Rotating cores are the most common solution to this problem. This is one of the most common setups. The rotating core features a gear, bearing journals and a retracting thread, in addition to the molding thread.
The basic operation is to rotate the core and retract it at the same time and at the same rate as the core is unscrewing from the cap on the other end. This requires several other components. Usually a rack is used to rotate the gear and an adjustable lead nut is used to time the retraction. The rack requires an air cylinder or some other means of drive. The core will require bearings of some type to allow free movement and guidance of the core during the retraction. The mold must have a feature to keep the cap from turning while the core is turning and retracting from inside. This is usually done with anti rotation lugs, which are cut into the ejection sleeve or into the stripper ring.
As you can see there are quite a few parts and some very critical timing involved in order to make a simple thread. It’s called a simple thread because the threads on a rotating core must also run all the way off the end of the core. If not the core would strip the threads off as it retracted from the cap.
The current design had some interesting advantages. The entire core assembly is much shorter than previous designs. This allows it to be used in smaller presses, presses that could not support a rotating setup. It simplified the mold construction—no racks, no gears, no drive components that a rotating setup uses. All of the motions needed are performed by the normal open and close motion of the press. The finishing of the molding area has been greatly simplified. No special fixtures, no long instructions, just grind it! This presents a potential cost-effective alternative to a rotating core setup.
Plastic injection molding of internal threads presents some problems. In general, injection molding of internal surfaces—such as a medicine vial—is relatively easy. A slight draft on the cores’ surfaces allows the plastic vial to be stripped from the core by means of a stripper ring. The part slides off the core easily, the draft reduces the drag quickly and the soft part can be stripped without deformation. Even with shallow details on the core—such as engraved lettering and fill lines found on dosage cups or laundry detergent caps—the part can still be stripped by conventional means. When it comes to a threaded internal surface like a bottle cap a different method must be used.
Some threaded caps can be stripped. Changes are made to the flank angle of the thread, or the threads are relatively shallow. Certain plastics are easier to stretch over the threads and better at retaining the original molded shape after ejection from the mold. When the threads are deep and the material is not so flexible, the cap must be removed in a different way.
Rotating Core
Rotating cores are the most common solution to this problem. This is one of the most common setups. The rotating core features a gear, bearing journals and a retracting thread, in addition to the molding thread.
The basic operation is to rotate the core and retract it at the same time and at the same rate as the core is unscrewing from the cap on the other end. This requires several other components. Usually a rack is used to rotate the gear and an adjustable lead nut is used to time the retraction. The rack requires an air cylinder or some other means of drive. The core will require bearings of some type to allow free movement and guidance of the core during the retraction. The mold must have a feature to keep the cap from turning while the core is turning and retracting from inside. This is usually done with anti rotation lugs, which are cut into the ejection sleeve or into the stripper ring.
As you can see there are quite a few parts and some very critical timing involved in order to make a simple thread. It’s called a simple thread because the threads on a rotating core must also run all the way off the end of the core. If not the core would strip the threads off as it retracted from the cap.
Collapsible Core
The collapsible core offers some unique advantages to the traditional rotating core. The concept of the collapsible core is to first collapse the core to a small enough size to be able to clear all the molded features of the piece part, and then retract the core from the molded part. It sounds and looks a lot like a magic trick.
The basic core uses two components, an actuator pin and a collapsing sleeve. The sleeve is made up of several fingers of two types: (1) inner fingers and (2) outer fingers. The fingers are arranged in a cylindrical formation and all the fingers are bent toward the center of the cylinder. The inner fingers are narrower and bent further toward the center than the outers.
The actuator pin is a cylindrical shaft used to hold the fingers in place and to form the top section of the molding surface. When the actuator pin is inserted from the back of the core, the inner fingers are forced outward against the outer fingers. As the pin is further inserted, the inner and outer fingers move to their molding position and form a solid sleeve around the actuator pin.
When you look at the core from the molding end of the assembly, the fingers look like pie sections placed next to each other in a circle around the pin. The way it works is that when the pin is removed the smaller (inner) pie sections will move to the area that was occupied by the pin. This produces gaps between the bigger (outer) pie sections. The outer sections move toward the center until the gaps are taken away.
The actions all happen at the same time due to the spring loading of the fingers against the pin, and the corresponding angles on the flanks of the fingers allow them to slide against each other as the core is expanded or contracted. A tapered area inside the bore area of the sleeve allows the core to open at a controlled rate.
So a collapsible core has on average 12 fingers and a pin to form the inside section of a threaded cap. Each finger has three surfaces that need to seal out the plastic during the molding cycle. That’s 36 surfaces that need to come together within tenths! A pretty daunting task considering all of the fingers are bent to produce the spring action needed to close the core.
This brings us to the main drawbacks of collapsible cores. They may flash. Plastic moves into any gap that is large enough. The resulting flash can make the molded parts unacceptable. Plastic can build up on the flanks of the fingers and require cleaning. Spring pressures seem to be inconsistent. Current finishing processes use complicated fixtures and detailed instructions on how to finish the molding details without affecting the sealing abilities of the core. Some say that the cost of the core and its unreliable sealing ability and complicated finishing process make them a last resort.
Flash-Free Collapsible Core
The applications of a flash-free collapsible core are nothing short of incredible. Undercuts, O ring grooves, tabs, ribs, bosses, locking tabs, and threads are easily produced. Straight or even back-tapered bores are no problem.
Actually, all of the features listed could be combined on one core! Since the core contracts before the part is removed almost any feature can be molded into the side of a bore. No rotation is necessary.
Core Development
During the development of the core, the emphasis was on the sealing ability of the fingers as well as the simplification of the finishing process. Several prototypes were produced using three different manufacturing procedures.
A customer provided actual in mold testing at its facility and recorded the results. The first core flashed badly. A design flaw was discovered. Major design changes were made after this first
attempt and a great deal of attention was put on the bending procedure. It seemed to be the key to a successful seal.
Two more prototypes and numerous changes to the bending fixture produced a working core. It was good, but it could be better. The best features of each of the prototypes were combined into a new design and three different processes were tried on three more prototypes.
To speed things up all three designs were run at the same time. A new and highly accurate bending fixture was created and used on all three. Initial testing in molding produced perfect results. More innovations and design changes made the core more robust and reliable.
The customer was very happy. Yet another larger core was produced using the latest design and submitted for testing. It performed flawlessly. As an afterthought it was asked if the core could be sampled using nylon instead of the intended polypropylene. The customer agreed and once again the core succeeded. The nylon was molded at 1800 psi with no flash. The core was produced using the standard procedure. It was not tweaked to run nylon; it just seals that well.
ROTATIONAL MOLDING can be defined as a high-temperature, low-pressure, open- mold, intermittent,manufacturing pro- cess that uses heat and biaxial rotation to produce hollow, one-piece plastic parts.To rotationally mold a product such as a roll-out refuse container, a mold that defines the shape of thepart to be produced is mounted on the arm of a molding machine . This machine is capable of biaxially rotating and moving the mold through the four phases of the process.To perform this process a predetermined amount of plastic material, in the form of a liquid or a powder,is loaded into the mold’s cavity. The machine then simultaneously rotates the mold in two directions and moves the mold into the heating chamber. The mold is heated and all the plastic material adheres to the inside surface of the cavity. While it continues to rotate, the machine moves the mold out of the heating chamber and into the cooling chamber, where the plastic is cooled to the point that the formed plastic part will retain its shape. The machine then moves the mold to the open station, and the mold stops rotating. The mold can then be opened and the molded part removed. The mold is then recharged with plastic mat erial and the process can be repeated.
Types of Mold Cavities and Cores
In injection molds, there are many types of cavities, and cores can be categorized. There are some main types as bellow
1. Fixed Cavities and Cores:
Standard Cavities and Cores: These are the primary components of the mold that define the shape and features of the molded part. They are typically machined from hardened steel and remain fixed in the mold throughout the production process.
Simple Cavity/Core: This type of cavity and core is used for producing parts with straightforward shapes and minimal internal features. It consists of a single cavity and core component.
Multi-Cavity/Core: In this configuration, multiple cavities and cores are arranged within the mold to enable the simultaneous production of multiple parts in each molding cycle. It increases production efficiency and reduces cycle time.
2. Interchangeable Cavity/Core:
Interchangeable Cavity/Core: Interchangeable cavities and cores allow for flexibility in producing different parts using the same mold base. They can be replaced or swapped out to accommodate various part designs.
Replaceable Cavities and Cores: These are cavities and cores that can be easily replaced or interchanged in the mold. They are used when different part variations or designs need to be produced using the same mold base.
Sliding Cavities and Cores: Sliding cavities and cores have the ability to move or slide within the mold to create complex part features, such as side holes or threads. They are often used for producing parts with internal threads, sliding mechanisms, or other intricate features.
Insert Cavities and Cores: Insert molding involves placing pre-formed inserts or components into the mold, and the cavities and cores are designed to securely hold and encapsulate these inserts during the molding process.
3. Collapsible Core:
This type of core is used for producing parts with intricate internal features or threads. It allows for the creation of hollow or collapsible sections within the molded part.
4. Unscrewing Cavity/Core:
Unscrewing cavities and cores are utilized for producing threaded or screw-type parts. They can rotate or unscrew to release the threaded part from the mold.
The specific types of cavities and cores within each category can vary depending on the complexity of the part design and the specific requirements of the injection molding process. The choice of cavity and core types is determined by factors such as part geometry, parting line considerations, material flow, and ease of mold assembly and disassembly.
The basic principle of rotational molding involves heating plastic inside a hollow shell-like mold, which is rotated so that the melted plastic forms a coating on the inside surface of the mold. The rotating mold is then cooled so that the plastic solidifies to the desired shape and the molded part is removed. There are many methods that can be used to achieve the essential requirements of mold rotation, heating, and cooling. It has been estimated that about 40% of the rotational molding machines in use in the U.S. are home-built. Of the remaining 60%, about 70% are more than ten years old, and 40% are more than twenty years old. The percentage of home-built machines is even higher in some other parts of the world, but there is a move toward the purchase of new machines as molders start to concentrate on their core business in order to survive in very competitive markets. The data acquisition systems and process control on commercial machines also make them attractive and compare very favorably with what is available in competing technologies such as blow molding, thermoforming, and injection molding.
Most people with general engineering skills tend to take the view that a rotational molding machine is not a complex piece of equipment. While few individuals or molding companies would contemplate building a blow molding machine or an injection molding machine, there has been no such reluctance to build rotational molding machines. This has worked well for some small companies in that it has allowed them to meet internal needs or satisfy a local niche market, but this do-it-yourself approach has also harmed the image of the industry. Home-built machines by their nature often do not have much investment in safety features or aesthetics and are highly individual in appearance and performance.
The build vs. buy strategy depends on many circumstances and quite often relates to the nature of the business and the local market. The uniqueness of the part can dictate this decision. A company may be in an engineering business not directly involved in plastics, but it currently purchases hollow plastic parts. It may take a business decision to manufacture these in-house. From its general engineering expertise such a company can be quite capable of making a simple machine to rotate, heat, and cool a mold for making the parts. The machine will be product specific but will be as good or better than anything that the company could buy for its needs, and will certainly be less expensive.
Another common scenario is where a company manufactures products from fiber-reinforced plastic (FRP) and/or thermoformed plastic, but desires to broaden its product range. Rotational molding is a closely allied manufacturing method and from the company’s expertise in working with plastics, it is no great challenge for it to make a rotational molding machine for new products that are similar to its existing lines, in order to broaden its customer base. There are also many examples of individuals or companies that use tanks or containers for dispensing or storing insecticides and chemicals, and they decide to manufacture their own storage containers because these are regarded as being too expensive or have limited availability. Or there may be confidentiality associated with the product. If the part being rotationally molded requires special polymers, special treatment, or special processing conditions, the logical business decision may be to construct a special machine specifically for that particular part.
In circumstances such as those described above, it may well have proved advantageous to build rotational molding equipment in-house. The trend in the industry is, however, toward high technology with more sophisticated molds, improved machine controls, internal cooling, and mold pressurization. Commercial machines will undoubtedly offer economic advantages in terms of faster cycle times and more economic operation, so that it will be difficult for molders to remain in competitive market sectors without having this type of equipment.
The basic principle of rotational molding involves heating plastic inside a hollow shell-like mold, which is rotated so that the melted plastic forms a coating on the inside surface of the mold. The rotating mold is then cooled so that the plastic solidifies to the desired shape and the molded part is removed. There are many methods that can be used to achieve the essential requirements of mold rotation, heating, and cooling. It has been estimated that about 40% of the rotational molding machines in use in the U.S. are home-built. Of the remaining 60%, about 70% are more than ten years old, and 40% are more than twenty years old. The percentage of home-built machines is even higher in some other parts of the world, but there is a move toward the purchase of new machines as molders start to concentrate on their core business in order to survive in very competitive markets. The data acquisition systems and process control on commercial machines also make them attractive and compare very favorably with what is available in competing technologies such as blow molding, thermoforming, and injection molding.