Surface defects of Elastomer Injection Parts
Surface defects of Elastomer Injection Parts
One of the reasons for the surface problems of TPE injection molded parts is the poor quality of raw materials, which is also related to the uneven dispersion of formula. Secondly, it is also related to the setting of some parameters in the injection molding process.
1. Whitening phenomenon: refers to the migration of stabilizer and other additives to the surface of molded products, and the surface presents a white phenomenon like spraying powder.
Reason: it is mainly caused by excessive coordination of stabilizer or incompatibility with polymer. Incompatibility can be solved by adding compatibilizers related to Nanjing plastic Thai. The stabilizer with good compatibility with polymer shall be selected or the amount of stabilizer shall be controlled in the best range.
Secondly, it is also very effective to replace it with a stabilizer with high relative molecular weight. In addition, there are stabilizers that function by migrating to the surface of molded products. For example, antistatic agents, lubricants, etc. For this stabilizer, it is very necessary to choose a stabilizer that is difficult to whiten even if it migrates.
Thermoplastic elastomers rarely have the problem of whitening when used in general environment. However, in the occasions of high temperature, humidity and long-term outdoor use, it is very necessary to add heat-resistant stabilizer (antioxidant) and weather resistant stabilizer in order to improve durability. Especially at high temperature, the selection of stabilizer is also very important because it is easy to cause migration.
2. Stickiness: compared with resin, the surface of soft thermoplastic elastomer is more prone to stickiness.
Reason: it is mainly caused by the migration of low molecular weight polymers, plasticizers and stabilizers to the surface due to aging. However, in any case, the substances related to this can be easily determined by analyzing the sticky components by means of infrared spectroscopy (IR).
The main reason for Tackiness is that the molding temperature is too high and the polymer forms low relative molecular weight substances due to thermal decomposition. Although the set temperature of the forming machine itself does not reach the thermal decomposition temperature, the shear heat generation in the forming process sometimes makes it reach the high temperature temporarily.
Solution: as a countermeasure, it is very effective to reduce the forming temperature, reduce the shear, and purify the inside of the hydraulic cylinder of the forming machine with nitrogen. In addition, when the molding machine is suspended, the rubber is left in the mold cavity of the molding machine in a molten state for a long time, and sometimes it will become sticky due to thermal aging. In addition, products used at high temperature are prone to stick. Therefore, the selection of stabilizer and softener types and the determination of dosage are very important.
3. Aging phenomenon: the mechanical properties of the products are significantly reduced and the appearance quality becomes worse.
Reason: compared with inorganic materials and metal materials, polymer materials have poor heat resistance and UV resistance linearity, resulting in product aging. The mechanical properties of most products are obviously reduced and the appearance quality becomes worse due to aging.
Solution: the aging phenomenon can be restrained to a certain extent by combining heat resistance, weather resistance and other stabilizers and adding ultraviolet absorber and light stabilizer. Problems and countermeasures related to injection molding.
4. Air hole: concave hole occurs in the molded product, which is mainly caused by shrinkage during the cooling process of the molded product in the mold. In addition to the requirement that the material must fill the mold cavity, rapid cooling is also very necessary. Specifically, it is to increase the holding pressure (secondary pressure) and reduce the set temperature of resin and mold.
In addition, it also has a great dependence on the shape of molded products. Since pores are easy to appear in thick parts, the model design with glue injection port or glue flow channel around such products should be adopted.
5. Burr: This is caused by resin overflow from the mold cavity. For rubber injection molding, it is normal to have burrs, but it is not normal for resin or thermoplastic elastomer.
The reasons are:
Compared with rubber compound, the fluidity of molten resin or thermoplastic elastomer is higher and the injection pressure is lower;
Contact and cooling with the mold can cure and terminate the flow in an instant. Therefore, resins or thermoplastic elastomers are usually not prone to burrs.
As a countermeasure, we must first reduce the filling amount, reduce the pressure holding and shorten the pressure holding time. In addition, it is very necessary to repair the die with poor dimensional accuracy and clearance on the parting surface. When the projection area of molded products is large and the clamping force is relatively lower than the injection pressure, burr sometimes occurs, so a larger molding machine must be used.
6. Flow trace: stripes with different gloss appear on the surface of molded products.
Generally speaking, in the injection molding of resin:
Narrow spaced recording stripes;
There are relatively wide spacing stripes in the same phase on the surface of the molded product;
There are three types of narrow interval stripes of ectopic phase 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 glue injection port, increasing resin temperature and injection rate, increasing molding temperature, mold temperature or reducing injection speed. It is effective to increase the injection speed and mold temperature.
7. Poor demoulding performance: poor demoulding performance means that the molded product is difficult to take out from the mold or completely deformed in the taking out process. Adhesive materials are easy to cause this problem, but the method of adding release agent to the material or coating release agent on the mold before forming can be improved.
Insufficient cooling (insufficient curing) of molded products is also prone to such problems, so it is very necessary to fully cool the molded products. In addition, unreasonable mold design will also become the reason why it is difficult to demould, especially in the parts easy to adhere to the mold such as the glue injection port and the glue inlet channel, it is very effective to increase the pull-out angle of the glue injection port and widen the glue inlet channel.
8. Silver Stripe: radial stripe with the glue injection port as the center, which is caused by the gasification of water or volatile components in the material. Among them, the air involved in the plasticizing process or left in the mold will also lead to this phenomenon. Therefore, it is very necessary to fully dry the hygroscopic material before forming and reduce the forming temperature of the material easy to produce decomposable gas.
9. Lack of glue: the phenomenon that the end of the mold cavity is not filled is called lack of glue. This is mainly caused by improper forming conditions such as insufficient glue filling, but this phenomenon can also be caused by insufficient exhaust or uneven glue channel (multi cavity mold).
10. Scorch: refers to the aging phenomenon like scorch in the part not filled to the end and not filled with the mold cavity. This is mainly due to insufficient exhaust, heat insulation and compression caused by air or generated gas, which immediately increases the temperature significantly (i.e. thermal aging on the surface of molded products). Improving the exhaust mode is a better solution. In case of light degree, reducing the injection speed can also be solved.
11. Uneven color: when the parent mixture of thermoplastic elastomer particles and dry mixture is used for coloring, it is easy to appear the phenomenon of uneven color and luster of formed products, uneven mixing or poor combination. As a countermeasure, use appropriate coupling agent and compatibilizer. It is effective to increase the screw back pressure and strengthen the mixing of packing.
