This article will take an in-depth look at thermoplastic molding.
The article will bring more detail on topics such as:
This chapter will discuss the definition and process of thermoplastic molding.
Thermoplastic molding is a manufacturing process that works to create fully functional parts by injecting plastic resin into a pre-made mold. Thermoplastic polymers are more widely used than thermosetting polymers in injection molding. The main reason for this is that thermoplastics are plastics that can be repeatedly softened by heating and solidified by cooling, making them highly recyclable materials.
Materials left over from a previous molding process cycle are re-grinded and added back to an injection chamber along with virgin pellets. This addition is limited to 30% of the bulk material, as it can degrade the plastic's original physical properties.
Thermoplastic molding has several sub-categories, such as rapid injection molding, which is best utilized in fine-tuning prototypes before a product is given the go-ahead for production. Another sub-category of thermoplastic injection molding is production injection molding; this is best utilized for full product runs. Developers get to use the thermoplastic injection molding process for many applications, as it can produce anything from cell phone cases to car door panels with good accuracy and surface finish.
This process is the industry standard for producing plastic parts. This reason allows developers to be certain that they are putting out a quality product if they go this route in the development process. The thermoplastic injection molding industry has changed over the years: initially, it aimed mostly at producing combs and buttons. Now it has transformed into the production of a wide range of products for many industries, including automotive, medical, aerospace, consumer products, toys, plumbing, packaging, and construction. Thermoplastic injection molding, or injection molding, uses certain materials' thermoplastic properties to inject a softened plastic into a mold to create an impression. This thermoplastic molding technique provides the best quality for large and very large series productions.
The principle of plastic injection is based primarily on the use of heat to soften a thermoplastic (TP) material in the form of pellets. The softened material is kneaded and injected into an endless screw. The rotation of this screw and the pressure added after that can insert the material into a mold. This inserted material takes on the impression of the mold and is then cooled; after it cools, the object takes on solid form and can be removed from the mold. The removed mold can then be reused for another process of injection. This method is ideal because it allows the duplication of many identical objects of high quality. This fact is made possible due to the ability of the mold to be reused with the right material. This process is why thermoplastic injection is particularly suited to large or very large series production.
Another advantage is that designed parts require little or no subsequent machining. However, this method has a cost because it requires the design of a steel mold, which caters to the duplication of many series parts. This disadvantage is why it is not recommended for designs of small series. An example of a preferred solution is silicone molding.
The way thermosets are made differs from thermoplastics in several aspects, and both the categories require varied treatment during the injection molding process. Included below are a few differences when molding thermosets and thermoplastics.
| Thermosets | Thermoplastics |
|---|---|
| When producing parts, the cold material is injected into a hot mold | When producing parts, the plastic material is melted and injected into a mold |
| Can’t be remolded or reshaped | Can be remolded and recycled |
| Forms a permanent chemical bond | 100% reversible, as no chemical bonding takes place during the process |
| Comparatively difficult to surface finish | Thermoplastics result in accurate, flexible, and pleasing surface finishes |
| Does not require high heat and high pressure compared to thermoplastics molding | Requires high heat and high pressure |
| Thermosets are made by condensation polymerization | Thermoplastics are made by additional polymerization |
| The production process includes compression, transfer, and casting | The production process includes injection molding, extrusion, and blow molding |
| Some of the end products that come from thermosetting injection molding include: Handles of tools, billiard balls, insulation, computers parts, television parts, any electronic equipment, gardening equipment, tools, sprockets, and cooking utensils | Some of the end products that come from thermoplastic injection molding include: Vacuum cleaners, toys, machine screws, gear wheels, kettles, packaging film, sacks, power tool casings, toasters, gas pipes, and fittings |
| Disadvantages of thermosets are: unable to be recycled, and they release emissions referred to as volatile organic compounds (VOCs) | Disadvantages of thermoplastics are: they are expensive, easily melt when heated, and are hard to prototype |
The thermoplastic injection molding process cycle takes place in an injection molding machine which mainly consists of the injection unit, clamping unit, and the mold.
In the clamping step, the halves of the mold are shut before the molten plastic is injected. They are then held after the molten material has dwelled in the cavities. This process occurs in the clamping unit, which is responsible for several functions, such as applying sufficient clamping force to resist the injection force. It does this while keeping the mold halves shut during the injection step until the dwelling step is reached.
The clamping unit is also responsible for ejecting the molded part after the dwelling step, closing and opening the mold plates between molding cycles, and holding the mold plates in a proper alignment.
The clamping unit is made of:
In the injection step, the raw plastic pellets are melted and then transported to the mold. This takes place in the injection unit. The injection unit is responsible for several things, including supplying molten plastic to fill the mold cavities. The volume of plastic injected into the mold is referred to as a shot. The volume of the shot depends on the part's volume.
The injection unit supplies heat which melts and homogenizes the plastic pellets before injecting them into the mold. In addition, it supplies enough injection speed and pressure to push the molten plastic and fill the cavities of the mold.
The injection unit comprises:
Once the molten plastic is moved into the mold, it is allowed to settle inside the cavities. The holding pressure is used to replace the injection pressure in this step to compact the molten plastic during its solidification. The cooling process proceeds once the molten plastic comes in contact with the surface of the cavities. Cooling is made possible by a coolant system inside the mold. Shrinkage of the part may occur during cooling; therefore, an additional melt is allowed to flow to compensate for shrinkage that occurs during cooling. After cooling it for a required time, the mold halves are separated, and the molded part is ejected.
The cooled part is separated from the mold in the ejection process stage. The ejection system, which is found in the clamping unit, eases the removal of the molded part from the mold cavities. The ejection system is made up of an actuating ejector bar that acts to push the ejector plate with ejecting pins. The ejecting pins drive out the solidified part from the open mold plates at the end of the molding cycle. Enough ejecting force must be supplied because the part adheres to the mold during cooling. A mold release agent is used to help in removing the molded parts from the mold cavities. The agent can be reapplied before the start of the clamping step after a few molding cycles. It may also be permanently fixed on the surface of the mold cavities.
Trimming is the last step in the production of injection-molded plastics. Here, excess plastics resulting from the flow of the molten plastic are cut from the molded part. Then each molded unit is separated from the rest of the molded parts. The process of trimming takes place with separate equipment. During injection of the molten plastic, the mold channels, which are the sprue, runners, and gates, are filled. The molten plastic in those channels can then solidify together with the melt inside the cavities. Flashes may also form on the edges of the part. After cooling, the surplus plastic materials adhere to the part that must be cut.
The mold mainly comprises two plates fastened to the clamping plates. The rear mold half is linked to a movable plate which allows the opening and closing of the mold. It is also adjacent to the ejection system of the clamping unit. Before the start of the molding cycle, the two plates must be free and clean from contamination. The mold cavity is the shaped section in the mold plates that provides the plastic parts their final form. When the molten plastic flows into the cavity, it fills up the shape of the hollow space and gains its volume.
The front mold half contains most of the volume. One or more cavities can be found in a mold. The parting line is a line found in the closed mold halves that reflects their separation. A parting line may be a straight line or a curve in complex tooling designs. Air can vent in the parting line more easily, so the molten plastic tends to move in this region. Some finished parts may have a visible line or curve, which reveals that the two sides of the part are formed on different plates.
In the mold channels:
Other mold tool features include air vents that eliminate entrapped gasses inside the mold and the cooling channel, which facilitates the dissipation of heat to a coolant.
The injection molding parameters include:
This chapter will discuss the different types of injection molds and the common types of thermoplastics used in injection molding.
An injection mold is selected depending on the part geometry, production volumes, budget, and tool design. The type of injection mold may affect the manufacturing cost and the quality of the components.
Single cavity molds are manufactured to produce one part per cycle. These molds are a cost-effective approach when part volumes are low because they are less expensive to build. They have a shorter lead time. The disadvantage of these molds is a higher piece part cost in molding production.
Multi-cavity molds produce more parts per cycle. Some of the included advantages are increased capacity, and lower piece part costs since more parts can be produced within a comparable cycle time.
A family mold contains a single mold base with two or more different cavities, allowing for the production of two or more different parts. All the produced parts can be formed simultaneously, or shut-offs can be used to isolate production to selected cavities. For great results in molding, the parts should be similar in shape, resin, size, and anticipated volumes.
Having similar parts is especially important if the intention is to run all parts at the same time. Automation may be needed to separate the pieces during or after production. A family mold can provide cost savings and versatility if mold cost is a driving factor and volumes are low.
Unscrewing molds are commonly used to create threaded holes within a plastic part. Unscrewing molds are automated with small drive systems, including rack & pinion, electric, or hydraulic motors. These drive systems are tied into the process and rotate threaded features to extract undercut features. Threads can either be internal or external, and the extraction is tied into the press cycle. Multi-shot or multi-component tooling allows a product designer to use two or more materials that are not similar on one part within the same cycle.
There are many reasons why different materials would be needed, including different physical properties or simply appearances. Unscrewing molds often use multiple manifolds within one tool. Multi-shot tooling can be an ideal solution for complicated products or color changes within a product line. This requires multiple machine nozzles, hot runner systems, and mold rotation.
Hot runner components utilize a temperature-controlled manifold to reduce or eliminate runner scrap from the cycle. Injection points can be outside or directly inside the part. Cycle times are increased as a sprue system, also called a runner system, is a determining factor in the cycle of the mold. Eliminating the runner saves expensive wasted material.
The controller must be sized to match the manifold in the mold. Through material and cycle time savings in the long run, they often compensate for themselves. This is particularly true in uses that need expensive engineering-grade resins or high annual volumes.
Cold runner molds are more traditional tools that utilize sprues and runners to gate into the part. This is the least complicated form, but it may result in larger amounts of wasted material and slower running cycles. Depending on the area of use, a percentage of wasted material may be re-grinded and re-processed for future use. However, this can affect the physical properties of the resin. Dealing with more advanced, high-cost materials such as medical and engineering-grade resins may lend to using an efficient hot runner. This also applies to the inability to re-grind components in an application.
Insulated runner molds resemble more traditional cold runner molds. They use cartridge heaters or other methods of heating to form a surrounding layer of molten resin. This results in forming an insulated cull to create a similar effect to a hot runner system. This approach is less expensive than using a hot runner, which requires a temperature controller. It also allows for faster color and material changes. The disadvantage is that insulated runners are not suitable for all types of materials and therefore do not work well with more demanding engineering-grade resins.
Three plate molds are grouped as cold runner tools. By adding a third plate to the runner system, one can locate the injection point in virtually any location on the tool. The process is usually less expensive than adding a hot runner system. However, this type of mold can be more difficult to automate as it often has large and unwieldy runners.
Some of the common thermoplastic injection moldings are:
Acrylonitrile Butadiene Styrene (ABS) is an opaque thermoplastic and amorphous polymer. Acrylonitrile butadiene styrene is a terpolymer, a copolymer consisting of three distinct monomers; in this case, they are acrylonitrile, butadiene, and styrene. These three combined can create a product that is flexible, lightweight, and moldable into many items we use in our everyday lives.
The advantage of ABS is that it allows various modifications to be made to improve impact resistance, toughness, and heat resistance. When molding is performed at a high temperature, the gloss and heat resistance of the product are improved.
Molding at a low temperature results in the highest impact resistance and strength. In addition to molded plastics, acrylonitrile butadiene styrene is used in drain pipe systems, golf club heads, and automotive parts.
Polyethylene is a thermoplastic polymer that contains a variable crystalline structure. It is one of the most versatile and popular plastics and is used in an extremely large range of applications depending on the specific type.
Polyethylene is found in two common plastics: high-density polyethylene (HDPE) and low-density polyethylene (LDPE). It has high ductility levels, tensile strength, impact resistance, resistance to moisture absorption, and recyclability.
A polyethylene material with a higher density produces plastic that is stronger, more rigid, and more heat-resistant. The primary applications of this material are plastic bags, plastic films, containers, and geomembranes.
Polycarbonate plastics refer to a naturally transparent amorphous thermoplastic. These thermoplastics are put to use in the production of a variety of materials that require impact resistance and transparency, e.g. bulletproof glass.
Polycarbonate can experience large plastic deformations without cracking or breaking. As a result, polycarbonate is commonly used for greenhouses, eyewear lenses, medical devices, automotive components, and cellular phones.
The applications and benefits of thermoplastic molding include:
Thermoplastic molding is used to create various things such as bottle caps, wire spools, packaging, automotive dashboards, and pockets combs. This molding technology allows the production of small series with the proper material. The uses of thermoplastic injection are highly varied in the plastics industry, particularly in the automotive, packaging, medical and electronics fields. A thermoplastic injection can be ideal for the regulatory requirements and constraints in a field requiring many tests and certificates, such as the medical sector. The parts produced are mechanically equal to series production. In addition, the thermoplastic molding manufacturing technique is used to produce parts that are very small, such as electronic components, or very large, such as car body parts.
In providing good quality for the part, using the right material for the injection gives the parts made by thermoplastic molding the robustness and mechanical properties of series parts, making it ideal for carrying out tests and marketing a small series. Molds produce goods of excellent quality, and injection molding or thermoplastic injection can use very small or very large molds depending on needs while also maintaining high precision. This is why this method of rapid prototyping is used in the medical and automobile fields.
Thermoplastic molding is a manufacturing process that works to create fully functional parts by injecting plastic resin into a pre-made mold. Thermoplastic polymers are more widely used than thermosetting polymers in injection molding. The main reason is that thermoplastics can be repeatedly softened by heating and solidified by cooling, making them highly recyclable materials. Materials left over from a previous molding process cycle are re-grinded and added back to the injection chamber along with virgin pellets. An injection mold is selected depending on the part geometry, production volumes, budget, and tool design. The type of injection mold has the ability to affect the manufacturing cost and quality of the components.
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