All About Plastic Injection Molding
Transcript of All About Plastic Injection Molding
-
8/9/2019 All About Plastic Injection Molding
1/33
Plastic Injection Molding
www.engmatl.com
-
8/9/2019 All About Plastic Injection Molding
2/33
Injection Molding
3 major functional units; injection, mold, clamping
-
8/9/2019 All About Plastic Injection Molding
3/33
Plastic Injection Molding
is a manufacturing technique for making parts fromthermoplastic and thermoset materials
In contrast to the extrusion (which makes continuousparts of constant cross section), injection molding make
discrete parts (with complex and variable cross section) Molten plastic is injected at high pressure into a mold,
which is the inverse of the desired shape.
The mold is made from metal, usually eithersteel oraluminium
widely used for manufacturing a variety of parts, from thesmallest component to entire body panels ofcars
-
8/9/2019 All About Plastic Injection Molding
4/33
The key to success in injection molding are to have;
Proper machine for good melting and injecting of theresin
The proper resin to appropriate part performance A good mold for part definition and removal
Proper operation for efficient molding cycle (mold cycledepends on the design of the mold and manufacturingparameters)
Plastic Injection Molding
-
8/9/2019 All About Plastic Injection Molding
5/33
The most commonly used thermoplastic
materials are: Polystyrene (low-cost, lacking the strength andlongevity of other materials)
ABS oracrylonitrile butadiene styrene (a co-polymeror mixture of compounds used for everything fromLego parts to electronics housings)
Nylon (chemically resistant, heat-resistant, tough andflexible - used for combs)
Polypropylene PP (tough and flexible - used forcontainers
PVC (more common in extrusions as used for pipes,window frames, or as the insulation on wiring where itis rendered flexible by the inclusion of a highproportion ofplasticiser).
-
8/9/2019 All About Plastic Injection Molding
6/33
Injection Unit
Purpose: to liquefy the plasticmaterials and then inject theliquid into mold
Resin is introduced throughhopper
Some machines can haveseveral hoppers (to fed filler,colorants, other additives)-Injection molding act as mixer
However, due to limited sizeof barrel, mixing capability ispoor
-
8/9/2019 All About Plastic Injection Molding
7/33
From hopper hole (feed throat)
Barrel made of heavy steel cylinder to
withstand the pressure and temperature
involved in melting the resin
2 types of system used in injection
molding;
Reciprocating screw- similar to extruder screwbut with unique reciprocating action
Ram injector
Injection Unit
-
8/9/2019 All About Plastic Injection Molding
8/33
Design of screw- similar to an extrusion screw 3 sections;
Feed section- to advance the resin
Compression section- to melt the resin
Metering section- to homogenize the resin and pump it forward
The screw of injection molding machine is shorter thanextruder, L/D ratios are 12:1 and 20:1
Low L/D ratios suggest the mixing is less efficient in theinjection molding machine
The compression ratio (diameter of root at feed zone to
the diameter of root at metering zone) often in the rangeof 2:1 and 5:1
Low compression ratio means less mechanical action isadded during melting process
Injection Unit
-
8/9/2019 All About Plastic Injection Molding
9/33
Important measure of the size of an
injection molding is weight of resin that
can be injected, called shot size
Typical shot size range from 20g to 20 kg
Since shot size depends on the density of
the plastic, PS has been chosed as the
standard for rating the machine
Injection Unit
-
8/9/2019 All About Plastic Injection Molding
10/33
Reciprocating Screw Injection
Molding Machine Resin is melt by mechanical shear and thermalenergy from heaters
The molten resin is conveyed to a space at the
end of the screw- collects in a pool Here, the mold is closed
-
8/9/2019 All About Plastic Injection Molding
11/33
The entire screw move forward and pushes themolten resin out through the end of barrel
To ensure the resin does not flow backward, a
check valve ornonreturn valve is attached to theend of screw
Normally the screw will stay in the forwardposition, until resin began to harden in the mold
Reciprocating Screw Injection
Molding Machine
-
8/9/2019 All About Plastic Injection Molding
12/33
Retraction of the screw, create space at the end of thescrew
Cooling of the part in the mold, until it can be removed
While the part is cooling, the screw turns and meltsadditional resin
Reciprocating Screw Injection
Molding Machine
-
8/9/2019 All About Plastic Injection Molding
13/33
Advantages
More uniform melting
Improved mixing or additives and dispersionthroughout the resin
Lower injection pressure
Fewer stresses in the part
Faster total cycle
Reciprocating Screw Injection
Molding Machine
-
8/9/2019 All About Plastic Injection Molding
14/33
In this type of injection molding, the resin is fed from a
hopper into the barrel, and heated through thermalenergy from the heaters
The molten resin is collect in a pool in a barrel celledinjection chamber
The molten resin is then push forward by the action of
plunger (ram or piston) To five better mixing, the molten resin is pushed past a
torpedo/spreader, impart shear to the melt
Ram Injection- Injection Molding Machine
-
8/9/2019 All About Plastic Injection Molding
15/33
Moulds
Designing and making mold for injectionmolding is more complicated than making
extrusion die
Mold Parts mold is placed in between
stationary plate and the moveable plate
-
8/9/2019 All About Plastic Injection Molding
16/33
The connection from the injection unit to themold is through the nozzle
The channel that run through the stationary plate
of the mold is called the sprue channel (materialthat is in the channel is called the sprue)
The solid sprue is removed from the finishedpart assembly after the part is ejected from the
mold Resin flow from the sprue through the runner
(connecting channel) to the mold cavities
Moulds
-
8/9/2019 All About Plastic Injection Molding
17/33
Mold Bases
Assembly of variousmold parts
Mold bases can be
purchased as entireunits, then the
cavities are cut from
A & B plates
-
8/9/2019 All About Plastic Injection Molding
18/33
Runners
Distribution system for the resin from the sprueto the cavities
Flow characteristics (viscosity), temperature and
other factors are important in determining therunner diameter and length
If the diameter of the runner is too small or thelength is too long,the resin can freeze in the
runner before the mold is completely full If the runner system is too large, excess material
would be ejected and too much regrind created
-
8/9/2019 All About Plastic Injection Molding
19/33
If the resins have a high viscosity, larger runnersare needed compared to low viscosity resin
The optimum flow of the resin through therunner system depends on the shape anddiameter of the channel
Round channel give the best flow characteristicsbut difficult to machine
Machining cost can be reduce by machining one
side of the mold plates Better shape where the depth of the channel is
at least two-thirds the size of the width and thesides are tapered between 2 to 5.
Runners
-
8/9/2019 All About Plastic Injection Molding
20/33
Runner Channel Shapes
-
8/9/2019 All About Plastic Injection Molding
21/33
Secondary Runners
Secondary runnerchannel are used formulticavity molds
The flow into thesecondary channelshould bestreamlined (angle in
flow direction) The streamlined
minimizes shear onthe resin
-
8/9/2019 All About Plastic Injection Molding
22/33
Gates
The end of runner and the entry path into
the cavity
The gate shape can also affect the fillingof the cavity, dimension and properties of
the parts
Gate is the most restricted point in
injection molding system, i.e. for
reinforcement and filler + polymer systems
-
8/9/2019 All About Plastic Injection Molding
23/33
Gate Design
Small rectangular opening atthe end of the runner channel,connect to the edge of cavity
Edge gate can be below theparting line if the channel andpart are also below the partingline
Or it can be symetricaly aboutthe parting line, if the runnerchannel and part are at bothside of parting line
Edge gate
-
8/9/2019 All About Plastic Injection Molding
24/33
Submarine Gate
Starts from the edge of therunner, and goes into the cavityedge at an angle
It narrows to a point as it movesfrom the runner to the cavity
The advantage; separation ofthe parts and the runner isautomatic
Disadvantage; gate cannot beused for some resins becauseof high shear
Submarine Gate
-
8/9/2019 All About Plastic Injection Molding
25/33
Tab Gate
By connecting the runner directly
into the cavity with no reduction
in runner cross-section
Used for very large parts where a
reduction in flow would disturb
the resins flow pattern and might
result inadequate flow into the
cavity
-
8/9/2019 All About Plastic Injection Molding
26/33
Fan Gate
Made by reducing thethickness and not thediameter of the runner
channel as it goes into thecavity
Used for intermediate size,and when reinforcement inthe resin cannot flow throughthe edge gate
-
8/9/2019 All About Plastic Injection Molding
27/33
Ring Gate
Used to make hollow
cylinder parts
The ring gate covers theentire top of the cylinder
part so that the resin flow
is downward into the wall
of the part
-
8/9/2019 All About Plastic Injection Molding
28/33
Cavities
Are actual molding locations
Resin enter the cavities through gate, fills
the cavities, and cools to form the solid.The parts are ejected and finished
Cavities are the heart of the molding
process, and must be precisely prepared
The shape of the cavities determines the
shape of the part
-
8/9/2019 All About Plastic Injection Molding
29/33
Materials & Product Consideration
Almost all thermoplastic can be injectionmolded
Resin with low melt viscosity is required;
so that the flow through runner, gate,cavity easily done with minimuminjection pressure
Resin with injection molding grades havelow molecular weight and narrowmolecular weight distribution
-
8/9/2019 All About Plastic Injection Molding
30/33
Part Shapes
Hollow parts can be created by allowing
the moveable plate to protrude into the
cavity of the stationary plate
-
8/9/2019 All About Plastic Injection Molding
31/33
Threads can be placed on the inside of a
part by using a core pin that is inserted
into the cavity where the threads are
desired
Part Shapes
-
8/9/2019 All About Plastic Injection Molding
32/33
A hollow part with a hole on the side is evenmore complicated (the core pin is used)
The core pin slide into position after the mold isclosed
The core pin seals against the surface of themoveable plate, prevent flow of resin into thearea
Part Shapes
-
8/9/2019 All About Plastic Injection Molding
33/33
Authors request: Please add more useful
information & re-upload the file.
THANKS