Injection Blow Molding Basics

Injection blow molding is a manufacturing process that combines the accuracy of injection molding with the minimal waste and rapid processing of blow molding. Injection blow molding can be used with plastics and is generally used to make hollow shapes such as bottles. The manufacturing process is more expensive than other blow molding techniques, so larger containers are generally fabricated using extrusion blow molding. The final products of injection blow molding have no seams, and can be glass clear.

Injection Blow Molding Process

The injection blow molding process has three phases: Injection of the material into the preform mold; blowing of the preform, or parison, into the product mold; and ejection of the product from the machine. A commonly used machine configuration is a three-stage blow molding machine, which rotates through 120 inches for each phase of the process. In the injection phase, molten plastic or glass is injected into the preform mold over a core pin, which forms the interior of the parison and finishes the interior of the neck. At this point, the neck of the container is fully formed. The parison is removed from the mold and inserted into the blow mold. The parison is inflated inside the blow mold by pressurized air. After a cooling period, the product is ejected from the machine, tested for leaks, and moved to another station for labeling or finishing.

Designing for the Injection Blow Molding Process

Designing products to be fabricated using injection blow molding includes selecting a thermoplastic resin for the product application, designing the mold geometry to take into account material shrinkage during cooling, calculating the right amount of material to be injected and choosing the right pressure to inflate the parison in the mold. Tolerances in the core rod and preform mold design are critical so that the core rod sits exactly in the center of the mold and the resulting parison has constant wall thickness prior to blowing.

Products Fabricated Using the Injection Blow Molding Process

Injection blow molding is used to create hollow products such as bottles, and is generally only used for small containers such as medicine bottles or single-serve beverage containers. The injection phase of the process can create a neck with very fine tolerances, with designs such as screw threads or threads for childproof caps. Unlike standard injection molding, undercuts can be incorporated into an injection blow molded design for some products, such as mascara bottles.

Chrome Vs. Nickel Plating

Chrome and nickel are metals used to plate machine parts, such as rollers and cylinders, with a scratch-resistant surface that protects them from wear and tear. They are also used on bathroom and kitchen fixtures.

1.                            Chrome

Chrome comes in standard and hard versions. It is applied in varying thicknesses according to the required purpose. Thinner coatings are used on objects that are not exposed to much abrasion. Thicker coatings offer more protection from abrasion and corrosion. If a machine part is frequently exposed to water, experts at Phoenix Electroplating recommend an undercoat of nickel plating, as chrome is porous.

Nickel

Nickel is used to prevent corrosion, particularly when applied prior to chrome plating on objects. It is also hard-wearing and is widely used on machine parts in the oil and gas industry, the automotive industry, in making molds for plastics and in food processing machines.

Bathroom and Kitchen Fixtures

According to Rejuvenation, suppliers of home fixtures, nickel was the standard finish for kitchen and bathroom fixtures made from the 1800s until the 1930s. Chrome then overtook nickel in popularity. Nickel is warmer in appearance and creates a more authentic, antique look, but Rejuvenation says the two finishes blend harmoniously in one room.

How to Clean Chrome Plated Plastic

Any chrome surfaces actually consist of an extremely thin layer of chrome plated onto another material. Regardless of what that other material is, from plastic to steel to aluminum, this chrome plated is what needs to be addressed when the time comes to clean it. You can always buy a special chrome cleaner, but there are several items in your home that work just as well and are far less expensive.

1.                            Toothpaste Method

1

Spread a thin layer of toothpaste on the surface of the chrome plating. Use the opaque white toothpaste, not one of the gel varieties.

2

Rub the toothpaste-covered chrome plating with a soft, clean cloth in small circular and swirling patterns. There is no need to apply a lot of force.

3

Take a fresh cloth and wipe away the toothpaste, revealing a shining, clean surface.

2.                            Vinegar Method

4

Pour undiluted distilled white vinegar into a spray bottle.

5

Spray the surface of the chrome plating with the undiluted vinegar.

6

Wipe down the surface with a clean cloth.

3.                            Baby Oil Method

7

Dampen a portion of a soft cloth with baby oil.

8

Rub the baby oil into the chrome plating in the same manner as you did with the toothpaste.

9

Wipe off excess baby oil with a fresh cloth or a still fresh portion of the same cloth.

10

Wipe down the newly cleaned area with soapy water to remove any greasy feeling from the baby oil, if desired.

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The Stages of Blow Moulding

today virtually all cast plastic parts are manufactured using blow molding technology.

Blow molding is a process used to cast plastic parts that are hollow. Since 1942 when the first polyethylene bottle was produced using blow molding technology, blow molding has become the standard manufacturing process for making cast plastic parts. Blow molding involves the fabrication of a tube made of molten plastic called parison. The next stage of the process is to shape the tube into the desired form using one of several blow molding processes.

Heating The Resin

The first step in any blow molding process is to heat the resin. Thermoplastic resin is heated to about 400 degrees. Once the resin reaches its molten state, it is extruded over a die head. The die head forces the thermoplastic resin to flow around it, thus creating a hollow center in the resin. The resin has now formed a tube called parison.

Molding

The parison is then placed inside a pre-determined mold. The interior shape of the mold cavity is what will ultimately determine the shape of the finished casting. The mold is then closed around the parison. At this point in the process, the parison is still very warm. The mold however, is cooled with water.

Inflation

Compressed air is then blown into the mold. The air fills the center of the parison tube and forces the sides to expand, thus conforming to the shape of the mold interior. The parison is still very warm at this point to allow it to expand without breaking. As the warm parison expands to meet the cold walls of the mold, it freezes off and becomes rigid.

Cooling

Once the warm parison has cooled off, it solidifies inside the mold. Once properly cooled, the parison will maintain the form in which it was cast. The mold can now be opened and the finished plastic component removed.

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Dangers of Injection Molding

Many plastic products are pressed by an injection molding machine.

Many common plastic items are created using an injection molding machine, and manufacturers point to the machine's efficiency and speed in pressing out plastic products. Much of the injection molding process is automated, and in many cases, the machine operator's role is merely to monitor it. While operation requires minimal training, there are several potential dangers to running an injection molding machine.

Heat

Depending on the type of material used, the melting point for plastic ranges from 250 to 650 degrees Fahrenheit. An injection molding machine will heat up enough to not just melt the plastic, but allow it to flow smoothly into the mold. Burns are a hazard, whether from the machine's heated surfaces, melted plastic or from the freshly molded product. In addition to eye and face protection, heat-resistant gloves may be needed while operating the machine.

Caught In the Press

On an injection molding machine, the platens join together to form the actual mold, and the plastic is molded under pressure before it is ejected. This constant movement creates a hazard, as fingers can be crushed or amputated in the press. Long hair and jewelry can also get hung up in the machinery, severely injuring the operator. Reaching into a molding machine to free up a stuck part is particularly dangerous.

Peripheral Machinery

While not part of the injection molding process, waste plastic is often thrown into a grinder that can shred the scraps--or anything else that gets into the grinder--in seconds.

Fumes

Plastics and polymers are made from different chemical compounds that, when melted, may give off hazardous fumes. An injection molding machine should only be operated in a well-ventilated area.

Flying Objects

While most products made via an injection molding machine are ejected in a controlled manner, there is still the possibility of flying objects. Eye and face protection should be worn when operating the machine.

Slipping and Falling

Good housekeeping is essential when running an injection molding machine. Scraps of plastic can quickly collect on the surrounding floor area quickly, and it is easy to lose your footing and fall. New plastic is often shipped in pellet form, which can be hazardous if spilled.

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Steps of Injection Molding

·                                  

Many popular plastic items are produced by injection molding.

Injection molding has continued to grow since the late 19th century. Capable of producing small items such as combs, it is also used to create parts for airplanes and medical supplies. It is hard to imagine the world without the products it produces. The process was patented by John Wesley Hyatt and his brother Isaiah in 1872. Today, injection molding is used to produce about 30 percent of all plastic products. The process is relatively simple, but expensive. Thus it is usually only used to mass produce items.

Instructions

1

Clamp the mold shut. This will hold the mold in place while the mold is filled with melted plastic. It will also keep the mold still while the plastic cools.

2

Inject the melted plastic into the mold. The plastic starts out as polymer resin pellets which are poured into a large open-bottomed hopper. A motor turns the auger, feeding the pellets into the cylinder where they are melted and turned into molten plastic, then pushed into the mold. The auger injects the melted plastic into the mold at a pressure between 10,000-30,000 pounds per square inch. The auger then holds the plastic, forcing more plastic in to fill the mold completely. This guarantees that the final product will not contain any gaps. A gate closes keeping the plastic inside the mold while it cools. Molds are usually either water ic belting. 35+ years' experience.

3

Drill small holes into the mold, if it is cooled by water or another liquid. The cooling period accounts for about 85 percent of the molding process. The temperature of the water is usually between 33 and 60 degrees Fahrenheit. Water below freezing can be used. However, glycol, or a similar additive, needs to be used to keep the water from freezing. The major disadvantage to using water to cool the mold is the buildup of condensation.

4

Loosen the clamp and open the mold. Remove the plastic part that was just created. Then clean the part, removing any excess plastic.

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 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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key point of injection mold design (2/2)

  2.8 one hinge

2.8.1  for PP material toughness, can be designed hinge and product to be an integration.

2.8.2 as a film hinge should be less than the size of 0.5mm, and to maintain a uniform ,

2.8.3 When integrally injection hinge design, the gate only can be designed at one side of the hinge .

 

  2.9 inserts

2.9.1 In the injection molding ,inserts increase the local strength , hardness, dimensional accuracy and setting small screw holes ( shaft )  to meet a variety of special needs. Will also increase the cost of the product .

2.9.2 Insert typically may be copper ,or other metal and plastic.

2.9.3 insert embedded in the plastic part should stop rotation and anti- pull design structure . Such as: knurled , holes , bending, flattening and so on.

2.9.4 insert should be appropriately thick plastic around to prevent stress cracking of plastic parts .

2.9.5 design inserts, should fully consider the positioning in the mold ( holes , pins, magnetic )

 

  2.10 logo

   Product identification is generally set in the relatively flat surface of the product , and the use of convex form , select the consistent face of mold opening, or may cause the part injury .

 

  2.11 precision injection molded parts

  Due to heterogeneity and uncertainty injection shrinkage , significantly lower than the metal injection molding precision parts, should Select the appropriate standard tolerances (OSJ1372-1978)

 

  2.12 deformation of injection molded parts

  Improve the rigidity of the injection of product structure, reduce deformation . Try to avoid flat structure , a reasonable set of cuffs , uneven structure. Set reasonable reinforce ribs .

 

 2.13 GAIM

2.13.1 using gas-assisted injectionmolding, can improve product rigidity and reduce distortion.

2.13.2 using GAIM avoid sink mark.

2.13.3 using gas-assisted injection molding, can save raw materials, shorten the cooling time .

 

  2.14 welding ( hot plate welding, ultrasonic welding , vibration welding )

2.14.1 welding , can improve the coupling strength.

2.14.2 welded to simplify product design.

 

3 . Reasonable to consider the contradiction between process and product performance

Must be integrated into contradiction product appearance , performance and design of injection molding processes Sometimes sacrifice some process to get a good appearance or performance.

3.1 if sometimes mold design can not avoid injection defect, try to design and make the defect on the hidden place of the parts.

 

 

Injection molding design points

1 , the direction of mold open and mold parting line

2 , draft angle

3 , thickness of the part wall

4 .ribs

5 , chamfer, rounded corners and holes

6 , core-pulling mechanism and avoid

7 , plastic deformation

8 , one hinge

9 , insert

10 , GAIM

11 , considering the technology and performance parts

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