Injection Mold Design
Part 4: Injection Molding Curriculum
Injection Mold Design
Mold Design
Common terms used to describe the tool used to
produce injection molded parts. Injection Molds are
constructed from hardened steel, pre- hardened steel,
aluminum, beryllium-copper alloy.
In general, steel cost more to construct, but their
longer lifespan will offset the higher initial cost over a
higher number of parts made before wearing out.
Injection Molds / Tool can be manufactured either by
CNC machining or by using electrical discharge
machining processes.
Design
The consists of two primary components,
the injection (A plate) and the ejector (B
plate).
Design Consideration
Shrinkage allowance: Depends on shrinkage
property of material core and cavity size.
Cooling circuit: In order to reduce the cycle
time, water circulates through holes drilled in
both the core and cavity plates.
Ejection gap: The gap between the ejector
plate face and core back plate face should
hold dimension within the core. It must allow
component to be fully removed from the mold.
Design Consideration
Mold polishing: The core, cavity, runner and
sprue should have good surface finish and
should be polished along material flow
direction.
Mold filling: The gate should be placed such
that the component is filled from the thicker
section to thinner section.
Draft: Required in both the core and cavity for
easy ejection of the finished component.
Design Steps
2. Parting line
3.
Core Cavity
3. Number of Cavity
Ejector system
Cooling System
Mold Structure: Parting line
A dividing line between a cavity plate and a core
plate of a
mold.
- Make a parting line on a flat or simple-curved
surface so that flash cannot be generated.
- Venting gas or air.
Two plate mold
One parting line
Three plate mold
Two parting lines
Melt Delivery
Gate
Delivers the flow of molten plastics.
Quickly cools and solidifies to avoid backflow after
molten plastics has filled up in the cavity.
Easy cutting from a runner
Location is important to balance flow and orientation
and to avoid defects.
L = 0.5-0.75 mm
h(thickness) = n.t
W= n A1/2
30
Runner cross section
Runner cross section that minimizes liquid resistance
and temperature reduction when molten plastics flows
into the cavity.
Too big
Longer cooling time, more material, cost
Too small
short shot, sink mark, bad quality
Too long
pressure drop, waste, cooling
Hot runner, runnerless mold
Runner balancing
Balanced
Not balanced
Defects
Molding defects are caused by related and complicated
reasons as follows:
* Malfunctions of molding machine
* Inappropriate molding conditions
* Flaws in product and mold design
* Improper Selection of molding material
Weldline
This is a phenomenon where a thin line is created when different flows of
molten plastics in a mold cavity meet and remain undissolved. It is a
boundary between flows caused by incomplete dissolution of molten
plastics. It often develops around the far edge of the gate.
Cause
Low temperature of the mold causes incomplete dissolution of the molten
plastics.
Solution
Increase injection speed and raise the mold temperature. Lower the molten
plastics temperature and increase the injection pressure. Change the gate
position and the flow of molten plastics. Change the gate position to prevent
development of weldline.
Flashes
Flashes develop at the mold parting line or ejector pin installation point. It is a
phenomenon where molten polymer smears out and sticks to the gap.
Cause
Poor quality of the mold. The molten polymer has too low viscosity.
Injection pressure is too high, or clamping force is too weak.
Solution
Avoiding excessive difference in thickness is most effective.
Slow down the injection speed.
Apply well-balanced pressure to the mold to get consistent clamping force, or
increase the clamping force.
Enhance the surface quality of the parting lines, ejector pins and holes.
Short shot
This is the phenomenon where molten plastics does not fill the mold cavity
completely. and the portion of parts becomes incomplete shape.
Cause
The shot volume or injection pressure is not sufficient.
Injection speed is so slow that the molten plastics becomes solid before it
flows to the end of the mold.
Solution
Apply higher injection pressure. Install air vent or degassing device.
Change the shape of the mold or gate position for better flow of the
plastics.
Warpage
This deformation appears when the part is removed from the mold and
pressure is released.
Cause
Uneven shrinkage due to the mold temperature difference (surface
temperature difference at cavity and core), and the thickness difference
in the part. Injection pressure was too low and insufficient packing.
Solution
Take a longer cooling time and lower the ejection speed. Adjust the
ejector pin position or enlarge the draft angle. Examine the part
thickness or dimension. Balance cooling lines. Increase packing
pressure.
Sink marks
ts
ts < t
-Equal cooling from the surface
-Secondary flow
-Collapsed surface
Sink Mark
Considerations in design of injection molded parts
Guideline (3) gate location determines weld lines
weld lines
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm
Injection Molding: molds with moving cores and side-action cams
- If the geometry of the part has undercuts.
Mold Structure: Undercut, Slide core
Designing injection molds:
Typical Features
[source: www.idsa-mp.org]
Designing injection molds:
Typical Features
(a) Shut-off hole:
no side action required
(b) Latch:
no side action required
(c) Angled Latch:
Side action cam required
