How to Calculate Molding Machine Sizes

Producing quality plastic products requires a molding machine that has been properly sized.

Molding machines come in a wide variety of clamp tonnages. The clamp tonnage refers to the amount of force that the machine can exert on a mold while the molten plastic is being injected. The plastic is injected into the mold at high pressure in order to ensure that the mold is completely filled before the plastic hardens. The pressure of the injection when coupled with the surface area of the mold creates a force that must be counteracted by the molding machine. The molding machine must be sized to handle this force to produce acceptable products.

Instructions

1.                            Sizing a Molding Machine

1

Determine the surface area of the part that is being molded in square inches. If it is a multiple cavity mold, the surface area associated with each cavity along with the runners that feed each cavity should be included in the total surface area.

2

Determine the planned injection pressure in pounds per square inch (psi). The injection pressure will be a function of the material that you are using and the part wall thickness. A material supplier can help determine the injection pressure.

3

Determine the pressintensification ratio for the molding machine. This is a function of the machine's screw and the hydraulic piston that acts upon it. The machine supplier can provide this information if it is not already known.

4

Multiply the injection pressure by the intensification ratio. This provides the plastic packing pressure in psi.

5

Multiply the plastic packing pressure by the part surface area. This results in the total pounds of force that will be exerted on the mold in pounds.

6

Divide the pounds of force by 2,000 (2,000 pounds = 1 ton). This provides the tons of force required to be offset by the molding machine's clamp tonnage. This gives the minimum clamp tonnage size needed for a molding machine for the specific product in question.

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How to Generate Runner Sizes for Plastic Injection Mold Design

The runner feeds the molten plastic from an injection machine into the mold to form the desired plastic part. The length of the runner is completely dependent on the part design, so there are no specific rules regarding overall length. It is the diameter of the runner that impacts part quality and cost and therefore it should be sized with some care. Once the runner freezes, no more plastic can be injected into the part. The runner should be sized to make sure that part receives sufficient plastic without excessive fractional heating, while freezing quickly enough to minimize cycle time.

Instructions

1.                            Calculating Runner Diameter

1

Determine thickest cross-section of the desired part.

2

Where part design allows, locate runner nearest to the thickest cross section of the part. If the part is of a uniform thickness, locate the runner near the center or along the longest edge, as part design allows.

3

Runner diameter should be between 75% to 100% of the part thickness where the runner is located.

4

Based on your injection machine type, determine the acceptable pressure drop for an injection cycle.

2.                            Calculate Injection Pressure Drop

5

Calculate runner volume. V = pi x r^2 x L, where pi = 3.14159, r = runner radius and L = runner length.

6

Calculate volumetric flow rate (Q) based upon your pre-determined fill time. The fill time is a function of material type, part thickness, and desired cycle time. The molder must know this independent of this exercise. Q = V/t, where t = seconds of fill time.

7

Calculate shear rate (S). S = 4Q/(pi x r^3)

8

Calculate the pressure drop (P). P = (S x m x 2L)/r, where m = melt viscosity of the plastic. The melt viscosity is a function of S, melt temperature, and the material. The molder must know this independent of this exercise, although many plastic suppliers can help with this.

9

Compare pressure drop to predetermined acceptable pressure drop. If improvement is desired, change the runner diameter and repeat the exercise. Repeat calculation until an acceptable runner diameter is found.

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How to Clean Injection Molds

Injection molding is an industrial plastic-making process.

Injection molding is a production process in which plastics are heated until liquid and sprayed into a mold where they cool and are given the final shape of the manufactured product. As the project cools, it shrinks away from the mold walls, allowing it to be ejected. To keep this process working efficiently, molds need to be systematically and regularly cleaned. Although most molds are cleaned haphazardly and by hand, there are also a number of technological solutions available.

 

Instructions

1.                            Cleaning Molds by Hand

1

Disassemble the mold using a screwdriver to disconnect the parts. Separate the parts that are most susceptible to corrosion and contaminate build-up. These include the front and rear clamp plates, bubbler plates, bushings, water lines and wear plate grease grooves.

2

Spray the pieces of the mold with a cleaning spray, or place them in a solvent bath. More abrasive cleaning chemicals may work more quickly, but they will also cause wear and tear on the mold that can shorten its life.

3

Scrub the mold with a stiff brush to remove the contaminants, then wipe dry.

2.                            Cleaning With Dry Ice Pellets

4

Connect a dry-ice pellet blasting system to an air line near your mold. A double-line system introduces the pellets at the spray nozzle, while single-line systems introduce them at the hopper and offer a more powerful spray.

5

Disassemble the mold and strongly secure the parts to be cleaned. The force of the dry-ice pellet impacts can easily knock over unsecured parts, possibly damaging them.

6

Ensure the area is clear. These guns spray dry ice at 900 feet per second, which can cause severe injury.

7

Put on safety gear. This includes a face mask, gloves and an apron as well as ear protection that can block the 102-decibel noise produced by the sprayer.

8

Turn on the air line and squeeze the trigger to spray the parts. The dry ice will dissolve on impact, taking residue with it. This method, however, may not be very effective at rust removal.

3.                            Cleaning Molds Ultrasonically

9

Disassemble the mold and immerse the parts in a detergent solution. The exact type of detergent depends on what kind of contamination is to be removed. Heavily alkaline solutions like sodium hydroxide are very effective with most contaminants, but are dangerously caustic. Mildly acidic solutions are better for rust, but will turn steel gray.

10

Connect an ultrasonic generator to a shop power line. This generator transforms regular current into the very high frequencies necessary for ultrasonic cleaning.

11

Attach transducers to the bottom and sides of the tank. These transducers will vibrate at the high frequencies coming in from the generator, which produces tiny bubbles in the water that impact the mold pieces with force enough to strip away contaminants, while leaving the metal mold undamaged.

12

Set the timer on the generator, and walk away. The cleaning process will proceed on its own.

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The Plastic Manufacturing Process

1.                            Plastic Injection Molding Process

Injection molding is one of the main methods by which parts are manufactured from plastic. The first step in the injection molding process is to feed plastic pellets into the hopper, which then feeds the pellets into the barrel. The barrel is heated and contains a reciprocating screw or a ram injector. A reciprocating screw is typically found in machines that produce smaller parts. The reciprocating screw crushes the pellets, making it easier for the plastic to be liquefied. Toward the front of the barrel, the reciprocating screw propels the liquefied plastic forward, thereby injecting the plastic through a nozzle and into the empty mold. Unlike the barrel, the mold is kept cool to harden the plastic into the correct shape. The mold plates are held closed by a large plate (referred to as a movable platen). The movable platen is attached to a hydraulic piston, which puts pressure on the mold. Clamping the mold shut prevents plastic from leaking out, which would create deformities in the finished pieces.

2.                            Plastic Extrusion Molding Process

Extrusion molding is another method of manufacturing plastic components. Extrusion molding is very similar to injection molding and is used to make pipes, tubes, straws, hoses and other hollow pieces. Plastic resin is fed into a barrel where it is liquefied. A rotating screw propels the liquefied plastic into a mold, which contains a tube-shaped orifice. The size and shape of the tube determines the size and shape of the plastic piece. The liquefied plastic then cools and is fed through an extruder, which flattens the plastic and forms the piece into its final shape.

Issues That Arise in the Plastic Manufacturing Process

A number of complications can arise during the plastic manufacturing process, including burned parts, deformities, surface imperfections and brittle parts. Parts become burned when the molds are not kept cool or if the melting temperature in the barrel is too high. Additionally, if the reciprocating screw becomes jammed or is not rotating fast enough, liquefied resin will remain in the barrel too long and become scorched. Surface imperfections and deformities occur when the surface temperature of the mold is uneven, if the molds are not clamped tightly enough or if the melting temperature is too high. Brittle pieces are formed when not enough liquefied resin is injected into the mold or if the plastic hardens before the mold can be filled. Regular testing and calibration of injection and extrusion molding machines is critical to ensure that the process runs smoothly.

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Using a Valve Gate for Injection Molding

How to Use a Valve Gate for Injection Molding

A hot runner system with a valve gate can lower the production cost of molded plastic parts.

A hot runner system offers some significant advantages in injection molding. A key component of such a system is the valve gate. The valve gate shuts off the flow of molten plastic to the mold at a specific time right at the part surface. This allows a part to be molded without a sprue or runner remaining on the part. Eliminating the sprue or runner improves the appearance of the part, may shorten cycle times, eliminates the need for a runner removal operation and eliminates the waste associated with a discarded runner or sprue.

How to use a Valve Gate in Molding

1

Size and select a valve gate in tandem with the hot runner system. The hot runner system provider will aid in the selection process. In many cases, the mold maker will coordinate this effort. The material to be molded and the overall part design will be the fundamental keys to sizing a valve gate.

2

Install the mold with a hot runner system and the valve gate in an injection press. The electronic controls for the valve gate will need to be tied to the press timer that times the injection cycle. Additionally, valve gates are often pneumatically or hydraulically actuated. Ensure that proper controls are set up to actuate the valve.

3

Determine the time in the injection cycle when the gate should be closed. This will be a function of the rate of material flow and the size of the part. Once the mold has been sufficiently filled by molten plastic to produce acceptable quality, there is no need for additional plastic to be injected. This is when the gate can be closed and the melt procedure for the next cycle can begin.

4

Set the actuator on the valve gate to shut when the injection timer reaches the time determined in the previous step.

5

Set the actuator on the valve gate to open when the injection timer is reset at the beginning of the next cycle.

6

Commence operating the injection press as per normal operating conditions.

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Two Different Types of Injection Molding

Injection molding is a manufacturing process for producing high volumes of finished parts at a low cost per part. Various plastic or rubber parts are produced in a wide range of colors. The injection molding machine injects heated molding material to the part mold through input channels. The two principal types of injection molding machines use either a cold runner channel or a hot runner channel.

Cold Runner Machines

Cold channel injection molding machines cool the entry channel, or runner, after each part is molded and ejected. During each molding cycle a part is produced with material in the runner channel. After the part is ejected, the runner is waste material and must be separated from the molded part. Runner waste is reground and reused or thrown away. Disposed material affects part cost. Reground and reprocessed material may affect part quality. Changing part colors in a cold runner machine is fairly easy since each ejected parts carries the material with it. Cold runner machines offer the advantages of cheaper mold designs, lower maintenance costs and lower operator skills.

Hot Runner Machines

Hot runner injection machines keep the runner portion of the mold hot. This reduces or eliminates runner scrap material, which may reduce part costs. Hot runnermachines are more expensive than cold runner machines. Hot machines require more skilled operators and require costly maintenance. Changing colors in hot runner machines is difficult because material is hard to remove from the runners. Hot runner machines eliminate wasteful runner scrap and the need to separate them from molded parts.

Changing Injection Molding Technology

Until recently, all injection molding machines were hydraulic devices. Newer machines are operated by an all-electric process or a hybrid combination of both technologies. Each injection molding process has an advantage. Hydraulic injectors offer high injection rates. Electric injection machines produce precision parts accurately and consistently. Hybrid machines are best suited to part positioning accuracy and repetitive parts. In a paper presented in May of 2008 to the Energy Technology Conference at New Orleans, Amit Kanungo and Eric Swan, senior engineers for RLW Analytics stated, "though it is a challenge to choose a right machine for molder's processing needs, by most accounts in the near future, all-electric machines will dominate the injection molding industry in the U.S."

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How to Find Quafiyied Plastic Injection Molding Companies

How to Find Qualified Plastic Injection Molding Companies

Selecting the best plastic injection molding company from the suppliers out there can be a difficult task. Let's take a look at how to find the best molding company for you budget by following these steps.

Instructions

1

First, in order to begin evaluating plastic injection molding companies, it is important to know what you need them to mold for you. Specifically, you will need to have a fully defined 3D Model and engineering drawing of the plastic component you wish to have molded. Be sure to fully specify draft angles and surface textures in drawing.

It is important to also specify things like maximum allowable flash.

2

Next, it is critical to realize that for most companies seeking an injection molding supplier, for right or wrong, the largest factor tends to be cost. Going oversees to places like China or Korea usually results in a lower piece cost than staying in the United States. However, this savings in cost per molded part can be offset by delays in getting the product to market.

Based on the project your company is working on, determine if there are absolute restrictions on price as this will be important to know.

3

Next, when evaluating a potential injection molding company, it is very important to learn about the history of the company and their capabilities. How long have they been in business and what industries do they typically mold parts for?

If you have a part that requires extremely tight tolerances, an injection molding company in the health or aeronautics industry may work out better than a company that molds toys.

4

After you learn about the background of the company, investigate their capabilities. What type of molding capabilities do they have? Do they have any specialties such as 2-shot molding or insert molding?

What type of presses do they use and how old are they? Are they able to handle the volumes or quantities that you require?

As alluded to earlier, lower molded component cost does not always translate to an overall savings. Depending on the quality control of the molder, their quality may be sub par resulting in delays in the schedule or worse...returned product from the customer.

Take some time to understand how the plastic injection molding company monitors and ensures quality. Do they do control charting? Are they ISO certified? Ask to see examples of quality records/documentation for some of their recent projects.

If some unacceptable product is found during an audit, what is their corrective action plan? Do they quarantine the last X hours of product?

5

Lastly, the location of the supplier can also be very important. Where are they located and how will this impact your working relationship? Having an injection molding supplier "right in your backyard" so to speak is great as it allows you to make day trips to the supplier when issues arise. Contrast this to going overseas to an Asian supplier where it will cost thousands of dollars per person per trip to visit.

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Research and Forecast of Injection Molding Machine Industry in China, 2014-2018

The injection molding machine belongs to the plastic machinery type that has the largest output and application volume in China, as well as being the main force of China's plastic machinery export. In 2013, the export volume of injection molding machines was 22,946 sets with the export amount of USD 932 million; in 2012, the export volume was 24,830 sets, so the export volume in 2013 declined, but the export value in 2013 increased 3.78%, which fully shows that the average price of export products of China's injection molding machines is gradually growing. With the increasing demand for plastics, plastic machinery industry has huge market potential. From the global perspective, the top three plastic machinery types are injection molding machine, extruder/extruder production line and blowing molding machine, which account for more than 80% in the plastic machinery total output value. Among them, injection molding machine accounts more than 50% of the total output value of these three main types of plastic machines. 

In 2013, the output of China's injection molding machines was about 82,000 sets, rising by 4.21% compared with 2012. On the whole, the growth of China's injection molding machine industry gradually slowed during the year of 2007 to 2013, but the output was still rising. During 2007 to 2013, the average growth rate of output of China's injection molding machines was 4.77%. 

As this report estimates, the market supply volume of China's injection molding machines will reach 86,175 sets by 2014, 89,612 sets by 2015 and about 101,095 sets by 2018

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How to Texture Injection Molds

Injection molds receive a finish to eliminate the machining marks from the mold. High-speed machines typically do this polishing. In some cases, a textured finish is desired. A chemical etch procedure creates the texture based on a pattern. This process usually is manual.

Mold Texturing

1

Create a design of the desired texture as a photocopy or digital file.

2

Transfer the design to a negative. This can be easily done with a scanner. Most scanners have the capability to scan to a negative output file. Or, if the file is already digital, many photo editing software packages have the option to convert a file to a negative.

3

Transfer the negative to a flexible elastomeric material with the desired texture. This is not a do-it-yourself activity for the average person. In general, commercial equipment, often a laser engraver, is required to complete this step. A commercial provider of this type of service should be considered.

4

Placing the textured elastomeric material on the mold surface and add the chemicals to etch the mold. The exact chemical used depends on the specific metal of the mold. It is generally one of the stronger acids.

5

Completely wash the mold of all chemical residues.

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Minimize Sink Marks in Injection Molding

How to Minimize Sink Marks in Injection Molding

Sink marks are caused when a plastic part is not thoroughly packed out. This results from less plastic entering the mold than the volume of the mold was designed to hold. As the plastic cools, it shrinks. When insufficient plastic is injected into the mold, the thicker cross sections will sink as there is not enough plastic there to completely hold the desired structure. The three main reasons for this are poor gating location, poor processing conditions and insufficient gate or runner diameter.

Instructions

1.                            Poor Gating Location

1

Thin sections of plastic will freeze off before thick sections. Determine if the part has uniform wall thickness.

2

If the wall sections are uniform and the gate is relatively central to the part, move on to the next possibility.

3

If the wall sections are not uniform, make sure that the part is gated into the thickest section. This allows the thicker sections to fill with material before the thin sections freeze off and starve them.

2.                            Poor Processing Conditions

4

Running a mold too cold or molding a part with too short of a cycle time can cause a part to be insufficiently packed. First, weigh five parts produced at the current molding conditions and determine the average part weight.

5

Increase the cycle time by 10% and produce five additional parts.

6

Weigh the parts, and determine if there is a noticeable increase in the average part weight.

7

If there is an increase in weight, return to step two and repeat the experiment. Continue repeating until there is no significant change in part weight.

8

If after step two there is no change in part weight on the first attempt, repeat the series of experiments with a different process variable, such as mold temperature. Increase mold temperature by 10 degrees Fahrenheit and repeat the test.

3.                            Insufficient Gate or Runner Size

9

Insufficient gate and runner diameters may allow the runner to freeze before sufficient plastic is injected into the mold. To check for this, first determine the designed volume of the part.

10

Determine the specific gravity of the plastic resin that is being used. The material supplier can give that information if it is not readily known.

11

Calculate the designed part weight. The part weight in pounds is found by multiplying the part volume in cubic inches x specific gravity x .0361. Weigh a set of five parts and determine the actual average weight of the part.

12

If the part weight is below the calculated part weight, increase the gate or runner diameter by one standard size. Mold new parts and weigh.

13

Repeat the process as needed. Once the actual part weight reaches the calculated weight, optimal gate and runner size have likely been

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Advantages of Injection Molding

Injection molding is a method of creating plastic products. Most plastic products are made using injection molding. This process is attractive to manufacturers because of multiple advantages it provides over other plastic molding methods. Overall, injection molding is simpler, more reliable, more versatile and more efficient than most other types of injection molding.

High Pressure

Plastic injection molding operates at a very high pressure, meaning that the melted plastic is pressed harder against the mold than in other molding processes. This provides the advantage of a greater amount of detail in the design of the piece being molded, included complex geometry or detailed features.

Speed

The injection molding process is also very fast compared to other molding processes. A full injection molding process takes very little time, which allows for far more products to be generated from the same mold. Therefore, you get a lot more mileage out of a single mold. Injection molding is highly efficient.

Automation

Injection molding is a highly automated process. Most injection molding is done by machines that a single operator can run. Therefore, very little overhead will have to go into paying the labor force in a plastic injection molding process.

Monomer-Free

Injection molding is a monomer-free process. Monomers are a type of plastic that create a lot of fumes, which many people have a negative physical reaction to. Therefore, any molding process that employs monomers require a good deal of ventilation. If you use injection molding, you will not have to worry about these problems.

Multiple Plastic Types

When you use injection molding, you have the option of molding two different types of plastic at once. This process is called co-injection molding. Therefore, you do not have to worry about access to a particular type of plastic.

Thermoplastics

Injection molding makes use of thermoplastics. Thermoplastics can be injected into the mold, then the excess can be remelted and used again. Therefore, injection molding can save you on materials. Thermoplastics can also come in a wide variety of densities or strengths.

Fillers

Injection molding provides you with the option of using fillers in your molds. Fillers are used to reduce the density of the plastic as it is molded as well as provide greater strength to the product once it is molded. This is another method of reducing the total cost of the process.

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