MOULDING PROCESSES

MOULDING PROCESSES

The moulding process can be very confusing so to make sure you are receiving the best process for your required application we have explained each process in easy to digest sub titled areas.

Before we begin, we would like to thank Active Plastics Moulding in New Zealand for instigating the creation of this article.

THE INJECTION MOULDING METHOD EXPLAINED

We take plastic granules, and these are melted until fluid enough to fill a mould under pressure. Once injected into the mould it is clamped shut and then the molten plastic is forced under a lot of pressure to make sure all cavities are completely filled. This results in exact copies.  Once cooled, the shape is released ejected with an air blast, rod or plates. This is known as one cycle, once a cycle is completed, the process begins again.

EXPLAINING THE MOULDING MACHINE

The moulding machines are very much like a traditional kitchen mincer in principal as they contain a screw within a barrel. The rotating screw forces the plastic along the barrel and on the outside of the barrel are the important heater bands.  These heater bands make sure that the plastic granules are heated to the correct temperature to melt the plastic. Polymers have many different melting temperatures and the heater bands are crucial part of the melting process. Too high and it can lead to plastic degradation and too low can result in un-melted plastic within the shape.

HOW A PLASTIC INJECTION MOULD TOOL WORKS

The mould tool is a vital part of the process.

  • It is a canal that allows the plastic to flow from the injection cylinder to the mould
  • It disperses all air from the mould, leaving no air bubbles to lessen surface imperfections
  • It controls the cooling of the mould until set.
  • Its ejection system will push the finished shape from the mould

THERMOPLASTIC MATERIALS AVAILABLE

The simple and easy to read table shows clearly the arrays of wide array of thermoplastic materials used in the injection mould process. The list is not the only materials available as there are certain blends designed to cover multiple components to the best performance quality.

Each group of plastics can be enhanced by adding fillers such as glass or talc for strength, or additives to improve much more stable UV limits, better fire retardant and also antimicrobial properties where sterile elements are required in a shape.

BLOW MOULDING

Blow moulding is a blanket term but includes the processes below:

  • Blown Film – mainly used for Packaging, bags, sheets or sacks.
  • Extrusion Blow Moulding – this method is mostly used in the production of bottles
  • Injection Blow Moulding – used for the production of containers such as jars or bottles for superior dimensional and visual properties
  • Injection Stretch Blow Moulding – used for highly designed and intricate containers

EXTRUSION

Unlike injection moulding where individual parts are produced, Extrusion is where the molten plastic is forced through a die to form certain shapes in an ongoing cycle.  Once the profile exits the die it is still molten and is then conveyed into a water bath that contains the guides and sizing formers. Once cooled the final shape is realised. Extrusion is typically used for:

  • Cable conduit
  • Rainwater pipes and guttering
  • Gas and Water pipes
  • Medical drip and catheter tubes
  • Window, door and conservatory profiles
  • Drainage pipe
  • Curtain track
  • Automotive door seals
  • Fascia and soffit

ROTATIONAL MOULDING

Rotational Moulding or Roto/Rotamoulding is mainly used for plastic products that are hollow inside. The powdered, unheated polymers are put into the mould (charging) which is then rotated and heated in a large specialised oven allowing the melted plastic to spread into all cavities of the mould equally (heating and fusion).  The mould is then cooled using air or water, or both (cooling). Once cooled the part is taken from the mould the final shape realised (demoulding). This is a method used for larger products such as:

  • Children’s playhouses or plastic slides
  • Water tanks
  • Canoes and kayaks
  • Pallets
  • Manhole inspection chambers
  • Fuel tanks
  • Traffic cones

VACUUM FORMING

The process of vacuum forming is where plastic sheets are heated until pliable, this is then placed over a mould and sucked into shape. Thermoforming is a very similar process, but air and weight pressure can be applied and can have speedier cycle times, more widely used for food packaging.  This process is used for both smaller and larger products such as:

  • Boat hulls
  • Machinery guards
  • Refrigerator linings
  • Baths and shower trays
  • Exterior signs
  • Parts of vehicle cabs
  • Yoghurt pots

RIM or REACTION INJECTION MOULDING

This process, also called structural foam moulding is where two or more different materials are put into the chosen mould and the reaction that happens chemically, makes the material expand and completely fill all cavities in the mould.  There is usually some finishing required such as painting to the external surface. In the main this process is used for:

  • Office furniture
  • Automotive parts
  • Point of sale display components
  • Pallets
  • Sporting goods
  • Containers

DIP MOULDING

This is a very similar moulding technique to candle dipping where a heated shape, or mould is dipped in the specified plastic paste or liquid powder. The mould is then baked, and the part is peeled off.  The mould can be dipped and baked multiple times before peeling if needed.

Typical Dip moulding applications:

  • Surgical gloves
  • Handles for hand tools
  • Plugs
  • Caps
  • Covers

COMPRESSION MOULDING

Compression moulding is used in the moulding of precision thermosets. It is also called dough moulding.  The plastic powder granules are heated and compressed from the top of the mould into parts that require a more detailed finish. The usual material is Phenolics (“Bakelite”), some polyesters, urea formaldehyde, melamine formaldehyde to form parts such as:

  • Electrical switches and sockets
  • Electrical parts
  • Radio and appliance knobs
  • Automotive exterior panels (especially commercial vehicles)

PULTRUSION

Pultrusion is a very similar method to Extrusion but it uses a polymer that is reinforced with high performance fibres, such a nylon, glass or carbon fibre that are passed through a guide to align the shape and the reinforcement before heating.  The end result is that the fibres are fully co-extruded with the polymer. Usual uses of the pultrusion method are:

  • Tent poles
  • Ski poles
  • Flagpoles
  • Handrails
  • Bridge structures
  • Aerospace components
Categories: Business

About Author