1. Causes of Dents and Pores in Thermoplastic Elastomer Injection Molding Products

1.1 Causes of Pore Formation in Thermoplastic Elastomer Injection Molding Products

In the production process of injection molded products, product dents and pores are the most common adverse phenomena. The plastic injected into the mold shrinks in volume after cooling, while the surface of the later cooled part, which hardens first, will produce bubbles on the outside. The so-called porosity refers to the material in the mold that solidifies from the surface, which is relatively insufficient for the total volume of the mold. Due to this reason, there are vacuum holes that typically appear in areas with thicker products, welding materials, and injection materials. However, this situation occurs in thermoplastic elastomers, which is relatively rare. The principle is very simple. After curing, low hardness thermoplastic elastomers do not form undeformed shells and eventually collapse. However, unless the hardness of the thermoplastic elastomer is greater than 90a, the thickness of the product is relatively thick, the material flow is excellent, and the exhaust is not smooth.

Due to the pores generated by moisture and volatiles, they usually diffuse to all parts of the product. The shape of pores is generally small and even difficult to detect. However, the specific gravity of the tested material should be very significant. In fact, the entire material is in a foaming state, similar to the situation where a wasp nest is compacted. In this situation, the material usually does not dry on site, and some additives in the material will decompose and evaporate.

If transparent products have pores, that's a problem. If there are pores in the opaque product, it is not suitable to see them in the product. However, in some applications, such as casters, if there are holes, it can seriously affect the service life of the casters, as shown in the following figure:

For casters, although the exhaust is good (as shown in the above figure, there is a lot of overflow at the exhaust), there are large holes above the weld seam. We speculate that the reason for this void is that two streams of material are moving in a circular motion. If the speed is fast enough, the material flow must move along the circumference tangent rather than close to the circumference. At this time, at the intersection of two material flow, that is, the intersection of tangents, there is one place where the material cannot reach, that is, the cause of the hole.

1.2 Causes of dents (collapses) in injection molding of thermoplastic elastomers

The so-called dent refers to a noticeable dent on a part that cools slowly in the direction of bubble contraction; Thermoplastic elastomer materials with larger shrinkage are also prone to dents. When changing the forming conditions to eliminate dents, the solidification conditions should be set along the shrinkage direction. That is to say, when the mold temperature and cylinder temperature decrease, the injection pressure increases, but attention should be paid to the possibility of residual internal stress.

Generally speaking, the shrinkage rate of low hardness thermoplastic elastomers is greater than that of high hardness thermoplastic elastomers. The shrinkage rate of high sulfur thermoplastic rubber is greater than that of low sulfur thermoplastic rubber. The more fillers, the smaller the shrinkage rate of thermoplastic elastomers. Overall, the shrinkage of TPU and TPEE is much smaller than that of TPe-s and TPV.

Because dents are best kept inconspicuous, when they do not affect the appearance, they will be processed into corrosive patterns on the mold, such as particles, granular shapes, etc

If the mold material is impact resistant polystyrene hips (a type of polystyrene PS), reducing the mold temperature can also effectively reduce the finish. However, once these methods have dents, it is difficult to repair the polished product.

2. Solution to Dents and Pores in Thermoplastic Elastomer Injection Molding Products

2.1 From the perspective of forming process

Instantaneous: Increase injection pressure, extend injection holding time, reduce cylinder temperature and mold temperature, fully dry materials caused by moisture and volatiles, and force cooling in areas where dents occur.

Short term: Fill the flow edge where dents appear. When the material at the location of the dent passes through the narrow area, the part becomes thicker.

Long term: thickness differences in design products should be completely avoided. For stiffeners that are prone to dents, narrow and long shapes should be as short as possible. Add gates, main flow channels, diversion channels, and nozzle holes. Improve exhaust.

2.2 From the perspective of material forming performance

Minimize the shrinkage rate of the material as much as possible to effectively avoid dents (shrinkage marks); If unnecessary, the flowability of thermoplastic elastomer materials should not be too good; Choose appropriate material additives to avoid high-temperature decomposition or excessive volatilization.

3. Other reference materials

3.1 For materials with high shrinkage during molding, such as polyethylene PE, polypropylene PP, etc., even with a small amount of reinforcement, dents may occur. The shrinkage rate of low hardness TPV and tpe-s is 2-4%, with more prominent dents.

3.2 When the temperature drops to no dents, if there is still pressure on the material inside the cavity, it should be considered that no dents will occur. The pressure around the material in the mold is static pressure, no matter where it is.

The pressure near the gate is very high. If the material has a wide edge, the pressure difference near and far from the gate is relatively small compared to the overall pressure difference due to the transmission of pressure to all corners, and there will be no dents, resulting in a product without residual internal stress.

When some materials enter difficult areas, high pressure will be generated in this area, while pressure in other areas will decrease, resulting in dents. The residual high pressure in this area refers to the high external stress of the product. Under ideal conditions, the increase in material temperature and mold temperature leads to better material flow, while the injection volume decreases under static pressure.

3.3 When the forming conditions change, the combination of temperature, pressure, and time should be carried out in advance, and the results should be known as soon as possible. Firstly, after a long period of time, it is easy to detect small changes in stress. It should be noted that the results of temperature changes should be obtained after injecting the material, while the reproducible results should be obtained after the temperature drops.

3.4 To determine the cause of the porosity, one can only observe whether the bubbles in the plastic product appear instantly or after cooling during mold opening. If they appear immediately when the mold is opened, most of them are material issues. If it occurs after cooling, it is a problem with the mold or injection molding conditions.