermoplastic elastomer has higher fluidity and lower injection pressure;
Contact with mold and cool down to solidify in an instant and stop flow. Therefore, resins or thermoplastic elastomers are generally less prone to burrs.
In addition, for mold alignment with poor dimensional accuracy and gaps in parting surface, its repair is very necessary. When projected area of molded product is large and clamping force is relatively lower than injection pressure, burrs may sometimes appear, so a size molding machine must be used.

6. Flow traces:

Streaks with different gloss appear on the surface of molded product.
Generally speaking, there are:
Recording stripes with narrow intervals;
There are relatively wide spaced stripes in same phase on the top and bottom of molded product;
There are three types of relatively narrow spacing stripes with different phases appearing on the surface of molded product.
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. It is effective by increasing injection speed and mold temperature.

7. Poor mold release:

Adhesive materials can easily cause this problem, but it can be improved by adding a release agent to material or replacing release agent on mold before molding.
In addition, unreasonable mold design will also become reason for difficulty of demolding, especially in parts where mold is easy to stick, such as injection port and glue channel. It is very important to increase pull-out angle of glue port and widen glue channel.

8. Silver stripes:

Phenomenon of radial stripes centered on injection port is caused by vaporization of moisture or volatiles in material. Among them, air involved in plasticizing process or remaining in mold can also cause this phenomenon. Therefore, it is very necessary to fully dry moisture-absorbing material before molding and reduce molding temperature of material that easily generates decomposable gas.

9. Lack of material:

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

10. Burnt:

It is mainly due to insufficient exhaust, air or generated gas causes adiabatic compression, temperature rises immediately (ie: thermal aging on the surface of molded product).
Improving way of venting is a better solution. In the case of a mild degree, reducing injection speed can also be solved.

11. Uneven color:

When using a master mix of thermoplastic elastomer particles and dry blends as pigments for coloring, it is prone to uneven color of molded products, uneven mixing or poor combination;
As a countermeasure, use a suitable coupling agent and compatibilizer. It is effective to increase screw back pressure and strengthen filler when mixing.

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