Injection Molding and Injection Mold Nomenclature: PART 1

REPORT 1

OBJECTIVE: TO STUDY INJECTION MOLDING & INJECTION MOLD NOMENCLATURE

MAIN REPORT: 

1. FREEHAND SKETCH OF AN INJECTION MOLDING MACHINE:

 

An Injection Molding Machine has three main units consisting of several parts in each unit that are as follows:

A) Injection Unit: The Feed Hopper with Hopper Dryer, Injection Ram/Reciprocating Screw, Heating Bands, Backflow Preventer/Non-Return Valve, Motor

B) Molding Unit: Locate Ring, Sprue Bushing, Upper Plate, Stripper Plate, Cavity-Plate, Cavity-Block, Core-Plate, Core-Block, Die Locker, Ejector Pin, Spacer Block, Guide Pillars, Bottom Block, Lower Plate

C) Clamping Unit: Tie Rods, Ejectors, Crossheads, Clamping Cylinder, Hydraulic Cylinder

                              

1.1- Feed Hopper: Feed Hopper is a large, pyramidal, or cone-shaped container used in an injection molding machine to hold particulate matter or flow-able plastic material of any kind that can be dispensed from the bottom when needed. Nowadays, these Feed-Hoppers are equipped with Hopper-Dryers that can be used to remove moisture from hygroscopic and non-hygroscopic plastic pellets before exiting from the hopper’s outlet into the Injection-Unit. Without the Hopper Dryer, the moisture will remain attached to the plastic pellets when they’re fed into the injection unit, and eventually, that moisture will evaporate and turn into a gaseous state which can delay the discharge of the mold cavity and cause marks in the final products known as ‘Silver Marks’. These Silver-Marks not only affect the appearance of the plastic parts but also reduces their mechanical strength of them as well.

Feed Hopper's Image & Line-Diagram: 

     

 

Placement of the Feed-Hopped in Injection-Molding Machine:

 

Materials that can be fed into the Injection-Unit through Feed-Hopper:

 

1.2- Injection Ram/Reciprocating Screw: This screw is a helically flighted hard steel shaft that rotates within a plasticizing barrel to mechanically process and advance the plastic being prepared.

The rotating drive system for the screw can be powered by a hydraulic or electric motor. The use of electric motors tends to increase the melt-processing efficiency and thus the production rate.

In the Modern Injection Unit, Plastics fill only the space around the shaft of the screw that evenly distributes molten-plastics around the screw.

Molten Plastic around the Reciprocating Screw:

 

The screw has 'Flights' that wrap around the shaft. As the screw rotates, these flights transfer the raw material forward through the barrel.

 

Plastic pellets moving forward due to the Rotation-Movement of the Screw:

 

The flights also serve to mix the plastic. The screw action agitates the melting pellets within the flights to create a uniform mixture.

 

The Screw’s action itself heats the plastic throughout. The temperature profile is found to be distributed among three zones that are dependent on the plastic type. The first zone is called the ‘Feed-Zone’. The second zone is called the ‘Compaction Zone’ and the third zone is called the ‘Metering Zone’.

The Feed-Zone is typically 50% of the length of the screw. The shaft’s diameter increases along the screw so that the distance between the barrel’s wall and the shaft decreases.

The flights, then, squeeze out air as they move the plastic forward and they shear the pellets and press them against the barrel’s wall. The shearing of plastic pellets against the barrel’s wall results in the creation of frictional forces between the pellets and the wall that heats the plastic throughout.

The Screw Induced Shear supplies the majority of the heat needed to melt the plastic which is between 60 to 90 percent while the rest of the heat is extracted from the Heater-Bands.

 

Thermal Transition of Plastics:

Feed zone has the largest screw diameter and hence has the lowest temperature of the three zones. The Compaction zone has an increasing screw diameter from the small diameter in the Feed zone to the larger diameter in the Metering zone. The metering zone has a constant diameter and gap between the screw and barrel.

The length and diameter of the screw and flights are important parameters for Injection-Molding. The L/D ratio nowadays typically ranges between 19:1,20:1 or 21:1 whereas the ‘Flight-Depth’ ratio is typically between 2 and 3.

 

The diameter of the Screw is gradually decreasing towards the nozzle:

 

Shear forces are being generated between Plastic pellets and the barrel's walls:

 

1.3- Heating-Bands: Heater-Bands are ring-shaped devices that clamp around a cylindrical element such as the barrel in the Injection-Unit. Heat transfer from Heater-Bands occurs via the ‘Conductive Method’ as they clamp around the outer diameter of the cylindrical barrel providing fast-acting indirect heat that is desired to fulfill the heating needs.

There are three main types of Heater-Bands: Mica, Ceramic & Mineral Insulated of which Mica Band Heaters are the most commonly used within the plastics industry for Extrusion Injection Molding and Blow Molding.

 

1.4- Backflow Preventer/Non-Return Valve: The Molten-Plastic flows past the front of the screw through indentations or ‘Flutes’. When there’s enough plastic to fill the mold at the front of the screw, it rams forward like a plunger injecting the plastic into the mold.

The plastic cannot flow backward because when the screw pushes forward, a ‘Check-Ring’ is shoved against a ‘Thrust-Ring’ to block that backward movement of the molten plastic. This forces the plastic into the mold.

Types of Nozzle Tips in Screw: 

• General-Pupose 
• Reverse Taper
• Free Flow

Nozzle Tip design and size will impact Molding-filling, Process behavior, and Stability. If the Orifice is too small, the required injection pressure to inject material through the nozzle may be high. This would reduce the available machine pressure needed to fill and pack the mold. If the nozzle tip is in contact with the cold mold, so it will get cold quickly. A small nozzle orifice could freeze the polymer too soon which would impact the ability to properly pack that part. In this scenario, the cold plug could also be injected into the next cycle, potentially causing process and part quality-related surface defects. 

 

1.5- Motor Drive Control Unit: A Drive Control Unit for an Injection Molding Machine has Servo-Motors as driving sources. The drive control unit prevents the injection molding machine from getting out of control when the servo motors are no more capable of following the operation and prevents the operation speed of each function of the injection molding machine from exceeding the pre-set speed. The servo motors are driven by outputs of servo circuits that include an error register that stores the difference between a shift instruction from a controller and a practical shift amount of the servo motors. The controller reads the value stored in the error register and compares it with the predetermined value. When it is determined that the value is greater than the predetermined value, the controller temporarily ceases to produce the shift instruction.

 

1.6- Locate Ring/Register Ring: It is a circular member fitted onto the front face of the mold over the ‘Sprue Bush’. Its purpose is to register or locate the mold in the correct position on the injection machine, to ensure proper alignment between the nozzle and the sprue bush, thereby eliminating leakage by assuring that nozzle on the molding machine will interface with the sprue busing allowing the plastic to flow into the mold.

In other words, it is used to ensure that the ‘Gate Sleeve’ of the Injection Mold and the nozzle position of the Injection Molding Machine is level and completely coincide when the injection molding machine is on the moveable mold.

The locating ring has a cylindrical inner surface that fits in with the corresponding surface on the die. The surface is used to align the mold during assembly and to support the mold during the injection. The outer surface of the locating ring is textured or serrated to provide traction for the clamping system. The locating ring also has a series of holes for mounting the mold to the machine. These holes are located on the periphery of the ring and are spaced to align with the corresponding holes on the machine moving platens.

There are different types of locating rings to choose from and the best type for a particular application will depend on factors such as the size and shape of the object being molded. In some cases, multiple locating rings can be used to achieve the desired effect.

• Linear Rings: Linear-Rings are the most common types of locating rings. They are simple and reliable, but they can be difficult to adjust if the injection mold needs to be repositioned. The friction ring relies on friction to hold the mold in place. They provide good retention and are easy to adjust, but they wear out over time.
• Frictional Rings: Friction-Rings rely on friction to hold the mold in place. They provide good holding power and are easy to adjust, but they wear out over time.
• Hydraulic Rings: Hydraulic-Rings use hydraulic pressure to hold the die. They provide good holding power and are easy to adjust, but they require a more complex injection setup.

       

 

1.7- Sprue Bushing: The ‘Sprue Bush’ is the connecting member between the machine nozzle and the mold face and provides a suitable aperture through which the material can travel on its way to the mold cavity. Sprue bushings are often made of hardened steel or a copper base alloy. They may also be lined with carbide for wear and corrosion resistance, and faster heat transfer rates. Carbide-lined sprue bushings are well suited for use with abrasive resins. Rigidity is important since the sprue is often the point at which pickers or other industrial robots grasp molded parts for removal.

A draft angle is also needed to be considered to reduce the chance of damaging the part due to friction which otherwise would cause wear and tear during release. By introducing a small draft angle, it is ensured that a uniform, smooth unscratched sprue is developed.

 

Types of Sprue Bushings:

• Cold Sprue Bushing: Cold Sprue Bushings are inserted into the mold and provide a channel between the molding machine nozzle and the mold cavity. They are unheated and leave a sprue that must be removed, often by a secondary operation if appearance is a consideration.
• Hot Sprue Bushings: Hot Sprue Bushings are inserted into the mold and provide a hot channel between the molding machine nozzle and the mold cavity. A heating element contained within the bushing is used to keep the resin in molten condition within the bushing.

Sprue bushings typically have a spherical radius of 0.50” or 0.75”, however, flat-type bushings that have a flat surface instead of a nozzle radius is also available.

Sprue Bushing Parameter Specifications:

• Tip Hold Diameter:  The diameter of the inlet hole on the nozzle seat. This is the hole that mates to the nozzle tip and where the material enters the sprue bushing. The tip hold diameter may also be referred to as the gate diameter.
• Overall Length: The overall length of the sprue bushing does not include the nozzle.
• Shank Length: It is measured from the underside of the bushing head to the end of the sprue bushing. It also doesn’t include the measurement of the nozzle.

Types of Sprues:

• Inclined
• Parabolic
• Hyperbolic
• Conical-Helix

     

                           

 

1.8- Fixed Clamping Plate/Top Plate: The Fixed Clamping Plate or Top Plate is used to hold the fixed side of the mold to remain attached to the fixed plate of the Injection Molding Machine. This plate consists of locating ring, eye bolt, and sprue bush.

1.9- Stripper Plate: It is a plate that is used to push a part of an Injection Mold Core. In other words, it removes the part off of a core, preparing the mold for the next shot. It is only used in the 3-plate molding process where the runner stripper plate’s function is to cut resin from the nozzle on top of the sprue bush and pull the runner by a runner locking pin.

1.10- Cavity Plate: Also known as the ‘Fixed Mold Plate’, It remains fixed to the cavity-side of the Mold-Unit that holds a Cavity-Block made of High Carbon High Chromium Steel(HCHCR), Leader Pins, Support Pins, Puller Bolts, and Angular Pins. Here, one port of the gating system is in contact with the injection molding machine. The fixed part of the flow channel is provided with a heating element so that the material in the fixed mold flow channel maintains a molten state through the fixed part. When the mold is closed, it is connected to the mouth of the injection molding machine. Thus the flow passage extension portion must have sufficient length and it is also ensured that no molten materials leak out during molding opening.

1.11- Backup Plate/Support Plate: It is used to support the cavity plate, attach the hole for the return pin’s spring, and cool the channel when in cavity plate can not make it.

1.12 Guide Pillars: Guide Pillars are used for aligning the ‘Core-Half’ & the ‘Cavity-Half’. Guide Pillars are always present on the cavity side of the mold and guide bushes are present on the core side. The length of the guide pillars should always be more than the core length so that it aligns before the core and cavity align themselves.

1.13 Core Plate: This plate eventually gets attached to the cavity plate and then the material is injected into the region between the core and the cavity. After the material is injected and solidified after cooling, then the core plate is released so that it can move back to its initial position away from the cavity plate.

1.14 Ejector Retainer Plate: It is used to hold the ejector, Z-pin, and Shoulder Bolts, and provide space for the ejector leader pin and support pillar.

1.15- Ejector Pins: Once the part is formed and the core plate moves back the ejector pins are used to eject the part that has been molded. The size of the ejector pins is around 6mm. The part of the ejector pin near the core is transition fit and after some distance, relief is provided for the ejector pins. The length for which transition fit is provided is called ‘Guidance of the Ejector Pins’ and this length is around 12mm.

1.16- Crossheads:  The clamping Mechanism for an Injection Molding Machine got a ‘Toggle Link’, a Crosshead connected to the toggle link, and a driving device for moving the crosshead. To drive the crosshead, the ball spline shaft is provided parallel to a ball screw. Also, the ball spline nut provided in the crosshead is attached to the ball spline shaft so that the crosshead may be controlled by the ball spline shaft.

1.17- Return pins: The function of the return pin is to bring back the ejector pin to its initial position. Return spring consists of a spring that is compressed when the ejector pin moves and hence when the spring eventually relaxes, the ejector pins can be brought back to their initial intended position.   

1.18- Spacer Block: It is mounted between the moveable clamp plate also known as the bottom plate and the moveable cavity plate to provide space and allow the ejector plate to move when ejecting the part. The required length of the spacer block depends on the ejector stroke that is needed to eject the intended product.

1.19- Ejector Back Plate: It is driven forward via hydraulic actuators. It pushes the ejector pins and returns pins, connected with ejector rods.

1.20- Rest Buttons: It is provided behind the ejector back plate to reduce the friction that will take place if the back plate is directly placed beside the moving plate.

1.21- Moveable Clamping Plate or Bottom Plate: The moveable clamping plate or bottom plate holds the moveable side of the mold like the Spacer Block, Support Plate, Cavity Plate, and Ejector Mechanism to the moveable platen of the Plastic Injection Machine.

 

DIAGRAM OF A MOLD UNIT:

EXPLODED VIEW OF THE VARIOUS PARTS OF THE MOLD UNIT: 

 

IMPORTANCE OF DRAFT ANGLE DURING EJECTION: 

If a part has walls that are exactly 90 degrees, it’ll be very difficult to eject that part as its inner walls will scrape the core half of the mold. Also, the vacuum will be difficult to break because air cannot readily enter.

 

However, if the walls are slightly tapered-even just one or two degrees, it becomes much easier for the part to be removed because once the part moves slightly, the walls are no longer in contact with the core half and air can rush in.

  

                                 

 

2. CAVITY & CORE WITH FREEHAND SKETCHES: 

CAVITY: Cavity is known as the ‘Female-Section’ of the mold that creates the external shape of the plastic component. It belongs to the fixed-half section of the mold-unit. It is usually formed by removing metal from the mating surfaces of the two halves. Molds can contain a single cavity or multiple cavities to produce more than one part in a single shot.

CORE: Core is known as the ‘Male-Section’ of the mold that creates the internal shape of the plastic component. It belongs to the moving-half section of the mold-unit.

FREEHAND SKETCH OF CORE, CAVITY & PART:

 

 3. IDEAL WALL THICKNESS FOR THE INJECTION MOLDED COMPONENT: 

MATERIAL	RECOMMENDED WALL THICKNESS
ARCRYLONITRILE BUTADIENE STYRENE(ABS)	0.045in. - 0.140in.
POLYCARBONATE(PC)	0.040in. - 0.150in.
POLYAMIDE(PA)/NYLON	0.030in. - 0.115in.
POLYESTER(PES)	0.025in. - 0.125in.
POLYETHYLENE(PET)	0.030in. - 0.200in.
POLYPHENYLENE SULFIDE(PPS)	0.020in. - 0.180in.
POLYPROPYLENE(PP)	0.025in. - 0.150in.
POLYSTYRENE(PS)	0.035in - 0.150in.
POLYURETHANE(PUR)	0.080in. - 0.750in.
ACRYLIC	0.025in. - 0.500in.
ACETAL	0.030in. - 0.120in.
LIQUID CRYSTAL POLYMER	0.030in. - 0.120in.
LONG-FIBER REINFORCED PLASTICS	0.075in. - 1.000in.

 

Importance of Wall-Thickness: Thinner walls cool faster, shortening the cycle time of the mold, and the amount of time it takes to make each part. If a plastic part can cool faster after the mold is filled, then it can safely be ejected sooner without wrapping, and because time on the injection machine costs money, the part is less expensive to produce.

In the cooling cycle, the outer surface of a plastic part cools first. Cooling causes contraction: If the part is of uniform thickness, then the entire part will shrink away from the mold uniformly as it cools, and the part comes out smoothly. if the part has thick and thin sections next to each other, then the molten center of the thicker area will continue to cool and contract after the thin areas and surfaces have already solidified. As this thick area continues cooling, it keeps contracting, and it can only pull material from the surface. The result is a little dimple on the surface of the part called a sink mark.

Some ways to achieve uniform wall thickness:

• Walls in any plastic-molded part should be no less than 40-60% of that of adjacent walls, and all should fit within recommended thickness ranges for the selected material.
• Part geometries such as long unsupported spans, sharp internal corners, and poorly designed bosses should be avoided, regardless of wall thickness.
• Ribs should be used to strengthen tall walls where needed.
• When dealing with sharp external corners, a radius should be placed inside corners with part design permission to make them stronger and alleviate the stress that creates wrap.
• Bosses should follow molding design guidelines rules of properly designed walls of 40-60% of the surrounding area to avoid sink marks.
• Lastly, draft angles should be introduced as 1 degree of draft per 1 inch of cavity depth is a good thumb rule and keeps the draft consistent throughout the workpiece to prevent internal stresses that lead to wrap and curl.

   

 

4. THE MINIMUM PRECISION THAT CAN BE EXPECTED FROM AN INJECTION MOLDED COMPONENT:

Injection molding typically produces parts with tolerances of ± 0.250 mm (0.010") to ±0.500 mm (0.020’’). Tighter tolerances down to ± 0.125 mm (0.005’’) are also feasible in certain circumstances but they increase the cost drastically. This level of accuracy is enough for most applications and comparable to both CNC machining and 3D printing.

   

 

5. STAGES OF INJECTION MOLDING PROCESS: 

A) CLAMPING STAGE: Before injecting the molten plastic material into the mold’s cavity, both halves of the mold have to be closed tightly. This is done with the help of a ‘Clamping-Unit’. Both halves are then attached to an injection molding machine and one half can slide. Material is then injected as the clamping unit pushes the halves together and both halves are held tightly while the material is injected. Larger machines with more clamping power take a longer time to close and clamp the mold.

 

B) INJECTION STAGE: Now, Plastic pellets are fed through a feed-hopper into the Injection Molding Machine. These pellets then move towards the mold by the Injection Unit. The heat provided by the Heater-Bands and the heat generated by the friction from the shear forces being generated between the plastic pellets and the barrel walls serves together to melt the pellets. The volume of material that is injected at once is called a ‘Shot’. The injection time is over when the respective mold is filled completely from the molten material. It is hard to calculate the exact time duration for injection as the flow of the plastic is always changing and dynamic. Injection time can also be estimated by other factors such as Injection pressure, Power, and Shot Volume. The Injection Pressure can range between 35-140Mpa. The rate of injection and the pressure which can be reached are determined and controlled by the hydraulic system in the machine.

 

C) COOLING STAGE: The plastic material inside the mold begins to cool as soon as it makes contact with the interior mold cavity. As the plastic cools down, it hardens and takes the desired shape. The part may shrink during cooling. After the cooling phase is over, then the mold is opened for the ejection of the component.

 

D) EJECTION STAGE: The final stage for the Injection Molding Machine. As the mold opens, the part is pushed out via Ejector Pins. Force must be used as sometimes the part shrinks to a certain degree and tends to stick to the surface of the mold cavity. The mold can be closed again after the ejection process is completely over and then only the next shot can be injected for the process to begin again.  

 

6. INJECTION MOLDED COMPONENT WITH EXPLATION: 

The runner system is the channel that guides the melted plastic into the cavity of the mold. It controls the flow and pressure with which the liquid plastic is injected into the cavity and it is removed after ejection by snapping off.

 

THE RUNNER SYSTEM CONSISTS OF THREE MAIN SECTIONS:

A) SPRUE: The sprue is the main channel through which all the melted plastic initially flows as it enters the mold. Sprues typically have a diameter of about 6mm and taper with a 5-degree angle. This angle on the sprue helps it to release itself from the sprue bush of the mold when it opens, so it can be ejected and recycled. To achieve faster production and reduce the quantity of regrind material, the sprue is kept in hot condition with an external heater. This system is called a Hot Sprue System or a Hot Runner System.

 

B) RUNNER: The runner spreads the melted plastic along the face where the two halves of the mold meet and connects the spur to the gates. There may be one or more runners, guiding the materials towards one or multiple parts. The runner system is cut off from the part after ejection. This is the only material waste in injection molding. 15-30% of which can be recycled and reused. In a mold, if the runner is too small, this can result in the cavity being filled very slowly, which is not desirable. We recommend a large runner with a diameter between 3mm and 6mm. The circular profile of a runner reduces the friction and loss of heat during the injection of the material into the mold. For multi-cavity molds, balancing the runner to fill all the cavities is critical. The flow length between the machine nozzle to the cavities must be balanced and equal for all the cavities to ensure every cavity is filled with the same pressure and speed.

 

C) GATE: The gate is the entry point of the material into the cavity of the mold. Its geometry and location are important, as it determines the flow of the plastic. It leaves a small blemish on the surface of the final part. The gate is the most critical part of the mold design as it must freeze before the part, runner and sprue. This helps in keeping the cavity packed with sufficient pressure and avoiding sink marks on the surface of the part during cooling. The component sticks to the cavity if excessive pressure is used in the gate while injection of the material.  The position of the gate is best when it is located on the thickest portion of the component. This helps in reducing the heat loss, and premature freezing of the gate and lowers the pressure that is required to fill the entire mold. The placement of the gate also determines the flowlines or weld lines on the component. These flowlines or weld lines can be avoided if the material temperature is maintained suitably. The wrong location of the gate or any reduction in the material temperature will form a weld line/flow line defect on the surface of the component.

 

D) COLD SLUG WELL: Its primary function is to act as a trap, or ‘well’, for the cold slug. It’s the small nub of plastic that cools and solidifies inside the nozzle tip during the cooling phase of the injection cycle. This occurs because while the nozzle is heated, the tip of the nozzle is pressed against the cold, water-cooled mold steel. The clog slug that forms at the nozzle tip is useful to act as a small plug to prevent drool during ejection and prevents that nub of plastic to go into the next part’s material. It is useful for pulling out the sprue from the cavity when the mold opens.

 

FREEHAND SKETCH OF SPRUE, RUNNER & GATE & INJECTION MOLDED COMPONENTS: