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Plastics Molding Processes

 

Overview

The plastic molding process is known to the industry for over a hundred years creating many different types of shapes and designs. Plastic molding also lets companies create mass-market consumer durables at affordable costs reaching billions of people across the world. From basic toothbrushes to large auto and aerospace parts, all made possible through plastic molding. However, to satisfy costs, production & usage needs, the industry has invented different types of plastic molding processes to suit every industrial and consumer requirements. In this article, we are going to discuss various plastic molding processes currently available in the industry.

 

Types of plastic molding processes

  1. Compression Molding
  2. Extrusion blow Molding
  3. Injection Molding
  4. Low-Pressure Injection Molding
    1. Structural foam
    2. Gas Assisted IM
  5. Pressure Forming
  6. Reaction Injection Molding
  7. Rotational Molding
  8. Thermoforming

 

Compression Molding

Compression molding uses matching closed cavity molds under high pressure. The material which is in the form of powder, sheet, semi-liquid etc. is placed into the heated mold. When the mold is closed, the material takes the form of the mold shape under high compression pressure. Once the part is solidified under continued heating, mold is opened to remove the formed part. Fillers and other reinforcing materials are added to the thermosets. The alignment of the fibres and length determines the physical properties of the part.

Materials – EPDM, NBR, PVC nitrile, polypropylene, ABS

Compression molding schematics. Photo credit: Wikipedia

 

 

Compression molding

 

 

Advantages

Limitations

Applications
  • Lower mold cost
  • Lower mold stresses
  • Medium part complexity
  • Strengthening ribs are possible
  • Medium to high-quality surface finishes
  • Dimensional stability at high temperatures
  • Flash needs to be cleared
  • Longer cycle and curing time
  • Cost impact if side cores are required
  • Trimming operations to clear holes and other features
  • Automobile - Seals, grommet, insulations, dampers
  • Industrial - O-rings, hoses, gaskets, electrical insulation
  • HVAC - Tubing, hoses, seals, gaskets, insulations

 

 

Extrusion blow molding

Extrusion blow molding is the process of making hollow parts out of polymer-based raw materials. The essential components of the process are the die head, pressing screw, extruder and mold. The entire set-up is installed on an automatic production system. The heated raw material in viscous form is extruded out into thin flat or circular sheets. Once the sheet is extruded enough to sit inside the mold, then the two halves of the mold are closed, and the air is blown at high pressure into the mold. The heated sheet expands due to the air pressure and takes the shape of the mold.

Materials – HDPE, LDPE, PET

1 Feed from the extruder. 2 Melted plastic. 3 Extruder head. 4 Air tube. 5 Preform (parison) / tubular shape of hot plastic. 6 Mould. 7 Air pressure. 8 (End) product. Source credits: Wikipedia

 

 

Extrusion blow molding

 

 

Advantages

Limitations

Applications
  • Overall good quality parts
  • Can achieve a high rate of production
  • Costs lower than injection molding
  • Easily incorporate material handling features
  • The best process for hollow parts
  • Medium design complexity
  • Variation in wall thickness
  • The significant volume of scrap materials
  • Toys
  • Car AC ducts
  • Engine air intake ducts
  • Large storage tanks & bins
  • Containers for consumer goods

 

 

Injection Molding

Injection molding is the process of manufacturing thermoplastic parts from raw material powder or pellets. The pellets are heated inside the extruder until turned liquid, then forced into the mold cavity thru gates and runners. The mold is cooled by circulating water into in-built cooling channels to accelerate the solidification process. Then mold opens and part is ejected out of the cavity. The parts are designed according to injection molding design methodology for ease of manufacturing.

Materials – ABS, polypropylene, nylon, Acrylic, Polycarbonate, PC-ABS, POM

 

Injection molding schematics. Photo credit: Wikipedia

 

Injection molding process

 

 

Advantages

Limitations

Applications
  • Complex geometries could be created
  • Family molds
  • Capability to incorporate bosses, ribs, hinges, and lugs into the design
  • In-mold decoration, labelling, texturing
  • Very high production capability running into millions
  • High initial mold making cost
  • Regular mold maintenance
  • Cycle time depends upon the part size and weight
  • Consumer goods
  • Mass market products

 

 

Low-Pressure Injection Molding

 

Structural Foam

The structural foam process is generally used for thermoplastic parts with a cellular core and is made by the injection molding process. A special blowing agent is mixed with the raw material and the rotating screw rotates to move the material forward. Along the path, the mixture is heated, and the chemical agent reacts to form the gas. The entire mixture of gas and bubbles is held in the cylinder under pressure to avoid expanding. Then the mold is then initially filled with the material in smaller quantities to achieve the desired weight needed. Later the pressure is released, and the expanding gas bubbles fill the entire mold completely with the mixture. The mold is cooled down and the part is ejected. The overall injection pressure is much lower than the standard injection molding process.

Material – ABS, PP, PVC, Polyethylene

 

Low-pressure Injection molding schematics. Photo credit: Researchgate

 

Low-pressure IM Structural foam. Photo credit: LGSovermold

 

 

Advantages

Limitations

Applications
  • Fewer material usages
  • Reduced molding pressure
  • No sink marks
  • Enhanced acoustic properties
  • Increased part strength
  • Counter pressure could enhance surface finish & may not need painting
  • Part tolerances close to standard injection molding
  • High cycle time 2~3min
  • Transparent parts are not possible
  • The extra cost of blowing chemical
  • Medium to large size parts
  • Painting is required to hide swirl marks
  • Large complex shapes with high stiffness
  • Car consoles
  • Fan shrouds of automobiles

 

 

Gas-assisted injection molding

Gas-assisted injection molding is to create parts with hollow cavities inside and a low-pressure process than standard injection molding. In this process, a gas injector is fixed on the main nozzle of a standard injection molding machine. The liquid plastic enters the mold partially filling the cavity under normal plunger pressure. Then the gas pressure is applied which enters the cavity centre containing liquid plastic to the outer periphery having softer plastic, thus creating the hollow voids inside the formed part. The created parts are lighter in weight, with lower shrinkages, stress and warpage.

Material – ABS, PP, PC, HIPS, HDPE

Gas-assisted injection molding. Photo credits: PSImoldedplastics

 

Gas-assisted injection molding