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Injection Molding Process
The plastic injection molding process is a manufacturing method for producing custom plastic parts. The service takes place at injection molding companies often referred to as a custom molder. Before the process begins, an experienced mold maker must construct a mold or tool in order to produce a part. The construction of the mold will include 2 halves and contain all the geometry and features that make up the part specifications.
Once the mold is constructed, it is then loaded into an injection molding machine where the process begins. A hopper is use to hold and feed plastic virgin material into the barrel of the machine, where it becomes molten. From here, a reciprocating screw will continue to feed and mix the proper amount of material. After this, the screw will ram inject the material into the mold cavity. Once the material enters the mold, it begins to cool and harden to conform to the geometry of the mold. After the material cools, it is safe to remove the part from the mold, and this completes the cycle.
The injection molding cycle is as follows;
- Material Enters Barrel
- Material melts and mixes
- Volume of material (Shot sizes in barrel is created)
- Mold closes
- Injection of the plastic into the mold cavity
- Molten material cooled (during this process steps 1-3 are preparing for next cycle)
- Mold Opens
- Part Ejects
- Jump to step 4
Calculating an injection molding cycle is as follows;
Cycle = Mo+Mc+I+C
Mc = Time to close the mold (this is the time it takes to actually close the tool)
I = Time to inject material into the mold
C = Cooling Time (Time to solidify molten material)
To = Time to open a mold and eject the part (these can overlap and together make up total open time)
Design Characteristics of Plastic Injection Molded Parts
Custom components for the molding process, should be designed and engineered by an experienced industrial designer or engineer. Producing a dimensional and stable part requires many factors to be considered. Failure to follow the design guidelines for injection molding can end up with undesirable results. Many factors to consider are as follows;
There are many design guidelines for plastic parts, these are just to name a few. See (Design Guidelines) for more information.
Mold Making Companies
Some injection molding companies offer an in house mold making service. However, due to the lack of in house capabilities, there are many stand alone mold making companies throughout the united states and the world that offer this service. The majority of the highly skilled plastic molders and mold makers reside in Michigan, Texas and Florida. Michigan offers the majority of the injection molding services because of its relation to the automobile industry, which is one of the oldest industries utilizing custom plastic injection molded parts.
None the less, If the injection molding company of choice does not offer an in house mold making service, then an outside tool-making company will have to be utilized. Although outsourcing the tooling is more common than not, it usually drives up the price, lead time and prolongs serviceability because of multiple sourcing and added management.
Once tooling sourced, an experienced mold maker will carefully select aluminum, steel or both to construct the best performance for the injection mold as possible. There are many factors to consider when engineering a mold. These factors include; cooling (temperature control), cost, high or low volume life cycle and geometry complexity. Following the design guidelines for plastic injection molding will help keep costs down.
Injection Molding service Companies use two main methods to manufacture molds: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds with a knee mill. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.
The electrical discharge machining (EDM) spark erosion process has become widely used in mold making. Most injection molding companies have EDM service in house, as it is essential to the mold build process of complex molds. EDM allows the formation of injection molded shapes which are difficult to machine, such as square corners or ribs.
Custom Injection Molds can be very expensive to manufacture. One of the key factors driving up cost is the tool material selection. Typically molds constructed out of steel would cost more than double than that of aluminum, due to the much slower machining process. High volume projects typically require steel because of its wear resistance. However, with the advancement made in alloy blends, higher grade aluminum such as QC-7 and QC-10 aircraft aluminum has closed the gap on the durability between steel and aluminum. These molds can produce hundreds of thousands of parts in a life cycle, with no wear.
Aluminum tools also offer quick turnaround and faster cycles because of better heat dissipation for cooling. This can result in cost savings on the production parts as well, because most of the shot cycle consists of cooling time. The aluminum tool can also be coated for better wear resistance to fiberglass reinforced materials and add life to the tool and engineered with steel components for high wear areas of the tool.
The cost of plastic injection molds depends on a very large set of factors ranging from number of cavities, size of the parts, complexity, life of the project, surface finishes and many others. The initial tooling cost can be high, however the piece part cost is low, as with greater quantities the overall price decreases. With the expanding global market, many injection molding service companies have found ways through technology and processes and quality systems such as ISO-9000, to drastically reduce the cost of tooling and parts..
Talk with us at Xcentric and see how we can save you time and money on your next project.
Injection Molding Equipment
There are two types of injection molding machines, Electric and Hydraulic. While in the past hydraulic machines ruled the industry, today electric presses are fast becoming the molding machine of choice. The reason for this is because electric presses use far less energy and are nearly 100% repeatable. While the cost of an all electric machine is typically 30% higher than a hydraulic presses, higher demand is closing the gap on cost. It is estimated that in the next 20 years, hydraulic machines will be a thing of the past, as more injection molding companies are making the switch to stay competitive.
Presses come in a wide range of sizes knows as tonnage. Injection Molding machines are rated by tonnage, ranging from less than 5 tons to 6000 tons. The tonnage is the reference of the amount of clamping force that the machine can exert. The clamp force keeps the mold closed during the injection molding process. The clamp force required is determined by the projected area of the custom part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. Polymers that have a high melt temperature and or low viscosity, will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed.
Plastic injection molding is the preferred process for manufacturing mass produced custom plastic parts for all industries. Some of these components include;
- Electronic Housings
- computer components
- Bottle Caps
- Automotive Interiors
- Top and Bottom Covers
- Many More…..
Some advantages of injection molded parts are;
- High Tolerances
- Large Material Selection
- Low Cost Production Piece Price
Some disadvantages of plastic injection molding include;
- Expensive Tooling Investment
- Surface Defects (such as blemishes)
- Thermal Expansion Issues (primarily on materials with high shrink rates)
Most polymers can be used for injection molding, including all thermoplastics, in addition to, some thermoset, and some elastomers. There are tens of thousands of different plastic materials available to choose from. Because of the wide range of applications and needs, when choosing a polymer it is best to speak directly to a plastic sales engineer. It is also advised to talk direct to the manufacture of the pellets rather than an injection molding company. Some of the characteristics that can be of interest are;
- High Heat Applications
- Tensile strength
- Clarity (clear Materials)
- Food Grade
- Medical Grade
- Fillers for Reinforcement such as fiberglass or mineral
Different types of Processes for Injection Molds
Although most injection molding processes are covered by the conventional process description above, there are several important injection molding service variations including:
- In-mold decoration and in mold lamination
- Insert molding
- Low-pressure molding
- Micro molding
- Multi-component (over-molding)
- Family Molds
- Thin-wall molding
- Rubber injection molding
- liquid silicone rubber
Metal inserts can also be injection molded into the work piece. It is best to choose a company that has experience in insert molding, as there are many variables to consider.
Talk with us at Xcentric and see how we can save you time and money on your next project.