To successfully injection mold a plastic part, cycle time should include a drying time for the resin.  Drying is an essential part of the process as moisture content plays an important role in part quality.  Too much moisture in the resin can lead to voids, streaking or even degradation of the polymer.  Every material is affected by moisture and therefore must go through a drying process before molding.

The amount of dry time necessary depends on multiple factors.  The type of plastic is the biggest factor.  Some plastics, such as Polystyrene, require only 1-3 hours at a low temperature of 80ºC.  However a material like PET would require a maximum of 12 hours at 140ºC.  These recommendations can be found by asking the plastics manufacturer.

Plastics are considered to be either hygroscopic or non-hygroscopic which describes how they are affected by moisture.  Hygroscopic materials, such as PET, absorb moisture within the pellet where it forms a molecular bond with the material.  Non-hygroscopic materials, such as PS, do not absorb any moisture but allow the moisture to adhere to its surface.  Both materials should go through a drying process but hygroscopic materials will often require a longer drying time. 

Some materials are Hydrophobic, meaning that they are repelled by water, such as oil or greasy substances.  The opposite of this is Hydrophilic, meaning that they have an affinity to water, such as sugar or salt.  Plastics do not tend to be either Hydrophobic or Hydrophilic.  Interestingly, soap is both which allows it to dissolve in both water and oil.

There are different ways of drying your material.  The moisture content can be removed right in the machine, but only if the degassing stage is present in the plasticating zone of the machine.   However, few machines have this capability and so external dryers are used in most cases.

Dryers can be either hot air or desiccant dryers.  A hot air dryer contains heaters and an air blower which typically replaces the standard hopper on the injection unit.  Air is pulled in to the dryer, heated and then blow up through the plastic pellets.  This air evaporates the moisture and is then pushed back into the environment.

Desiccant dryers use small desiccant beads to absorb moisture, similar to the packets found in clothing or handbags.  Desiccant can absorb high amounts of moisture without a significant structural change.  In a desiccant dryer, the plastic pellets are loaded into the drying hopper and then air is pulled through them into the dryer through a filter filled with desiccant beads.  The desiccant beads will absorb the moisture and the dry air is heated and re-circulated through the plastic resin.  This process is repeated as the dry air will then pull more moisture out of the plastic before it is again pulled through the desiccant filter.

Steinwall, Inc. has chosen to create a "Cocoon", or white room, where the presses are located to control the temperature and humidity of the press environment.  The dryers are located outside the cocoon to allow the humidity inside to remain at 55% no matter the outside weather or time of year.  This ensures that materials will always be at the proper moisture content for molding.

[1] Olmsted, Davis Practical Injection Molding, Marcel Dekker, Inc, 2001

[2] Osswald, Turng, Gramann, Injection Molding Handbook, Hanser Verlag, 2008

[3] Johannber, Injection Molding Machines: A User’s Guide, Hanser Verlag, 2008