- Rotational molding and blow molding
are methods we've over viewed a little bit,
but now it's time to talk specifics.
In rotational molding,
plastic powder is measured into the mold,
and the entire mold is placed in a heating oven.
The machine then rotates the mold
along one or more axes,
letting the material coat all of the walls
to a nominally even coating.
Rotational molding is well suited
for vessel containers for liquids,
and for parts that have specific outside
measurements required.
The benefit of this method is primarily cost,
as it's cheaper than injection molding.
Rotocast parts are a great choice
for parts that don't have interior geometry,
and don't have strict wall thickness requirements.
Rotoparts can be robust and have a reduced tooling cost,
though sometimes the material cost per unit is higher.
Rotomolded parts can be entirely self contained,
meaning they don't have to be assembled later.
Rotopart tooling still has to follow mold guidelines though.
Draft, flash, and drag marks for mold separation
are still factors to consider.
Also, temperature control is extremely important
to all thermoplastic forming,
especially for rotational molds,
as consistent temperature for forming
and cooling cycles make the best parts.
Inconsistent heating could result in voids,
warping, or other defects.
Examples of parts that benefit from this process
over injection molding include plastic road barriers
that are filled later with sand,
flotation, or insulated cases,
that are then filled with foam,
and large liquid containment vessels, like this.
Other examples of rotomolded parts
you might have seen include,
playground equipment, liquid tanks, and coolers.
Blow molding is an umbrella term for molding a thin,
hollow container using air to blow it into shape.
Predating machine use, and still done today,
glass blowing by hand was the original method
for making clear drinking bottles,
inflated with the lungs of the glass blower.
Currently, glass blowing is done on an industrial scale,
using machines to blow air into the mold,
stretching the glass.
Plastic blow molding shares some similarities.
It also uses air to press the plastic outward,
contacting the mold surface,
usually in a thin, watertight layer.
Material can be introduced into the mold in a few ways.
Extruded tubes or blobs of malleable hot plastic
can be dropped in, pinched, and inflated.
Or, you can use something called a preform.
Preforms are injected molded parts made of thermoplastics
that follow draft considerations
in the direction of the cap.
All material introduced needs to be hot and stretchy,
and is blown up like a balloon.
Blow molding shares similarities with rotational molding
in that its primary purpose is to create vessels,
but in general, blow molded parts have much thinner walls.
A common example of this are water bottles, like these.
If you're using a preform,
the initial part is placed into the mold,
heated, and then blown up with air like a balloon.
The plastic is pressed into the walls of the mold
while it's still hot,
and then begins a rapid cooling process
to maintain its shape.
Once the cooling process has completed,
sometimes as short as a few seconds,
the finished part is ejected into the machine
and the cycle repeats.
Blow molds usually include design details,
like ridges to enhance bottle style
and improve the structural strength.
Keep in mind that all design details
will need to follow molding rules,
including draft, and havw no sharp edges,
as thin plastic is even more vulnerable to tearing.
Blow molding is a great choice for round containers,
because round profiles have inherent draft,
which will minimize scuff marks.
If you look closely at disposable water bottles,
the bottom usually has its own parting line.
This pull helps create the bottom features you see,
like the concave lift and recycling information.
While both of these methods result in hollow containers
that could work for your project,
always consider the scale, thickness,
and end use of your solution.