Mold Design

LECTURE PRESENTATION

Reading Assignment

Recommended Reading

Design Principles

Cooling Channels

Extrusion Dies

Gate Design

Overmolding

See Also: Forming and Molding Plastics

Outline

The Injection Molding Process

Injection molding is the most widely used method of producing parts out of
thermoplastic material. The molten thermoplastic material is injected into the
mold at high pressure. Once this material cools and solidifies, the mold opens
and the part is ejected. During the injection molding cycle, the mold serves
several purposes, including:

• Determining the finished shape of the part
• Venting trapped air or gas during injection
• Acting as a heat exchanger to draw heat from the part to aid in
solidification
• Providing a means of ejecting the part from the mold

Mold Components

All molds contain a number of common design features. These features include:
• Mold base
• Mold cavity
• Mold core
• Sprue bushing
• Runner system
• Gates
• Vents
• Cooling system
• Ejector system, plus other components
The mold base is an arrangement of steel blocks manufactured to specific
dimensions. Mold bases may be purchased from commercial mold base manufacturers
or produced in-house by moldmakers.

The basic mold base consists of two halves. The ‘A’ half, which is also referred
to as the stationary half, or the injection half, and the ‘B’ half, which is
also referred to as the moving half, or the ejector half. The mold cavity which
creates the outer image or surface of the part is usually mounted on the ‘A’
half of the mold, while the mold core which reproduces the inner image of the
part is typically mounted on the ‘B’ half of the mold. Collectively the cavity
and core halves are known as the ‘cavity set’.

Mold bases, cavity and cores are commonly made from special mold steels or from
other materials such as beryllium copper, stainless steel, aluminum, brass, kirksite, and epoxy. The softer mold materials are generally used for prototype
molds and limited production runs.

All plastics have their own shrink factor, meaning they shrink at a certain rate
as they cool and solidify. Depending on the type of material to be injection
molded, moldmakers must take it’s shrink factor into consideration when
producing the cavity set. For example, if a material shrinkage is calculated to
be one-hundredth of an inch for a part six inches in length, a total of sixhundredths
of an inch must be added to the mold design to compensate for
shrinkage.

Additionally, draft angles or tapers are machined into the side walls of the
cavity set to facilitate part removal from the mold. These tapers typically
range from 1° to 2° per side. Once completed, cavity sets may be heat treated to
protect them from the harsh injection molding environment. Molds may also be
coated or plated with wear resistant surfacing material, such as nickel and hard
chrome.

The interfacing plane between mold halves is called the parting line. Depending
on the complexity of the part, there may be several such parting lines. Proper
alignment of the mold halves is accomplished by using leader pins and bushings.
The mold halves are mounted on platens which are components of the injection
machine. Most injection machines have three platens:

• The stationary platen, which holds the ‘A’ half of the mold.
• The movable platen, which holds the ‘B’ half of the mold and moves back
and forth on the injection machine’s four tie bars.
• The rear stationary platen, which holds the other end of the tie bars,
thus anchoring the entire system.

A locating ring on the mold centers to a hole on the stationary platen. This
then allows the nozzle of the heating cylinder to seat firmly against the sprue
bushing on the ‘A’ half of the mold. The sprue bushing directs the molten
material from the heating cylinder nozzle into the mold’s runner system.
A mold’s runner system is the channel or network of channels through which the
material flows to reach the cavity set. Surface runners are the most common
runner design, and are half-round channels machined into the surfaces of each of
the mold halves.

Once the molten thermoplastic flows through the runner system it reaches the
cavity set through an interface called the gate. The mold gate restricts and
controls the flow of plastic into the mold. Passage through the gate causes a
frictional rise in material temperature, extending the materials flow into the
mold.

Common types of gates include:

• The edge gate, which is usually located on the parting line, and is the
most common gate type.
• The submarine gate, which brings material under the parting line to fill
the cavity from below.
• The tab gate, which redirects material flow into the mold.
• The ring gate, which is used in molding round or cylindrical parts.
• The fan gate, which is used to spread material quickly over a large area.

To remove trapped air and process gases during injection, a mold venting system
is needed. The number and size of the vents are determined by part geometry,
material type, viscosity, and the rate of injection. These vents are ground on
the parting line of the mold.

The hot thermoplastic remains in the mold under pressure until it cools. This
cooling is usually achieved by water circulating in channels machined into the
mold. Proper cooling contributes to controlled part shrinkage, part strength and
quality. Overall, the speed of the injection molding cycle is controlled by the
efficiency of the cooling system.

Once the parts are sufficiently cooled and solidified, the mold opens and an
ejector system, usually in the form of knockout pins, is used to aid in part
ejection. Ejector systems are mounted on the ejection side of the mold and are
typically activated by pneumatic or hydraulic cylinders. In addition to knockout
pins, other ejector methods include stripper plates, stripper rings, and air
pressure ejection. Sometimes a sprue puller is used to remove molded plastic
from the sprue bushing as the part is ejected.

Component Design Guidelines

  • Dimensional Stability Issues
  • Shrinkage
  • Wall Thickness
  • Warpage
  • Ribs and Profiled Structures
  • Gussets and Support Ribs
  • Bosses
  • Holes
  • Radii & Corners
  • Tolerances
  • Coring
  • Undercuts
  • Draft Angle

Dimensional Stability Issues

Thermoplastics can be amorphous, semi-crystalline or liquid crystalline polymers.

Amorphous polymers become rubbery, then glassy as they solidify.  They have a wide softening range with no distinct melting temperature.

  • Moderate heat resistance
  • Good impact resistance
  • Low shrinkage

Semi-crystalline thermoplastics

Shrinkage

Wall Thickness

Warpage

Ribs and Profiled Structures

Gussets and Support Ribs

Bosses

Holes

Radii & Corners

Tolerances

Coring

Undercuts

Draft Angle

Mold Design Guidelines

  • Mold Machine
  • Mold Construction
  • Multi-Cavity Molds
  • Cold Runner Systems
  • Hot Manifold/Runnerless Molds
  • Gate Design
  • Mold Cooling
  • Ejection System
  • Tooling Material Selection
  • Surface Finish
  • Venting

 

Injection Mold Types

Typical injection molds designed to meet specific production requirements
include:

• The cold-runner two-plate mold, which consists of two plates with the
cavity and core mounted in them. The sprue, runners, and gates, along with
the part are molded simultaneously, and then separated after ejection.

• The cold-runner three-plate mold, which includes a stripper plate that
automatically separates the sprue, runners and gates from the parts during
ejection.

• The hot-runner mold, which uses an electrically heated manifold that
maintains material temperature in the runners at the same level of the
material in the injection cylinder. The runner system is contained in a
plate of its own and does not open during ejection of the part. This is
also known as ‘runnerless’ molding and can decrease cycle time by 25
percent or more.

• The insulated-runner mold, which uses large diameter runners and no
heaters. During injection the outer layers of plastic in the runner
solidify and insulate the inner material, keeping it at molding
temperatures.

Mold Maintenance and Storage

Over time and repetitive use, mold components and surfaces will degrade. The use
of inserts and laminated construction for mold surfaces subjected to high wear
is recommended. Rust is also a major factor. Cleaning and lubrication are
critical measures between manufacturing cycles and for short-term and long-term
storage.

 

(SME FMP Video Notes)

 

Dealing with Undercuts

Collapsable Cores

Undercut Release Lifters

 

Gate Types

Manually trimmed gates
Sprue gate
Edge gate
Tab gate
Overlap gate
Fan gate
Film gate
Diaphragm gate
External ring
Spoke or multipoint gate

Automatically trimmed gates
Pin gate
Submarine (tunnel) gates

Hot runner gates
Valve gates

straightpull.gif

The A-Side is fixed, the B-Side does the clamping.  Remember, the plastic will shrink when it cools.  You want it to stick to the B-Side so it will pull out and be ejected off with the ejection pins.

Recommended wall thicknesses by resin type

Resin INCHES
ABS 0.045 – 0.140
Acetal 0.030 – 0.120
Acrylic 0.025 – 0.500
Liquid crystal polymer 0.030 – 0.120
Long-fiber reinforced plastics 0.075 – 1.000
Nylon 0.030 – 0.115
Polycarbonate 0.040 – 0.150
Polyester 0.025 – 0.125
Polyethylene 0.030 – 0.200
Polyphenylene sulfide 0.020 – 0.180
Polypropylene 0.025 – 0.150
Polystyrene 0.035 – 0.150
Polyurethane 0.080 – 0.750

 

Design Guide: Performance and Value with Engineering Plastics (Includes mold design)

CAD Models from Mold Hotrunner Solutions – (link)

CAD Models from Progressive Components (link)

Mold Wizard tutorial – (link)

Moldmaking Technology Magazine (link)

Goldsberry, C. Speeding up Mold Design and Manufacturing. Injection Molding Magazine, November 2009 pp. 22-24

Moldmaking Technology Magazine

EOS Selective Laser Sintering

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