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Dimension Precision and Surface Quality of Injection Moulded Parts

Dimension precision of injection moulded parts
The overall size of injection moulded part depends on the plastic fluidity. Plastic with good fluidity can moulding relatively large-sized injection moulded part. When thin-walled injection moulded parts or plastic with poor fluidity (such as glass fiber reinforced plastic) performing injection moulding, the size of injection moulded parts should not be too large, so as to prevent the melt from failing to fill the cavity or forming weld marks, which will affects the appearance and structural strength of injection moulded parts. In order to ensure good moulding, we also should consider from the forming process and the wall thickness of injection moulded, such as increasing the moulding temperature, increasing the forming pressure and the wall thickness of injection moulded parts. In addition, the size of injection moulded parts is limited by the nominal injection amount, locking force and template size of the injection moulding machine. The conformity degree of between the injection moulded parts obtained after moulding and the size in the product drawing is called the dimension precision of the injection moulded parts. Accuracy depends on the moulding process and the materials used. Under the premise of meeting the requirement of use, the dimension precision of injection moulded parts should be designed as low as possible.

There are many factors affecting the dimension precision of injection moulded parts. First of all, the manufacturing precision of mould has the biggest influence on it. The second is the fluctuation of plastic shrinkage. Third, in the injection moulding process, the mould wear and other reasons cause the constant change in mould size, will also affect the size of injection moulded parts. Fourth, the change of technological conditions, the change of flash thickness and the demould slope will also affect the precision of injection moulded parts.

Surface roughness of injection moulded parts
If the surface roughness of injection moulded parts is low, the appearance quality of it will be high. For the appearance quality, in addition to avoid these injection moulding defects such as cold scar, cloud grain, and blister, etc. The main determined factor is the surface roughness of mould forming part. The surface roughness of injection moulded is generally 1.6-0.2um. For the injection moulded part with no surface roughness requirements (such as the normal core), the surface roughness of injection moulded part will be consistent with that of injection mould. While for the injection moulded part with surface roughness requirements (such as normal cavity), the surface roughness of injection mould is generally 1-2 grades higher than the surface roughness of injection moulded parts. Therefore, in order to reduce injection mould manufacturing cost, it just requires the designed surface roughness of injection moulded part meet the usage requirements.

What should be concerned is that during injection moulding, the surface roughness of injection mould increases continuously due to the wear of mould cavity, and the surface roughness of injection moulded parts also increases. So the mould cavity should be polished after a period of use to recover. For transparent injection moulded parts, it requires the surface roughness of mould cavity is the same as that of mould core.

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