Rotational molded products sometimes exhibit some defects. Generally speaking, how can we address these defects?
1. Bubbles or holes
(1) Causal analysis
During the rotational molding process, the material inside the mold gradually melts, flows, and adheres to the hot inner surface of the mold as the mold rotates during heating. The air inside the mold expands due to heating, causing the pressure to rise. The air gradually flows out of the mold through the vent holes until the air pressure inside and outside the mold reaches equilibrium, and vice versa. At the same time, a certain pressure is maintained inside the mold cavity. During the melting and densification of the resin, the gas trapped between powder particles is pushed towards the free surface of the plastic melt. However, due to the surface tension of the melt, the gas is not sufficient to escape from the melt surface, easily forming bubbles, which can lead to bubbles on the inner surface and air inclusions on the outer surface of the product, and in severe cases, large holes. If the melt has good fluidity, the mold heating rate is slow, and the mold vent holes are unobstructed, the gas in the melt can escape smoothly. Conversely, if the melt has poor fluidity, the gas in the melt tends to remain trapped, causing defects in the product. When the mold is not tightly closed, a portion of the gas in the mold cavity will flow out of the mold through gaps at the mold closing position during heating, resulting in air holes or bubbles inside the corresponding part of the product in the mold. During the cooling process of the mold, if the mold is not tightly closed, there will be an air pressure difference between the inside and outside of the mold, and air will enter the mold through gaps at the mold closing position (parting surface), causing air holes on the outside of the product.
The formation of pores is also related to the shape of powder particles. When polyethylene (PE) powder particles have elongated tails or hair-like structures, they can form bridges during the packing process, trapping more air. Especially in the corners of the mold, the bridging of powder can lead to the formation of larger pores.
(2) Solution
Adjust the vent tube or a long strip made of metal wire with similar function to be rolled to an appropriate distance inside the mold. The vent tube is generally made of thin-walled metal fluoroplastic tube, and its diameter is determined by the product size and material properties. (Generally, for thin-walled products, a hole diameter of 10-12 mm is set per cubic meter of mold.) The length of the tube should ensure that its end extends to the center of the mold cavity or to a suitable position according to the depth of the product cavity. To avoid resin powder overflowing from the exhaust port when the mold rotates, the inside of the vent tube should be filled with glass wool, steel wool, graphite powder, etc.
Slowly heat up the mold, increase the furnace temperature (melting temperature), or extend the heating time to ensure that the material is fully melted and the gas is discharged.
Apply a Teflon (polytetrafluoroethylene) coating on the inner surface of the mold to replace various mold release agents and maintain the dryness inside the mold.
If the issue is caused by the insert, preheat the insert and the surrounding area.
During the product and mold design process, the following measures conducive to eliminating bubbles or pores should be fully considered: using materials with a higher melt flow rate (MFR), using materials with lower density, improving the uniformity of mold wall thickness, extending natural cooling time, delaying spray (water spray) cooling, and ensuring that the ribs or protruding parts on the product are not too narrow or too high (corresponding to the grooves on the mold not being too narrow or too deep).
2. Poor resin coating
(1) Causal analysis
Rotational molding products typically feature numerous metal inserts that form part of the product through rotational molding, enhancing the local strength of the product. During rotational molding, the insert acts as a part of the mold, increasing the wall thickness of the mold at that location. This makes it difficult for the end of the insert to reach the same temperature as the mold, leading to poor resin coating on the insert. Especially for large inserts, if the structural design of the insert is not reasonable, resulting in poor heat transfer performance and failure to reach the same temperature as the mold, it is more likely to cause uneven resin coating or failure to meet design requirements, reducing the bonding strength between the insert and the product. The rotational speed for rotational molding is usually lower, unlike centrifugal casting used in the production of cast nylon products. When the insert is too high relative to the surface of the product, there is a higher chance of poor resin coating. Generally, the plastic wall thickness on the insert varies greatly from the wall thickness of the product, which is directly related to the poor heat transfer performance and excessive thickness of the insert during rotational molding. When the position of the insert is too close to the adjacent side surface of the product, it can block the flow of material, resulting in less material accumulation at that location or incomplete bridging between the insert and the side surface. This can lead to defects such as large holes in the product or poor coating of the insert. It is particularly important to note that the good heat transfer performance of the insert is not only due to its material itself, but also to its structure, which should be designed to ensure good heat transfer performance. For example, the cavity should not be too large, or large cavities should be sealed with metal during rotational molding. This should be especially considered when designing large inserts.
(2) Solution
Ensure that the insert has a good heat transfer structure, and try to eliminate factors that are detrimental to the heat transfer of the insert.
On the premise of meeting the rotational molding conditions and the strength requirements of the insert, the height and volume of the insert relative to the surface of the product should be minimized.
The depth and width of the anti-rotation or anti-pull grooves on the insert are suitable for rotational molding requirements.
During rotational molding, preheating the inserts according to the situation can achieve better results, especially for large inserts.
