American Plastic Molds are the essential tooling that enables the production of plastic parts. They are made of a variety of materials that influence the quality and life expectancy of the final product.
The runner system (a network of channels that lead to gates) and nozzle (and sprue bushing) design is crucial for proper material distribution.
Injection molding is a highly productive plastic fabrication process that involves injecting liquid or semi-molten plastic into a mold. It’s ideal for the manufacture of complex plastic products and parts that require a high level of precision and repeatability.
The molten plastic is forced into the mold cavities by an injection screw or plunger. As the plastic enters the cavity, it expands and fills every crack and crevice in the shape of the mold. The pressure of the injected plastic causes molecular bonds to break, allowing the molecules to bend and twist until they take on the final shape of the product.
Once the plastic has fully cooled, it’s ready to be removed from the mold. To prevent the plastic from tearing as it’s pulled away, the injection mold contains ejector pins. These pins are circular and placed in the ejection half of the injection mold. The ejector pins push the molded plastic through the gates and runner system when it’s ready to be ejected.
Injection molded plastics can be made from both thermoplastic and thermosetting polymers. Thermoplastics, which are more common in injection molding, can be melted and reshaped without losing their original physical properties. Thermosetting plastics, on the other hand, must be completely solidified before they can be used again.
When designing injection molded plastic parts, engineers take a number of factors into account. They want to ensure that the design is easy to manufacturable, and they follow specific injection molding guidelines. These include avoiding features that are difficult to machine, designing walls to be as thin as possible for strength and cost savings, using rounded edges where possible, adding living hinges to create flexible joints, using ribbed supports to add strength, and making sure that all ejection mechanisms are in place before a plastic part is injected into the mold.
Clamping pressure is another important factor in the injection molding process. Also referred to as “clamp tonnage,” it refers to the amount of force required to clamp the two plastic mold halves together during the injection process. The clamping force is based on the projected area of the finished plastic part, and it is usually determined by multiplying that area by 2 or 3. Incorrect clamping pressure can cause not only defects in the resulting plastic parts but also expensive damage to the injection molding equipment such as cracked platens, fractured hydraulic cylinders, crushed mold vents, and misaligned ejection mechanisms.
Blow Moulding
Blow molding is a process used to produce hollow-bodied plastic workpieces. It was first developed in the 1800s, and uses compressed air to blow molten plastic like a balloon and shape it into a finished product. This method can be used to create anything from bottles to automobile gas tanks. Originally, craftsmen would heat glass to its melting point, then blow through it to inflate the glass into its final form. The technique was adopted for plastic and has since been used to mass-produce a number of different products.
Unlike injection molding, blow molding does not require a separate runner system to transport the plastic through the mould. This makes it ideal for producing large, thin-walled products such as soft drink bottles and automotive gas tanks. However, it is also possible to use this method to create taller, thicker products, such as plastic containers and pipes.
In order to produce a plastic part using this method, the manufacturer must first extrude a thick tube of plastic called a parison. The parison will have enough material to form the final shape of the plastic product, and then it is sealed into a two-part mold and pressurized with air. This air pressure pushes the parison against the walls of the mold, which helps to close it into its final shape. The plastic is then allowed to cool and solidify in the mold.
Once the plastic has cooled, it is removed from the mold and any excess materials are trimmed away from the edges of the workpiece. The process can be repeated to create multiple copies of the same part, and this is the basis of a major portion of the plastics recycling industry.
The ability to reuse plastics is a key advantage of this method, as it significantly cuts down on the amount of waste created by the manufacture of new plastic products. This is because the material can be recycled many times without losing its initial properties, meaning that it can still be made into a new plastic product after being used for several applications.
Cast Moulding
This is a relatively simple casting method using liquid metal. It can produce a high volume of parts that are very precise without the cost of more premium methods like injection molding. It can also work very well with a complex design and even combine several different elements into a single product.
Cast moulding can be a great way to get your prototypes on the market quickly and economically. It’s ideal if you want to test out your concept before investing in a more costly production run. In fact, many companies use this type of casting for low-volume production runs as it is very efficient and costs less than injection molding.
The original object is coated with a sealant to prevent the rubber mold from sticking to it, usually a solvent-based polyurethane resin or methyl ethyl ketone peroxide (MEKP). It’s then brushed on the surface of the model and allowed to set, or cure as we say. During the curing process, temporary walls of cardboard or clay need to be placed around the object to contain the liquid rubber.
After a set amount of time, the temporary walls are removed and the liquid is poured out to create the cast. This is usually done by hand or with a metered system (with someone on hand to keep watch). Once the casting is made, it needs to be cooled before it can be removed from the mould.
The final product is then shaped and sized to fit the intended use. This can be done either with hand tools such as files and sandpaper, or with industrial machinery that uses high-pressure water jets to remove any rough surfaces. The cast is then finished off by buffing, sanding and polishing to give it a glossy finish or the more rustic look of natural stone.
It’s important to be aware that some of the materials used in this process pose health and safety issues. For example, plaster powder and some of the liquid rubbers are irritant and may cause breathing difficulties. It’s important to take the necessary precautions and always read the label carefully. Those who are serious about their mouldmaking and casting should consider buying their equipment from a specialist supplier rather than a regular art/hobby shop. In London for instance, I recommend Tiranti’s in Warren St or 4D modelshop near Tower Bridge (details in the Suppliers list). Both have a helpful advice service and offer excellent value for money for high-quality products.
Rotational Moulding
Rotational molding (also known as rotomolding) is one of the most popular ways to produce hollow plastic products. It uses an oven-like chamber to rotate the mold biaxially, allowing heat and pressure to distribute throughout the entire hollow structure of the product. The process offers several advantages over other forms of plastic fabrication, including consistency in wall thickness, high stability, and ease of maintenance.
The rotomolding process is relatively simple, with the exception of the long processing times and the need for special equipment to regulate and keep a stable temperature inside the hollow mold. The melted polymer must be heated to the proper temperature and held for the correct length of time, otherwise the polymer will degrade, decreasing impact strength. It’s also important to ensure that the molten plastic has a consistent flow so it can fill every crevice of the hollow mold, particularly those areas where the walls are thicker.
Once the molten plastic has been held for an adequate amount of time, it’s allowed to cool before being separated from the mold. A cooling system may be used to facilitate the process, but it’s crucial that the mold isn’t cooled too quickly or it could cause the new plastic part to shrink and warp.
Unlike other types of plastic molding, rotational molding produces very little waste during production. There are no sprues, runners, or gates that need to be cut off from the finished part, so the majority of the molded plastic is able to be retrieved and recycled. This, in addition to the shorter cycle times, helps reduce costs and waste associated with other molding methods.
The rotomolding process can be used to produce many different types of products, from toys and medical devices to furniture and car components. However, it is especially well-suited for creating hollow containers and lids. For this reason, rotomolding is often used in the food industry to manufacture disposable bottles and lids for liquids such as juices and soft drinks. It’s also commonly used to create industrial drums, cylinders, and buckets. In addition to the traditional two-plate tooling, it’s possible to add inserts and ribs to the rotomolded parts to improve their structural integrity.