A common issue faced by both moulders and mould-makers is the presence of air traps or bubbles in transparent or translucent moulded parts, often caused by improper venting and variations in wall thickness. This problem can be time-consuming to resolve, leading to frustration for customers, mould-makers, and moulders alike. Below are some common solutions adopted for avoiding air bubbles in plastic parts, along with their drawbacks.
Providing vent pins - this could result in pin marks on the component which are also undesirable.
Increasing depth of air vent grooves - could result in flash as well as change the flow pattern as it creates a pressure variation.
Changing gate size or type - results in definite change of flow pattern thereby creating unequal pressure distribution which could ultimately lead to weld lines.
Making cold slug well - results in excess material, not suitable for high volume application due to scrap generation.
Changing process parameters, high speed or high pressure filling - lot of trial and error results in material wastage with very minor improvements.
In the recent past, our company has made moulds and provided moulded parts in high volumes (approx 2 million per month) for LED bulbs, namely the diffuser cover. In lesser volumes, we have produced street light lens covers as well. In the LED lighting application, since there is high intensity light passing through the component, the final product usage itself is in itself a test of the part's volumetric defects. Therefore, there is no margin of error which we would otherwise have in the case of opaque parts.
Though air traps are not a significant moulding defect in most cases, it takes precedence in the mind of the customer in the case of transparent parts since it is clearly visible and creates a perception of a substandard product. In some cases, it also creates a hot spot wherein the trapped gases may expand and 'pop' if the area is subjected to intense heat. Due to either of the reasons stated above, there is a fair chance the parts with air bubbles will be rejected.
To overcome this, moulders struggle with trying different parameters whereas mould-makers try to identify the best places to vent the trapped air by making numerous design changes. The permanent solution is always found after exasperating delays and a loss of customer faith.
Through our numerous trials in our earlier products, we experimented a solution that could provide a reference point for further improvements. The concept was to ensure that the mould cavities were devoid of any trapped air either before or during moulding. There are very high cost equipment such as the MoldVac etc which help in achieving this. We used the same technique but using low cost devices to achieve the same effect. A step by step account is as follows:
A venting groove is made around the entire profile area.
The profile area is connected to the venting groove by smaller grooves made by grinding the surface of about 0.03-0.04 mm (for PC).
Holes are made from venting groove to the back plate and opened to the sides of the mould.
A simple vacuum generator such as below is installed to each of the side venting holes.
These vacuum generators work on the Venturi principle. Compressed air around 8-10 bar pressure needs to be provided at inlet which generates about 80% of the input pressure in vacuum pressure. Depending on the layout of the air channels, either one or more of these vacuum generators may be used with common input supply from your compressor.
The air on/off into the generators may be controlled via the machine air manifold which is synced to the machine controls. Timing may be adjusted to ensure vacuum is in effect before, during or through the entire cycle time. For best results, we used the vacuum generation from mould close upto the injection.
The results we observed were:
99% reduction in air bubbles, traps.
Lower filling pressures.
Smoother functioning.
As you will see above, there is drastic improvement. We feel there is still a lot of fine tuning to be done and different applications would throw out a separate set of challenges but as a low cost solution, this worked better than expected.
We are Vidhata Plastics India Private Limited, mould manufacturer, injection moulding and extrusion supplier for a wide, diverse clientele providing creative and innovative solutions for the moulding industry.
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