Injection Molding and Injection Mold Nomenclature: PART 2

REPORT 2

OBJECTIVE: TO STUDY INJECTION MOLDING AND INJECTION MOLD NOMENCLATURE

 

MAIN REPORT: 

1. TYPES OF INJECTION MOLDING MACHINES: 

An injection Molding Machine is a special machine designed for manufacturing mostly plastic products. Mostly. It comprises two sections that are Injection Unit & Clamping Unit. Customized Mold Units are then added by the manufacturers befitting their requirements. In general, an injection molding machine has faster processing ability and fair economical advantages. When choosing an Injection Molding Machine, firstly, the nature of the injection product must be discussed in detail after which sizes of the mold and the product should be identified as larger products that require larger and more sophisticated injection molding machines. Requirements such as mold opening stoke, mold clamping force, injection volume, screw compression ratio, injection speed, surface finish to the product, etc should be taken into consideration and analyzed thoroughly.

There are many types of injection molding machines that are generally classified based on many differentiating factors as explained below:

A) CLASSIFICATION BASED ON THE TYPE OF DRIVE:

HYDRAULIC INJECTION MOLDING MACHINE: Here, all process-relevant movements including injection, opening, and closing of the mold, and recovery are executed via Hydraulic Motors or Pistons. They are an ideal choice when large-volume parts are needed to be produced. It can be used to produce complex products that are needed in the automotive industry with a perfect surface finish. It can be used to produce both thin and thick-walled parts and performance can be enhanced using optional hydraulic accumulators which is a pressure storage reservoirs that can be used to maintain pressure, store and recapture energy, reduce pressure peaks, reduce pressure peaks, power, and dampen shock, vibrations, and pulsations. It is capable of using larger shot sizes when needed. It can deliver exceptionally high ‘Clamping Force’ up to 55000KN. It has a lower base price than an all-electric machine and is capable of delivering a high return on investment because of its high availability and lower cost for spare parts. They’re highly resistant to wear and tear and good for saving energy too as they can use ‘Servo-Hydraulics’ when needed. If the machine is not producing parts, the motors will remain idle and will not use energy. Although their downside is these machines experience variations in their process throughout the year, even throughout the day as the weather is always changing and has an effect on the machine’s temperature and humidity. As hydraulic oil gets cooler, its viscosity increases, and as hydraulic oil gets warmer, its viscosity decreases. The way hydraulic oil performs also changes as it degrades over time which means that the way the oil moves and lubricates will eventually be entirely different than its initial starting date and hence these machines may need to be adjusted periodically throughout the day.

 

II) ALL-ELECTRIC INJECTION MOLDING MACHINE: Electric Injection Molding Machines consume energy only when required for a given action and their motor is configured to match the desired requirements only unlike traditional hydraulic injection molding machines which consume energy continuously even when in an idle state. These injection molding machines are digitally controlled with high-speed and highly efficient servo motors to drive the whole process where each axis is controlled by an independent motor for injection, extrusion, clamping, and ejection. This delivers a faster, cleaner, more repeatable, and energy-efficient injection molding process where energy consumption can be reduced by 50-75% compared to a hydraulic injection molding machine. Out of all types of injection molding machines, electric-type machines have the highest accuracy and repeatability as these machines use servo motors with a 0.1 +/- rotation to actuate their movements. This type of movement allows the process to completely negate the fluctuations and inaccuracies of the movements actuated by hydraulic pressure and their unavoidable pressure curves. Standard hydraulic molding machine controls are considered “Open Loop” as they send signals to valves to adjust pressures, but don’t actually have direct control over the physical movements of the machine and thus cannot make all of the minor adjustments that an all-electric machine can make in real-time. All electric injection molding machines are considered to have “Closed Loop” controls as they control every movement with physical gear turns of their servo motors and can constantly monitor and control their positions and speeds up to very tight tolerances regardless of the ambient conditions. All electric injection molding machines are the perfect fit for manufacturing the most complex, difficult, and thin-walled parts. One can inject very accurate shots while maintaining precise repeatability through the day, night, and during all seasons.

 

III) HYBRID INJECTION MOLDING MACHINES: Hybrid Injection Molding Machines have the characteristics of high efficiency and economic benefits. It is a model that integrates the advantages of hydraulic type/all-electric type and is transformed. These molding machines offer the superior clamping force of hydraulic machines and the precision, repeatability, energy savings, and reduced noise of electric machines. On the shop floor, this translates into improved performance for both thin and thick-walled parts. These hybrid machines use servo motors coupled to a hydraulic pump to circulate the oil used to provide the hydraulic pressure which actuates the components of the machine. Servo pump allows for continuous adjustments to the actual power requirements, low emissions, less noise, and energy savings. Servo motors have a faster response time than standard hydraulic pumps, which leads to faster cycles and also offers more precise controls and more consistent processes due to encoders that track each turn to the exact rotation position. They require less maintenance and experience less downtime than both hydraulic or all-electric injection molding machines. They offer increased diversity of part designs, use of a two-clamp system over toggle, median upfront costs of the three machine options, provide long-term savings, closed-loop process for quicker response times, and lower temperatures that require less cooling, leading to longer oil and machine life, and faster return on investment. 

 

B) CLASSIFICATION BASED ON DIFFERENT AXIS OF OPERATION:

I) VERTICAL INJECTION MOLDING MACHINES: These machines are oriented to operate on the ‘Vertical-Axis’ and are available in Hydraulic, All-Electric, and Hybrid executions based on requirements. Here, the two halves of the mold move vertically, up and down, to open and close. The injection mechanism is typically located at the top of the mold. Vertical molds are ideally suited to insert molding and over-molding due to the configuration of the mold. Inserts are naturally held in place by gravity, rather than having to be built into the cavity or use other methods to remain in position and hence play a large role in filling the mold cavities, along with the injection pressure which helps with the filling time and consistency of the operation. The design of vertical molds and their use of rotary tables means two bottom halves and one top half can be used in tandem so that pre- and post-molding operations can be occurring while parts are being filled. This is especially useful in operations such as insert molding or over-molding, where inserts or substrates must be loaded before resin injection. As a result, there is less of a need for manual operation and intervention, as well as higher efficiency, increased productivity, and reduced costs. Here, products do not automatically fall out of the mold after being ejected and hence must be extracted by hand or robotic arm.

 

II) HORIZONTAL INJECTION MOLDING MACHINES: Horizontally oriented injection molding machines are historically the most common type of machines used for the injection molding process but the orientation itself does not guarantee any superiority over vertically injected molding machines. Here, the mold opens and closes along a horizontal axis. This configuration requires appropriate injection pressure to fill the mold cavities properly and to ensure packing and cooling needs are met as desired by the manufacturers.  These machines are built with more cavities than their vertical counterparts and hence, they can produce more parts per cycle. This type of injection molding machine is easy to operate and repair. Parts can be automatically dropped after being ejected out of the mold cavity although it does require larger floor space. These machines are ideal for cylindrical parts and are advantageous for standard molding that doesn’t require the use of inserts or overmolds. The overall efficiency of the process and Cycle-time for parts are consistent. If inserts are needed to be used then these machines seem to be difficult to work with are proves to be rather inefficient.

 

C) CLASSIFICATION BASED ON APPLICABLE RAW MATERIALS:

I) THERMOPLASTIC INJECTION MOLDING MACHINE: In the case of Injection Molding Machines that are used to produce parts made out of thermoplastic materials, plastic pellets of thermoplastic materials are melted and injected into a mold to create a part. Once this part cools down, the mold opens, and the part drops out. Commonly, thermoplastics that are processed using this process include PC, PP, ABS,  PA, PE, TPU, HIPS, etc.

 

II) THERMOSETTING INJECTION MOLDING MACHINE: Thermoset Injection Machines are typically more difficult to find than their counterparts. The primary difference between Thermoplastics and Thermosetting plastics comes down to heat and chemical resistance. In the case of thermoset injection molding machines, plastic pellets are melted and injected into an extremely hot mold to create a part. This process ensures that the part is cured or hardened to such an extent that it can never be melted again to recycle. Plastic parts made by this process are used in situations where the part must be able to withstand high temperatures, especially in higher voltage electrical applications. This process is generally slower and hence equates to a higher cost depending upon the properties of the materials. Thermoset plastics that are made using this process include alkyds, epoxy, phenolic, polyimides, thermoset polyester, thermosetting polyurethane fiber-reinforced polymers, and melamine.

 

III) POWDER INJECTION MOLDING MACHINE: Also known as Ceramic Injection Molding(CIM) or Metal Injection Molding(MIM). Here, the raw materials are precision-treated fine metal or ceramic powders and bonding materials that can be used to produce portable products, smart devices, electronic products, etc. These machines are generally intended to produce smaller parts with higher precision involving complex appearance, especially when dealing with metals. These parts will have the characteristics of high strength and high density. After the metal powder and bonding material of the powder injection machine are processed, they will become small molding raw materials similar to plastic particles.

 

D) CLASSIFICATION BASED ON CLAMPING STRUCTURE:

I) OUTWARD TOGGLE TYPE INJECTION MOLDING MACHINE: These injection molding machines feature minimum deformation of the mold pattern and provide accurate flatness. Here, the injection unit uses a low-friction linear slide design the injection stability increases substantially for any product. As the machine structures are rigid, it reduces deformation to a minimum while ensuring consistent product accuracy. It has an extra-long mold opening stroke and is excellent for products requiring higher surface accuracy.

 

 

II) SINGLE-CYLINDER/DOUBLE-CYLINDER/ FOUR-CYLINDER DIRECT PRESSURE INJECTION MOLDING MACHINE: Direct Pressure injection molding machine is divided into various models such as single-cylinder, double-cylinder, and four-cylinder according to the number of hydraulic cylinders. The single-cylinder type hydraulic machine requires a larger area and hence the production of parts leads to more difficulty when used in a larger machine as accuracy is a very important factor for production. Insufficient precision will directly affect the accuracy of the mold clamping. These single-cylinder direct-pressure injection machines are gradually being replaced by multiple-cylinder direct-pressure injection machines. The double-Cylinder injection consists of two cylinders that although will be slower than single-cylinder injection machines but will deliver higher injection pressure. The design of the four-cylinder direct pressure machines is different as it has an automatic balance correction function and hence it does not affect the accuracy of the clamping. Also, the scope of application is relatively wider. When compared to ‘Toggle Type’ injection machines, the direct pressure type has the advantages of long life, direct mold clamping, and no resistance during travel and hence requires less lubrication and results in low maintenance costs. Their durability factors make them a better choice than the toggle-type injection molding machines.

 

III) COMPOSITE DIRECT PRESSURE INJECTION MOLDING MACHINE: The components injection by the compound direct pressure injection machine are the same as the direct pressure type. The machine with a longer length has a larger mold opening force, and a longer clamping stroke can be designed though maintaining the durability of the components is a great challenge.

 

E) CLASSIFICATION BASED ON INJECTION STRUCTURE:

I) MONOCHROME INJECTION MOLDING MACHINE(SINGLE-COMPONENT INJECTION): Monochromatic Injection means that the entire plastic component is of the same colour during the injection. A monochromatic Injection Machine can also be used to inject products with two different colours. The colour injection molding machine uses the first colour to inject the first semi-finished product, and then uses the second colour to process the semi-finished product to complete the production.

 

 

II) MULTI-COLOUR INJECTION MOLDING MACHNE(MULTI-COMPONENT INJECTION): The Multi-colour Injection Molding Machine can simultaneously inject plastic components with more than two different colours. The most widely used multi-colour injection molding machine is the two-colour injection molding machine. It is an application in combination with LSR silicone rubber. Multi-colour injection molding offers lesser environmental impact and higher efficiency. 

 

 

2. DROOLING: In the nozzles of the injection molding machine, when the nozzles are moved away from the sprue gate after injection molding has been carried out, trickles of plastic comes out from the nozzle’s orifice. This is said to be caused by the residual pressure of fused plastic stored between the nozzle and the screw. This trickle of plastic is generally called drooling or trickling. 

A) REASONS FOR DROOLING:

1. High nozzle tip temperature
2. High melt temperature
3. Insufficient gate cooling
4. Insufficient melt pressure release
5. Longer pressure holding time
6. Longer injection time
7. Damaged, worn, corroded, replaced, or reworked gate

 

B) HOW TO STOP OR REDUCE DROOLING:

• Using a Needle-valve:  A needle valve is provided within the nozzle in which the needle valve is biased by a spring, the drooling may be prevented even when back pressure is applied, but high plastic pressure is required to open the valve during the process of injection and thus entails a considerable resistance.
• Decompressing the Mold: It is a machine control setting that moves the screw back a small distance away from the plastic remaining in the barrel at the end of the holding phase. The purpose of doing this is to release any pressure remaining in the plastic after the holding phase. Decompression is important before the screw rotation starts and after the screw rotation has ended. 1-2% of the total screw back distance is sufficient for decompression. When packing and holding pressure is applied, the check ring becomes firmly sealed against the matching screw surface also known as the thrust ring portion of the screw. When the holding time ends, there is still plastic pressure holding the check ring against the thrust ring surfaces. If the screw immediately begins rotating, pressure will cause extra wear on the two mating surfaces. Decompression moves the screw back and releases the pressure on the check ring before screw rotation begins which results in relieving pressure at the nozzle tip and hence preventing drooling along with other benefits as well.

 

3. INJECTION CAPACITY OR SHOT CAPACITY: Injection capacity, also known as ‘Shot-capacity ’, is the maximum volume of material that is required to be injected into the mold to manufacture the product within one operational cycle. While calculating the shot volume/weight, the weight of the actual product, spruce, runner, gates, etc are taken into consideration. Generally, the ‘Shot-weight’ is always lesser than the ‘Shot-capacity’. It is dependent on factors like material density & machine screw diameter. 

 

    

 

4. PLASTICIZING CAPACITY: Plasticizing Capacity is the maximum quantity of a specified plastic material that can be raised to a uniform and moldable temperature in a unit of time.

 THE PLASTICIZING RATE  IN INJECTION MOLDING:

A) Material: ABS

Screw recovery time: 7.2 secs

Total shot weight: 132g

No. of impressions: Single

Plasticizing Rate = Total shot weight of  the shot / Screw recovery interval(secs)

P.R = (132/7.2)g/sec

P.R = 18.33 grammes per second for ABS

 

B) Material: PVC

Screw recovery time: 5.02 secs

Total shot weight: 6.42g

No. of impressions: 16

Weight of sprue and runner: 8.68g

Plasticizing Rate = Total shot weight of  the shot / Screw recovery interval(secs)

Total weight of 16 impressions: No. of impression(16 in this case)*Weight of each impression(6.42 in this case) = 102.72g

Total weight including feed system: 102.72g + 8.68g  = 111.4g

Plasticizing Rate: 111.4/5.02 = 22.19g/sec

 

5. INJECTION PRESSURE: Injection Pressure is the pressure that is applied over the molten plastic material in front of the screw inside the barrel section from the injection unit to fill the mold cavity. Injection pressure molding balances against the machine’s clamping pressure and is calculated based on the size and shape of the part, as well as the size of the gate opening. During the initial injection, the mold is empty, so there’s little resistance and the cavity fills quickly but if it fills too quickly, it can cause various types of defects which are as follows:

• The molten resin can spray itself out of the gate in a jet, rather than flow smoothly. The resin will hit the opposite tool wall, cool, and solidify, making a defective part.
• Resin under excess pressure will shear, meaning that the chemical bonds can get destroyed and make the part defective.
• High injection pressure can force the mold cavity to open at the parting line which can result in the creation of flashing on the part and more importantly, it can seriously damage the mold tool. This happens because the mold fills very quickly at first when it’s empty when there’s low internal resistance but when the mold cavity is almost 95% filled, the resistance spikes and internal pressure increases significantly can lead to the destruction of the tool or damage the machine and hence pressure must be scaled back quickly to the changeover point.

 

6. CLAMPING PRESSURE: Clamping Pressure is a measurement of the force needed to hold the mold closed during the injection. If this pressure is not set high enough then the mold can get forcibly opened prematurely due to excess injection pressure leading to flashing on the desired part. The mold opening and closing are controlled by a motor mounted at the rear of the machine. The moveable part of the mold is called the B-side, and it’s mounted on a moving plate called a platen. This platen slides back and forth on thick metal tie bars as the mold is opened and closed for each cycle. The tie bars not only guide the opening of the platen but also act as springs. When closed these bars must be preloaded with a tension higher than the injection pressure will exert. Toggles act as locking levers to make this happen.

Importance of Clamping Pressure:

Clamping pressure is the first variable that should be calculated by the injection molding engineer since it will determine the capacity of the machine to be used to get the best results. This is important for product developers because the choice of the machine affects cycle times, parts per hour, and cost.

Calculation of Clamping Pressure: To determine how much force the platen must hold, the process engineer will calculate using the following parameters:

• The area of the finished part’s footprint.
• The flow length is the distance from the gate of the mold to the furthest edge of the part.
• The average wall thickness of the part.
• The number of cavities in the mold.

 

7. MAXIMUM DAYLIGHT: A daylight in an injection molding machine is the space or distance between the fixed platen and the moving platen during open and close clamping on an injection molding machine. The minimum and maximum daylight in the injection molding machine determines the sizes of the items it can make. The maximum daylight has to accommodate the height of the moving half of the mold, the height of the molded part, the required clearance for removal of the molded part, and the height of the fixed half of the mold.

Formula to calculate the daylight for Toggle Injection Molding Machine:

Maximum Daylight: Maximum Mold Height + Mold’s Open Stroke

Minimum Daylight: Minimum Mold Height + Mold’s Open Stroke

 

Formula to calculate the daylight for Hydraulic Injection Molding Machine:

Maximum Daylight = Minimum Mold Height + Mold’s Open Stroke

 

8. TYPES OF INJECTION MOLDS:

A) BASED ON THE FEEDING SYSTEM:

• Hot Runner Injection Mold
• Cold Runner Injection Mold

B) BASED ON CAVITIES:

• Single Cavity
• Multi Cavity
• Family Cavity

C) MOLD OPENING:

• Two Plate Mold
• Three Plate Mold

 

9. TWO-PLATE MOLD: Two Plate Mold is the simplest and most reliable mold. It has simpler construction and generally has fewer moving parts including a fixed plate and a moving plate. Here, Sprue, Runners, Gates, and Cavities are all on the same side of the mold. Cold Runner and Mold Cavities are on the same plate. Easier to design and build. Has one ‘Parting Line’ and 2 main parts known as A-side and B-side.

It is more straightforward to run in production as it is easier to operate and due to that reason it is also cheaper to manufacture compared to its counterparts. It has minimal operational problems that lead to a longer lifespan and requires less maintenance. Also, the cycle time is shorter which helps to reduce the cost of products and improve the efficiency of production. This type of mold can drop the manufactured plastic parts freely and after molding is done.

Although it is necessary to cut off the gate manually for this kind of mold it is also easier to choose a gate shape and location. The gating method includes a side gate, direct gate, or submarine gate.

 

10. THREE-PLATE MOLD: Three Plate Mold is also known as ‘Small Gate Mold’ and is more complex than a Two-Plate Mold. It has an additional mold plate between the top clamp plate and cavity plate that is called a runner plate or stripper plate. It allows the runner scrap and plastic molded parts to be released separately. The three-plate mold opens three times in total. The first opening is to break the gate and pull the sprue, so the feeding runner can be removed easily by hand or by a robotic arm. The stripper plate will move after puller bolts are pulled by the cavity plate. The second time it opens is called a runner opening to release the molded parts. This opening will make space between the stripper plate and the cavity plate. The third time it opens to release the product automatically and is suitable for automation. The three-plate molds generally have guide pins on the front mold. The cost of the three-plate mold is generally higher, injection pressure is also higher. Nozzle recovery cost increases. With three plates, the gate can be designed in an ideal position for comparison and convenience, and the gate will fall off by itself, leaving little traces. Three-plate mold is required when product appearance requirements are high, and the obvious glue point cannot be seen. It eliminates the process of cutting the nozzle. From the perspective of the mold structure, the guide pillars of the three-platen mold are all on the front mold whereas the guide pillars of the two-platen mold are on the back mold. From the usage point of view, products with a small nozzle inlet method use a three-plate mold while a large nozzle inlet method uses a two-plate mold for plastic products.  

 

11. HOT RUNNER MOLD: It is an alternative melt delivery system for the cold runner. Its function is similar to that of the cold runner, as it provides a path for the molten resin to flow through a highly technical and carefully engineered system. The most important difference is that a hot runner never allows the molten resin to solidify. The distribution network is completely encased in a steel manifold and never exposed to air. The manifold distributes molten resin to nozzles that provide access to the gate and cavity. The manifold and nozzles are regulated with thermocouples and heaters to maintain the temperature of the molten plastic. Where a cold runner can be likened to an open-air downtown street, the hot runner is more like a subway tunnel. While a hot runner increases mold price, it should be considered an investment because of its ability to eliminate waste, reduce cycle time and decrease part variation. Several options are available for gate design and temperature control, depending on resin and application. Gates can be controlled thermally with hot-tip nozzles or mechanically with valve-gate nozzles. These have their design requirements for how they interact with the mold. Temperature controllers can be used for process monitoring, control, and fine adjustment. Hot runners eliminate the need to handle the cold runner before, during, and after processing. Manual labor or robotics are not needed to remove solidified runners or handle regrind, resulting in reduced cost, less equipment to maintain, better use of precious floor space, and valuable time savings. With no runners to manage, this eliminates de-gating and facilitating sorting. For processors, these cycle time advantages can translate into significantly increased output.

Depending on the application, many factors impact the suitable design of the hot runner system. These include gate type (wear, cooling, tolerance, and dimensional design), manifold requirements (flow balance, thermal balance, melt channel sizing, mold size, and the number of cavities), and processing variables (pressure, injection speeds, mold and processing temperatures, resin type, and system timing).

Hence, it can be said that hot runners allow for increased productivity and system performance, producing better part aesthetics, greater flexibility, and improved process monitoring. In addition, they improve energy efficiency and eliminate scrap plastic, resulting in faster cycle times and reduced part costs. They also deliver melt to the mold with high gate quality, excellent cavity-to-cavity balance, fast color changes, total design freedom, and greater customizability. Hot runner solutions help processors achieve the highest levels of quality and part volume at the lowest part cost. They are available in both hot tip and valve gate configurations, optimized for a processor’s specific application.

 

12. ADVANTAGES OF RUNNER MOLDS: 

• NO RUNNERS LEADS TO LESS WASTE: In the hot runner system, any residual substrate that ran through the runner chambers but not into the mold cavity remains heated and liquified and thus runs straight through the mold and back into the ‘pool’ of readily available molding material to use on the next cycle. In cold molding, this runner was either discarded(meaning material waste) or, in a more time-consuming process, reground and recycled into that pool of material.

 

• FASTER CYCLE TIMES: The runner reclamation process takes time and creates a potential loss of resources if this time investment is not made. By eliminating all runner concerns, the hot runner system can eliminate these finishing processes and potentially speed up cycle times. Remember that heating and cooling the mold in a hot runner process also takes additional time- time that is often offset by the savings associated with eliminating the runner.

 

• A CLEANER FINISHED PRODUCT: Removing runners from cold molded parts, whether through robotics or manual removal, involves a mechanical removal process. If a clean break or shear is not achieved, it can potentially affect the acceptability of the finished product, depending on the cosmetic and operational requirements. Hot molding can also help to eliminate many issues like burn marks or other surface irregularities that can occur where the temperature of the mold and the substrate is not as closely controllable or monitored.

 

• PROVIDES BETTER SURFACE FINISH AND DIMENSIONAL ACCURACY: Hot-Runners allow greater control over the part quality, dimensional accuracy, and surface finish. The gate location is no longer limited to the perimeter and can be moved to direct melt flow in a way that better influences critical dimensions or other features affected by filling characteristics. A shorter total flow length may be possible, which can reduce cavity fill pressure requirements and other process elements that impact part warpage. The projected total area and clamp tonnage requirement may also be reduced.

 

13. INSULATED RUNNER MOLD: Insulated runner systems are inexpensive alternatives to traditional hot runners, where the mold is not heated, but the runner channels are extremely thick and stay molten during continuous cycling. This feature provides the flexible gating advantages of a hot runner switching out the added cost of the manifold and drops of the heated hot runner system. Properly designed insulated runners, with thermally controlled gate offers several advantages over hot runner mold systems, including:

• Dependable Reproduction: It is ensured that No “Dead-Spots” is present inside the mold.
• Uniform Temperature Distribution: Thermal insulation forms through melt deposited at the wall.
• Minimal Heat Loss:  Reaches Thermal Equilibrium quickly.
• Inexpensive: Molds are less expensive to build and maintain compared to the hot runner system.
• Reduces energy cost: Low energy input is required at the time of start-up.
• Consistent parts: More consistent volume of polymer per part can be obtained.
• Easy to maintain: Can be cleaned quickly.
• Faster Molding Cycles
• Improved Part Finish
• Decreased Tool Wear

 

14. HOT MANIFOLD MOLD: Hot manifold mold consists of a heated manifold system with two plates that are also heated. The manifold helps maintain a consistent temperature by keeping the molten thermoplastic in the runners at the same temperature as the heating cylinder. The heated runners deliver the molten plastic to nozzles that fill the core mold to form the final part.

Two main categories are: Externally heated and internally heated. The externally heated systems are well suited to polymers that are sensitive to thermal variations while internally heated systems offer better flow control.

 

15. TRANSFER MOLDING: 

The transfer molding process combines the principle of compression and transfer of the polymer charge. Transfer molding is similar to compression molding except that instead of the molding material being pressurized in the cavity, it is pressurized in a separate chamber and then forced through an opening and into a closed mold. The advantages of transfer molding are that the preheating of the material and injection through a narrow orifice improves the temperature distribution in the material and accelerates the crosslinking reaction. As a result, the cycle times are reduced and there is less distortion of the molding. The improved flow of the material also means that more intricate or complicated shapes can be produced.

Molding Sequence:

• The required amount of polymer charge is weighted and inserted into the transfer pot before the molding process.
• The transfer pot is heated by the heating element above the melting temperature of the polymer charge.
• The liquid charge is gravity filled through the sprue to the mold cavity.
• A “piston/plunger/ram and cylinder” arrangement is built in the transfer pot so that the resin is transferred into the mold cavity through a sprue.
• The plunger is also preheated in the transfer pot.
• The plunger is used to push the liquid polymer charge from the transfer pot into the mold cavity under pressure.
• The mold cavity remains closed as the polymer charge is inserted.
• The mold cavity is held closed until the resin gets cured which means the material sets hard to the cavity shape after a certain time (cure time) has elapsed.
• The mold cavity is opened and the molded part can be removed once it has hardened with the help of an ejector pin.
• The sprue and gate attached to the molded part have to be trimmed after the process has been completed.

Types of Transfer Molding Processes:

• Pot Transfer Molding

• Plunger Transfer Molding

 

16. VACUUM MOLDING: Vacuum molding is also known as “Vacuum Forming” which is done using a single mold and a vacuum pump. The heated sheet is placed into the mold, and a vacuum is applied to place it properly into the mold of the desired shape.

A) The entire process consists of six processing steps mentioned below:

I) Load: A sheet is placed in a clamped frame and is held down on all four sides.

II) Heat: After loading, the sheet is ready to be indexed to the heating station, depending on the machine design the sheet will be indexed to the heaters or the heaters will move towards the sheet. In either case, heaters above and below the sheet heat the plastic to its forming temperature typically from 285˚F(140˚C) to 375˚F(190˚C) depending on the type of plastic. As the sheet is heated, it loses strength and begins to sag. To experienced vacuum forming personnel, the amount of sag and the shape of sag is a useful indicators of how evenly the sheet has been heated and how well the plastic can be formed. For some parts designs, the sheet is intentionally heated unevenly to improve the plastic distribution in the formed part.

Molds for production are usually made of aluminum that is easy to machine to form a solid block or be cast from a pattern. Aluminum is an excellent conductor of heat and hence it cools the plastic parts efficiently. Prototype molds and molds for low production quantities might be made out of wood, fiberglass, and even plastic.

III) Form: A sheet is placed over a mold due to which the air under that sheet is trapped. Then, that plastic sheet is heated in a clamp frame until it becomes rubbery. A vacuum is used to remove the air from under the sheet. The air under the sheet leaves the mold through vents. To remove a large amount of air, the mold has gaps in the assembly. Drilled vent holes are used in locations where small volumes of air are likely to be trapped. Atmospheric air pressure stretches the sheet into mold. Some regions stretch more than others and this causes uneven sheet thickness.

If the part is made in a female mold, the thickest plastic is present at the top flange where the sheet first touches the mold surface and begins cooling. The sheet becomes thinner as it is stretched towards the bottom of the cavity. The sheet is the thinnest at the base of the molded part.

If the part is formed on a male mold, then the mold moves up towards the heated sheet. Then the plastic first touches the metal at the projection and this plastic is held there as it begins cooling. The rest of the sheet thins as it is stretched towards the flanged region. The sheet is sealed around the mold and the vacuum is applied. Atmospheric pressure presses the sheet onto the mold. This sheet distribution is the opposite of the previous mold. It is thick and the base of the formed part and thinning along the side walls towards the flange. The flange itself does have varying thickness, thin at the corners, and thicker towards the clamp frame.  

Drape vacuum forming is also a process where the plastic sheet is draped over a male mold and the sheet stretches as it is pulled.

Another way to stretch the sheet into a deep section is called plug assist. In a fixed plug assist, a plug is attached to the upper half of the mold. The mold closes and the plug stretches the sheet. A vacuum then pulls the sheet onto the surface of the mold. The plug has pre-stretched additional plastic where it is needed. The shape of the plug affects the sheet distribution to improve the part thickness along the bottom and in the corners. There’s very little control over the part’s thickness.

IV) Cool: The formed part is then allowed to cool down after the forming process is done.

V) Part Removal: After the part has cooled down sufficiently it is then removed from the vacuum former.

VI) Trim: In the end, any excess part is trimmed to obtain the desired finish. Processes that can be used to trim the part are Trimming with a vertical band saw, Trimming with a drill press and slitting saw, Trimming with an overhead or table-mounted router with a guide pin/bearing cutter/ slitting saw, Trimming with roller press, Trimming with a horizontal band saw, Trimming with a 3 or 5 axis router or Trimming with a robot.

 

B) Types of Vacuum Forming Processes:

• Drape Forming- Male

• Snap-Back Vacuum Forming – Male

• Billow Snap-Back Vacuum Forming- Male

• Straight Vacuum Forming- Female

• Plug Assist Vacuum Forming- Female

• Plug Assist Pressure Forming- Female

• Billow Plug Assist Vacuum/Pressure Forming

• Pressure Forming- Heavy Gauge Sheet

 

C) Materials that can be formed using Vacuum Forming Process:

• Acrylonitrile Butadiene Styrene ABS
• Polyester Copolymer PETG
• Polystyrene PS
• Polycarbonate PC
• Polypropylene PP
• Polyethylene (sheet and foamed sheet) PE
• Polyvinyl Chloride PVC
• Acrylic PMMA

D) Machines used in Vacuum Forming:

• DIY Vacuum Forming Machine
• Table-Top Vacuum Forming Machine
• Single Heater Vacuum Forming Machine
• Double Heater Vacuum Forming Machine