Injection Molding 101
Many people have contacted me over the years as a result of this website wanting to know more about the plastic injection molding process. They either have an idea they want to bring to market, or are simply curious about the plastic injection molding process.
Before we go further, let me explain that there are four key components to the plastic injection molding process (part design, material selection, tool construction and the plastic molding process). Because it is my intent to keep this at the "101" level, this information will focus on the various types of molding processes along with some tooling information at the end.
Although we are very capable and skilled at the part design and material selection aspects of molding, those disciplines are primarily reserved for the use of Hunter Douglas Inc. We will evaluate using our resources in part design and material usage for outside business on a case-by-case basis.
If you are in need of help with part design and prototyping, check out our Alliances section. Prototyping is an industry all in itself. Once the prototypes have been developed and the design is proven stable, you are ready for production tooling and the production process to begin.
There are a variety of different types of plastic molding, and injection molding is just one of those types. Some others that I am familiar with are the following:
While the above are some of the different types of plastic molding, we are what you would term an "injection molder." There are two types of injection molders in the market; "captive" injection molders and "custom" injection molders.
A captive injection molder would be an injection molder whose sole or primary purpose is to provide plastic parts to its parent company/owner. Some examples of captive injection molder would be Hunter Douglas, General Electric, Rubbermaid, etc. These companies are all large enough that they have vertically integrated the injection molding process. Their demand for plastic components is so high that they have decided to make them themselves rather than source them from an outside supplier.
A custom injection molder is one who injection molds parts strictly to outside customers. They do not mold parts for themselves. We specifically developed this website for the purpose of growing outside business. Therefore, you might consider us a hybrid, or an injection molder who is both captive and custom. There are many custom injection molders in the marketplace, and they serve a variety of industries such as automotive, electrical, medical, etc.
If you have read this far, then you must be interested in plastic injection molding, so I will give it the most attention.
Now that we have reviewed the various types of plastic molding processes, let's turn our attention to the mold itself, which is another critical component of the injection molding process.
Injection molding 101 should give you a broad understanding of what we can do and how you can approach the process of getting your material on the market. If we can be of service to you, or if you have any questions or comments, please contact us.
In rotational molding, or rotomolding, a product is formed from fine powder within a closed mold that is rotated in a heating chamber and then a cooling chamber. While the mold is slowly turning a tumbling, it is heated in an oven by forced hot air, and as the mold wall heats, the resin begins to stick to the inside of the mold, forming a hollow part.
- May be used for hollow, completely closed objects such as balls, toys, containers and industrial parts, including armrests, sun visors, fuel tanks and floats
This is a method of processing in which a parison (hollow tube) is forced into the shape of the mold cavity by internal air pressure. There are three types of blow molding:
Extrusion Blow Molding:
Extruder is operating continuously, and the output either feeds an accumulator or flows through the die as an endless parison. Basic equipment consists of an extruder, crosshead die (and accumulator), clamping arrangement, and mold.
Injection Blow Molding:
This type is suited for containers that have very close tolerance threaded necks, wide mouth opening and highly styled shapes. In the basic process, plastic melt is injected as a parison to a preform cavity, forming the preform around a core rod. A completely finished injection molded neck is formed at this station. The preform is indexed to the blow station where it is blown through an opening in the core rod into the final shape.
Reheat Blow Molding:
For critical containers for foods, cosmetics, carbonated beverages, etc., this process gives many resins improved physical properties. In biaxial orientation, parisons are stretched lengthwise by an external gripper, or by an internal stretch rod, and the stretched radically by blow air to form the finished container against the wall of the mold. This aligns the molecules along two planes - a configuration that gives substantially better barrier properties.
- Used primarily for containers, toys, packaging units, automobile parts and appliance housings
- Similar to rotationally molded parts, but we better controlled variation of wall thickness
- The original process was developed to make glass bottles.
Thermoforming is an ancient technique. Ancient Egyptians found that animal horns and tortoise shells could be heated and formed into a variety of vessels and shapes. When synthetic plastics became available, thermoforming was an early application. In the United States, John Hyatt thermoformed celluloid sheets over wooded cores for piano keys. Thermoforming is a broad encompassing term with many techniques. Some of the most commons techniques to thermoform are straight vacuum forming, drape forming, matched-mold forming, pressure-bubble plug-assist vacuum forming, plug-assist vacuum forming, plug-assist pressure forming, solid phase pressure forming (SPPF), vacuum snap-back forming, pressure-bubble vacuum snap-back forming, trapped-sheet contact-pressure forming and air-slip forming. Today, thermoformed items surround us.
- Signs, light fixtures, ice-cube trays, ducts, drawers, instrument panels, tote trays, housewares, toys, refrigerator panels, transparent aircraft enclosures and boat windshields
- The packaging industry relies heavily on thermoforming.
- Cookies, pills and other products are commonly packaged by blister packaging.
- Single portions of butter, jellies and other foods also appear in blister packs.
The extent of my knowledge on die-casting deals primarily with zinc die-casting. We have four 50-ton Hydrocast zinc die-cast machines that are considered small machines in the die-cast industry. We primarily use them to make small gears that need to hold up with strong wear resistance.
I mention die-cast here because it is very similar to plunger injection molding, only that the material is metal rather than plastic. We use Zamac as a base material, which is a type of zinc. It melts at approximately 800 degrees. This is substantially higher than the melt index of plastic. The molten metal is then injected into a die-cast mold where it is held for a period of time. Once the part has cooled sufficiently, it is ejected off of the live half of the tool and drops into the parts bin.
PLASTIC INJECTION MOLDING
Plastic injection molding is a repetitive process in which plastic is melted or plasticated and injected into a mold containing a cavity in the shape of the desired article. In thermoplastics, the mold is kept at a temperature below the solidifying point of the plastic, causing the injected polymer to freeze, thus forming the article. When processing thermosets, the material is kept below the temperature at which it would cause solidification due to its exothermic reaction until it enters the cavity. In turn, the cavity temperature is kept high to cause the melt to solidify. There are two types of injection molding:
Plunger Injection Molding:
The design of the plunger machine is ideally suited for molding thermoset molding compounds and bulk molding compounds. In the plunger machine, the molding compound is fed into the heating cylinder (barrel). The plunger or ram forces the compound through the cylinder where it is heated by conduction of heat from the cylinder wall. As the material is forced forward, it passes over a spreader or torpedo within the barrel that causes mixing. The plunger forces the material though the nozzle and into the mold.
Reciprocating screw injection molding (What we do):
An extruder type screw rotates within a cylinder, which is typically driven by a hydraulic drive mechanism. Plastic material is moved through the heated cylinder via the screw flights and the material becomes fluid. The injection nozzle is blocked by the previous shot, and the action causes the screw to pump itself backward through the cylinder. During this step, material is plasticated and accumulated for the next shot.
When the mold clamp has locked, the injection phase takes place. At this time, the screw advances, acting as a ram. Simultaneously, the non-return valve closes off the escape passages in the screw and the screw serves as a solid plunger, moving the plastic ahead into the mold. When the injection stroke and holding cycle is completed, the screw is energized to return and the non - return valve opens, allowing plastic to flow forward from the cylinder again, thus repeating the cycle.
The mold base is an assembly of precision steel plates that holds or retains the cavities or cores in a mold. It provides a means for melt to be injected into the cavities and a means to eject the solidified parts from the mold.
The mold itself is a hollow form or cavity into which molten plastic is forced to five the shape of the required component. The term generally refers to the whole assembly of parts that make up the section of the molding equipment in which the parts are formed. Molds can be made out of a variety of materials; stainless steel, aluminum and even wood. The following are some types of molds commonly used and their characteristics.
- Tool used to form hollow plastic products, such as bottles and cans
- Generally made of aluminum, molds can have either water jackets, cast in tubing, or drilled cooling lines
- Isolated areas, such as threads or pinch edges, can be inserted in steel for longevity.
- Molds are equipped with a method for injecting air into the cavities.
- Developed to provide for injection of thermoset material either directly into the cavity or through a small sub-runner and gate into the cavity
- It may be compared to the hot-runner molds with the exception that the manifold section is cooled rather than heated to maintain softened but uncured material.
- The cavity and core plates are electrically heated to normal molding temperature and insulated from the cooler manifold sections.
- A reproduction of an item by electro-deposition of a metal over a model or mandrel that is later removed
- Made from iron, nickel, and copper, with nickel being the most prevalent.
- Designed to produce several different and often unrelated parts
- It is most often used when a processor need to make only a small amount of different parts for a particular application, and all parts are made of the same material.
- Specific core/cavity inserts can be dropped into the mold base, and they can be easily changed without long production stoppage.
- A mold in which the runners are kept hot and insulated from the chilled cavities
- Plastic freeze-off occurs at gate of cavity; runners are in a separate plate so they are not, as is the case usually, ejected with the piece.
- Tools used to form a product using the injection molding process
- Types include hand, two-plate, three-plate, insulated runner, hot-runner and structural foam molds
- Materials commonly used are tempered aluminum, steel, pre-hardened steel, hardened tool steel, stainless steel, beryllium copper and kirksite
- A simplified mold (usually a single cavity) routinely used when part quantity requirements are low
- Used for molding new products for the testing for the product or the mold itself.
Reinforced plastic mold:
- Designed to from a structure manufactured with any epoxy- or polyester-based resin with reinforcing fibers, strands, metallic, ceramic or mineral extenders
- Hard tooling is manufactured from electroformed nickel, cast aluminum or machine steel.
- Use for long run, high-surface finish requirements and high-cure temperatures
- Soft tooling is manufactured from cast plastic, rubber, or reinforced plastics.
- Molds for reaction injection molding may be constructed from epoxy, nickel aluminum or steel, depending upon the expected life of the mold
- They should be designed to constrain a foaming pressure of 30-50 psi.
- For molding with a dense outer skin, it is important to use a temperature-controlled metal mold.
- Molds are manufactured from electroformed nickel, vapor deposited nickel and cast aluminum
- The thickness of the parts is controlled by heat sinks fabricated into the tool
- The tool must be temperature controlled so it will cure the resin within it.
Spray metal and/or melt mold:
- Constructed similarly to those used for conventional injection molding
- Majority are built from forged aluminum plate generally 6061-T651 grade
- Tools that form pre-extruded sheets by means of temperature and vacuum and/or pressure
- Generally made from aluminum, either cast or machined
Definitional credit due to the Thermoplastic Troubleshooting Guide, General Polymers a division of Ashland Inc. and the Third edition of Industrial Plastics Theory and Applications by Richardson and Lokensgard