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What is Rotational Molding? 
Rotational Molding (sometimes shortened to rotomolding) is a
plastic process for manufacturing usually larger plastic
items.  It is not a new process as it has been around for
over 50 years, but it is only in the relatively recent past
that mold makers have developed the technology to make molds
sophisticated enough to manufacture something as complex as
a boat; in the past it was only possible to make simpler
shapes like agricultural tanks, water tanks and less complex

Rotational molding is basically a process in which a
predetermined amount of finely ground plastic material
(usually polyethylene but not always) is put into a hollow
metal mold.  This mold (usually two halves but sometimes
made with 3 or more major parts), with the material in it,
is put into an oven/machine which heats the mold, while at
the same time rotating it around two or more axes.  As the
mold rotates on these axes the heat melts the plastic
material which adheres to the inside of the mold.  When all
of the plastic has melted and adhered to the walls of the
mold the mold then goes into a cool-down process.  Then the
mold is taken apart and the newly formed part is removed. 
This plastic part is hollow on the inside as the amount of
material originally put into the mold does not completely
fill it.   

The rotomolding process, because it makes parts which are
hollow, combines simultaneously the bottom outer part (hull)
together with the topside (cockpit and deck).  This means
that not only are the top and bottom parts made at the same
time but that no subsequent process is required to fit and
attach them together.  Compared with other boat
manufacturing processes this is a large time saver, and
since the rotational molding cycle can take less than an
hour the manufacturing economics are positive.  There is of
course, high capital investment in the machine and mold(s)
but the process is repetitive with minimal equipment

Polyethylene as a plastic material is not new as it also has
been around for over 50 years.  As a material it is produced
in a variety of formulations and is used to make common
everyday products such as food storage containers, bottles
of all kinds including the common every day milk jug.  As
such, one of its most important characteristics is its
impact resistance – items made from polyethylene seldom
break when dropped or are impacted by other items.  This
gives a boat made from polyethylene a big advantage in the
rough and tumble world of marine use.  However, because it
is relatively soft polyethylene does scratch easily.  But
because color can be blended into the material itself
incidental scratches are not as unsightly as they are in the
gel coat or paint of fiberglass.
We are frequently asked how punched through holes are
repaired.  Fortunately this happens so seldom that it is not
a big problem.  A hole can be repaired, however, by
“welding”.  It does require a special plastic welding tool
(we have one in our shop) but we have never had to repair an
accidentally punched through hole; instead we use it to
repair manufacturing defects such as holes that
inadvertently get drilled in the wrong place.

As excellent as polyethylene is as a boat manufacturing
material it has its trade-offs.  Because polyethylene is not
as stiff and hard as say metal or fiberglass it is
frequently not sturdy enough to take concentrated loads. 
Highly concentrated stress points of things like screw
locations or supporting attachment points have to be
considered carefully.  Rotomolded part engineers frequently
call for metal fasteners to be molded into the plastic part
as stress location terminals.  There are also other design
ways of dealing with high stress areas such as distributing
the loads over a single wide area or over through several
points instead of just one.  Also, metal screws in a
material such as polyethylene have better holding power if
long screws with many threads are screwed into material
thick enough so a lot of threads are engaged.

Because Nauticraft has its own rotomolding machine and is
committed to the process for making our boats, we take
advantage of this position to design and rotomold ancillary
parts such as the seats, pedal drive units, rudders, and
steering handles as well as the Escapade windshield arch. 
These smaller molds are mounted together on a “unit frame”
which enables them to all be molded at the same time.  This
makes it convenient for the production of these very
necessary parts.

Polyethylene is also relatively inexpensive as boat building
materials go.  It is also light in weight, having a density
less than water itself.  This means that a polyethylene boat
in a catastrophic mode of being filled with water will still
float unless weighted down by something heavy.  This is
psychologically a big advantage to those of us concerned
about safety out on the water.

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