Injection Molding Processing: The Basic Guide to Using Auxiliary Equipment

General

Injection Molding Processing: The Basic Guide to Using Auxiliary Equipment

What is injection molding?

Injection molding is the most widely used form of plastics processing worldwide. The process involves the injection of heated, liquefied plastic into a temperature-controlled mold under high pressure.

After the plastic fills the mold, it cools and solidifies into finished part(s) which can be easily removed when the machine opens up the mold. Each production sequence, from the closing of the mold to the injection “shot” to opening and removal of the part, is a “cycle.”

Injection molding processing is done in specialized injection molding machines, or IMMs, which are sized according to tons of mold clamping force.  These may range from “micro” IMMs of just a few tons that produce extremely small parts in small molds to very large IMMs exceeding 3,000 tons of clamping force that can hold very large molds and make extremely large parts, such as automobile front fascia or bumper covers.

Injection Molding Animation

What are the key applications for injection molded parts?

Major applications for injection molded parts include:

  • Automotive parts
  • Consumer electronics
  • Packaging and beverage bottling
  • Industrial components
  • Consumer products
  • Medical devices
  • Construction and building

What is the Role of Auxiliary Equipment in Injection Molding Processing?

Injection molding machines make high-volume, high-quality part production possible.  But without the help of the auxiliary equipment that surrounds them, IMMs couldn’t mold or produce anything.

Think about it.  Auxiliary equipment is vital to:

  • Properly preparing and drying valuable raw materials, with minimal waste
  • Reliably delivering materials to injection molding machines, without contamination or moisture regain
  • Assuring accurate and reliable temperature control for product quality and energy efficiency
  • Maintaining process consistency over long production periods
  • Proper removal and handling of finished parts
  • Efficient management and re-use of scrap material

Injection Molding Machine with Conair Auxiliaries

Types of Auxiliary Equipment that support Injection Molding Processing

Pre-Injection Molding

Storage

It all starts when material is delivered by railcar, truckload, or Gaylord. Moving large bulk deliveries of material often requires specialized auxiliary equipment such as railcar- or truck- unloading systems.

These systems must accept very large quantities of resin very quickly, move this resin over distances up to 1,000 feet, then lift it high enough to rapidly fill large material storage silos. Alternatively, you can move smaller loads and fill smaller surge bins using a cycling loader, which can push or pull bulk material quantities over shorter distances as required.

Conveying

To move material (virgin pellets, post-consumer recycled pellets, regrind) from in-plant storage locations to molding machines and auxiliary equipment like dryers or blenders, processors rely on vacuum conveying systems. These systems, which draw their motive power from a vacuum pump, often include a plant-wide network of conveying lines; material loaders, receivers, and valves; resin selection tables and line proofing systems, and automated controls.

To deliver value for a plastics processor, conveying systems must not only move required amounts of material to the proper locations, but must do so without damaging it (i.e., pellet breakage, dust/fines, pellet smearing, angel hair, streamers) due to excessive or uncontrolled conveying speeds.

Dust or fines from broken pellets or flakes, as well as angel hair and streamers, waste valuable material and cost processors money in two ways.

First, this damaged material is often trapped as in conveying system filters and must disposed of as trash (just six pounds/day equals one ton of material per year!).  Secondly, when dust/fines or angel hair gets into the IMM plasticating barrel, it typically heats up and melts much faster than undamaged pellets, resulting in burned or blackened specks that can render parts cosmetically unacceptable.

To convey materials in high volumes, but at “safe” speeds that prevent damage, consider upgrading from older dilute-phase conveying systems to a slower-speed, dense-phase conveying system like Conair Wave Conveying.

Mixing and Blending

The appearance, performance, strength and quality of many injection-molded parts depends on a consistent blend of raw materials:  virgin pellets, pelletized post-consumer material, scrap regrind, colorant, and/or performance or property additives (e.g., UV stabilizer).

Delivering such blends consistently and continuously is the job of feeding, mixing, and blending equipment.  Blenders typically hold multiple ingredients in top-mounted hoppers or feeders, using digitally controlled “recipes” to measure each ingredient with a gravimetric weighing system before releasing it into a central mixing chamber for batch preparation.

When it comes to blending equipment, precision, repeatability, operating simplicity, and reporting are everything, since even small ingredient changes from batch to batch (e.g., colorant percentage, virgin/PCR/regrind ratio, additive content) can not only raise material or input costs, but also produce quality/performance changes – and possible scrap – in downstream production.

Drying

Whether material drying is done with a large central dryer, a multi-hopper drying cart, a machine-side dryer or a mobile drying unit, this auxiliary equipment is about much more than removing moisture.

Of course, consistent moisture removal is essential for processing any hygroscopic resin (i.e., PET, polycarbonate, ABS, nylon, etc.).  That is because internal moisture in these resins can cause voids, discoloration, holes, or structural weakness in injection molded parts, resulting in diminished performance and appearance properties and, ultimately, scrap.

The other great advantage of consistent drying—for all resins—is that it “homogenizes” the material, enabling you to feed your injection molding machines a steady flow of material that’s at a constant temperature.  When you consider how hot material can get sitting in a silo during the summer—or how cold it could be in the winter—taking the opportunity to pre-dry and homogenize incoming material can make a big difference in part quality all year around.

Auxiliary Equipment Used “Before” Injection Molding

Process Requirement or Task: Auxiliary Equipment:
Material, ingredient, or batch storage Silos, storage bins, surge bins, hoppers
Material conveying Vacuum conveying systems:

  • Conveying Controls
  • Pumps
  • Conveying lines
  • Loaders, Receivers, Valves
  • Material Selection Tables
  • Mobile conveying systems
Material drying, pre-heating, or dehumidifying Drying systems

  • Central
  • Machine-side
  • Mobile or mini systems
  • Dehumidification systems
Material weighing, ingredient dispensing, and batch blending
  • Feeders
  • Blenders
  • Mixers
  • Weigh Scales

Injection molding machine

During Injection Molding

During the injection molding process, other types of auxiliary equipment manage heat transfer, not only within the mold that produces parts, but also in the hard-working elements of the molding machine, especially the hydraulics.

Heat transfer is at the heart of the entire injection molding process.  The injection molding machine’s screw and barrel heaters add heat to material to melt it so it can be injected into molds.  But after the mold “shot” is completed, removing heat from the mold is essential to cooling and hardening the plastic part.

Temperature Control Units (TCUs), usually located at machine-side, circulate a steady flow of coolant through passages in injection molds to help them maintain correct, consistent internal temperatures. When the IMM delivers a “shot,” the flow from the TCU ensures that mold surfaces are warm enough to allow the molten plastic to fill the mold completely.  After the shot occurs, the TCU’s flow carries away the burst of heat from the shot, helping to cool both the mold surface and the part that will be removed when the mold opens.

TCUs manage coolant temperature using an internal, thermostatically controlled circuit that can automatically heat or chill the circulating coolant as needed.

With TCUs usually dedicated to mold temperature control, the heat transfer essential to cooling other production equipment (i.e., IMM pumps, motors, and hydraulics; desiccant dryer process air and regeneration circuits; etc.) is usually tasked to other, larger auxiliary systems:

Portable chillers are mobile units that can be placed machine-side to deliver up to 50 tons of cooling capacity, with water or glycol-based coolant flow delivered direct to primary or auxiliary equipment, or diverted to support TCU internal cooling circuits.

Central chillers provide facility-scale cooling and can be used in single or tandem/multiple arrangements to provide the required capacity, which is  which is  distributed throughout the plant using a network of cooling lines.

Recirculating water systems can be a good option for processors with access to clean, affordable water supplies.  Cooling capacity for these systems is supplied by forced-draft or adiabatic cooling towers.

Auxiliary Equipment Used “During” Injection Molding Process

Process Requirement or Task: Auxiliary Equipment:
Mold temperature control

Mold heat removal to cool plastic, solidify parts

Temperature Control Unit (Thermolator®)
Process equipment cooling:

  • IMM and auxiliary equipment
  • Motors, hydraulics, pumps
  • Drying systems
ChillersCentral Chillers

Forced-draft or adiabatic cooling towers

Recirculating water systems

Injection molding machine in cleanroom

Post-Injection Molding

After each injection molding cycle, auxiliary equipment also plays an important role.

When molds end at the end of injection-molding cycles, more and more processors are using Cartesian and multi-axis robots to automate everything from sprue picking to finished part inspection and packing.

Size-reduction equipment, including machine-side granulators or central granulators, are essential to minimize process waste and maximize your raw material investment.  Granulating scrap parts, as well as sprues and runners, enables you to realize “closed loop” reclaim of virtually every bit of material that enters your plant.

A properly sized granulator will not only accommodate the material waste that is generated, but also reduce it into small and consistently-sized granules or flakes that can be stored, conveyed, and blended with virgin pellets or other feedstocks for reprocessing.

Auxiliary Equipment Used “After” Injection Molding

Task: Auxiliary Equipment:
Part or Sprue Picking Robots
Reprocessing Scrap Parts Granulation Systems:

  • Beside-the-Press
  • Central Granulation Systems
  • Shredding Systems

Conclusion

The injection molding of plastic resins would not be possible without the use of multiple types of auxiliary equipment before, during, and after the injection molding process.

  • Before injection molding:
    From the moment a resin arrives at a plant site, auxiliary equipment is vital to store it, move it, dry it, mix/blend it with other ingredients, and deliver it — free of moisture or contamination and at the proper temperature – to injection molding machines.
  • During injection molding:
    After resin is melted and ready for injection, mold temperature control, provided by circulating coolant from temperature control units ensure stable conditions for part production, while supplemental cooling from chillers prevents process equipment from overheating.
  • After injection molding:
    Robots efficiently pick finished parts and sprues from molding equipment, while granulation equipment size-reduces scrap parts into valuable material for reprocessing.

In performing all of these vital roles, properly functioning auxiliaries address and eliminate a wide range of material, temperature, and process inconsistencies that can cause many common injection molding problems.   Thus, they make a tremendous contribution to continuous, high-quality production.