View A Video on Injection Molded Plastics - A Quick Introduction
Injection molding can use a wide variety of starting materials, including most polymers. Plastic injection molding gives the injection moulders the freedom to choose exactly what the best material for the final molded plastic part would be and requires a lot of calculation as to composition of the resin as well as temperature and pressure in the mold. All these variables can affect the final quality of the plastic. Injection molding uses thermoplastics, called thermoplastics injection molding, thermoset materials or elastomers, depending upon the intended use of the final product. Thermoplastics and thermosets harden completely after cooling in the plastic injection mould; elastomers however, retain a sense of elasticity and so are used to produce items such as rubber bouncing balls. Other variations of plastic molding, or insert molding, include gas assist injection molding which uses an inert gas, such as nitrogen, to create a hollow portion in the mold, and to force the molten plastic into the mold cavities. Another is reaction injection molding; reaction injection molding machines require one further step which is the addition of a curing agent in the mold. This is required for the specific product material to maintain shape and strength once removed from the mold.
The basic machinery used in the process of injection molding consists of a hopper - where the plastic pellets are placed prior to being heated. The plastic material is then fed into a heating unit where it is heated and mixed until molten, at which point dyes or other chemical agents can be added to change the final appearance and feel of the product. Next, an injector, or screw, forces the molten plastic into the mold cavity under hydraulic or mechanical pressure, to ensure that there are no air pockets. Most molds will allow air bubbles to escape to avoid damaging the final product. Cooling liquids are sometimes used around the mold chamber of the machine to extract heat from the plastic and speed up the cooling process, thus saving time in overall manufacturing. The two halves of the plastic injection mold are then pulled apart to release the plastic part. Ejecting pins or rods may have to be used to remove the plastic part from the mold. The entire process can take a matter of seconds or as many as a few minutes to get one complete plastic part. Injection molders are not limited by manufacturing capabilities when it comes to material choice, and are able to fabricate a wide range of product shapes and sizes. If a mold can be made for an object, plastic molding can be used to produce a product. Everything from car bumpers to intricate medical equipment can be made using the techniques of melting the appropriate material, forcing it into a pre-made cavity and allowing it to cool.
Compared to other forms of plastic parts production, injection molding is a cost effective manufacturing method. It can be used for high production runs, and in fact the more parts that are made from a mold, the more cost-effective it is. Once a mold has been made for a certain product, it can be used again and again to produce the same object with close tolerances. In injection molding, there is little or no change in the manufacturing process between cycles, and so often the plastic part extricated from the mold will need little finishing. Furthermore, because the molten plastic is forced only into the space of the cavity, there is very little material waste apart from joining lines and negligible other amounts. The production of minimal scrap saves further money and is a responsible manufacturing practice. Furthermore, what plastic is wasted and considered scrap can often be melted and recycled. Labor costs for the injection molding process are minimal as there is little need for human interference in the injection process.
There are important considerations to be made when discussing plastic injection molding however. Firstly, all the plastic material that is forced into the mold needs to be removed at some point, therefore mold design needs to be carefully considered. A complex rigid, square design might result in cracks or stress marks on the final product when trying to extricate it from the cavity. There also needs to be rods or pins in place to help remove the plastic part from the mold. Plastic injection molds are costly and so it is imperative to ensure that the plastic part can be removed from it once cooled. Another consideration concerning removal is the existence of joining lines, or imperfections in the final products that may require finishing services before being used. The finishing required will usually be minimal, but it is dependant on the quality of the starting material and mold. Finally, while injection molding plastic is a cost effective method of producing plastic goods, custom injection molding will be more costly due to the necessary production of a specific plastic injection mold for the custom product. If however, the quantity of product manufacturing is high, the cost of the plastic injection mold will balance out over time. With so much variance in injection molded plastics, it is helpful to consider the industries which make use of it such as automotive, medical, consumer and household goods.
Injection Molded Plastic Types
- Custom injection molding is an injection molding process in which the mold is not pre-made, but is formed specifically for the consumer's application.
- is a two-step process in which
either the color or the material is injected first. Upon the hardening
of the material, a second color or material is injected into or around
the first shape.
- is a process in which
inert gas, such as nitrogen, is forced into the melt while it is entering
the mold,
packing the plastic into the cavities. Gas-assisted injection molding
reduces cycle time, part weight, warpage and stress to the cooled parts,
as well as minimizing other problems.
- is the process in which plastic is injected into a mold to form a plastic
tube. The
tube is then blown into a cavity mold to form a hollow part.
- is a process by which plastics parts and products are formed.
- Injection moulders are the tools and
die machines used to mold molten plastic.
- is an injection molding process in which plastic
gets injected into a cavity surrounding an insert piece just prior
to molding. Plastic insert molding results in a single piece encapsulated
by the plastic.
- pieces are parts that are formed by pouring heated plastic into molds.
- is a process by which plastic is heated into a malleable form and pressed into molds.
- Plastic injection mould processes are used to fabricate parts with a range of complexities, from simple shapes to precision parts of geometric complexity
- fabricate plastic products by injecting molten plastic into molds or dies.
- Plastic molding is one of the most common methods of part manufacturing.
- is a process that involves multiple layers with
different orientations, providing more uniform properties to the parts
than if
they were molded from a single direction
- Rapid injection molding is primarily used in prototyping and the low-volume production of plastic parts since the production time is drastically shorter than conventional molding process.
- Reaction injection molding is an
economical option commonly used for larger, more complex polyurethane plastic and
rigid foam parts produced in small quantity.
- Thermoplastics injection molding is the most common method used to process thermoplastics, due to its ability to fabricate parts with a range of design variances, from simple shapes to high precision parts of geometric complexity.
Injection Molded Plastic Terms
- A plate that provides support
for the mold cavity block, guide pins, bushings and so forth.
- The space inside a mold into which the material is injected.
- The amount of material needed to fill a mold during a
single cycle.
- Channel through which a cooling medium flows
to control the temperature of the surface of the mold. Cooling channels
are located within the body of the mold.
- Plastic
used to package products with short shelf lives, such as bottles for
milk, juice, water and laundry products. Unpigmented HDPE bottles
are translucent and have good stiffness.
- Devices that use the force of fluids to move
the mold in the injection molding process.
- A plastic that is used
predominantly in film applications due to its toughness and flexibility.
LLDPE is the preferred resin for injection molding because of its superior
toughness and is used in items such as grocery bags, garbage bags and
landfill liners.
- A series of steel plates,
which contain cavities into which plastic resin is injected to form a
part.
- A relatively simple compound that can react to form a
polymer.
- A test that measures the ability of molten plastic
to flow.
- The temperature at which crystalline portions
of polymers melt. A material becomes soft and completely amorphous once
it reaches the Tm.
- A term that refers to the measure of resistance
to flow. For all materials, the viscosity decreases as the temperature
increases.
- A term that means
many units. These units are linked together to form chain-like structures
that give polymers unique properties.
- A very strong substance that has the lowest
density of the plastics used in packaging. PP has a high melting point,
making it ideal for hot-fill liquids.
- A very versatile
plastic that can be rigid or foamed and has a relatively low melting point.
General purpose polystyrene is clear, hard and brittle.
- Any of a class of solid
or semi-solid organic products of natural or synthetic origin, generally
of high molecular weight with
no definite melting point. Most resins are polymers.
- Materials that can be melted by heating and then
re-solidified by cooling. Blends of thermoplastics can be prepared by
melt-mixing.
- Materials that
"cure" to form solids when heated. Unlike thermoplastics,
solid thermosets do not melt when heated, so they are very useful for
high heat applications.
- Device that uses mechanical links to move the mold
during the formation of injection molded plastics.
