Plastic Injection Molding Manufacturing

Plastic Injection Molding


Plastic Injection Machine Diagram


  1. Molten plastic resin is forced into a mold
  2. The Mold, filled with molten plastic material, is then cooled to allow part to maintain its shape
  3. The mold opens and the part is ejected from the mold
  4. Mold is closed, allowing the cycle to be repeated

Quality Factors

The following 4 elements are crucial parts of making a Quality Part:

  • Design
  • Tooling
  • Material
  • Manufacturing

9 to 5 Seating has brought in-house many of its manufacturing processes to ensure direct control of the above elements.


Computer Aided Design (CAD), and Computer Aided Manufacturing (CAM) are relatively newer technologies that have enhanced the creation of part design and mold design. This technology is used to for:

  1. Creation of “to-scale” and “3 dimensional” drawings and renderings, which can be examined from different perspectives and for different analysis.
  2. Once the part design and part geometry is created, using sophisticated engineering programs such as Pro-E, the Mold Design and the subsequent CNC milling and mold fabrication electronic files are computer generated for the actual mold fabrication.
  3. The CAD/CAM files described above facilitate important part design and mold design analysis, mold temperature and material flow analysis, as well as many other necessary elements that determine the design quality of a past and its mold.

9 to 5 Seating has on its team a crew of highly skilled engineers who are well versed with the latest technologies in CAD/CAM engineering programs and techniques. This facilitates rapid prototyping of new product, as well as shorter lead times in new part developments.


A Plastic Injection Mold comprises of 4 or more blocks (plates) of steel and aluminum which are assembled together, moving along 4 Guide Bushings (one in each corner). Two of the steel plates in every mold help shape the molten plastic that is forced inside a mold into a finished plastic part. These two plates are generally referred to as “Core Plate” (with a convex image of the part machined onto its surface), and the “Cavity Plate” (with the Concave image of the part machined away from its surface). When the mold closes, the Core and Cavity plates are shut tight against each other, with only the thickness of the part separating the Core and Cavity images, allowing the molten plastic to flow inside the mold and fill this space.

Plastic Injection Mold 2


Fabrication of Quality Molds requires particular attention to the following elements:

  1. Quality of steel used for the Core and Cavity plates. Some abrasive materials, such as Fiber Glass filled Nylon, erode the surface of the mold, causing quality issues during the part manufacturing process.
  2. Accuracy of the Parting Line, that joins the Core and Cavity shapes, affects the finish quality of parts made. Use of CNC milling machines and other highly accurate machinery are key elements of manufacturing a high quality mold..

9 to 5 Seating owns and operates highly accurate CNC Milling Machines, EDM Machines, and other highly accurate tool manufacturing machinery. This facility reduces the time needed for mold fabrication, and ensures quality of the molds made.


Most of the plastic materials (or plastic RESINS) used in the office seating industry are either Nylon (PA), Polypropylene (PP), or ABS. These plastic resins are made to differing specifications, resulting in different properties, and made to varying quality standards, depending on the resin manufacturer, the intended application, use, and market.


Nylon (PA-6, or PA-66) is a very tough material and mainly used in Structural parts. It can be used without any additives (in the manufacture of castors, as an example), or compounded with various percentage of Glass Fiber (typically 30%) and used primarily in the manufacturing of star bases.

Polypropylene, especially when mixed with a flexible material (similar to rubber), can produce a smooth surface and a fine finish look. It is mostly used in non-structural parts such as seat or back covers, cover cans, armrest parts, etc.

ABS, if made to withstand High Impact, can be used in manufacturing structural parts, but yet it is soft enough to allow staples to be driven into it; and hence it is typically used in the manufacturing of seat or back pans, as a replacement to plywood.

Different materials have different properties, such as:

Virgin Plastic Pellets 2

  1. Strength
  2. Melt Temperature
  3. Material Flow inside a mold
  4. Shrink Factor
  5. Appearance

Using a high quality material, purchased from a reputable manufacturer and in accordance with the specifications and properties required (as specified during Part Design) is crucial in making a high quality part.

Materials used in by various manufacturers can broadly be divided into:

  1. Virgin Material – not previously used in any injection molding operation
  2. Recycled, or Regrind Material – made from material that has already been injection molded (once or more times), then chopped into small pieces again, and used with or without the addition of a percentage of Virgin Material.

In order to save costs, many office seating manufacturers chose to use Regrind, or Recycled Material. In its self, regrind or recycled material can be safely used in most applications, providing the source of the recycled material, the mix, and the properties of the resultant material are controlled.

ReGrind Plastic Pellets 3

Quality issues, stemming from the use of regrind or recycled material, can arise when:

  • Different materials, with different properties, are mixed together in a batch, preventing the material to melt uniformly. This will result in the mix of molten material NOT flowing evenly inside the mold, causing structural and appearance problems.
  • The source of the regrind material is not known. It is not possible to look at regrind material and readily determine how many times that material has been regrind. Also, recycle material suppliers tend to mix different types of material together and sell them as one.
  • Plastic resin will lose some of its properties and characteristics each time its temperature is raised to melting point and then cooled down. Therefore regrind material of unknown origin will not produce consistently the expected result.

9 to 5 Seating manufactures almost all its plastic components in-house, so that exclusively uses high quality resins that it purchases from world-class and reputable suppliers such as BASF.

Manufacturing Process

The manufacturing process of Plastic Injection Molding requires precision machinery, experienced operators, and continuous production. Today the level of technology and sophistication of state of the art machinery can be found in almost all corners of the world. The main reasons for 9 to 5 Seating to have brought in-house its plastic injection molding operation can be listed as follows:

  1. Control of Prime Raw Materials used by 9 to 5 Seating. Once a plastic part is made, it is difficult to examine the reliability of the resin that was used to make the part. By conducting all its plastic injection molding in-house, 9 to 5 Seating ensures that the expensive and the Prime Material that it has purchased go to make quality parts for its use.
  2. Before use, many of the plastic resins need to be dried of moisture. To do this, the resin should be placed inside electric dryers for many hours before being fed into the hopper of the plastic injection machines. This operation will result in time and money, and is often avoided by some manufacturers. As an example, if the Nylon material that is to be used to make start bases should not be dried enough before injection, air bubbles will form inside the star base, making it structurally weak and not suitable for its intended use.
  3. The main cost in plastic injection molding is incurred in the electricity used to operate the machines. Therefore, the shorter the cycle time in making a part, the less will be the manufacturing cost of that part. One practice to cut costs by some injection molders is to take the part out of the mold before it has been cooled enough. This will result in shorter cycle times, but will cause the part to change shape, twist and warp, if the core of the part should not have been cooled enough before the part is taken out of the mold. Toshiba plastic injection machine