Scientific injection molding

Scientific injection molding

Hot Runner (Runnerless) Molds

In the hot runner mold, the runners are kept hot in order to keep the molten

plastic in a fluid state at all times This is a “Runnerless” molding process and

hence the name Hot runner molds are similar to the three plate molds, except that the runner section of the mold is not opened during the cycle The heated runner plate (Manifold) is kept insulated from the rest of the relatively cooler mold

¢ No runner to separate from the molded parts

¢ No runners to either dispose of or regrind and reprocess

¢ Less possibility of contamination

¢ Hot drops carry consistent heat at processing temperature directly into the cavity

¢ Balanced Melt Flow

¢ Lower cycle (cooling) time — cooling time not runner dependent

¢ No robotics (or automation) needed for runner removal

¢ Possibly lower injection pressure

¢ No sprue/runner sticking problems

¢ Cleaner molding environment

rot Runner systems

750 Series — Complete System

Manifold Plate

Manifold Backing Plate

Plate Bolt

Insulating Wire Groove Air Gap

<7 Manifold Heater Ultra System Back Up

Insulator Pad Manifold

Sprue Bushing Center Insulator

Plate Cooling Guide Pin

**Patented

Injection Molding Tooling

Valve Gate Shut Off System

=

Injection Molding Tooling

Valve Gate Shut Off System

ed during the injection phase to avoid weld line formation The valve gates can be programmed to open

Figure 2.53 A series of mechanical, valve gates can be used to control the mold filling pattern for a mold cavity The valve gates can be opened after the melt flow front has crossed over the gate opening

and closed on demand, and can be sequenc

¢ Doubles the output from a single press

¢ Only 20 to 30 percent more clamping force required

¢ Three level and four level stack molds possible

¢ Additional shot capacity required

¢ May need more daylight

¢ Expensive

¢ High maintenance

¢ Technical expertise

Stack Mold

RWAY OLDS, Inc

PLASTIC MOLD SPECIALISTS

m High Performance DVD Package Stack mold

@ (2x4) Stack 7.7 second cycle w/layered cavities and air piston ejection

Machine Tie Bars Significant Reduction in production costs

Si on 7mm Dia Lid ; 500 Ton mc ;

Unit Dies (Master unit Die) MUD

Description:

This mold is engineered to have a "frame", which is permanently mounted in the machine, and

an interchangeable "insert" unit, which includes the A and B plates and the ejector mechanism

The operation of the mold is similar to an A series tool once the interchangeable insert unit is

secured in the frame

Attributes:

Similar to an A or B series mold, plate thickness are standard Typically used in short run (small

volume) production environments due to the ease of "mold" change

(+) Quick mold changes (improved production efficiencies)

(+) Versatile

(+) Easier maintenance (lower weight)

(+) Easier storage (less volume)

(+) Frequently used for short run production

(-) Expensive (Must buy frame and inserts)

(-) Runner typically dictates cycle time

(-) Two plate design only

Mold Metallurgy

Molders Demands Moldmakers Demands

Major categories of applications in molds

¢ Mold Cavity and Core unit components

¢ Mold base plates

¢ Special function components (Slides, gibs, wear plates)

Material selection considerations

¢ Type of plastics to be molded abrasive, corrosive etc 420 SS

¢ Number of parts to be molded (Alum, p-20, H-13, SS?)

¢ Surface finish of molded parts

¢ Cavity design requirements metal to metal contacts etc

¢ Method of cavity forming Machining requirements

¢ Method of heat treating

Applications of Mold steels

Type of Steel Typical Uses in Injection Molds

4130/4140 General mold base plates

P-20 High-grade mold base plates, hot-

runner manifolds, large cavities and cores, gibs, slides, interlocks

414 SS, 420 SS Best grade moki base plates (no plating

(prehardened) required), large cores, cavities and

inserts

P5, P6 Hobbed cavities

01 Gibs, slides, wear plates

06 Gibs, slides, wear plates, stripper

HN

H-13 Cavities, cores, inserts, ejector pins

and sleeves (nitnded) S7 Cavities, cores, inserts, stripper

rings

A2 Small inserts in high-wear areas

“Trade mark of Uddehoin Corp

Type of Stee!

A6 A10 D2

420 SS 440C SS

250, 350 455M SS M2

ASP 30°

Where Typical Mold Steels Are Used

Typical Uses in Injection Molds Cavities, cores, inserts for high-

wear areas

Excellent for high-wear areas, gibs,

interlocks, wedges

Cavities, cores, runner and gate in-

serts for abrasive plastics

Best all-around cavity, core and in- sert steel; best polishability

Small to medium-size cavities, cores,

inserts, stnpper rings Highest toughness for cavities, cores,

small unsupported inserts

High toughness for cavities, cores,

inserts

Small core pins, ejector pins, ejector

blades (up to % in diam)

Best high-strength steel for tall, un-

supported cores and core pins

CYCLE TIME CYCLE TIME CYCLE TIME

Comparison of mold materials

Thermal conductivity/ Hardness

Thermal conductivity (BTU/ft h °F)

Courtesy of Frosts Knivfabrik AB, Sweden

Faced with the limited thermal conductivity

of tool steel, this manufacturer of poly- propylene cutlery storage containers would have been required to incorporate complex

and costly cooling ducts in his mold

Instead, Ampco alloy inserts allowed

simple cooling channels to be adopted,

significantly reducing machining costs In

addition, cycle time was estimated at 25%

less than tool steel

Courtesy of Owo-Presswerk AG, Switzerland

Ampco alloy inserts are used in the mold cavity corners for 26-inch polystyrene television cabinets to obtain better cooling efficiency and easier ejection

Courtesy of Philips, Netherlands.

Runners control cooling time in small

Best to design runners as small as possible

Example:if the runner diameter is increased from 1⁄4

in to 5/16 in.; the percentage increase in material flow is 60 %

However, too small a runner require higher injection pressure and may cause surface defects on parts

Recommended

runner sizes

Runners

Full round runners are the most efficient for

minimizing heat loss and pressure drops

Trapezoidal runners are satisfactory when

dictated by design Half round runners are not

recommended The diameter of the runner for

various lengths of flow is shown in Figure IV

Secondary

Q

Primary Ỷ ) Cold Well D Sprue P ~ Ậ

L——~—— B†up, 1⁄2 Full Round —————4

Mold Center Line

Mold Balance

Not Recommended Suggested

Balanced runner design

Balanced runner design in a family

mold

Runners dimensioned by computer aided flow analysis

to balance pressure and flow

DFIM-40 Balanced Flow in a Family Mold

Melt Flipper Technology

VFS

This cutaway view shows the concept

of Beaumont’s solution, the Runner

Flipper The final product differs from

this particular design The Flipper

reorients the temperature and shear

distribution of the melt so that when it

splits at an intersection, equal

amounts of high- and low-viscosity

material are delivered to each cavity

These are the partially filled parts and runner after ejection from the mold It’s apparent that the melt fills the inside cavities before filling the outside cavities

Although the mold is four cavities, notice that the runner system

duplicates an eight-cavity design

CAN YOU SPOT THE DIFFERENCE?

HALF OF THIS MOLD USED BTI TECHNOLOGY, WHILE THE OTHER HALF DID NOT

WHICH SIDE WOULD YOU PREFER?

Types of Gates

oprue Gate used on large single cavity parts, cold slug issues Edge gate Large surfaces, thin wall, keep parts attached

Fan gate minimize surface imperfections, reduce stress

Sub gate (Tunnel gate) Automation

Diaphragm gate round part, avoid weld line

Flash gate similar to fan gate much wider, low warpage

Ring gate hollow tubular parts, helps with core shift

Tab gate stress free part and optical clarity acrylic lens

Sub gate into ejector pin no gate marks

96 PLASTIC PRODUCT DESIGN

| PIN POINT TAB | SUBMARINE FLARE

GATE OR CHISEL GATE SPRUE GATE

Figure 3-6 This illustrates the many types of gates used in the plastic industry

Gate Types

Gating Considerations

Land Length:0.040 max long land length creates

excessive pressure drop, part filling problem

Steel safe: Start small and increase as needed

Gate size: Larger the gate lower the stress

Gate placement: Cosmetic issues, Jetting

¢ To minimize jetting, splay and gate blush, the gates should

be located at right angles to the runner (Figure 1-10)

¢ Direct gating to a cosmetic surface may cause surface

result in part distortion

It is suggested that direct gates not be used in

shapes Fiber orientation may

GATE LOCATED RIGHT ANGLE TO THE RUNNER HELPS WITH MELT

HOMOGENIZATION, VENT OUT GASES THROUGH RUNNERAND

THEREBY BEST COSMETIC RESULTS

sprue Bushings & Sprues

Standard taper is 1⁄2 degree per foot

Orifice: generally not less than %4 in diameter

Must be 1/32 or 20% larger in diameter than machine nozzle orifice

Radius: % in radius most common, % in next size

Length: Never more than 4 inches

Extended nozzles are used where length of the sprue bushing exceeds 4 inches

Sprue outside diameter should be 25% larger than runner diameter

In many instances Sprue controls the cycle time

Who needs them?

Mold Venting

Why Vent?

Evacuation of latent air that is in the closed mold

e Allow evacuation of gases produced by low molecular weight polymers and

additives

Problems associated with poor venting

Weak weld lines

e Stress cracking in presence of chemical Internal bubbles and non-fill areas

High stress concentration

Sink marks

Longer cycle time

Mold deposit build-up

Down time

Proper venting starts with Part

Design

e Avoid part design with deep blind holes or deep thin ribs

e Avoid thick sections surrounding thin sections

Figure 1 Part Designs To Avoid

\

|

Blind Holes

“Delrin” acetal resin

“Zytel” nylon resin

“Minlon” engineering

thermoplatic resin 2 (0.05) 30 (0.75)

GRZ (glass-reinforced

“Zytel’ nylon) resin 2 (0.05) 30 (0.75)

“Rynite’”’ polyester resin 2 (0.05) 30 (0.75)

Ask material supplier

Land: As short as possible

Relief slot (vent channel):

Minimum 20 x depth

Amount: 30% of the perimeter of

the part

VENT THE RUNNER

YOU CAN'T HAVE TOO

300 Ton Machine : Machine Hour rate $/Hr 35.00

Current Cycle Time: 30 seconds

New Cycle time: 29 seconds

Cycle time savings: 2 seconds per minute

Cycle time savings: 2 Minutes per hour

Cycle time savings: (5000 hrs per year) 10,000 Minutes or 166 hours

Total $ amount savings: 166 x 35 = $ 5810

Total $ amount saved: (10 Machine shop) 58,100

Benjamin Franklin once said “‘Beware of little

expenses; a small leak can sink a great ship.”’

Mold cooling

Molding Cycle 80% is cooling time

Tool Design/Cooling CENERAL POLYMERS

i} How does cooling work ?

®There are three methods for exchanging heat

@The heat is then conducted through the mold to the water cooling channels

@There is a substantial amount of the heat that reaches the outside of the mold and is lost by radiation

Flow rate or Water 2

temperature? =

Reynolds Number = 3600 x GPM / Diameter x KV

GPM (water flow from hose to mold in gallons per minute)

Diameter of the waterline in inches

KV _ kinematic Viscosity of water at 78 F is 1.00

Reynolds number should be greater than 4000 to 5000

For Turbulent Flow

¢ Most plants do not have adequate water flow

Approximate flow rate needed

to produce turbulent flow™~ in drilled passages

Min flow rate for turbulent flow Pipe Size ID of drilled passage (gal/min)

Cooling Considerations

¢ The best cooling system in the world won't take

away heat any faster than the molded part will give it

up Most unfilled resins transfer heat at a rate 1/10 to 1/25 that of steel The outer walls of a thick part

insulate the mold from the heat trapped in the center

of the part

¢ The message here is that for very thick part, the

cooling system will have relatively little effect on

cycle time.

Cooling Considerations

Molding Cycle 80% is cooling time

Flow type Laminar or Turbulent

Flow rate GPM

Reynolds number of > than 5000 for

turbulent flow Thermal conductivity of mold steel

Plastic material’s Heat Content

Fig 10-27 Heat content of plastics

Optimizing Cycle Time

Table 3 Heat removal at different melt temps for HDPE

Overall productwity improvement 21.3%

: *

Cakeulato+e^« ba«eØ ca coos tron stated melt temp 0O 10T oart fœwvvoval terme

| siouinates CO ocr exc *S^ ! arntperd?t ha lxx^

source: Injection Molding Magazine article

Temperature

Revised de-molding tem

Typical de-moiding temp

Flow rate "ĐT =m

Minimum flow rate (GPM)

For good Reynolds Number (turbulent flow)

Minimum GPM = 3.5 x pipe I.D

¢ len %” lines in parallel

¢ All equal lengths into common manifolds

RECOMMENDED TURBULENT FLOW GUIDELINES FOR OPTIMAL MOLD TEMPERATURE CONTROL

Turbulent Flow Reference Chart

Approximate Minimal Flow (in gallons per minute) required for turbulent flow in drilled water passages based on Reynolds Number of 4000

Pipe Size Drilled Passage I D Flow Rate

Flow Rate Monitoring

Proper water management

Is the supply pressure adequate (50 psi min)?

ls the return pressure at least 40 psi less than the

supply? (10 psi)

NO-NO (oh! No!)