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The below article can be used for machine setters and machine operators of injection moulding machines.
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View our other articles form our setter and operators guide for injection moulders. Taken from our moulders catalogue (Toolbox Edition)
How to Calculate of Dwell Time for Drying Hygroscopic Materials
Calculation of dwell time for drying hygroscopic materials with regard to moisture removal using a desiccant dryer.
The Question – Is the material “wet or dry”?
Example 1
- Material being processed: PC/ABS blend
- Recommended drying time and temp: 4 hours @ 120oC
- Component weight: 39.45 g
- Impressions in mould tool: 2
- Weight of feed system: 8.6 g
- Cycle time: 32 seconds
- Dryer capacity: 50 kilos
Total shot weight = component weight x no of impressions + feed system weight
= (39.45 x 2) + 8.6
= 78.9 + 8.6
= 87.5 g of PC/ABS
Throughput of PC/ABS material in kilos/hr = total shot weight in grammes x 3600 / cycle time in seconds x 1000
= 87.5 x 3600
32 x 1000
= 9.844 kilos/hr
Dwell time in dryer = Capacity of dryer in kilos / Throughput rate of PC/ABS material in kilos/hr
= 50 / 9.844 = 5.07 hours
= 5.07 hours
Dryer is capable of drying the PC/ABS material in the recommended time.
CONCLUSION: Material will be suitably dried.
Example 2
- Material being processed: PMMA
- Recommended drying time and temp: 3 hours @ 85oC
- Component weight: 8.7 g
- Impressions in mould tool: 4
- No feed system (hot runner valve gated mould):
- Cycle time: 20.8 seconds
- Dryer capacity: 25 kilos
Total shot weight = component weight x no of impressions
= 9.8 x 4 = 39.2g
Throughput of PMMA material in kilos/hr = total shot weight in grammes x 3600 / cycle time in seconds x 1000
= 39.2 x 3600
= 20.8 x 1000
= 6.785 kilos/hour
Capacity of dryer = 25 kilos
Dwell time of PMMA material in dryer = Capacity of dryer in kilos / Throughput rate of PMMA material in kilos/hr
= 25 / 6.785
= 3.68 hours
CONCLUSION: Insufficient dwell time for material to remain in dryer. Material will be damp when processed.
EXAMPLE 3
- Material being processed: PA66 unfilled
- Recommended drying time and temp: 4 hours @ 80oC
- Component weight: 2.1 g
- Impressions in mould tool: 8
- No feed system (hot runner thermal gate):
- Cycle time: 9 seconds
- Dryer capacity: 100 kilos
Total shot weight = component weight x no of impressions
= 2.1 x 8 = 16.8 g
Throughput of PA66 in kilos/hour = total shot weight in grammes x 3600 / cycle time in seconds x 1000
= 16.8 x 360
= 9.0 x 1000
= 6.720 kilos/hr
Capacity of dryer = 100 kilos
Dwell time in PA66 material in dryer = Capacity of dryer in kilos / Throughput rate of PA66 material in kilos/hr
= 100/6.72 = 14.88 hours
= 14.88 hours
CONCLUSION: The excessive dwell time will result in the PA66 material be over-dried causing possible discolouring through oxidation and loss of flowability due to external lubricants being exhausted off. Will cause inconsistency in component manufacture.
