Plastic Injection MoldingProcess

Plastic: Injection MoldingIn its broadest terms, thermoplastic injection molding forms a part by forcing (injecting) a liquid resin (either as a hot, molten thermoplastic, or a thermo set that is still in liquid form) into a closed mold, under pressure, until the part has cooled or cured and can be ejected from the mold. This section will primarily deal with thermoplastic injection molding as opposed to thermo set injection molding which is covered in a different section. But, many of the design guidelines apply to both types of materials. This process consists of four distinct operations: 1. melting the resin 2. injection of the resin into the closed mold 3. cooling the resin inside the closed mold 4. opening the mold and ejecting the molded part

How does it work?

In its broadest terms, thermoplastic injection molding forms a part by forcing (injecting) a liquid resin (either as a hot, molten thermoplastic, or a thermo set that is still in liquid form) into a closed mold, under pressure, until the part has cooled or cured and can be ejected from the mold. This section will primarily deal with thermoplastic injection molding as opposed to thermo set injection molding which is covered in a different section. But, many of the design guidelines apply to both types of materials. This process consists of four distinct operations:1. melting the resin 2. injection of the resin into the closed mold 3. cooling the resin inside the closed mold 4. opening the mold and ejecting the molded partFirst, let's take a look at a schematic of a standard horizontal thermoplastic injection molding machine: These machines are designed to melt (or "plasticize") the resin, convey it under pressure into the clamped closed cavity, cool the hot part and eject it once sufficiently cooled. This system can be divided into 3 main sections: injection , mold and clamping. The fourth area concerns itself with the control, operation and coordination of these three systems.

Schematic of a standard horizontal thermoplastic injection molding machine

Injection System Process

Injection System At the heart of this system is the plastic injection plasticizing cylinder (also referred to as the extruder). The extruder consists of a barrel with heater bands outside and a rotating screw inside. The resin, in pellet form, feeds from the hopper into the heated barrel where shear forces, friction and the heat from the heater bands melts the plastic. The heater bands also keep the melted resin ("melt") at a constant temperature inside the barrel. Depending on the resin, this temperature can be between approximately 300F and 590F (150C and 310C). As it turns, the screw can move axially back and fourth ("reciprocate"). This allows the melt to accumulate in the front of the barrel as the screw retracts away from the front of the barrel. When the shot is delivered, the screw moves forward forcing the shot into the cavity. The screw is constantly turning to keep the resin melted and to plasticize additional material as the shot is delivered. Even after the cavity is completely filled, the screw continues to push forward to maintain the pressure in the cavity as the melt cools. Once the shot is complete and the plastic freezes off in the mold, the screw begins to retract ("recover") to accumulate the next shot of melt. The size of the barrel, the horsepower of the screw drive motor and other factors determine the speed of recovery and thus how many shots can be delivered in an hour. For high speed production, the injection molding machine is fitted with a "fast recovery" injection molding system that may involve an accumulator which is a separate heated barrel using a system of valves, the accumulator takes up the melted resin like a syringe and allows the extruder to begin its recovery earlier while the accumulator delivers the shot and maintains the pressure as the part cools.

Injection System

1Plasticizing the resinThe cycle begins with the extruder plasticizing the resin and accumulating it in the forward section of the barrel. The heater bands maintain the melt's temperature as the shot it built up. The mold is closed. The cycle is typically timed so that there is minimal time between the closing of the mold and the next shot

2.Injecting the resinOnce the shot is ready, a valve is opened at the nozzle and the melt is quickly injected into the mold. This part of the process only takes a few seconds. As the melt enters the cavity, the displaced air is vented out through the holes for the ejection pins and along the parting line. Proper filling of the cavity is dependant on part design as well as good gate location and design and proper venting.

3.Cooling the PartThis is the longest portion of the molding cycle. Once the cavity is filled, the part is allowed to cool. If an accumulator is not used, the extruder continues to push material into the mold and maintain the proper amount of pressure until the material cools (or "freezes"). This is all controlled by timers.

4.Ejecting the PartOnce the part has cooled enough (so that it will hold its shape out of the mold, and the ejection pins won't deform the part), the mold is opened. The moving platen has moves backwards and the ejector pins strike the rear plate (or "ejector plate"), ejecting the part. (There are many different ways to eject the part which are discussed elsewhere in this section.) At the same time, the extruder begins retracting ("recovering") to build up the next shot.

Designing for the Process

Designing plastic parts is a complex task involving many factors that address a laundry list of requirements of the application. "How is the part to be used?" "How does it fit to other parts in the assembly?" "What loads will it experience in use?" In addition to structural and functional issues, processing issues play a large role in the design of an injection molded plastic part. How the molten plastic enters, fills, and cools within the cavity to form the part largely drives what form the features in that part must take. Adhering to some basic rules of injection molded part design will result in a part that in addition to being easier to manufacture and assemble, will typically be much stronger in service.

The primary enemy of any injection molded plastic part is stress. When a plastic resin (which contain long chains of molecules) is melted in preparation for molding, the molecular bonds between the molecules are temporarily broken with heat and the shear forces of the extruder, allowing the molecules to flow. When the hot molten plastic is injected into the mold, it is done so under great pressures (up to 15,000 psi). This pressure forces the around into every feature and into every crack and crevice of the mold. As these molecules are pushed through each feature (a rib or a wall or a boss), they are forced to bend and turn and distort to form to the shape of the part.

You can see the stress in these part samples when viewed with polarized light. The left picture shows a part with small fillets which creates "hot spots" of stress. the right picture shows the same part with large fillets and the stress is spread out (even though it's not totally eliminated).Reducing Stress

Sharp CornersTurning tight or sharp corners is harder on the molecule than if it took a gentle turn with a generous radius. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to. All of these difficult transitions can build up shear stresses in the material. As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part. Part stress can cause warp age, sink marks, cracking, premature failure and other problems. Consider adding smooth transitions between features. Using rounds and fillets will help the material flow more easily through the mold and result in less stress in the part. While some stress in an injection molded part is to be expected (even the best designed part still undergoes the high pressures of molding), you should design your parts with as much consideration for stress reduction as possible. As with most design choices, there must be balance between what you want and what makes sense.

Anatomy of an Injection Molded Part

To help simplify the process, the basic features of an injection molded part can be divided to three main categories:nominal wall projections holes (and recesses) Dividing a part into these basic groups will help you to build your part in a logical manner while minimizing molding problems. As a part is developed, always keep in mind how the part is molded and what you can do to minimize stress

Basic Features of a Plastic Part

Determining the Best Wall Thickness

How thick should your nominal wall be ? This important decision is driven by several factors:Functional Requirements of the Part: What does the part have to do? If you need a lot of strength and stiffness, you may choose to make your nominal wall thicker. Bear in mind that you can also make a part stiff by adding ribs. If there is room on the inside of the part for ribs, you may be able to reduce the nominal wall. If part stiffness is a critical issue in your design, you may want to turn to finite element analysis (FEA) to more precisely determine exactly what wall thickness is required by your application. Nominal Wall = Uniform Wall: Maintaining a nominal wall in your base feature will assure you of good resin flow through the mold. Typically, your nominal wall shouldnt vary more than +/- 10%. But sometimes a complex part may require a varying thickness to meet the functional requirements of the parttry to avoid this because this variation can cause major problem in mold filling and part warpage. Since the hot plastic will always follow the path of least resistance (flowing into more open areas as opposed to smaller, tighter areas), the thinner walls of you part may not fill properly. The molder may have to increase the injection pressure to force those thinner areas to fill, but this can build even more stress into the part causing increased warpage. This is particularly the case with plastics that have a higher mold shrinkage factor.

The Nominal Wall

Every plastic part begins with a basic shape. This could be a simple box, enclosure, housing or anything you can think of. It could be a flat plane of material, or some basic structure on an internal component. This basic shape typically serves as the base feature for the part. Typically, other features (such as ribs, bosses, holes, etc.) will be added to this base feature. In addition to its outside shape, a wall thickness or "nominal wall" will be established. This wall will serve as the basis or how all other features added to this basic feature will be designed. This decision is a critical because as you will see, the dimensions for these other features will be driven by the nominal wall thickness. Of course, you can change the nominal wall as you develop the part, just be careful to remember to adjust the other features that are based on it if you do.

Variations on your wall thicknessIf your part is so complex that you need variations on your wall thickness, look for an alternative. You may want to core out the thick section and using ribs for stiffness. If you need to have both sides of your wall rib-free, you may want to consider alternative processes like structural foam or gas assist injection molding. At the very least, try not to make the transitions between thicker and thinner sections too abrupt. Try using a gradual transition or chamfered corners to minimize the dramatic change in pressures inside the mold. Molding Considerations: The thinner the nominal wall, the shorter the distance the hot melt can flow before it solidifies or "freezes off." Most molders have a rule of thumb they use to describe the relationship between nominal wall and flow length. This flow length is also dependent on material since different resins have melt, flow, and freezing characteristics. If your FEA analysis indicates that you need a particularly thick nominal wall to satisfy your structural requirements, perhaps upgrading your material to a stiffer variety will help you to reduce the nominal wall. Also, additives such as glass fibers can increase the stiffness of a part with the same nominal wall. But bear in mind, these additives can also affect the flow and shrinkage characteristics of the material and thus make your part design more difficult as opposed to easier. And, theres the issue of cost.

CostingCost Considerations: A parts cost is directly driven by nominal wall and cycle time. The thicker the wall, the more plastic the part requires. But since cycle time is largely a function of cooling, the thicker wall requires more time to cool. In addition to having a rule of thumb they use to describe the relationship between nominal wall and flow length, molders have a relationship between nominal wall and cycle time.Part Costing Material Cost = (part weight x $/lb) + scrap%Labor Rate = press rate ($/hr) / (part cycle x # of cavities)Part Cost = raw material + labor + packaging + mark-upset Up Cost = mold set-up cost / quantity of production run Therefore,Production Part Cost = Cost + Set-up CostSo, determining the proper nominal wall requires a balance between the parts function, the molding requirements and limitation as well as cost considerations. There is also an element of whats practical. Its best to consult with your mold at the beginning of the design process to discuss the nominal wall as well as the other features you are planning to add to your part.

The part on the left was the result of redesigning the part on the right which shows signs of stress.

DraftsMost plastic parts include features such as outside walls and internal ribs that are formed by opposing surfaces of tool metal inside a closed mold. To properly release the part when the mold opens, the side walls of the mold are tapered in the direction that the mold opens. This tapering is refereed to as "draft in the line of draw." This draft allows to part to break free of the mold as soon as the mold opens. This is particularly important on the inside of a part since plastic shrinks down around and grips the core (male part) of the mold as it cools. Without draft, ejecting the part can be very difficult. Normally plastic parts are ejected as soon as they have cooled and hardened enough to tolerate the force of the ejector pins as they push the part off the core without "denting" the part. If there is too little or no draft, the part has to cool longer to accept more force to push the part off (if at all). The longer you have to wait for the part to cool, the longer the cycle time, which increases the part cost.

Projections

Any feature that adds material onto the nominal wall can be referred to as a projection. Projections can include ribs, bosses, snaps, gussets, etc. and all can cause serious problems to your part if not implemented properly. Once again, just like in selection the appropriate nominal wall thickness, the application of projections have to strike a balance between function, mold ability and cost. Reinforcing Ribs: The most common projections used are reinforcing ribs. They can be used to provide stiffness while reducing the required nominal wall thickness. The taller the rib extends away from the base wall, the better the mechanical advantage and stiffening effect the rib has. However, tall ribs (or deep ribs if you look at it from the molds perspective) can be difficult to fill. This problem can be more so if you add draft to both sides of the rib. You may have the tendency to increase the thickness of the rib to allow for better filling in the mold, but this comes at a cost. As you increase the ribs thickness, the area where the rib intersects the base wall (or nominal wall) increases. This creates a section of material that is thicker than the thickness of the nominal wall. When this happens, you can get a sink mark on the outside of your part.

Projections

Sink Marks When the hot melt flows into the mold, this thick section doesnt cool as fast as the rest of the part because the thicker material becomes insulated by the outside surface of faster cooling plastic. As the inner core cools, it shrinks at a different rate than the already cooled outer skin. This difference in cooling rates causes the thick section draw inward and create a sink mark on the outside surface of the part. In addition to being unattractive, they also represent added stress that is built into the part.Rib Design The are some basic rules to follow that will help to minimize sink marks when adding ribs. If you picture the section of material where a rib meets the nominal wall, a circle can be inscribed into this area. This circle represents the thickest cross section where a sink could occur. As a general rule, this circle should not be larger than 1.2 to 1.5 times the nominal wall thickness (t). This translates into a rib thickness of about 50 to 60% of the nominal wall (.5t to .6t). Depending on the material, this rule can be bent a little. While high mold shrinkage materials like nylon or polypropylene are very sensitive to sink marks, lower shrink materials such as polycarbonate and polystyrene can tolerate a slightly thicker rib (say, 75% of the nominal wall). It is important to consult with you molder as you begin to plan your part design since they can best tell you what wall and rib thickness works best for your application.

Projections

As for rib height, the same common sense approach applies. Ribs that are too deep are more difficult to fill and can create quality problems with ejection. A good rule of thumb is to limit the rib height to five times the nominal wall with a minimum draft of per side. If more strength is required, consider a series of shorter ribs as opposed to one tall one. Please note the in all the illustrations shown the rib meets the base wall with a fillet. The fillet serves as smoother transition between the rib and the wall, which allows the plastic to flow easier in the mold. This smoother transition greatly reduces stress in the part as well as adds a great deal if strength to the part. Always take into account what fillet you will be using when you evaluate what rib thickness to use. Adding the fillet without this forethought can result in unexpected sink marks. A good rule of thumb is to limit your fillets to 25% of the nominal wall.Designing "Steel Safe" As a precaution you may want to choose to design your ribs thinner and shorter than you would normally. Your toolmaker refers to this as building your mold "steel safe". This means that the ribs are initially cut a little smaller than your target dimension. This way, first shots can be made of the part to see if the part is stiff enough with the thinner ribs without being to difficult to fill. If the part works, you have saved yourself unnecessary sink marks. However, if it is too difficult to fill or needs more strength, they can increase the rib thickness or height (or even add additional ribs) by cutting away more metal in the mold. This method is much less expensive than having to weld up the mold and re-cut the ribs thinner and shorter.

All Projections Are RibsThis basic philosophy of maintaining a nominal wall and minimizing the thick sections can be extended to all other projections that you may need to add to your part. In fact, some designers approach plastic part design by reducing the entire "piece part" into a collection of walls and intersecting ribs. This would mean that a box was just a flat wall with four additional walls added around the sides. A boss for a screw or threaded insert could be viewed as a rib in the form of a cylinder. One thing to keep in mind as you are placing your projections is how the mold might fill with molten plastic to form your part. Imagine the flowing plastic finding its way though the mold, filling the larger spaces first (the nominal wall), and then moving into the thinner areas (the ribs) until it reaches the farthest corners of your part. The thing to be aware of is that the hot plastic will be displacing air in the mold and that air has to have a way to escape, or the mold can't fill..

ExampleThis can be particularly true if you have a freestanding rib or boss in the middle of your part. Features such as this need a way to get the plastic in, and let the air out. If air is trapped, it will compress and create a burn mark on the rib, which probably wont fill anyway. The best solution is to try to tie your ribs into the side walls or other features to help convey the plastic and air through the part. Another solution is to transition to a projection from the base wall with a gusset or ramped rib. This allows plastic and trapped air to flow smoothly though the cavity. Once again, consulting a molder can be very helpful in your design process. Their experience can identify such problem areas early in the process before they become problems.Rib A will trap air in the top corner while rib B has a better transition to the base wall. Rib C is the best since it's tied into the side wall.

Recesses and Holes

If we view plastic part design as a building process, we began with a nominal wall that serves as the basic shape for our part. Then we added ribs and other projections to add rigidity and functionality. The third category of feature that we can add to our part is what we can take awayrecesses and holes. Recesses are depressions in the nominal wall while holes are well, holes though the plastic wall. This may sound like a simple thing and hardly worthy of a designers attention, but how you implement these features can dramatically affect the tooling costs, aesthetics and strength of your part.

Weld Lines

The metal that creates the hole or depression affects the flow of plastic into the mold. As the front of hot plastic flows into the cavity and hits the metal that forms the feature, the front is constricted (in a depression) and can disturb the smooth flow of material. The disturbance can cause an aesthetic defect on the opposing side from the depression.In the case of the hole, which is formed by metal pin or other feature, the front of hot plastic divides to flow around the pin. When the two fronts rejoins, there will be a line called a "weld line" or "knit line." Sometimes this line is visible, other times it is not. The weld line also causes a weakness in that area of the part because the fronts dont completely melt back together.

Weld line exampleSome of these problems can be minimized by proper location of the gate (the point at which the plastic enters the part), or changing the processing conditions. Also, there are FEA programs available to accurately predict where these weld lines will occur in your part so that you can anticipate any structural weakness that may result. Consult your molder if weld lines are a concern in your part.

Side Actions and Shut-Offs

While adding holes that are in-line with the direction the mold will open ("in-line with the draw"), creating holes in the side walls of a part is much more complex. These types of features require a "side action" in the mold and require much more forethought. As an example, imagine designing a part like an enclosure that required a hole in the side for a power cord. The first solution you should consider is how to eliminate the need for the hole in the first place. However, if you still need the hole, consider creating the hole with two parts using a "mouse hole" techniqueeliminating the need for the side action.The lowest cost option to create a complete through hole in the part is to use a straight "shut off" where a portion of the mold creates the hole by touching (shutting off) against an opposing part of the mold (i.e., the core half touches the cavity half). There are several ways to implement this technique with varying aesthetics, which may be a drawback since the side wall must be configured such that the mold can penetrate through the part and tough the other side. But the advantage is that the tooling cost may not be increased, and the cycle time should be affected as in the side core solution.

Alternative shutoffsAlternatively, if you still want a complete hole in the side wall of the part, then there a few ways to create this feature. The first is to use a "side core" which can take the form of a pin that is retracted pneumatically, hydraulically or with a cam mechanism. Depending on the complexity of the hole feature, cost restrictions and other factors, the molder and/or tool maker may suggest one over another. Generally, this feature in the tool has to be designed to move out of the way before the part is ejected, or move out of the way as the mold opens. This mechanism will add to the tooling costs and may add to the cycle time. Incidentally, these two techniques can be used to produce more than simple power cord holes. Snap-fits louvered vents, raised or embossed graphics and other features, can be formed to add more functionality to your part. You just have to be willing to pay for the added tooling and possibly a longer cycle time to get it.

Alternative shutoffsIncidentally, these two techniques can be used to produce more than simple power cord holes. Snap-fits louvered vents, raised or embossed graphics and other features, can be formed to add more functionality to your part. You just have to be willing to pay for the added tooling and possibly a longer cycle time to get it.

Shut-Offs - Part II

For high-volume production, snap fits provide an economical and efficient means of assembly. The most efficient are those formed as a straight shut-off between core and cavity. The exact configuration of your snap-fit will depend on many factors including the material and desired engagement and holding strength of the snap. There are several design guides published by resin suppliers with guidelines on snap-fit design. From a molding standpoint, all shut-off surfaces (mating surfaces of the the two halves of the mold) should meet at at least a 5 degree angle. This minimum angle assures a clean shut-off without excess mold wear. When designers are new to this design feature, visualization of the shut-off can be difficult. Below are some images of a modeled snap-fit feature. Click on each images for a larger version.

Parting Lines

A "parting line" is the line of separation on the part where the two halves of the mold meet. The line actually indicates a parting "plane" that passes though the part. While on simple parts this plane can be a simple, flat surface, it is often a complex form that traces the perimeter of the part around the various features that make up the parts outer "silhouette." Parting lines (denoted by P/L) can also occur where any two pieces of a mold meet. This can include side action pins, tool inserts and shutoffsParting lines cannot be avoided; every part has them. Whats important to understand is that regardless of where the plastic enters the mold ("gated"), most of the plastic will travel towards the parting line. Why? Because this is the easiest place for the displaced air to escape or "vent." This venting allows the part to fill without trapping bubbles in the mold (resulting in an incomplete part) or burning the part.The other important thing to be aware of is that how you design for the parting line can affect the cost of the mold as well as the aesthetics of your part. Depending on your application, you may not want the share edge that results from a standard parting line.

The melt will always head towards the parting line

Example-IIn example I, the standard parting line is formed where the core and cavity meet. While the internal edge of the part can be softened easily with a round machined into the base of the core, the outside edge is sharp. Aesthetically, this may be acceptable, but if this area of the part is handled, it could cut someone.

Example-IIIn example II, both edges are sharp. This may be preferable to someone who want to create a precise inside edge as well as a sharp outside parting line. For example, the mating edge between to halves of an assembly

Example-IIIIn example III, a full rounded edge is cut into the core. This full-rounded edge can also be called a "safety edge" because there are no sharp edges. The parting line lands precisely at the tangent point of the round as it transitions from the core to the cavity half. The use of this parting line does come with somewhat of a risk. If the mold is not matched perfectly as in "A" below, you could see a step in the parting line like in "B" or C" below. You may want to discuss this with your molder to better understand any possible problems that could occur

Example IFull Round ("Safety Edge")Parting Line Mismatch (arrow)

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