11 kinds of surface defects of TPE injection parts and their solutions

Time：2022-05-24 08:33:15 / Popularity： / Source：

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

1. Whitening phenomenon:

It means that stabilizer and other compounding agents migrate to surface of molded product, and surface shows a white phenomenon like being sprayed with powder.
Reason: It is mainly caused by excessive compounding of stabilizer or incompatibility with polymer. Incompatibility can be solved by adding related compatibilizers. Stabilizer with good compatibility with polymer should be selected or amount of stabilizer should be controlled within optimal range.
Secondly, it is also very effective to replace it with a stabilizer with a high relative molecular mass. In addition, there are also stabilizers that exert their functions by migrating to surface of molded product. For example, antistatic agents, lubricants, etc. For this kind of stabilizer, 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 it is necessary to add heat-resistant stabilizer (anti-aging agent) and weather-resistant stabilizer to combination to improve durability in high temperature, humidity, and outdoor long-term use. Especially at high temperature, because it is easy to cause migration, choice of stabilizer is also very important.

2. Sticky phenomenon:

Compared with resin, surface of soft thermoplastic elastomer is more prone to stickiness.
Reason: It is mainly caused by migration of low molecular weight polymers, plasticizers and stabilizers to surface due to aging. But no matter what case is, it is relatively easy to determine related substances by analyzing sticky components using analytical methods such as infrared spectroscopy (IR).
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 generation in molding process sometimes causes it to temporarily reach a high temperature.
Solution: As countermeasures, it is very effective to reduce molding temperature, reduce shear, and purify inside of hydraulic cylinder of molding machine with nitrogen. In addition, when molding machine is suspended, rubber compound remains in mold cavity of molding machine in a molten state for a long time, and sometimes it becomes sticky due to thermal aging. In addition, products used at high temperatures are prone to stickiness. Therefore, selection of stabilizers, softeners and determination of their dosage are very important.

3. Aging phenomenon:

Mechanical properties of product are obviously reduced, and appearance quality is deteriorated.
Reason: Compared with inorganic materials and metal materials, poor heat resistance and ultraviolet resistance of polymer materials cause product aging. Mechanical properties of most products are obviously reduced due to aging, and 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.

4. Stomata:

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 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 great dependence on shape of molded product. Since pores are easy to appear in thick parts, for such products, a model design with a glue injection port or a glue flow channel around it should be used.

5. Burrs:

This is caused by resin overflowing from cavity. Flashing is normal for rubber injection molding, but not normal for resins or thermoplastic elastomers.
Reason is:
Compared with rubber compound, molten resin or thermoplastic elastomer has higher fluidity and lower injection pressure;
When in contact with mold and cooling, it can solidify and stop flowing 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, reduce holding pressure and shorten holding time. In addition, it is very necessary to repair mold with poor dimensional accuracy and gaps in parting surface. When projected area of molded product is large and clamping force is relatively lower than injection pressure, burrs may sometimes appear, so a larger molding machine must be used.

6. Flow traces:

Streaks with different gloss appear on the surface of molded product.
Generally speaking, in resin injection molding there are:
Narrowly spaced recording stripes;
There are relatively wide spaced stripes in same phase up and down surface of molded product;
There are three types of relatively narrowly spaced stripes in which heterophase appears on upper and lower surfaces of molded product.
Solving 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, etc. It is effective by increasing injection speed and mold temperature.

7. Poor mold release:

Poor releasability means that molded product is difficult to remove from mold or completely deformed during removal. Materials with stickiness can easily cause this problem, but can be improved by adding a release agent to material or applying a release agent to mold before molding.
Insufficient cooling of molded product (insufficient solidification) is also prone to such a problem, so it is very necessary to sufficiently cool molded product. In addition, unreasonable mold design will also become reason for difficulty in demoulding, especially in parts that are easy to stick to mold, such as injection port and glue inlet, it is very effective to increase pull-out angle of injection port and widen injection channel.

8. Silver stripes:

Phenomenon of radial streaks centered on injection port is caused by vaporization of moisture or volatile components in material. Among them, air involved in plasticizing process or remaining in mold can also cause this phenomenon. Therefore, it is necessary to sufficiently dry hygroscopic material before molding and to lower molding temperature of material that easily generates decomposable gas.

9. Lack of glue:

Phenomenon of not filling end of cavity is called lack of glue. This is mainly caused by unsuitable molding conditions such as insufficient glue filling, but insufficient exhaust during molding or unbalanced flow channels (multi-cavity molds) can also lead to this phenomenon.

10. Charred:

It means that part that is not filled to end and part that is not filled with cavity has an aging phenomenon such as burning. This is mainly due to insufficient exhaust, adiabatic compression caused by air or generated gas, and temperature rises significantly in an instant (ie, thermal aging occurs on the surface of molded product).
Improving exhaust method is a better solution. In the case of a mild degree, reducing injection speed can also be solved.

11. Uneven color:

When using thermoplastic elastomer particles and dry mixture as pigment master mixture for coloring, it is easy to appear phenomenon of uneven color of molded product, uneven mixing or poor combination;
As a countermeasure, suitable coupling agents and compatibilizers are used. It is effective to increase screw back pressure and to strengthen filler when mixing.

Gud Mould Industry Co., Ltd