Basic Mold Functions
Professor Joe Greene
CSU, CHICO
Basic Mold Function
• Shaping the Product
– Product Shape
• Ducting the Plastic from Machine Cavities
– Runners
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Venting the Plastic
Cooling the Plastic
Ejecting the Product
Economical Considerations
– Mold Type and Number of Cavities
Shaping the Product
• Slides versus Lifters
– Slides move in die direction to
slide a piece of tooling away from
part that is used for holes, slots, or
openings
– Lifters move in die direction to lift
the part and free an undercut or
“in-turn” flange
Shaping the Product
• Product Shape
– Plain utility shape
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Plastic parts for plain containers, lids, toys, tool handles
Generous tolerances and radii, design changes to mold easy to make
Simple gate and runner system with inexpensive materials
Concerns include very flat areas (top of lids), undercuts, finishing (texturing
or polishing or engraving)
– Artistic shape
• Simple designs are similar to utility shape except that design change
approvals are more difficult if it involves cosmetic appeal. Molds may be
more expensive to accommodate.
• Concerns include changes to “free-hand” artwork with no CNC but models
– Engineering (functional) shape
• Function of product is key with tight tolerances and accurate dimensions
• Parts include computer housing, cassette box, car lamps, fascia, fender
• Changes to part design (ribs or bosses) affects other quality areas, e.g.,
Ducting the Plastic from Machine
• Cold runners
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Cost is less than hot runner due to no controls, heaters
Shot volume is more since more material is injected
Regrinding is done to reuse runner material
Labor is more due to handling of runners for regrinding
Power is more since more resin is injected.
Note: Some runners on small parts are 80% of part wt.
• Examples
– Sprue gate: sprue feeds directly to part which is trimmed
– Cold runners used for small and medium sized parts in
two or more cavities.
• Small (short) as possible and small cross section otherwise get a
Ducting the Plastic from Machine
• Cold Runner Examples
– Two Plate molds-
• Runner is in the parting plane of the mold and is ejected
together with the products.
• Mold is simple and selected where mold cost is paramount.
• Typically edge, fan, or diaphragm gated requiring trimming.
• Tunnel or submarine gates are self-degating when mold opens.
– Three plate molds• Runner is in a different plane than the parting plane of the mold
and is ejected separately from the products as the molds opens.
• Gating is usually Outside Center Gated where gating is
provided anywhere into the product except into the rim or side
wall. Inherently self-degating. More expensive than 2-Plate
• Inside Center Gated (ISCG) are used for cup-shaped products.
Ducting the Plastic from Machine
• Hot Runner
– Cost is more than cold runner 2-Plate and about the same
as 3-plate mold
– Shot volume is less since all of the plastic is injected.
– Regrinding is not done since no runners exist.
– Labor is less since no gates are removed.
– Power is less due to no sprues, runners, or gates.
– Sensitivity to dirt is more since any dirt lodged in a gate
will stay there since it isn’t molded in the part. Valve
gates tend to be less sensitive to dirt.
– Start-up from cold usually takes 10 to 15 minutes
– Color changes are easy and fast
Ducting the Plastic from Machine
• Hot Runner Examples
– Hot sprue is gated directly into cavity of single cavity
– Multicavities are possible from 2 to 96 cavities.
– Insulated runner molds are a different type of hot runners
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Single cavity is similar to a hot sprue
Multicavity molds have a max of 12 to 16 cavities
Used with commodity plastics (PE, PP, and PS)
Disadvantage is difficult start-up procedure
Advantages are
– simple construction, low cost, and high efficiency
• Molds for PE do not have heaters as do PP and PS molds
• Caution in some hot runners may get dead spots and stagnant
plastic areas where resin can decompose and decontaminate
Removing Air from The Cavities
• Single Gate
– In a well-designed mold with tight parting-line the air in the mold
is trapped as plastic replaces it.
– As resin is injected, the sir can be compressed in pockets within
the cavity, causing the air to heat up rapidly and then burning the
plastic and leaving voids in part
– Need vents to relieve the air
• Natural locations: ejector pins, sleeves in the core
• Especially in deep ribs and recesses
• Uneven Walls
– Plastic flow faster along heavier section or rib than in a narrow
section and will double back toward gate, trapping air
– Vent with vent pins, ejector pins, or vent slots
• Multiple Gates
– Flow fronts can combine to form a weld line and trap air
Cooling the Plastic
• Mold is a Heat Exchanger
– Most of the energy from the plastic must be removed
• Plastic enters hopper at room temp, TR (25°C)
• It is heated to just below melt temperature, TM (150°C to
250°C), in the barrel and then sheared to temperature above TM
to the operating temperature, TO (TM+10°C).
• Plastic is injected into mold, cools to 25°C below TG where it is
ejected at TE
– Keys to a good mold is quality and efficiency of the cooling
layout. The more efficient the cooling layout the faster heat is
removed and the shorter the cycle time.
• Design choices for the cooling channels are the number, the
diameter, the mold material thermal conductivity, and the
location of the channels_ distance from surface and spacing
between channels.
Cooling the Plastic
• Measurements of efficiency
– Temperature difference, T, between incoming plastic and ejected
product
– Temperature difference between plastic and the cooling water
– Temperature difference between the IN water supply to mold and
OUT water return from the mold.
• If no rise in temperature in the cooling water upon exit, then the channel
diameter is too large or the flow is too slow (laminar flow).
• Typical rise is temperature is 3°C to 5°C rise in temperature.
TIN
Mold
TOUT
Ejecting the Product
• Key to ejecting the product is reliability
– Mold must function 100% of the time without getting a part stuck
• Cost of 1 part in 10,000 failing can be substantial
– Assume, 16-cavity mold with 10-second cycle time and 1 part per 10,000 fails
» 3,600 sec/hr ÷ 10 sec/shot = 360 shots/hour x 16 parts/shot= 5,760parts/hr
» Failure of 1part in 10,000 pieces where the machine stops. The it will stop
every 10,000pieces/5,760 parts/hr = 1.74 hours machine stops.
• Failure causes
– Catastrophic failure of mechanical components of machine or mold
– Maintenance problems with power, water, or air service to machine
– Breakage of under-sized (too small) ejector pins,
– Misalignment of ejector and stripper plates causes bending of pins or
wear of ejector pin holes.
– Blocked air circuits for vacuum or air system
– Even distribution of air flow with air actuated ejectors
– Insufficient stroke of mechanical ejectors
– Undercuts or insufficient mold stoke during setup
Economically/Suiting Requirements
• Mold Type and Number of Cavities
– Production requirements of the mold
• Concept mold (test mold) where 10 or 20 parts are required.
– Shape is important, but mold function, and surface finish are not. Short
lead time is important so shortcuts can be made in the construction by
not having cooling lines and automatic ejector system. Can be soft steel,
aluminum, epoxy molds or rapid prototype molds.
• Prototype molds where a couple of thousand are required.
– Shape is important, fast timing on mold is important, function should be
demonstrated with cooling lines and ejector systems
– Surface finish and cycle time can be relaxed as a requirement.
» Assume machine is used 90% of the time or 5,400 hours per year
» Would require at least three cavities, though 4 cavity would be better
Economically/Suiting Requirements
• Mold Type and Number of Cavities
• Production mold where 1 million parts are expected in life of mold
– Shape is important with tolerances on dimensions, function is critical to achieve
low cycle time with the use of cooling lines, ejector system, surface finish
– Example to find number of molds or cavities
» 4 million pieces required per year for 3 years at 10sec-cycle
» 3,600 sec/hr at 10 sec/part = 360 parts/hr
» 4 million parts at 360 part/hr = 11,110 hours
» Assume a year has 5,400 hours by 24 hrs/day x 5 days per week x 50
weeks per year = 6,000 hours per year
» Assume machine is used 90% of the time or 5,400 hours per year
» Would require at least three cavities, though 4 cavity would be better
Economically/Suiting Requirements
• Machine Hour Time
– Multicavity molds maximize the output of a mold and a machine
• Molds cost less than a machine, thus, choose as many cavities as possible as
the machine throughput (Use the 80% rule for machine capacity)
• Example,
– As the number of cavities increase, the incremental savings become smaller and
will slowly approach zero.
– With a 4-cavity tool at a 10-sec cycle, the mold produces: 1,440 pieces/hr
» 3,600 sec/hr ÷ 10 sec/shot x 4 pieces/shot = 1,440 pieces/hr
– Machine cost = $100 / hour ÷ 1,440 pieces/hr = $0.0694/piece
– With an eight cavity mold, 2,880 pieces/hr at $0.0347/piece
– With a 16-cavity mold, the piece cost would be $0.0174/part
• Advantage of Multicavity molds
– Reduces piece cost of parts, reduces time required to use molding machine,
requires fewer molding machines to be used
• Disadvantage of Multicavity molds
– Risk of having production on fewer number of molds, which might break-down
– Cost of 8-cavity is about twice the cost of a 4-cavity
Economically/Suiting Requirements
• Multicavity on big molding machine versus single cavity on
smaller sized (Table-top) molding machines
– Trade-offs
• Compare costs of a multicavity tooling on one big machine versus single or
dual cavity on smaller machines.
• Compare floor space, part costs, labor costs, etc…
• High Productivity within Short Time
– Same product as previous example is required as a promotional
item, for a “one-shot deal” and is needed ASAP.
• Required 4 million pieces in 6 months instead of 12 months
– Use two 4 cavity molds on two machines or get an 8-cavity mold
• Piece cost is most affected by part eight (50% to 70% of total costs)
– Thinning the wall is common to reduce part cost.
Molding Different Shapes
• Rule of thumb:
– Mold should be made for one type of product
– Different shapes have different molding characteristics
• Exception
– Some products, e.g., toys and some engineering products_ which
require the same or multiple quantities of a number of different
shapes of the same plastic and the same color, may be produced in
a family mold.
– Advantage: Economical
• Volumes aren’t big enough for a multi-cavity mold for 1 part, so several different
parts are combined in one mold using the same material.
• Molding cycle is same for all parts
• Parts can be assembled after molding
– Disadvantage
• All parts from mold must use same plastic, even if it is not the best material for some
of the parts.
• One cavity may produce bad parts and need repair causing downtime
Multiple Color or Material Molds
• Molds consist of 2 or more cavities and cores which are fed
from 2 or more injection units.
– Over-molding of one material over another
– One resin is injected into a cavity and core, then the mold is rotated
and the second resin is injected in the new cavity with an identical
core.
– Example,
• Toothbrush handles with 2 types of plastic on them
• Typewriter keys with the letters or numbers molded in the plastic.