Medical Device Injection Mold Components Thermoformer For Machine
Molding Cycle Time
The sequence of events during the manufacturing is called the
cycle. It begins when the mold closes and the polymer is injected
into the mold cavity. When the cavity fills, the mold maintains a
holding pressure to account for material shrinkage.
Then, the screw turns, and the next shot moves to the front of the
screw. This causes the screw to react, and, once cooled, the mold
opens and the plastic part removed.
A formula can be used to determine the cycle time of injection
molding. The time it takes to make a part using injection molding
is calculated as:
Total time = 2M + T + C + E
(2M) = Twice the Mold Open/Close Time
(T) = Injection Time (S/F)
(C) = Cooling Time
(E) = Ejection Time (E)
(S) = Mold Size (in3)
(F) = Flow Rate (in3/min)
The mold closing and ejection times of injection molded parts can
last from less than a second to a few minutes, depending on the
size of the mold and machine. The cooling time, which dominates the
process, depends on the maximum thickness of the part.
About Injection Molding
Injection molding is the most common modern method of manufacturing plastic parts.
It is used to create a variety of parts with different shapes and
sizes, and it is ideal for producing high volumes of the same
plastic part. Injection molding is widely used for manufacturing a
variety of parts, from the smallest medical device component to
entire body panels of cars. A manufacturing process for producing
plastic parts from both thermoplastic and thermosetting materials,
injection molding can create parts with complex geometries that
many other processes cannot.
The first step of getting a plastic part injection molded is to
have a computer-aided design (CAD) model of the part produced by a
design engineer. The three-dimensional (3D) CAD model then goes to
an injection molding company where a mold maker (or toolmaker) will
make the mold (tool) that will be fitted into an injection molding
machine to make the parts.
Molds are precision-machined usually from steel or aluminum, and
can become quite complex depending on the design of the part.
Plastic materials shrink at different rates when they cool, so the
mold has to be constructed with consideration for the shrinkage
rate of the material being used for the parts. In other words, a
formula is applied in the construction of the mold to slightly
increase the size so that when the plastic shrinkage occurs, the
part will be to the dimensional specifications of the CAD model.
Plastic injection molding is a manufacturing process where resin in a barrel is heated to a
molten state, then shot into a mold to form a part in the shape of
the mold. The resin begins as plastic pellets, which are gravity
fed into the injection molding machine through a funnel-shaped
hopper. The pellets are fed from the hopper into a heated chamber
called the barrel where they are melted, compressed, and injected
into the mold’s runner system by a reciprocating screw.
As the granules are slowly moved forward by a screw-type plunger,
the melted plastic is forced through a nozzle that seats against
the mold sprue bushing, allowing it to enter the mold cavity
through a gate and runner system. The injection molded part remains
at a set temperature so the plastic can solidify almost as soon as
the mold is filled.
The part cools and hardens to the shape of the mold cavity. Then
the two halves of the mold (cavity or “A” side and core or “B”
side) open up and ejector pins push the part out of the mold where
it falls into a bin. Then the mold halves close back together and
the process begins again for the next part.
Plastic Injection Molds
Steel injection mold
Injection molds, or mold tooling, are the formed halves that come
together in the injection molding machine to be filled with molten
plastic and produce the plastic parts in their image. The cavity
side, or “A” side, is typically the half which will form the “best”
surface of the part, and the core side, or “B” side, will typically
show the visual imperfections caused by ejector pins because the
parts get ejected from this half.
Injection molds are manufactured by machining or by Electrical
Discharge Machining (EDM). Standard machining was the traditional
method of building injection molds with a knee mill. Technology
advanced the process, and Computer Numerical Control (CNC)
machining became the predominant method of making complex molds,
with more accurate details, and in less time than the traditional
EDM is a process in which a shaped, copper or graphite electrode is
slowly lowered onto the mold surface, which is immersed in paraffin
oil. Electric voltage applied between the tool and the mold causes
spark erosion of the mold surface in the inverse shape of the
electrode. EDM has become widely used in mold making - many
injection mold companies now have EDM in-house. The process allows
the formation of molds which are difficult to machine, such as
those with features such as ribs or square corners. It allows
pre-hardened steel molds to be shaped without requiring heat
Compared to other plastic manufacturing processes such as CNC
machining or 3D printing, injection molding has a high up-front
investment because the tooling is expensive. However, for large
production runs of thousands or even millions of identical parts,
injection molding is typically less expensive in the long run,
despite the high initial tooling investment, because of a lower
piece price at high volumes. In addition, it is a much faster
manufacturing process than the others mentioned.
Molds can be made of pre-hardened steel, steel that is hardened
after the mold is produced, aluminum, and/or beryllium-copper
alloy. The choice of mold material is determined in part by the
number of parts to be produced.
Multiple-Cavity Molds (Family Molds)
Family molded parts
In addition to a single plastic part being produced in a molding
cycle, the mold can also be designed to produce multiple numbers of the same part in a single shot. A tool with one impression is often called a single impression
(cavity) mold, whereas a custom injection mold with two or more
cavities of the same part is referred to as a multiple impression
(cavity) mold. The number of impressions in the mold is often
incorrectly referred to as cavitation. Some extremely high-volume
molds – like those for bottle caps – can have over 128 cavities.
A multiple impression (cavity) mold may also be referred to as a
“family” mold. However, a family mold is more accurately defined as
one which can produce multiple, like-sized parts in the same
quantity, color and material. Family molded parts are often part of
an assembly, such as a mating top half and bottom half.
Example of Overmolding
Overmolding is a plastic injection molding process which is very useful for
producing multi-material parts with some unique properties. For
instance, this process can be used to add a second part, of a
different material, for a handle or grip. Picture a power tool with
a grip made out of a softer material than the body of the tool.
Similarly, a game controller can be manufactured with different
textures of plastics on the body of it.
Basically, a previously injection molded part of one material (the
substrate) is re-inserted into an injection molding machine, and a
different material (the overmold) is injected to form a new layer
over the first part.
Some specialized injection molding machines have two or more
injection units that can “overmold” in a single molding cycle. For
example, “two-shot,” or “multi-shot” injection molds first mold a
basic shape in a base color of plastic material, then the second
material, of a different color, is injected into the remaining open
spaces to produce a one-piece, multi-color plastic part.
Overmolding is not as straightforward as injection molding a part
out of a single material, and it is not without its limitations.
One consideration is that the two materials must be compatible,
chemically and thermally. Since plastic materials have different
melt temperatures, the substrate material must have a higher melt
temperature than the overmolding material, otherwise the original
part would melt and deform when the overmolding material is
Metal, ceramic or plastic pieces can be inserted into the molten
thermoplastic to form multi-material, robust parts with additional
functionality. For example, threaded metal inserts can be molded
into the parts to allow them to be attached to other parts.
Insert molding can reduce cost by embedding secondary parts into
the plastic injection molding process, as opposed to installing the
parts after molding. By integrating the inserts at the time of
molding, the parts become more robust compared to staking the
pieces in post-molding. Of course, the insert pieces must be able
to withstand the high temperature and pressure of the injection
Insert molding is naturally a more complex process that standard
injection molding, so some injection molding companies are more
experienced in the process than others. For low-volume production
runs, a machine operator may load the inserts into the mold by
hand, prior to the plastic injection cycle. For high-volume
production runs, however, it is common to use automated machinery
to place the inserts into the mold.
Optimizing the injection molding process is essential because it
affects cost, quality, and productivity. Some of various
optimization checks include:
• Optimize the holding time by conducting gate seal or gate freeze
• Conduct a cooling time study to optimize the cooling time for an
injection molded part
• Pressure drop studies determine if the machine has enough
pressure to move the screw at the set rate
• Perform viscosity curves to determine injection speeds
• Vary the melt temperatures and holding pressures to optimize the
When an injection molding job is being set up for the first time
and the shot size for that mold is unknown, a molding trial will be
conducted to get everything “dialed in.” The mold technician will
usually start with a small shot weight and fill the mold gradually
until it is 95 to 99% full. Then a small amount of holding pressure
is applied, and the holding time is increased until gate freeze off
(solidification time) occurs on the injection molded part. Gate
solidification is important because it determines cycle time, and
cycle time is a crucial determinant in the efficiency, and
therefore the economics, of the production process. If the parts
have sink marks, the holding pressure will be increased until they
are minimized and the part weight is achieved. Once the settings
are settled in and the injection molding machine is making good
parts, a setup sheet is produced for standardizing the process for
future production runs.