1 - basm injection molding & advanced process control spe automotive tpo global conference 2009

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Injection Molding & Advanced Process Control

Injection Molding & Advanced Process Control

SPE Automotive TPO Global Conference 2009

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Polymer Basics

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Material selection is part of the design process and can directly cause failure

A key element in a successful plastic part program is the selection of theappropriate material for the application. The process of material selection is complicated by the fact that there are literallythousands of polymers, blends, alloys, and compounds available. The selection process can be simplified by comprehending: The performance requirements of the part How materials are classified Testing methods

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Transfer System

Silo Silo

Esterifier 1°

Esterifier 2°

High Polymerizer

Low Polymerizer

Pump

Manifold Extrusion

Polymer:

From the Greek Poly meaning many, and Meros (mer), meaning part(s)

Chemical Composition: Example

Polyethylene: Crack crude oil to ethylene then polymerized (linked together) to form 500-1,000,000 ethylene units

POLYMERIZATION PROCESS

Feedstock:

Crude Oil

Natural Gas

Agriculture

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Polymer:

From the Greek Poly meaning many, and Meros (mer), meaning part(s)

Chemical Composition: Example Polyethylene: Crack crude oil to ethylene then polymerized (linked

together) to form 500-1,000,000 ethylene units

Key Points:

Polymers can have the same carbon backbone but drastically different properties

Polymers with Oxygen (O), Nitrogen (N), Chlorine (CI), Bromine (Br), or Fluorine (F) probably have to be dried

Time and space do not allow us to review each polymer composition and its properties. An excellent source for basic polymer descriptions can be found in the first pages of the yearly Modern Plastics Encyclopedia.

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Chain Lengths At room temperature

C1 = Methane, a gas C6 = Hexane, a liquid C17 = Kerosene C50 = Wax C500-1,000,000 = Polyethylene

For strength

Long molecules are preferred

For processability

Short molecules are preferred Easy flow Low viscosity Transmits pressure more hydrostatically

Long molecules Difficult to flow High viscosity

Viscosity changes dramatically with changes in flow rate Dynamic: flow rate during filling Static: flow rate during pack and hold Large static pressure losses along the flow path

Resin manufacturing must compromise/balance chain length vs. strength vs. flow.

Most polymers that find commercial use as plastics or rubbers have molecular weights between 10,000 and 1,000,000.

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Molecular Weight (Mw):

Molecular Weight = (Molecular Weight of Repeat Unit x Number of Units)

In practice, when polymers are made, chains or the number of units linked together varies

Within the same polymer family molecular weight is a measure of molecular length:

High Molecular Weight = Long Molecules = Stronger

Low Molecular Weight = Short Molecules = Weaker

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Molecular Length

Polymers are mixtures of various length molecules Not all chains are the same length Molecular weight distribution

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Batch to Batch Variations Cause Changes in Viscosity

Effects of Molecular Weight Generally as molecular weight increases, physical properties increase

at the expense of processing ease Short chains - easy flow Long chains - difficult to flow

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Polymer chains are strong Aramide fibers (flack jackets, tires)

Chains can be broken by improper processing Too much heat Too much time at elevated temperatures (drying and melt) Too much shear force Presence of a chain-breaking chemical (e.g., water)

Polyester Polycarbonate Nylon

Proper drying is essential for these materials

Polymer Chain Degradation

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Degradation

Improper processing can degrade polymer chains and provide poor part performance

Chain degradation will provide a regrind that is easier flowing (lower viscosity)

Shorter molecules = poor strength

How can this be detected?

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Polymers Plastics

Plastics = Polymer + Additives

Additives:

Impact modifiers (rubber) Fillers Nucleating agents Flame retardants Mold release agents Antioxidants

Wetting agents

Plasticizers Fiber reinforcements Heat stabilizers Lubricants Antistatic agents Coloring agents Slip aids

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Additive Degradation:

Additive chain size is small relative to the polymer May become volatile during processing which can

result in: Accumulation in vents, mold surfaces, etc. Change in physical properties (often an

increase) Stiffer flow (higher viscosity plastic)

Change in Additive Content is a Prime Difference Between Virgin and Early Generations of Regrind.

Material is going to vary folks – that's no excuse!

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Polymer Morphology

Two families of thermoplastics

Amorphous Semi-crystalline

Random Structure Broad Melting Point Often Solvent Sensitive Poor Fatigue/wear Low Shrink No Molecular Cuddling More Forgiving on dimensional

Control

Ordered Structure Sharp Melting Point Solvent Resistant Excellent Fatigue Wear High Shrink High Molecular Cuddling Problematic on

dimensional Control

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Cooling and Crystallinity Extremely important for semi-crystalline materials Crystals are non-existent above melting temperature Low mold temp, faster cooling, smaller/less crystals, less shrinkage, but may

not be suitable for application High mold temp, slower cooling, larger/more crystals, more shrinkage Mold and Melt temperatures are critical to maintain dimensional tolerances

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Decoupled MoldingSM Strategies

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Train1.JPG

Train2 a fill pack only.jpg

Train2.jpg

Helping Injection Molders

Succeed

Injection Molding Approaches

Traditional Molding: The original method of molding

Fill and pack on first stage

Use only enough first stage pressure to pack the mold

DECOUPLED MOLDINGSM: Use abundant first stage pressure

Separate the filling or velocity phase of molding from the packing or pressurization phase

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Train2 a fill pack only.jpg

Definition:A process control method that addresses how the machine controls are used to fill and pack plastic into the mold.

Objectives: Injection speed reacts only to the machine’s velocity setting(s)

Constant and repeatable injection speed shot to shot, year to year, regardless of any effective viscosity changes

Reduce part variation due to effective viscosity variations

Gain the ability to fill fast to minimize effective viscosity variations

Make the process capable of always making good parts

Reduce cycle time due to increased fill rates

DECOUPLED MOLDINGSM

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Plastic Temperature

Screw Configuration

Barrel & Nozzle Heats

Screw R.P.M.

Back Pressure

Feed Throat Condition

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Barrel 1a 001

The Screw

Inside the barrel is a screw.

Most injection molding screws are divided into three zones each with its own task: Move plastic - Feed Zone (conveying) Melt plastic - Compression Zone (transition) Mix plastic - Metering Zone

The difference between the zones is in the channel depth which creates a space for plastic.

The channel depth on the feed end of the screw is larger than the depth of the metering end. The ratio of depth difference is called Compression Ratio.

Example - 3:1 Compression ratio screw means the channel depth in the feed zone is three times larger than the channel depth in the metering zone.

When the screw is rotated, drawing plastic into the barrel, it is conveyed forward and compressed. This action converts screw work into shear heat.

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Compression Ratio

The proper compression ratio screw for the application is a key component for proper plastic melting and mixing.

Low - 1.5 to 2.5:1 - Shear sensitive materials (PVC)

Medium - 2.5 to 3.0:1 - General purpose

High - 3.0 to 5.0:1 - Crystalline materials (Nylon)

How do you know if you have the correct compression ratio for your job?

What other features of a screw are important?

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Screws come in different lengths and diameters. The flight length in relationship to the outside diameter of the flights is called the L/D. A high L/D indicates a long screw relative to its diameter; therefore, the plastic will have a greater

distance to travel from the feed port to the nozzle

The time to travel this distance is called the residence time

Conversely a low L/D is a short screw which will provide faster throughput and a lower residence time

The profile of a screw describes the number of turns in each zone from the feed zone through the metering zone.

Example: 10-5-5 (20/1 L/D screw) = 10 turns feed, 5 turns transition, 5 turns metering

The exact screw type and configuration needed depends on the application; however, generally speaking:

Amorphous resins need long transition sections Crystalline resins need long feed sections

What is a good guide for residence time?

How can we know what it is?

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Mixing Screw.jpg

Mixers

Mixing Screw

Breaks up tornado-like flow of melted plastic in the screw flights Is placed in the metering zone to mix melted plastic Must not cause high localized shear Provides more uniform melt temperature Provides more uniform color dispersion Faster purging / quicker color change

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Screw RPM determines the maximum shear rate of the plastic

SCREW RPM: Should be as slow as possible withoutincreasing cycle time.

Reduces maximum and overall shear rate

Shears material more uniformly

Reduces fiber breakage in filled material

Provides more melt homogeneity

Think circumferential speed when moving to another machine

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Injection cylinder works as a pump pushing oil back to tank Changes the amount of heat added by shear Changes temperature of the melt quickly (temporary) The change will not be totally sustained unless zone temperatures are

also changed Changes the amount of shear mixing or homogenization May increase fiber breakage in fiber-filled materials

Back Pressure

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Feed throat temperature effects melting and moisture splay

Must be cold enough to prevent clumping and hot enough for dew not to form (105 °F or 35 °C minimum)

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1. Set all zones at the desired melt temperature

2. Run the machine on cycle

3. Take the melt temperature

4. Adjust the center and rear zones to achieve desired melt temperature

5. This may not be possible on machines with an incorrect screw for the material being molded

6. Set the rear zone to make the plastic stick to the barrel and to obtain faster screw recovery (temperature at which minimum screw recovery time is achieved with a constant RPM)

If screw recovery problems exist, do step 6

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plasticbarrel1 - 2004.jpg

Plastic Flow

Injection Speed and Distance

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plastic flow rate key - 2004.jpg

During filling two things must be available:

Pressure in abundance (Ri)

Flow sufficient to achieve the desired flow rate (pumps or accumulators)

On electric machines available plastic pressure and injection rate are analogous to R

i and flow rate

Force=Pressure a 001.jpg

Force can be intensified in a hydraulic system

The determining factor for force intensification is the square inch area on which the hydraulic pressure is applied

If the piston in a cylinder has more area than the area where the force is being applied then the output force will be greater than the input force

Pressure increases inversely proportional to the area ratios.

For example, if two pistons of different size are connected by a rod, the pressure existing on the smaller area will always be greater This principle also applies to the cap side and the rod side of a normal double

acting piston

psip?

A1

= ?A2

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NonNewtonian.JPG

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THE RUNNERThe main purpose of the runner is to distribute the plastic to all the cavities in such a way as to fill all the cavities at the same shear rate and direction, at the same temperature, and at the same pressure gradient.

Better Better

Best

Two are almost as good

Good

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Hot Runner Manifold System - 2 Drops

The runner can be kept liquefied through the use of a hot runner manifold.

The mold a1 - 001.jpg

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If the mold does not treat the plastic material the same in each cavity. This fact may render the process uncontrollable.

This is why no mold can be more controllable than a single cavity mold.

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How can we complete the fill of these two glasses at the same rate (time)? What must be adjusted?

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Heated systems require the use of a valve gate to seal the gate.Of critical importance is the number of gates and their location.

What is the primary reason for using a valve gate?

C:\Documents and Settings\rod\Desktop\Phillips-Orbis-Nissei Tech Day 2007\Gear_Gizmo.avi

Complex 8-Cavity Independent

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Always remember that the weld lines are the weakest area of a plastic part

Varies with material

Difference in strength is greater in fiber-filled material

Always determines the strength of screw bosses and depressions where knit lines are present

Improper venting enhances the effect

Weld line strength varies with how the part is packed

Most part failures occur at the weld lines

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Parts psi a 002

16,490 psip

9,628 psip

8,345 psip

5,245 psip

Dynamic Pressure Loss

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DECOUPLED MOLDINGSM

Decouple injection speed from first stage hydraulic pressure. Make sure abundant hydraulic pressure is available to the injection cylinder - at least 10% more than the peak

hydraulic pressure observed at the injection cylinder gauge Use the speed control to exclusively control injection speed

Any adjustment of fill speed should change the fill time The peak hydraulic pressure must not reach the maximum setting

Separate filling the cavity from packing and/or holding Fill to 95-99% full (not packed)

Fill at a constant “CONTROLLED” rate. Fill time should remain constant Peak hydraulic pressure should vary as viscosity varies from all causes

Fill as fast as far as possible. May need to be relatively slow

Lens molding Shear sensitive material like PVC Poor mold venting (not acceptable in the long run)

The best fill set up fills the mold quickly with as few speeds as possible to make good parts. Sometimes a slow start is necessary

Jetting in hot runner molds Reducing some types of splay or blush at the gate

Cut off or end filling by: Stroke Cavity pressure

Don’t flash! Speed doesn’t flash a properly built, properly clamped mold The sudden, complete filling of the cavity flashes the mold

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Injection Steps.jpg

As few as possible Normally no more than three (3) Ten (10) is overkill

How should these programming steps bedetermined?

Equal divisions over the shot size Each step size determined by the set-up person

Preferred Fewer steps Simpler setup

Should be able to cancel unused steps (KIS) Should be able to change overall fill speed with

one command

How many programmed injection steps are required?

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Purpose: To fill the cavity and produce part(s) that are 95-99% filled

Goal: Fill as fast as possible under control Separate speed from pressure Have adequate pressure available

During Fill: The injection unit is brought forward so the nozzle is in contact with the mold sprue

bushing The moving platen is brought forward, the mold halves meet and clamping force is

built When the proper force is built, oil is directed to the injection cylinder, pushing the

piston forward Since the screw is attached to the piston in the cylinder, the screw is also pushed

forward As the screw moves forward, without rotation, the ring on the non-return valve is

pushed back into the closed position and the screw acts as a plunger, pushing plastic out of the barrel

Fill

consistent with

quality

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The material is forced through the nozzle of the machine and into the part of the mold called the sprue bushing.

The stream of melt can be divided and distributed into different areas of the mold via the runner system

The plastic then flows through the gates and across the cavity(s)

At the end of fill, the part is 95 to 99% filled, or “short”

Speed of fill can be “constant” or “programmed”. First stage pressure

setting must never be reached.

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Plastic Pressure

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Packing Pressure Over TimePacking Velocity to a Cavity PressureHolding Pressure & Time

Plastic Pressure Gradient

(Static Not Dynamic)

Pack and Hold

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Decoupled ІІ Molding

Objective: fill the mold fast, transfer by screw position when the cavities are 95-98% full. The ram inertia is used up just before the cavities fill out, and 2nd stage hold pressure is used to complete the filling and pack out the parts

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Example of Decoupled II Molding (2-Stage Control)

Better

But Not

The Best

What effect will a +/- 10% viscosity change have inside the mold? Can parts be contained if a viscosity change happens which

requires re-centering the process?

How?

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What is the post gate peak cavity pressure variation? __________What is the end of cavity peak cavity pressure variation? __________Formula: ((High - Low) divided by Original) X 100 =% change

Decoupled II Process

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23.76%

34.10%

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Plastic is compressible 1/2 to 3/4 percent per 1,000 psip.

This means overall dimensions will change on average 1/2 to 3/4 percent per 1,000 psi of plastic variation.

REMEMBER:A short shot is zero pressure in the cavity

at the end-of-cavity.

Controlling pressures in the mold cavity is the

key to minimizing part variation.

Other factors: Direction of flow vs. transverse to flow direction Long fillers such as glass Crystallinity

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Pressure Loss

10,000

.500± .002

Material Highly Nucleated Polypropylene ABS

Morphology Crystalline Crystalline Amorphous+10% viscosity change +20% viscosity change -10% viscosity change

What is the Pressure Loss?

What are the New Dimensions?

Post Gate psip 12,000

EOC 2,000 psip

Group Practice

11000 12000 11000

.495 Short .4975

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Short Shots

Flash Sinks Dimensional

Variations Warp Gloss Gradient Strength

Pressure Cause

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Objective: fill the mold fast, profile by screw position. When the cavities are 85-95% full, transfer to a slow, controlled velocity pack stage. Packing is complete when the cavity pressure or screw position transfer completes packing the part(s)

Decoupled III: Three stage molding

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Decoupled III Molding

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Process Pivot Point 2a.jpg

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Example of Decoupled III Molding

The BEST!

What effect will a +/- 10% viscosity change have on the end of cavity pressure of this mold? How much better is

this than Decoupled II in the previous example?

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What is the post gate peak cavity pressure variation? __________What is the end of cavity peak cavity pressure variation? __________

Decoupled III Process

11.81%2.82%

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Mold CoolingHow do we know when cooling

in a mold is adequate?

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Cool The Plastic Design for mold temperature uniformity

The hottest spot in the cavity determines the cycle time

Locate coolant lines to adequately cool critical area Inside sharp corners Around the gate, runner and sprue Thick part sections

Cooling lines should be in the mold inserts

Sliding cores, as well as lifters, must be cooled

Coolant channels and connectors should be standardized

All coolant circuits should have provisions to detect flow problems for process control

COOLING IS THE LONGEST PORTION OF THE MOLDING CYCLE

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How do we know when the cooling or heating system in our home is adequate?

When every area is at the temperature we want it to be

In order to optimize cooling we must understand what limits it

Heat and heat content

Heat conductivity and flow

We are filling something with

heat, then removing it.

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This total heat content multiplied by the shot weight is transferred into the mold each shot.

The mold is a leaky heat

bucket.

Heat Content of Each Shot

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If the cycle is interrupted heat continues to flow out of this leaky heat bucket called a mold until the mold reaches room temperature.

During startup heat flow starts when the plastic contacts the surface of the mold. The temperature difference between the plastic and the mold is the thermal pressure causing heat flow from the plastic into the mold.

To understand how fast heat can flow, we must understand thermal conductivity and thermal impedance or resistance.

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Heat Flow

Heat flows through matter like water seeps through soil

Temperature difference is a measure of thermal pressure

In the winter the temperature difference between the inside of a wall and the outside is the thermal pressure making heat flow out through the wall

The ability of a material to conduct heat is called it’s thermal conductivity

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To compare polypropylene to tool steel:21 .070 (from Table 5) = 300 times more conductivity12” tool steel size 300 of conductivity = .040” thickness of polypropylene

12” x = Equivalent Thickness

To compare tool steel to air:21 .014 (from Table 5) = 150012” 1500 = .008” of dead air space has the same thermal impedance as 12” of tool steel

BTU’s have a hard time passing through plastic or

air.

1300

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A dirty mold surface or reduced cavity pressure can reduce cooling rates by as much as 20%

A mold with scale in cooling lines can reduce cooling rate by as much as 50%

Scale

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General piping rule:

Use the fewest flow circuits that will provide turbulent flow in all channels with a maximum coolant T between the inlet and the outlet of 4°F (2°F on critical jobs)

Monitor or control the flow in all parallel flow channels

Without turbulent flow in the channels, the outer layers of coolant near the walls insulate the center of flow and efficiently reduce the volume of fluid available to carry the heat away

Plastic Cooling

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The ability to transfer heat is increased with turbulent flow as shown below:

Heat transfer can be done without turbulent flow. More turbulent is better than less.

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The following tables show the flow necessary in commonly used mold cooling lines to obtain turbulent flow.

FLOW THROUGH MOLD COOLING LINES FOR TURBULENT FLOW:

Note: Twice the flow rate is needed for 50/50 ethylene glycol/water mix for turbulent flow.

Flow in GPM for water to obtain turbulent flow:

RN = 5000

Where V = Kinematic Viscosity (Centistokes)

D = Diameter in inches

3160 x GPMV D

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Note: Twice the flow rate is needed for 50/50 ethylene glycol mix for turbulent flow.

The tables below show the flow necessary in commonly used mold cooling lines to obtain turbulent flow:

GPM NEEDED FOR TURBULENT FLOW:

Temperature / °FKinematic Viscosity

Water

50/50 Water Ethylene

Glycol32° F 1.79 3.650° F 1.31100° F 0.69140° F 0.47180° F 0.35200° F 0.31

140° F 100° F 50° F 32° F

1/8 5/16 0.25 0.50 0.75 1.25

1/4 7/16 0.40 0.70 1.25 1.50

3/8 9/16 0.50 0.75 1.50 2.00

1/2 11/16 0.60 1.00 2.00 2.50

Pipe Size I / D InchesTemperature

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Note that ethylene glycol increases the viscosity and doubles the flow necessary to achieve turbulent flow.

Ethylene glycol should not normally be used in cooling circuits but it is OK if abundant capacity exists. (Reduces efficiency)

Thermal conductivity of coolant changes with temperature.

Viscosity of coolant changes with temperature.

Calculating the potential heat transfer coefficient is complex and time consuming and makes understanding the finer points of heat transfer at the coolant interface difficult.

Computers Programs Make It Easy!

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When possible, use higher flow rates and coolant no colder than 50°F to obtain

required cooling.

System is more efficient

Eliminates need for use of ethylene glycol

Remember the potential cooling capacity depends on the total cooling impedance

of the system which includes:

The plastic

The plastic mold interface

The mold material

Any air gaps

The scale or lack of scale in the cooling channels

All must be optimized and maintained for continued constant cooling rates and

times.

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How to pipe a mold:Strategy

Treat each cavity the same

Pipe the mold the same each time

Separate cooling circuits for each cavity or: Use enough flow so that the temperature difference between in and out

will create essentially uniform temperature throughout the mold

Same size lines Same length lines Same size disconnect fittings Same manifold position to each mold circuit

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Critical dimensions for cooling channels are:

Diameter (should they be round?) Depth (the average distance from the center of a cooling channel

to the surface of the part) Pitch (the average distance between the cooling channels)

As in a building, the criteria for judging if the environmental system is adequate

is if all areas are at the desired temperature for the intended use or result.

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Temperature Map of the Molded Part

280°+

300°+

210°+

160°+70°+

190°+

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Uses least coolant Has largest pressure loss Largest temperature differential All channels get the same

coolant flow

How should cooling channels be connected?

Provides maximum cooling if coolant is available

Uses the most coolant Lowest pressure loss May waste resources Flow channels with highest

restrictions get least cooling

Series Parallel

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Combination of series and parallel

Best: A balance of the two

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This allows easy detection of cooling problems. Blocked line Insufficient available coolant Coolant interruption Coolant line breaks (auto shut-off)

Coolant can be prioritized to areas where it is needed. Hard to cool areas Smaller lines Areas far away from coolant

surface

Q = KA P

For a given flow rate Q there is a specific P if all channels are open. If not, A changes and P will change.K = A flow constantA = Area of all flow channels

If a line is plugged, piped wrong, or the flow to the manifold from the

central system changes, then the P or pressure difference will change.

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Trouble killing

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Plastic’s Point of View

Shorts shots mean zero pressures at the end-of-fill. There was no packing phase in the cycle. If no machine changes of pressures or flow rates were evident, this generally means a viscosity change has occurred. The viscosity is increased to a condition where the plastic will no longer flow.

Short Shots

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Causes for Short Shots

Plastic temperature is too low.

Injection rate is too slow

During fill, insufficient injection pressure was available to maintain fill speed set.

During fill, pressure is abundant and fill time is too slow.

Material is too high viscosity.

Possible Corrective Action

Measure melt temperature using the 30/30 approach.

Monitor fill time and compare it to correct fill time.

Raise first stage pressure.

Check for nozzle obstruction.

Purge into the air and observe the injection pressure.

Change material.

Increase pack and hold pressures to compensate for material increase in viscosity.

Short Shots, cont.

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Splay


Splay is plastic road kill!

Splay is a gas or liquid disbursed over the surface of a plastic part and manifests itself as tracks. Splay can be caused by several liquids or gases which can be moisture from undried material, trapped air, degraded polymer molecules, or degraded additives. Splay can be generated by moisture condensing on the surface of the mold and smearing across the mold surface by the plastic flow.


Dry material.

Increase back pressure to remove trapped air.

Change the screw type to remove trapped air.

Check material temperature and residence time to eliminate molecular degradation and additive degradation.

Keep moisture from condensing on the mold surface.

Remove sprue break.

Reduce or eliminate decompression.

Check L/D of screw; if lower than 16:1, try higher L/D or different screw design.

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Flash and Short Shots at the Same Time


This generally indicates a change in the pressure distribution during fill due to a dramatic viscosity change. It can also be caused by insufficient clamp tonnage.


Check clamp tonnage availability.Check viscosity (Injection Fill Integral).Check fill time.Check melt temperature using 30/30 approach.Check mold temperature.Determine that the material is correct.

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As material cools and shrinks in a mold, insufficient packing will become evident as either sinks on the outer portion of the part or voids in the center. Sinks and voids are evident in thick sections which are the last to cool or in areas extremely remote from the gate or very near the gate. Classic sinks in thick areas and away from the gate indicate insufficient packing and possibly increased viscosity of material. Sinks near the gate generally indicate lack of gate seal and possibly decreased viscosity, which is many times due to increased plastic temperature causing lack of gate seal. Decreased packing generally causes sinks away from the gate while increased packing can cause sinks at the gate end of the part if the increased pressure in the part causes the gate discharge after injection.

SINKS AND VOIDS

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Causes of Sinks and Voids

Increase in viscosity of plastic

Packing and holding pressures are too low (end of fill sinks and thick sections only).

Injection time is too short to allow gate seal.

Mold temperature is too high to affect gate seal.

Voids are sometimes mistaken for bubbles. Bubbles are trapped gas while voids are vacuum voids due to a lack of material during cooling. SEE BUBBLES for further information.


Improper melt temperature -check using 30/30 approach.

Raise pack and/or hold pressure.

Increase injection forward/hold time.

Lower mold temperature.

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Dimensional Variations

Dimensional variations in plastics are caused by changes in the pressure distribution throughout the cavity and, for semi-crystalline materials, changes in cooling rate. Dimensional variations in both types of material can also be caused by changes in the post mold, cooling, and stabilization environment. In order to best analyze dimensional variations, the problem should be specifically categorized as on the next page:

Good Part Template

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Part is too small all over:


Increase packing pressure to obtain required in-mold pressures.


Plastic pressure throughout the cavity is too low.

Good Part Template

Dimensional Variations, cont.



Part is too large all over:


The pressure distribution throughout the cavity is too high.


Reduce packing pressure to obtain cavity pressure required.


Part is too small near the gate:


The effective plastic pressure near the gate is too low while the pressure in the rest of the part is okay. This generally is caused by the gate being unsealed.


Increase injection forward/holding time or determine the root cause, such as possible increase of plastic temperature.



Part is too large at the gate:

Plastic’s Point of View Possible Corrective Action

The pressure distribution in the mold at the gate end is too large; the rest of the pressure is okay.

Reduce packing pressure; possibly change the pack rate if possible.

Check melt temperatures; may be too high.

Check material viscosity (Injection Fill Integral).

Allow discharge by reducing injection forward/hold time.



Part is too small at the end of cavity:

Plastic’s Point of View Plastic’s Point of View

Pressure distribution is okay at the gate but is reduced from normal at the end of fill. This generally indicates a change in plastic viscosity.

Check fill time.

Check melt temperature using 30/30 approach and adjust as appropriate.

Check viscosity (Injection Fill Integral). If viscosity is too high, increase fill speed until the viscosity is correct.



Part is too large at the end of cavity:

Plastic’s Point of View Possible Corrective Action

This means that the pressure distribution in the cavity is okay at the gate and too high at the end-of -fill. This indicates a lowering of viscosity.

Check melt temperature by using 30/30 approach.

Check fill time.

Check viscosity (Injection Fill Integral). If viscosity is too low, reduce injection speed until viscosity number is correct.




Inconsistent dimensions mean inconsistent pressure gradients throughout the part. This could indicate that on some shots, the gate is sealing or not sealing. Otherwise the pressure distribution in the cavity is varying on a shot-to-shot basis. If this is the case, check to see if there are large variations in parts shot-to-shot or if there are trends over time. Trends over time indicated temperature variations or material lot variations, whereas shot-to-shot inconsistencies indicating packing fluctuations over short periods are either caused by gate seal changes or pressure changes due to check ring leaks, etc.


Run a series of short shots to check the check ring consistency.

Monitor hydraulic pressure consistency.

Do a gate seal analysis and possibly increase injection forward time.

Monitor viscosity fluctuations (Injection Fill Integral).

Monitor in-mold pressure to determine variability.

Dimensional Inconsistency


Warp


Warp is an inconsistent deformation of the part making it nonconforming to the shape of the cavity. This is generally caused by some kind of stress induced in the part during filling, packing, or cooling. Warp is a complex phenomena and is often caused by a summation of many forces, a few of which are dominant.

In amorphous material, the effect of crystallinity does not exist. Therefore there is an extra dimension in the crystalline or semi-crystalline materials. Parts filled with long thin fibers, such as glass, have another factor; the orientation of which is not present in unfilled materials.


Crystalline Materials

In crystalline materials, a large portion of warp is crystallinity induced, caused by non-uniform cooling. In amorphous materials compressive stress gradients caused by non-uniform packing pressures throughout the part are generally predominant. In addition, the orientation stresses caused by flow and subsequent relaxation of the stresses during cooling can cause non-uniform stresses in parts. In analyzing warp problems, it is important to categorize them as crystalline or amorphous, fiber-filled or unfilled, and then proceed. In the semi crystalline material, comparing the first shot out of a cold or uniformly-temperatured mold to one run later as the mold warms up will give an understanding of whether or not this is the problem. If the first part out of the mold is not warped, then the issue of non-uniform cooling is probably the predominant situation.

Another quick check is to run the part using an amorphous material. Many times an easy flow ABS can be substitutes for polypropylene. If the ABS part does not warp and the polypropylene part does, crystallinity issues are probably the cause.


Redesign the cooling to minimize hot spots and cold spots in the mold.

Change the material to an amorphous resin.

Redesign the part to a more rigid structure using complex shapes or ribs (ribs are least desirable).

Warp, cont.



Crystalline Materials con’t.

Analyzing warp in parts made of high aspect (long) fiber-filled materials involves comparing parts molded with the fiber to that of the unfilled resin to determine the degree of change. Flow during fill orients fibers in the direction of flow, but they do not de-orient as many molecules do during cooling. Most often, warp caused by orientation of fibers can only be corrected by changing the flow directions in the part or changing the design of the part.

Warp, cont.


Amorphous Unfilled Materials

Generally, amorphous warp is caused by a combination of orientation stresses and a packing stress gradient. A packing stress gradient can be reduced by reducing viscosity, generally by increasing fill rates and/or temperatures.

Also, when Decoupled III is being used, optimizing pack speed can minimize the packing stress gradient.

Orientation stress can be reduced by raising the plastic temperature, slowing the fill, and/or reducing the cooling rate. If the packing stress gradient is the predominant factor, increasing speed will generally reduce warp. If orientation stress is the primary effect, increasing speed will cause the warp to worsen.

Another important factor is whether or not the gate is sealed. Many times the packing stress gradient can be reduced by intentionally not sealing the gate which will reduce the compressive stress gradient by allowing back flow, thus making the part flat. This is especially true in center-gated parts, both semi-crystalline and amorphous.


Increase fill speed.Decrease fill speed.Increase melt temperature.Increase mold temperature.

Reduce injection time to allow gate to back flow or discharge.

Warp, cont.


Knitline Weakness or Cosmetic Problems



A knitline is truly a glue joint in that the two plastic flow fronts join and do not generally re-entangle. The exceptions are some crystalline materials which are welded together above their melting point. In a classic knitline formation, the same problems apply to a knitline as that of making a good glue joint. The material must be at a low enough viscosity, the flow front must be clean, there must be sufficient pressure to cause bonding, and the pressure must be held for a sufficient time to allow the material to solidify. Also, there is often air entrapment. Knitlines must be well vented. If all of the elements are available, knitline integrity should exist.


Check viscosity (Injection Fill Integral) and correct if necessary.

Check melt temperature.

Check fill time.

Check mold temperature.

Be sure the part is properly packed.

Raise packing pressure. Be sure that the gate is sealed.

Check gate seal time.

Maintain proper cycle.

Be sure that the cavity is adequately vented at the knitline area.

Knitline Weakness or Cosmetic Problems, cont.

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