Molding Our World: The Process of Injection Molding

It may be interesting to know that all sign holders, brochure holders and other p.o.p. displays are injection molded using polystyrene sheet or pellets. So what is the origin of this process? It all began in 1868. At that time, John Wesley Hyatt injected hot celluloid into a mold and produced billiard balls. As the first one to do it, he patented an injection molding machine, which remained the used process until 1946, when James Hendry built the first screw injection molding machine. This new process revolutionized the plastics industry. Today, almost all molding machines now use screws to efficiently heat, mix and inject plastic into molds.

Barrels, Source: Complete Plastics

Using thermoplastic material, Injection Moulding is a manufacturing technique for making thousands of unique parts for millions of unique products, from tiny transistor radio parts to car parts. To facilitate elementary understanding, molten plastic is injected at high pressure into a mold; the mold is the inverse of the desired shape. Made from either steel or aluminum, the mold is precision-machined to form the features of the desired part. Sometimes brochure displays, racks, counter displays, donation / suggestion boxes and point of purchase (p.o.p) displays are made through the injection molding process.

Injection Molding, Source: Zone

Key facts:

• Molds are carefully designed to ensure that parts are not trapped in the mould
• Molds need to be completely filled before the molten resin solidifies to compensate for material shrinkage
• Molds separate into at least two halves so that extraction can occur
• A part needs to be shaped in such a way so that it will not be locked into the mold.• Parts that are “bucket-like” tend to shrink onto the core while cooling and, after the cavity is pulled away are typically ejected using pins.
• More complex parts are formed using different types of molds, which may require moveable sections or “slides”.
• Slides are inserted into the mold to create features that cannot be formed using only a core and a cavity.

The Injection Process

• First, heated plastic is forced under pressure into a mold cavity. It is then clamped together and begins to take the shape of the mold.

Resin Pellets, Source: RJM International

• Resin pellets are poured into a “feed hopper”, a large open bottomed container which distributes the granules down to the screw.
• The screw is operated by a motor that turns the screw, feeding the pellets into the screws grooves.
• Next, the depth of the screw’s flight decreases toward the end of the screw nearest the mold.
• As the screw rotates the pellets are moved forward
• They undergo extreme pressure and friction which generates most of the heat needed to melt the pellets.
• The screw “travel limit switch” sets the distance the screw moves.
• The hydraulic system pumps oil from the oil tank to firmly close the male and female mold parts that run along the tie bar
• Liquid resin is then injected into the mold.
• Water cools the mold and the heated plastic solidifies into the part.
• The mold opens and the part is ejected.If you are looking for a brochure display, rack, counter display, donation / suggestion box or point of purchase (p.o.p) display, we can assist!