The Fundamentals of Plastic Injection Molding

The Fundamentals of Plastic Injection Molding


About Injection Molding

Injection molding is the most common modern method of manufacturing plastic parts. It is used to create a variety of parts with different shapes and sizes, and it is ideal for producing high volumes of the same plastic part. Injection molding is widely used for manufacturing a variety of parts, from the smallest medical device component to entire body panels of cars. A manufacturing process for producing plastic parts from both thermoplastic and thermosetting materials, injection molding can create parts with complex geometries that many other processes cannot.

The first step of getting a plastic part injection molded is to have a computer-aided design (CAD) model of the part produced by a design engineer. The three-dimensional (3D) CAD model then goes to an injection molding company where a mold maker (or toolmaker) will make the mold (tool) that will be fitted into an injection molding machine to make the parts.

Molds are precision-machined usually from steel or aluminum, and can become quite complex depending on the design of the part. Plastic materials shrink at different rates when they cool, so the mold has to be constructed with consideration for the shrinkage rate of the material being used for the parts. In other words, a formula is applied in the construction of the mold to slightly increase the size so that when the plastic shrinkage occurs, the part will be to the dimensional specifications of the CAD model.

The Process

Injection Molding Process

Plastic injection molding is a manufacturing process where resin in a barrel is heated to a molten state, then shot into a mold to form a part in the shape of the mold. The resin begins as plastic pellets, which are gravity fed into the injection molding machine through a funnel-shaped hopper. The pellets are fed from the hopper into a heated chamber called the barrel where they are melted, compressed, and injected into the mold’s runner system by a reciprocating screw.

As the granules are slowly moved forward by a screw-type plunger, the melted plastic is forced through a nozzle that seats against the mold sprue bushing, allowing it to enter the mold cavity through a gate and runner system. The injection molded part remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.

The part cools and hardens to the shape of the mold cavity. Then the two halves of the mold (cavity or “A” side and core or “B” side) open up and ejector pins push the part out of the mold where it falls into a bin. Then the mold halves close back together and the process begins again for the next part.

History of Injection Molding

The plastic injection molding process is generally dated back to 1868, when John Wesley Hyatt of billiard ball maker Phelan and Collander was searching for a suitable replacement material for the ivory in billiard balls. Hyatt invented a way to inject celluloid into a mould that processed it into a finished form. In 1872 John and his brother Isaiah patented the first injection molding machine. This machine was relatively simple compared to the complex machines used by today’s injection molding companies. It consisted of a basic plunger to inject the plastic into a mold through a heated cylinder. The industry was slow to adopt the injection molding process, eventually beginning to produce plastic items such as collar stays, buttons and hair combs. Not until the 1940s did the concept of injection molding really grow in popularity because World War II created a huge demand for inexpensive, mass-produced products.

The plastics industry was revolutionized in 1946, when James Hendry built the first screw injection molding machine with an auger design, replacing Hyatt’s plunger. The auger mixes the injection molded material in a cylinder and pushes the material forward, injecting it into the mould. This allowed colored plastic, or recycled plastic, to be mixed in with the virgin material before getting injected into the mould.

Today, screw-type injection molding machines account for 95% of all injection machines. The industry has evolved immensely over the years due to technological advancements and machine automation. It has come from producing combs and buttons, to a multitude of custom injection molded products for virtually every industry including automotive, medical, construction, consumer, packaging, aerospace and toys.

Molds

injection mold

Injection molds, or mold tooling, are the formed halves that come together in the injection molding machine to be filled with molten plastic and produce the plastic parts in their image. The cavity side, or “A” side, is typically the half which will form the “best” surface of the part, and the core side, or “B” side, will typically show the visual imperfections caused by ejector pins because the parts get ejected from this half.

Injection molds are manufactured by machining or by Electrical Discharge Machining (EDM). Standard machining was the traditional method of building injection molds with a knee mill. Technology advanced the process, and Computer Numerical Control (CNC) machining became the predominant method of making complex molds, with more accurate details, and in less time than the traditional method.

EDM is a process in which a shaped, copper or graphite electrode is slowly lowered onto the mold surface, which is immersed in paraffin oil. Electric voltage applied between the tool and the mold causes spark erosion of the mold surface in the inverse shape of the electrode. EDM has become widely used in mold making – many injection mold companies now have EDM in-house. The process allows the formation of molds which are difficult to machine, such as those with features such as ribs or square corners. It allows pre-hardened steel molds to be shaped without requiring heat treatment.

Compared to other plastic manufacturing processes such as CNC machining or 3D printing, injection molding has a high up-front investment because the tooling is expensive. However, for large production runs of thousands or even millions of identical parts, injection molding is typically less expensive in the long run, despite the high initial tooling investment, because of a lower piece price at high volumes. In addition, it is a much faster manufacturing process than the others mentioned.

Molds can be made of pre-hardened steel, steel that is hardened after the mold is produced, aluminum, and/or beryllium-copper alloy. The choice of mold material is determined in part by the number of parts to be produced.

Steel molds will generally have a longer lifespan, so a higher initial cost will be offset by longevity – it will be capable of producing a higher number of parts before wearing out. Pre-hardened steel injection molds are less wear-resistant than those hardened by heat treatments after they are machined, so they are used for lower volume part requirements.

Depending on varying economic conditions and material origin, aluminum molds can in some cases cost substantially less than steel molds. Aluminum molds have a quick build time, and can produce faster cycle times because of better heat dissipation than steel. Beryllium copper can also be used in areas of the injection molds that require fast heat removal, or in areas where the most shear heat is generated.

Additional complexity can be added to injection molds in order to produce more complex plastic parts. In the basic process of injection molding, the two mold halves separate at the end of the molding cycle and the part is ejected. In this simple case, the part design cannot have any overhanging or undercut part features, because the mold halves would catch on each other when pulling apart.

So, to accommodate part features such as undercuts, molds can be augmented with side-pull mechanisms called slides. Slides move into a cavity in a perpendicular direction from the draw of the mold halves to form the undercut feature, then stationary angle pins on the stationary mold half pull the slides away when the mold is opened. The pins enter a slot in the slides, and cause them to move backward when the moving half of the mold opens, like a cam. Then the part is ejected, the mold closes, and the slides move forward along the angle pins as a result of the closing action of the mold.

Multiple Part Molds

family molded parts

Family molded parts

In addition to a single plastic part being produced in a molding cycle, the mold can also be designed to produce multiple numbers of the same part in a single shot. A tool with one impression is often called a single impression (cavity) mold, whereas a custom injection mold with two or more cavities of the same part is referred to as a multiple impression (cavity) mold. The number of impressions in the mold is often incorrectly referred to as cavitation. Some extremely high-volume molds – like those for bottle caps – can have over 128 cavities.

A multiple impression (cavity) mold may also be referred to as a “family” mold. However, a family mold is more accurately defined as one which can produce multiple, like-sized parts in the same quantity, color and material. Family molded parts are often part of an assembly, such as a mating top half and bottom half.

Overmolding

Overmolding

Overmolding is a plastic injection molding process which is very useful for producing multi-material parts with some unique properties. For instance, this process can be used to add a second part, of a different material, for a handle or grip. Picture a power tool with a grip made out of a softer material than the body of the tool. Similarly, a game controller can be manufactured with different textures of plastics on the body of it.

Basically, a previously injection molded part of one material (the substrate) is re-inserted into an injection molding machine, and a different material (the overmold) is injected to form a new layer over the first part.

Some specialized injection molding machines have two or more injection units that can “overmold” in a single molding cycle. For example, “two-shot,” or “multi-shot” injection molds first mold a basic shape in a base color of plastic material, then the second material, of a different color, is injected into the remaining open spaces to produce a one-piece, multi-color plastic part.

Overmolding is not as straightforward as injection molding a part out of a single material, and it is not without its limitations. One consideration is that the two materials must be compatible, chemically and thermally. Since plastic materials have different melt temperatures, the substrate material must have a higher melt temperature than the overmolding material, otherwise the original part would melt and deform when the overmolding material is injected.

Insert Molding

Threaded insert molded into plastic

Metal, ceramic or plastic pieces can be inserted into the molten thermoplastic to form multi-material, robust parts with additional functionality. For example, threaded metal inserts can be molded into the parts to allow them to be attached to other parts.

Insert molding can reduce cost by embedding secondary parts into the plastic injection molding process, as opposed to installing the parts after molding. By integrating the inserts at the time of molding, the parts become more robust compared to staking the pieces in post-molding. Of course, the insert pieces must be able to withstand the high temperature and pressure of the injection molding process.

Insert molding is naturally a more complex process that standard injection molding, so some injection molding companies are more experienced in the process than others. For low-volume production runs, a machine operator may load the inserts into the mold by hand, prior to the plastic injection cycle. For high-volume production runs, however, it is common to use automated machinery to place the inserts into the mold.

Molding Cycle Time

A formula can be used to determine the cycle time of injection molding. The time it takes to make a part using injection molding is calculated as:

Total time = 2M + T + C + E

Where:
(2M) = Twice the Mold Open/Close Time
(T) = Injection Time (S/F)
(C) = Cooling Time
(E) = Ejection Time (E)
(S) = Mold Size (in3)
(F) = Flow Rate (in3/min)

The mold closing and ejection times of injection molded parts can last from less than a second to a few minutes, depending on the size of the mold and machine. The cooling time, which dominates the process, depends on the maximum thickness of the part.

Process Efficiency

Optimizing the injection molding process is essential because it affects cost, quality, and productivity. Some of various optimization checks include:

• Optimize the holding time by conducting gate seal or gate freeze studies
• Conduct a cooling time study to optimize the cooling time for an injection molded part
• Pressure drop studies determine if the machine has enough pressure to move the screw at the set rate
• Perform viscosity curves to determine injection speeds
• Vary the melt temperatures and holding pressures to optimize the process window

Molding Trial

When an injection molding job is being set up for the first time and the shot size for that mold is unknown, a molding trial will be conducted to get everything “dialed in.” The mold technician will usually start with a small shot weight and fill the mold gradually until it is 95 to 99% full. Then a small amount of holding pressure is applied, and the holding time is increased until gate freeze off (solidification time) occurs on the injection molded part. Gate solidification is important because it determines cycle time, and cycle time is a crucial determinant in the efficiency, and therefore the economics, of the production process. If the parts have sink marks, the holding pressure will be increased until they are minimized and the part weight is achieved. Once the settings are settled in and the injection molding machine is making good parts, a setup sheet is produced for standardizing the process for future production runs.