Injection Molding - Plastic Information

Injection moulding (United States Injection Molding) is a manufacturing technique for making parts from plastic material. Molten plastic is injected at high pressure into a mould, which is the inverse of the desired shape. The mould is made by a mouldmaker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection moulding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. It is the most common method of production, with some commonly made items include bottle caps and outdoor furniture.

 

Injection Molding Process

Heated plastic is forced under pressure into a mould cavity; it is then clamped together and solidifies into the shape of the mould creating the part.

Resin pellets are poured into the Feed hopper, a large open bottom container, which feeds the granules down to the screw. The screw is turned by hydraulic or electric motor that turns the screw feeding the pellets up the screw's grooves. The depths of the screw flights decreases towards the end of the screw nearest the mould. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control around the pellets during the melting process. The screw travel limit switches set the distance the screw moves.

The hydraulic system pumps oil from the oil tank to firmly close the male and female mould parts, that run along the tie bar; the liquid resin is then injected into the mould. Since the moulds are clamped shut by the hydraulics, the heated plastic is forced under the pressure of the injection screw to take the shape of the mould. The water-cooling channels then assist in cooling the mould and the heated plastic solidifies into the part.

Plastic Molds

Considerable thought is put into the design of moulded parts and their moulds, to ensure that the parts will not be trapped in the mould, that the moulds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts, which can occur due to peculiarities of the process.

Moulds separate into at least two halves—called the core and the cavity—to permit the part to be extracted; in general the shape of a part must be such that it will not be locked into the mould. For example, sides of objects typically cannot be parallel with the direction of draw—the direction in which the core and cavity separate from each other. They are angled slightly; examination of most household objects made from plastic will show this aspect of design, known as draft. 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 more complex moulds, which may require moveable sections, called slides, which are inserted into the mould to form particular features that cannot be formed using only a core and a cavity, but are then withdrawn to allow the part to be released.

The resin, or raw material for injection moulding, is usually in pellet form, and is melted by heat and pressure shortly before being injected into the mould. The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut off and recycled. Some moulds are designed such that it is automatically stripped from the part through action of the mould.

The quality of the moulded part depends on the quality of the mould, the care taken during the moulding process, and upon details of the design of the part itself. It is essential that the molten resin be at just the right pressure and temperature, so that it flows easily to all parts of the mould. The parts of the mould must also come together extremely precisely, otherwise small leakages of molten plastic can form, a phenomenon known as flash. When filling a new or unfamiliar mould for the first time, where shot size for that particular mould is unknown, a technician should reduce the nozzle pressure so that the mould fills, but does not flash. Then, using that now-known shot volume, pressure can be raised without fear of damaging the mould.

Moulds are very expensive to manufacture therefore they are usually only used in mass production where thousands of parts are being produced. Moulds are typically constructed from hardened steel or aluminum. The choice of material to build a mould is primarily one of economics. Steel moulds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made in the mould before wearing out.

JIT Processes

Just In Time (JIT) is an inventory strategy implemented to improve the return on investment of a business by reducing in-process inventory and its associated costs. The process is driven by a series of signals, or Kanban (Jp. カンバン also 看板), that tell production processes to make the next part. Kanban are usually simple visual signals, such as the presence or absence of a part on a shelf. JIT causes dramatic improvements in a manufacturing organization's return on investment, quality, and efficiency.

 

Assembly
Tombyll Plastics can provide some assembly services to assist with the production process.  Contact us for details so we can discuss how we may help your company.

Packaging

Tombyll Plastics takes great care to package the finished product and deliver it on time.  If your company has special requirements we will incorporate those needs into the packaging process.  In some cases we will provide transportation and  local delivery of the product to your company in order to provide JIT services.

Partnerships
By working closely with our clients Tombyll Plastics has established quality relationships that contribute to the success of their businesses.  We work with our clients to help promote and market the products that are produced which contributes to the success of the participating businesses.  Inquire about partnerships and how our production process combined with our joint venture marketing efforts can make your company more successful.