Injection Molding Auxiliary Equipment: Dryers, Conveyors, and Chillers
Published
Auxiliary equipment decides whether the material reaching the screw is dry, uncontaminated, and correctly blended, and whether the heat the parts bring into the mold can be removed at a stable temperature. Each piece is sized from a calculation: dryers from throughput and the resin supplier’s drying conditions, conveying and blending from material flow and the number of materials, and temperature control and chillers from heat load and coolant flow. When a molding problem appears to come from the machine, the auxiliary systems are among the first places to check, before process settings are changed.
Dryers
Hygroscopic resins absorb moisture from the air, and molding them wet can cause splay on the surface and, in polymers such as polycarbonate, nylons, and polyesters, loss of properties from hydrolysis. Non-hygroscopic resins such as polyethylene and polypropylene pick up only surface moisture.
Drying conditions belong to the grade. The resin supplier’s data sheet or processing guide gives the drying temperature, the drying time, and, where stated, the maximum moisture content allowed at the press. Those values differ between grades of the same polymer, and between natural and filled or flame-retardant versions, so a single table of “drying temperatures by polymer” is only a reminder of which materials need attention.
| Dryer type | How it dries | Fit |
|---|---|---|
| Desiccant (dehumidifying) | heated air dried by a desiccant bed, then passed through the material | hygroscopic resins needing a low dew point |
| Hot air | heated ambient air | surface moisture on non-hygroscopic resins |
| Compressed air | dry compressed air expanded through the hopper | small throughputs |
| Vacuum and other low-residence designs | reduced pressure or other methods to shorten drying time | faster changeovers, where the supplier approves |
Sizing a Hopper
The hopper must hold the material long enough to reach the drying time at the rate the press uses it:
hopper capacity needed = throughput × drying time
For example, assume a press uses 40 kg of a hygroscopic grade per hour and the data sheet calls for 4 hours of drying. The hopper must hold at least 40 × 4 = 160 kg of material at drying conditions, plus margin for start-up and rate changes. A hopper sized for 100 kg would deliver material that has dried for only about 2.5 hours at that rate.
Over-drying is also a risk: some grades yellow or lose additives when held too long at temperature, so the supplier’s maximum time matters as much as the minimum. Moisture analyzers at the press give a direct check when the material is critical.
Conveying and Blending
- Central vacuum conveying moves material from silos, gaylords, or a dryer to many presses through dedicated lines.
- Machine-side loaders fill one hopper from a container or dryer.
- Blenders combine virgin resin, regrind, colorant, and additives. Gravimetric blenders weigh each component; volumetric feeders meter by volume and depend on consistent bulk density, so they need periodic checks by weight.
The failure modes are contamination and wrong material: a line that carries two resins without purging, a mislabeled container, or a blender left with the previous color. Material identification at every hopper and a purge procedure for shared lines prevent these errors, and blend accuracy matters directly to color, as covered in color matching with masterbatch or pre-colored resin. Conveying lines and pellet flow can build static charge, so grounding follows the equipment maker’s instructions.
Temperature Control Units
A TCU circulates water or oil through the mold to hold it at the setpoint.
- Water units serve mold temperatures within water’s range. Above the boiling point of water at atmospheric pressure, a water unit must be a pressurized design rated for that temperature, with its pressure relief and safety devices maintained; the maximum temperature is the one on the unit’s rating, not a general figure.
- Oil (thermal fluid) units serve higher mold temperatures, at the cost of lower heat transfer than water and hot-surface and fire precautions.
Hoses, fittings, and quick disconnects on hot circuits are rated for the temperature and pressure they carry. Flow through each mold circuit matters as much as the setpoint, as covered in cooling systems and temperature control.
Chillers and Cooling Towers
Chillers remove the heat that molds and machines put into the cooling water; cooling towers reject heat to the air, either from chiller condensers or, in some plants, directly from process water. Sizing starts from the heat load:
heat from the plastic = throughput × heat to be removed per kilogram
The heat to be removed per kilogram depends on the resin’s specific heat, the latent heat of crystallization for semi-crystalline grades, and the drop from melt temperature to ejection temperature; the resin supplier or a heat-load table for the grade provides it.
For example, assume a press molds 50 kg per hour of a resin from which about 400 kJ must be removed per kilogram (an illustrative value):
- Heat load: 50 × 400 = 20,000 kJ per hour, or about 5.6 kW.
- Water flow for a 3 °C rise across the mold: 5.6 ÷ (4.18 × 3) = about 0.44 kg/s, roughly 27 L/min.
- Three such presses: about 16.7 kW and roughly 80 L/min at the same rise.
Heat from hydraulic oil coolers, if they share the system, is added on top. The chiller is then selected for the combined load at the supply temperature and on the hottest expected day, with margin for growth; undersized cooling shows up as supply temperature drifting with the weather and part dimensions drifting with it. Water treatment for scale, corrosion, and biological growth protects the heat transfer that the sizing assumes.
Granulators
Beside-the-press granulators grind runners as they are produced; central granulators handle scrap from many presses. Blade sharpness, screen size, and separation of materials decide regrind quality, covered in regrind ratios and quality control. Granulators are hazardous cutting machines: guarding, interlocks, and lockout before clearing or changing blades follow the manufacturer’s manual and workplace safety rules.
Robots and Part Handling
Sprue pickers remove runners; part-removal robots place parts on conveyors, in packaging, or into downstream operations; insert loaders place inserts or labels in the mold. Their value is consistent cycle timing and handling, which steadies the process, and they are integrated with the press through its interface so the mold cannot close on the robot. Guarding follows the robot and machine makers’ safety requirements.
A Startup Checklist for Auxiliaries
- Drying conditions and hopper capacity match the grade and throughput.
- Material identity verified at every hopper; shared lines purged.
- Blend ratio verified by weight.
- TCU type and rating suit the mold temperature; flow verified on each circuit.
- Chiller and tower capacity cover the combined heat load.
- Granulator, robot, and conveyor guarding in place.
A process that runs well on Monday and poorly on Friday afternoon can have an auxiliary cause: a hopper running low on drying time, a blender drifting, or cooling water warming with the day.
Leave a Reply