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COMMON MOLDING MATERIALS
These are the most common plastic materials for injection molding:
TPE is a blend of plastic and rubber materials. It is processed like plastic but has the properties and performance of rubber. Known also as Thermoplastic Rubber (TPR), TPE can be stretched to moderate elongations and, upon the removal of this stress, returns to the material’s original shape. TPE can also be recycled repeatedly.
Compared to liquid silicone rubber (LSR), TPE is easier and less expensive to mold. Under sustained pressures, however, TPE is susceptible to creep, the tendency of a solid material to permanently deform. TPE also tends to lose its rubbery properties at higher temperatures and is significantly more expensive than other plastic injection molding materials.
Applications for TPE injection molding include:
Automotive applications include weather seals and shock dust boots. Medical applications include breathing tubes, valves, catheters, and ventilation masks.
TPU is a type of TPE that is often called out separately because of its popularity. It has a rubber-like elasticity, and can be used as a replacement for hard rubber because of its higher durometer. Injection-molded TPU also has good load-bearing capabilities and can withstand exposure to ozone.
Compared to TPE, TPU has superior resistance to extreme temperatures and chemicals. TPUs may be too hard for some injection molded parts, and tend to cost more, in part because drying is required before processing.
Applications for TPU injection molding include footwear, gaskets, caster wheels, and sporting goods along with cases or enclosures for electronics and medical devices. In medical applications, TPUs are sometimes used instead of polyvinyl chloride (PVC), a plastic that may cause skin irritation.
Thermoplastic polyurethanes come in commercial, medical, and industrial grades. There are three main classes of TPU materials: polyester, polyether, and polycaprolactone.
Polyethylene is the most commonly used plastic in the world and is a commercial polymer that can be selected according to its density. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) both provide chemical resistance, but they differ in terms of their hardness, flexibility, melting point, and optical transparency.
PE plastics include polyethylene terephthalate (PET, PETE), a material that is not defined by its density. Like LDPE, PET can be as clear as glass; however, designers can also select grades of PET or PETE with different levels of optical clarity. HDPE, LDPE, and PET all resist moisture and chemicals, but LDPE is softer and more flexible than HDPE, which is an opaque material.
Applications for polyethylene injection molding may be limited to indoor applications because of their poor UV resistance. Because they are unable to withstand high service temperatures, these plastic materials may not be suitable for some processing applications. Often, injection molded polyethylene is used in products such as housewares, toys, food containers, and automotive parts.
PE plastics come in numbered grades where higher numbers generally indicate higher densities. For example, HDPE 500 has a higher density than HDPE 300. Both grades have a higher density than LDPE, which also uses a numbering system for grades. BASF and DuPont are two of the leading suppliers of PE for engineering (as opposed to general-purpose) applications.
Polypropylene is the second most commonly used plastic in the world. It provides good chemical resistance, retains its shape after torsion or bending, has a high melting point, and won’t degrade when exposed to moisture or water. Injected molded polypro, as PP is sometimes called, is also recyclable.
Despite its comparative advantages, polypropylene degrades with UV light and is extremely flammable. At temperatures above 100° C (212° F), this injection-molded plastic dissolves into aromatic hydrocarbons, such as benzene and toluene, which are harmful to humans. Polypropylene plastic is also difficult to bond and paint.
Applications for polypropylene injection molding include
Polystyrene plastics are lightweight, relatively inexpensive, and resistant to moisture and bacterial growth. These commodity plastics also provide good chemical resistance to diluted acids and bases and have excellent resistance to gamma radiation, which is used to sterilize medical devices.
There are two main types of polystyrene: general-purpose polystyrene (GPPS) and high-impact polystyrene (HIPS). GPPS is brittle and has less dimensional stability than HIPS, which is compounded with butadiene rubber to enhance its material properties. GPPS also has a glass-like clarity whereas HIPS is opaque.
Grades of polystyrene include GPPS and HIPS. General purpose polystyrene (GPPS) is naturally transparent and relatively brittle. High impact polystyrene (HIPS) contains additives that increase toughness but with a loss of transparency. When polystyrene is produced as a foam, types include extruded polystyrene (XPS) and expanded polystyrene (EPS).
Applications for polystyrene injection molding include medical, optical, electrical, and electronic applications. With its higher impact strength, HIPS is often used with appliances and equipment, while injection-molded GPPS is used in plastic toys, cases, containers, and trays.
Both materials are flammable and susceptible to UV degradation.
Polystyrene (PS) is a commodity thermoplastic with an amorphous structure. It has excellent resistance to gamma rays and, therefore, supports sterilization by radiation. Polystyrene plastics can be either transparent or opaque, and unmodified polystyrene is clear, rigid, brittle and moderately strong.
In simple terms, Thermoplastic Rubber (TPR) is a material that has both the characteristics of rubber and plastic. With its elasticity and rubber-like material, TPR effectively disperses impact energy away from the bone, reducing the force of the impact. Each piece of TPR is securely stitched to the dorsal side of the glove. Ansell’s TPR designs are made to follow the anatomy of the hand so that full range of motion is maintained at all times, and work productivity is not jeopardized. TPR is molded, shaped, and sized to fit the purpose and function of the glove: to protect worker's hands by maintaining dexterity, flexibility, and comfort, without compromising protection.
TPR made its first appearance in the market in 1959. Since then, TPR has been manufactured for a variety of different solutions.
Other than gloves, TPR can be found in various automotive components, such as gaskets, bushings, and other molded pieces. You may even find TPR in footwear products and toys.
BENEFITS OF TPR
Since TPR has the characteristics and structures of both plastic and rubber, it has a wide range of benefits, including strong resistance to weathering and chemicals.
Additionally, it also withstands a broad temperature range and durometers (durometer scales are used to measure the hardness of rubber and plastic materials). TPR is widely used in many products and industries and is recyclable and reusable, which helps reduce costs in certain situations.
TPR VS TPE
Although TPR and TPE are both thermoplastics, TPR is made from the polymer SBS (styrene and butadiene), and types of TPE (thermoplastic elastomer) are typically made from the quaternary block co-polymers like SEBS (Styrene Ethylene Butylene Styrene).
Custom Mold Manufacturing has the capability of building and running molds for either thermoplastic resin or thermoset materials. Most of our molds are built using high-strength aluminum. These molds provide excellent results in the prototype/short-run production category. They are easy to work with and can be built in a lot less time than a steel mold. We have molds with multiple inserts, and slides.
Some of our signature molds are used to overmold plastic material around an internal component(s) placed into the mold, such as an electronic cable or electrical device, to provide a sealed, shaped body around the part, exposing only the electrical connection in the form of wires or contacts.
CMM offers tooling to those customers who may have their own molding machines. In this case, the molds are tested and proved before leaving the premises. Most of our molds are built for the Morgan Press G-55/G-100T injection molding machines, which are excellent short-run production molding machines. These machines are of the vertical type, which lend themselves very well to insert molding/overmolding, and are very easy to operate. Some of our customers own this type of machine, and we build the tooling for them. Since Decker Design Engineering operates two Morgan Presses, we can offer those customers not only the tooling, but all press setup parameters, making it easy for them to start running as soon as their mold arrives.
A few of our customers have multiple parts that require more than one mold to produce. What we have done is provide those customers with a Fast-Change frame and insert combination that is tailored--though not limited--to the Morgan Press. In this setup, the frame portion resides in the press, while the insert set, containing the core and cavity detail, slides in and out of the frame. This arrangement speeds mold changes. Some of our mold changes, using the Fast-Change arrangement, require only two minutes to convert.
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Custom Mold Manufacturing operates Morgan Press G-100T injection mold machines, for our own production pieces, each producing a 4-oz. (maximum) shot. These machines operate on 120 volts and require 160 psi maximum air pressure. [Morgan Industries website]