So you need a custom plastic part or product,
but you don’t know how to get it made.
Is it large or small? Should it be flexible
or stiff? Is it round, square, or some weird shape? Do you already have a mold,
or do you need to have one made?
This guide will explain the different kinds
of molding processes available today to help you discover the ideal process for
your product.
What Is Plastic
Molding?
Molding, also sometimes spelled moulding, is
the process of manufacturing by shaping liquid or pliable material using a
rigid frame called a mold or matrix.
When molding plastics, a powder or liquid
polymer such as polyethylene or polypropylene is placed into a hollow mold so
the polymer can take its shape. Depending on the type of process used, various
ranges of heat and pressure are used to create an end product.
The History of Plastic
Molding
Plastic molding began in the late 1800’s to
fill the need for plastic billiard balls as opposed to the commonly used ivory
billiard balls of the time. In 1868, John Wesley Hyatt invented a way to make
billiard balls by injecting celluloid into a mold. Four years later, Hyatt and
his brother invented and patented a machine to automate the process. This was
the first plastic
injection molding machine in existence and it used a basic plunger to
inject plastic into a mold through a heated cylinder.
In 1946, the screw injection
molding machine was invented by James Hendry, which replaced the plunger
injection technique. This is the technique most commonly used today.
Modern rotational molding also has a rich
history beginning in 1855 when rotation and heat were used to produce metal
artillery shells in Britain.
Plastics were introduced into the process in
the early 1950’s, when rotational molding was first used to manufacture doll
heads. And then in the 1960’s the modern process of rotational molding that
allows us to create large hallow containers with low-density polyethylene was
developed. In recent history, process improvements, better equipment, and
plastic powder developments have sped up the process of creating finished
products which has caused rotational molding to grow rapidly in popularity.
The Types of Plastic
Molding
The most popular techniques in plastic
molding are rotational molding, injection molding, blow molding, compression
molding, extrusion molding, and thermoforming. We’ll cover all these techniques
in this guide to help you discover the best process to make your part or
product.
Rotational Molding
Rotational Molding, also called rotomolding,
is a manufacturing process for producing large hollow parts and products by
placing a powder or liquid resin into a metal mold and rotating it in an oven
until the resin coats the inside of the mold. The constant rotation of the mold
creates centrifugal force forming even-walled products. Once the mold cools,
the hardened plastic is removed from the mold.
Very little material is wasted during the
process, and excess material is often re-used, making it economical and
environmentally friendly.
Common Uses for
Rotational Molding
Rotational molding is commonly used to make
large hollow plastic products like utility carts, storage tanks, car parts,
marine buoys, pet houses, recycling bins, road cones, kayak hulls, and
playground slides.
Rotational Molds Are
Highly Customizable And Cost Effective
The mold itself can be highly intricate to
facilitate the molding of a wide range of products. Molds can include inserts,
curves, and contours as well as logos and slots for plastic or metal inserts to
be placed after a product is molded.
Tooling costs are lower with rotational molds
than injection or blow molds. The results are lower start-up costs and
cost-effective production runs even when producing as few as 25 items at a
time.
Injection Molding
Injection molding is the process of making
custom plastic parts by injecting molten plastic material at high pressure into
a metal mold. Just like other forms of plastic molding, after the molten
plastic is injected into the mold, the mold is cooled and opened to reveal a
solid plastic part.
The process is similar to a Jello mold which
is filled then cooled to create the final product.
Common Uses for
Injection Molding
Injection molding is commonly used for making
very high volume custom plastic parts. Large injection molding machines can
mold car parts. Smaller machines can produce very precise plastic parts for
surgical applications. In addition, there are many types of plastic resins and
additives that can be used in the injection molding process, increasing its
flexibility for designers and engineers.
Injection molds, which are usually made from
steel or aluminum, carry a hefty cost. However, the cost per part is very
economical if you need several thousand parts per year.
With injection molding, tooling usually takes
12-16 weeks with up to four more weeks for production.
Blow Molding
Blow molding is a method of making hollow,
thin-walled, custom plastic parts. It is primarily used for making products
with a uniform wall thickness and where the shape is important. The process is
based upon the same principle as glass blowing.
Blow molding machines heat up plastic and
inject air blowing up the hot plastic like a balloon. The plastic is blown into
a mold and as it expands, it presses against the walls of the mold taking its
shape. After the plastic “balloon” fills the mold, it is cooled and hardened,
and the part is ejected. The whole process takes less than two minutes so an
average 12 hour day can produce around 1440 pieces.
Common Uses for Blow
Molding
Blow molding processes generate, in most
cases, bottles, plastic drums, and fuel tanks. If you need a hundred thousand
plastic bottles, this is the process for you. Blow molding is fast and
economical with the mold itself costing less than an injection molding, but
more than rotational molding … sometimes as high as 6 to 7 times as much as a
roto-molding tool.
Compression Molding
Compression molding is done exactly like the
name suggests. A heated plastic material is placed into a heated mold and then
pressed into a specific shape. Usually, the plastic comes in sheets, but can
also be in bulk. Once the plastic is compressed into the right shape, the
heating process ensures that the plastic retains maximum strength. The final
steps in this process involve cooling, trimming, and then removing the plastic
part from the mold.
Common Uses of Compression Molding
The best use of compression molding is the
replacement of metal parts with plastic parts. It is mostly used for small
parts and products in very high volume. The automotive industry uses
compression molding heavily because the final products are very strong and
durable.
The initial cost of a compression mold is
substantial, depending on several factors including the number of cavities, the
size of the parts, the complexity of the pieces, and the surface finish among
other things. But the cost of each individual part is low at high quantities,
so large quantities of parts are ideal for this form of molding.
Extrusion Molding
Extrusion molding is similar to injection molding
except that a long continuous shape is produced. Another difference in
extrusion molding is that the process uses a “die” not a “mold.”
Extruded parts are made by squeezing hot raw
material through a custom die. A simplistic visualization would be like
squeezing Play Doh through a shaped hole.
While other forms of molding use extrusion to
get the plastic resins into a mold, this process extrudes the melted plastic
directly into a die. The die shape, not a mold, determines the shape of the
final product.
Common Uses of
Extrusion Molding
Parts made from extrusion have a fixed
cross-sectional profile. Examples of extruded products include PVC piping,
straws, and hoses. The parts do not need to be round but they need to have the
same shape along the length of the part.
The cost of extrusion molding is relatively
low compared to other molding processes because of the simplicity of the die
and the machines themselves.
However, the nature of the extrusion molding
process limits the kinds of products that can be manufactured with this
technique.
Thermoforming
Thermoforming is a manufacturing process
where a plastic sheet called thermoplastic is heated to a pliable forming
temperature, formed to a specific shape in a mold, and trimmed to create a
usable product. Thermoplastic comes in a wide variety of materials, colors,
finishes, and thickness.
Thermoforming uses several different types of
molds and processes in order to achieve the final product. To create 3D
products, the mold is typically a single 3D form made out of aluminum. Because
thermoforming uses low pressures, molds can be produced for a low cost using
inexpensive materials.
Common Uses of
Thermoforming
Thin-gauge thermoforming is commonly used to
manufacture disposable cups, containers, lids, trays, blisters, clamshells, and
other products for the food and general retail industries. Thick-gauge
thermoforming includes parts as diverse as vehicle door and dash panels,
refrigerator liners, and utility vehicle beds.
On average, it takes about 8 weeks to get a
thermoform mold ready for production. The cost of a thermoform mold is based
upon the size of the part that needs to be produced. A mold for a small part
can cost as little as $20,000 while the cost of a larger mold can be upwards of
$50,000.
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