Plastic Molding Seminar Basic Plastic Molding Technology An

Plastic Molding Seminar Basic Plastic Molding Technology An MMI customized seminar on industry standard practices for injection molded plastic part design, tooling, materials and applications. 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

Seminar Objective The main objective of this seminar is to provide you with additional resources that can be readily applied to current and future projects and improve your overall effectiveness. We hope you to leave with the following: • An understanding of injection molding processing, mold tooling, and part design. • Familiarity with plastic terminology and concepts. • A grasp of plastic molding’s strengths and limitations. • Basic design guidelines for injection molded parts. • Knowledge of the available array of plastic materials, their key properties, and potential applications. • The ability to recognize injection molded applications.

Agenda We find that the following topics build off each other best in the following order. We start with describing the overall process and work our way down to mold design, then the actual part. The design/analysis tools and materials/applications sections tie in with each of the previous sections. 1. 2. 3. 4. 5. Processing Injection Mold Design Part Design and Analysis Tools Materials and Applications Appendices: Part Design Guide (GE) Material Data Sheets

Processing Injection Machine Layout Injection Stages 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

Injection Press

Injection Press Operation Stage 1: Startup Stage 2: Barrel Fill

Injection Press Operation Stage 3: Injection Stage 4: Cooling

Injection Press Operation Stage 5: Opening Stage 6: Eject

Injection Press Operation Stage 7: Close Stage 8: Reset

Injection Mold Design Basic Mold Components Runner Systems Hot Manifold Systems Cylinders/Slides/Lifters 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

Injection Mold Assembly The injection mold assembly includes A/B Plates, the Eject System, and any Hot Runner System (not shown). The assembly is fitted into the press between the Nozzle and Knockout Pins. Cavity: Open area shaped to create the part geometry. Sprue: Removed in secondary operation. Parting Line: Contact surface between A & B Plates. Return Pins: Push back eject plate upon mold closure. Cooling Lines: Used to regulate mold temperature. Nozzle & Knockouts: Included on the press - not part of the mold assembly.

Mold Components Basic Features of a (2 -Plate) Injection Mold Assembly

Cold Runner Systems are employed to distribute the material from the nozzle to different gate locations. A common situation that requires this feature is a Multiple Cavity Mold. • Gates are positioned at the material entrance point of the part cavity. Their size and geometry vary based upon material, flow length, and other factors. • The Sprue and Runners are removed from the part after molding in a secondary operation. • The runner system should be balanced in order to create consistent parts from different cavitities.

Hot Runner Systems are employed to keep the material hot up until the point of entry at multiple locations. • The Hot Manifold and Hot Sprue Bushings contain heaters that keep the melted plastic from hardening until reaching the mold cavities. • Saves part material (no cold sprue/runners to remove). • Increases tooling cost. • Increases setup time.

Example: Hot Runner System Cross-Sectional Cutout 4 Drop Hot Manifold System. This is a Cross-sectional view taken through 2 of the manifold legs to show the material chambers.

Die-Lock Mold designers must consider the method of removing the formed part from the mold. Often the part geometry requires that the mold open in more than the standard 2 directions. This section of the tool must be pulled in this direction in order to release the part. Die-Lock requires a Side-Pull. The three common methods are: • Cylinders • Slides • Lifters Note that the formed part is ‘trapped’ in the A-Plate due to the additional geometry on the top end. This unwanted condition is called a Trap or Die-Lock.

Side-Action: Cylinders The most straightforward method of incorporating side-action is by attaching a core block (red) to a cylinder mounted to the side of the mold Event Sequence: 1. Part forms in cavity. 2. Mold opens. 3. Cylinder pulls back core block. 4. Part Ejects. 5. Eject and Cylinder move back to original positions. 6. Mold Closes.

Side-Action: Slides Another more common method of side-action uses a slide that is activated automatically by the opening of the mold. A horn pin attached to the A-Plate acts as a guide. Event Sequence: 1. Part forms in cavity. 2. Slide is pushed back by the horn pin and springs as mold opens 3. Part Ejects. 4. Slide is pulled back into place by horn-pin as mold closes.

Side-Action: Lifters A third less common technique actually ejects the part with the core block. At this point the part can be pulled out by the operator. In this scenario, the core block is called a lifter. Event Sequence: 1. Part forms in cavity. 2. Mold opens. 3. Eject system pushes out part with core block attached. 4. Part is removed. 5. Mold closes.

Example: Side-Action

Part Design Wall Thicknesses Draft Radii Rib/Boss Design Undercuts 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

Wall Thickness - Coring Typical injection molded parts should have a uniform wall thickness to allow even flow and cooling. • Nominal wall thickness normally falls between 1/16” and 1/4” depending upon material, desired strength, and necessary flow length. • Thicker sections require longer processing times for cooling. • Sharp transitions in wall thicknesses can cause uneven shrinkage that leads to molded-in stress and warpage. • Coring is the process of removing thick sections from a part design.

Rib Design Ribs are added to give part strength, but introduce the following problem in maintaining even wall thicknesses. The intersection point automatically creates a thick section. The thicker the section ®the slower the cooling ®increased shrink This phenomenon results in sink marks on the outside wall. To avoid sink ribs should be constructed at 50% to 75% of the nominal wall thickness.

Boss Design Bosses, used to reinforce holes, also introduce thick sections and follow the same design guidelines as ribs. There are many methods available to optimize the plastic design of these features.

Draft is required on surfaces perpendicular to the parting line (or side-action features as applicable). Benefits • Aids in part ejection. • Prevents marks on side walls from tool separation. • Reduces tool wear. Issues • Often results in thicker sections – especially on tall ribs. • Tooling more complex.

Corner Radii Sharp, inside corners have a dramatic weakening effect upon part strength. Radii should be employed wherever possible. Benefits • Avoids stress concentrations. • Aids in flow of material during processing. • Reduces molded-in stress points. Issues • Often creates thick sections – especially on ribs. • Design clearances restrict use. Note: Unlike machining a solid, when building a mold cavity it is usually easier to put a radius on an outside part corner due to the cylindrical shape of the cutter.

Undercuts are features in the part that require a lifter, slide, or a through core. A through core is a telescoping finger that requires a window in the part. Undercuts require shutoff areas in which the tool plates must be able to open and close cleanly. This requires the following conditions: • Draft on both sides of shutoff. Undercuts • Exact matching of tool geometry. The potential effects of shutoff considerations on part design. • Widened holes in part to allow for tooling draft. • Draft of some sidewalls. • Necessity of some sharp part edges. In cases where the shutoff is not sufficient, plastic will slip between the mold plates creating a condition called flash.

Other Considerations Understanding the mold layout will help identify issues that do not typically appear on the part prints. The following mold features do appear on every injection molded part and should be moved away from critical surfaces. • Gates/Hot Sprue Points • Eject Pins • Parting Lines • Knit Lines (defined below) Gate When material flows around a feature such as a hole, it must rejoin on the other side. This creates a knit line, which will be somewhat weaker than the surrounding plastic.

Design and Analysis Tools CAD Solid Modeling CAM Machining Mold Flow Analysis FEA Dynamic Analysis 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

CAD – Solid Modeling CAD Solid Modeling has provided great benefits for plastic part and mold design that go well beyond advanced visualization abilities. • Obtain quick and accurate part mass and volume properties for costing purposes. • Models can be directly imported into FEA and dynamic simulation programs. • Provides ability to conduct Mold Flow Analysis. • Part model can be cut directly from the mold model to quickly create cavities for mold design. • CNC machining of mold can be programmed directly to the solid model of the mold.

Computer Aided Machining (CAM) Computer Aided Machining allows tool engineers to program the entire machining process on a computer before cutting the first chip. • Identify issues before machining begins. • Full planning before the physical material is available. • Machines can operate with minimal supervision – even after hours.

Mold Flow Analysis simulates how the mold will from the solid model. This allows optimization of the following mold and processing characteristics. • Cavity Layout • Runner Systems • Gate Design • Injection Pressures • Material Selection • Processing Times • Clamp Pressures

Finite Element/Dynamic Analysis Finite Element Analysis is becoming more accurate, more robust, quicker, and easier to apply year-by-year. It has become an invaluable tool to engineering companies that depend upon strong design capabilities. • Load-Deflection Analysis • Static Stress Analysis • Dynamic Loading Analysis • Motion Simulation • Noise, Vibration, Harshness Studies (NVH)

Advanced Techniques Molded-In Inserts Overmolding Snap Design Living Hinges 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

Plastic Materials & Properties • History of Plastics and Polymers • The Structure of Polymers • Types of Polymers • Material Selection • Thermoplastic Selection Tree • Some Examples 44650 Helm Court Plymouth, MI 48170 (734) 459 -5955

The History of Plastics • Polymers have been with us since the start of time • Natural Polymers – – Tar Shellac Tortoise Shell and Horns Tree Sap • Chemically modified in 1800’s – Vulcanized rubber – Gun Cotton – Celluloid • First Synthetic Polymers – Bakelite in 1909 – Rayon fibers in 1911

The Structure of Plastics • A Monomer is a single link in a chain • By joining these links together, materials are built with useful properties • Building monomers together forms a Polymer.

The Structure of Plastics • Plastics are Polymers • Polymer is from the Greek words “Poly” meaning many and “Mer” meaning parts • The most simple type of polymer is a hydrocarbon – a polymer created from a single monomer

The Structure of Plastics • Copolymers are created when the polymer is built from multiple monomer links • ICP – PP with a rubber phase • Acrylonitrile – Butadiene – Styrene (ABS)

The Structure of Plastics • Blending two or more polymers together without a chemical reaction results in the creation of an Alloy • The benefit of creating an alloy is to achieve properties better than either of the individual materials • Common examples of alloys frequently used are: – Polycarbonate/ABS (Dow Pulse, GE Cycoloy) – Polycarbonate/PBT (Xenoy) – PPO/PA, PPE/PA (Noryl, Noryl GTX)

The Structure of Plastics • Some polymers pack together tightly in a regular pattern • These polymers have a characteristic called SEMI-CRYSTALLINE • Materials that do not crystallize upon turning solid are called AMORPHOUS

Semi Crystalline • Exhibit a very sharp melting point • With a small increase in temperature they become liquid or melt • Melting behavior similar to a household candle • Provide superior properties, but exhibit high shrink as they cool and reharden • Common examples of semi crystalline materials are: – Nylon – Polyethylene – Polypropylene

Amorphous • Do not crystallize upon solidifying • Demonstrate a gradual softening as the temperature is increased • No specific melting temperature • Usually not as easily processed as a crystalline material • Most commonly processed just above the Tg – Glass Transition Temperature • Common examples of amorphous materials are: – ABS – Polycarbonate – Acrylics

Semi Crystalline vs. Amorphous

Types of Polymers Generally speaking, there are two types of polymers • Thermoset polymers are like concrete • One chance to liquify and shape • “Cured” using heat and pressure or a chemical initiator • Common examples – Phenolics – Epoxies – Cast urethanes • Thermoplastic polymers are like wax • Can melt and shape multiple times • Processed by heating and cooling • Common examples – Polypropylene – Nylon – ABS

Material Selection • Material selection should be made based upon an applications’ need for specific key properties – Temperature – Load Carrying Capability • Tensile Strength • Flexural Strength • Modulus or Stiffness – Impact Resistance – Chemical Resistance • Wash solutions • Chemical dip – Wear Resistance – Cost

Steps in Material Selection • Using an injection molded dunnage tray as our example • Carefully define the application – – – – – What load will the tray carry Will the tray undergo shaking or impact What temperature will the tray see Will the tray be exposed to chemicals or moisture Will the tray be used with electrical components What about wear resistance against the parts Is dimensional stability an issue Will the tray be used and/or stored outdoors Cost allocated to the project

Steps in Material Selection • Identify the importance of each requirement – Cost is not always the most important ! • Match the defined requirements to the best available material • Design the application with the defined material in mind – Plastic is not designed the same as metal !

The Thermoplastic Selection Tree