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Causes and solutions of bubbles or holes in rotational molding products

By admin In Art, News Posted 2023-03-29

At this stage, most of the rotational molding products orders from abroad immediately indicate that linear density low polyethylene (LLDPE) rotational molding special materials are used for rotational molding. The products are mainly different specifications of cleaning main machine case shell, mower case, cargo transport box, storage box, various mechanical and electronic packaging boxes, two-layer insulation box with foaming layer inside, steam insulation box with high rotational molding technical requirements Car parts, etc. In China, rotational molding technology and rotational mold processing technology have been rapidly improved, but there are still differences between China and Europe in the design stage of rotational molding products.

Rotational molding process plays a key role in the quality of products. When the product design and mold design and manufacturing are finished, the mold structure which is easy to cause product defects will not be changed at will. It seems to be more critical to find ways to deal with the defects of rotational molding products from the forming process. At present, there are few materials to introduce the design and process of rotational molding products in China. Commodity designers or engineering producers mostly rely on specific experience to design and produce, which usually brings unnecessary inconvenience to the design and production. The research and analysis of defect causes and solutions of rotational molding products have key practical significance for improving the technical level of rotational molding products and rotational molding mold structure design.


Defect analysis and solution of rotational molding products

1. Bubbles or holes

During rotational molding, the material in the mold melts, flows and adheres to the inner surface of the hot mold with the rotation of the mold. The volume of the gas in the mold rises when it is heated, and the standard air pressure rises. According to the vent, the gas gradually flows to the outside of the mold until the pressure inside and outside the mold is balanced, and vice versa. In addition, there is a certain pressure in the die cavity. In the process of resin melting and densification, the gas remaining in the middle of the powder particles is squeezed to the random surface of the plastic melt. However, because of the existence of the support force on the surface of the melt, the gas can not get rid of the bubble on the surface of the melt, which is easy to produce bubbles on the inner surface of the product and gas entrapment on the outer surface of the product. If the melt fluidity is good, the mold temperature is slow, and the mold vent is smooth, the gas in the melt can escape smoothly. On the contrary, the gas in the melt is easy to stay and produce product defects. When the die is not tightly locked, part of the gas in the die will flow to the outside of the die according to the gap of the die locking part during the heating process of the die, resulting in 7L gas or bubbles in the commodity in the opposite part of the die; if the die is not tightly closed during the cooling process of the die, the gas will enter the die according to the gap of the die locking part (analysis surface) The appearance of the powder particles is also related to the appearance of the pores. When polyethylene (PE) powder particles have long and thin tails or hair shape, more gas will stay in the deposition process. Especially in the corner of the mold, the bridging of powder will cause large holes.


Adjust the appropriate distance between the strip produced by the vent pipe or the metal random wire with the same effect and the inside of the die. The diameter of the pipe is determined by the product specification and material characteristics. (generally, the diameter of thick wall products is 10-12mm per cubic die). The length of the pipe depends on the depth of the die. The end of the pipe should be extended to the core management center or the suitable part. In order to prevent the resin powder from overflowing from the exhaust pipe when the mold rotates, the vent pipe needs to be filled with glass fiber, steel velvet, graphite powder, etc.

Mold moderate slow boat temperature, raise furnace temperature (melting temperature) or increase heating time, to ensure sufficient melting of materials and gas out.

Apply Teflon (polytetrafluoroethylene) coating on the inner surface of the mold instead of various release agents to maintain the inner dryness of the mold. If it is affected by the elements of the insert, the insert and some surrounding areas can be heated.

In the design process of commodities and moulds, the following measures are beneficial to eliminate bubbles or stoma: choose materials with high melt flow velocity (MFR), use relatively low density materials, improve the symmetry of the thickness of the mould, increase the cooling time of course, slow down the spray (sprinkling) cooling, and the convex or prominent parts of the products are not suitable for too narrow or too high (corresponding to the die). The groove on the surface should not be too narrow or too deep.

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