Desiccant Air Dehumidification in the Plastics Industry
Injection and blow moulding operations utilise thermoplastics which are heated to plasticity and shaped into forms using a mould. During this process, if moisture is present in the raw material it boils, releasing vapour that can cause both structural and cosmetic imperfections to the finished product.
To ensure that the end product is defect free, raw materials must be free of moisture before being processed. Many of today’s plastic resins are hygroscopic (high affinity for absorbing moisture).
To reduce forming time and increase production volumes, the majority of systems employ the use of chilled water to reduce the mould temperature. Generally, the cooler the mould, the faster the process cycle. However, operating at low mould temperatures will create problems with condensation, especially in summer. This can result in unacceptable water marks on the product as well as corrosion of expensive moulds and guide pins, necessitating repair or replacement.
The problem can easily be overcome by simply increasing the mould surface temperature. However, as this will result in longer cycle times and reduced output, it is not the ideal solution.
By incorporating a desiccant dehumidifier the air dewpoint can be controlled. Lower mould temperatures can then be achieved without the risk of condensation.
Desiccant dehumidifiers are also used when storing expensive moulds. There is no need to waste time and effort greasing the moulds prior to storage. If the relative humidity is maintained below 45%, moulds can be stored safely without risk of rust formation (irrespective of store temperature).