While some stresses in an injection molded part are to be expected, you should design your parts with as much consideration for stress reduction as possible. Some ways to do this are by adding smooth transitions between features and using rounds and fillets in possible high stress areas.

Shrink Rate Refers to how much the plastic material will shrink after cooled. This % of shrink is added to the part before the mold is designed. Every plastic material has its own shrink rate ranging from .001 per inch to as much as .060 per inch. Although most fall in between .004" and .021"

Commitment to quality is the foundation of ACO Mold’s production philosophy, we apply strict standards in every step of the process for consistent, high-quality injection molds and parts.

The minimum wall thickness that can be used depends on the size and geometry of the part, structural requirements, and flow behavior of the resin. The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0.080" – 0.160"). Thin wall injection molding can produce walls as thin as 0.5mm (0.020"). The chart below shows recommended wall thicknesses for common injection molding resins.

Designing plastic parts is a complex task involving many factors that address application requirements. "How will the part be used?" "How does it fit with other parts in the assembly?" "What loads will it experience in use?" In addition to functional and structural 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. Dividing a part into 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.

Choosing the proper wall thickness for your part can have drastic effects on the cost and production speed of manufacturing. While there are no wall thickness restrictions, the goal is usually to choose the thinnest wall possible. Thinner walls use less material which reduces cost and take less time to cool, reducing cycle time.

Heel Refers to the portion of an automatic custom injection mold that keeps the slide in the forward position when the molding machine is closed on the mold

Line of Draw The direction in which the two custom injection mold halves will separate from the plastic part allowing it to be ejected without any obstructions from metal creating undercuts

Setting up the mold to do injection molding test, verify the mold functionality and correctness of the molded part, keep all data in records.

These type of gates require an operator to separate the parts from the runners manually after each cycle. Manually trimmed gates are chosen for several reasons:

The mold or die refers to the tooling used to produce plastic parts in molding. Traditionally injection molds have been expensive to manufacture and were only used in high-volume production applications where thousands of parts were produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. Selecting a material for mold building is primarily a question of economics. Steel molds generally cost more to construct but offer a longer lifespan that will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear-resistant and are primarily used for lower volume requirements or larger components. The hardness of the pre-hardened steel measures typically 38 and 45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining, making them superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).

One way to avoid sink marks is to core out the solid sections of the part to reduce thick areas. If the strength of a solid part is required, try using cross hatched rib patterns inside the cored out area to increase strength and avoid sink. As a rule-of-thumb, make sure that all bosses and locating/support ribs are no more than 60% of the thickness of the nominal wall. Textures can also be used to hide minor sink marks.

Hand Load Aluminum or steel feature in a mold used to create undercuts in molded parts.  They are manually removed from the mold during the part ejection process.

Sink Marks Refers to areas of the molded part where it seems to be sunk in, due to un-uniformed wall sections, thick wall sections and rib/boss to thickness ratios being off

Vestige Material protruding from the gate area after gate runner has been removed from the injection molded part. This vestige is usually trimmed by the molding machine operator

Runner A channel cut into custom injection molds, in which plastic travels from the injection molding machine, through the sprue, through the runner and then through the gate ultimately filling the part

With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly pushed forward by a screw-type plunger, the plastic is forced into a heated chamber called the barrel where it is melted. As the plunger advances, 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 mold remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.

From incoming raw material auditing to final product inspection, our expert quality control inspectors adhere to established metrics to meet your expectation.

Each injection mold design must have a gate, or an opening that allows the molten plastic to be injected into the cavity of the mold. Gate type, design, and location can have effects on the part such as part packing, gate removal or vestige, cosmetic appearance of the part, and part dimensions and warping.

The edge gate is the most common gate design. As the name indicates, this gate is located on the edge of the part and is best suited for flat parts. Edge gates are ideal for medium and thick sections and can be used on multicavity two plate tools. This gate will leave a scar at the parting line.

Injection molding machines, also known as presses, consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. Molds are clamped to the platen of the molding machine, where plastic is injected into the mold through the sprue orifice. Presses are rated by tonnage, which is the calculation of the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection molding process. Tonnage can vary from less than 5 tons to 6,000 tons, although higher tonnage presses are rarely used. The total clamp force needed is determined by the projected area of the custom part being molded. This projected area is multiplied by a clamp force of 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/inch can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed. The required force can also be determined by the material used and the size of the part, with larger plastic parts requiring higher clamping force.

Aluminum molds cost substantially less than steel molds, and when higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation. They can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated.

The hot tip gate is the most common of all hot runner gates. Hot tip gates are typically located at the top of the part rather than on the parting line and are ideal for round or conical shapes where uniform flow is necessary. This gate leaves a small raised nub on the surface of the part. Hot tip gates are only used with hot runner molding systems. This means that, unlike cold runner systems, the plastic is ejected into the mold through a heated nozzle and then cooled to the proper thickness and shape in the mold.

Resolve mold/molding issues revealed by mold trial, typically it takes 1-2 rounds mold trial and modification before the mold is ready for mass production.

Cavity Refers to the upper half of the injection mold usually the show surface of the finished product but is mainly concave

Shear Refers to when plastic enters into the mold and the melt is maintained by friction produced by speed and pressure. Too much shear can cause the plastic material to burn, too little can cause the material to freeze off causing short shot

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Plastic injection molding is the preferred process for manufacturing plastic parts. Injection molding is used to create many things such as electronic housings, containers, bottle caps, automotive interiors, combs, and most other plastic products available today. It is ideal for producing high volumes of plastic parts due to the fact that several parts can be produced in each cycle by using multi-cavity injection molds. Some advantages of injection molding are high tolerance precision, repeatability, large material selection, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are an expensive upfront tooling investment and process limitations.

Gate Types There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates.

Gate Refers to where the plastic enters into the cavity of the mold. The two types of gates are as follows: 1. Automatically Trimmed Gates: Gates that incorporate features in the tool to break or shear the gate as the molding tool is opened to eject the part 2. Manually Trimmed Gates: Gates that require an operator to separate parts from runners during a secondary operation

When the hot melt flows into the injection mold, the thick sections don’t 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 to draw inward and create a sink mark on the outside surface of the part, or worse, completely warp the part. In addition to being unattractive, the mark also represents added stress that is built into the part. Other less conspicuous areas where sink occurs include ribs, bosses and corners. These are often overlooked because neither the feature nor the part itself is too thick; however, the intersection of the two can be a problem.

Ribs Refers to thin bladed features on a part that are used for strengthening wall sections and bosses. Also, used to minimize warp (#3 in Figure 1 below)

Thick sections take longer to cool than thin ones. During the cooling process, if walls are an inconsistent thickness, the thinner walls will cool first while the thick walls are still solidifying. As the thick section cools, it shrinks around the already solid thinner section. This causes warping, twisting or cracking to occur where the two sections meet. To avoid this problem, try to design with completely uniform walls throughout the part. When uniform walls are not possible, then the change in thickness should be as gradual as possible. Wall thickness variations should not exceed 10% in high mold shrinkage plastics. Thickness transitions should be made gradually, on the order of 3 to 1. This gradual transition avoids stress concentrations and abrupt cooling differences.

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 stresses can cause warpage, sink marks, cracking, premature failure, and other problems.

Slide Area of the custom plastic injection molds that is used for creating undercuts. Required for automatic injection molds

The sub gate is the only automatically trimmed gate in this list. Ejector pins are necessary for automatic trimming of this gate. Sub gates are quite common and have several variations such as banana gate, tunnel gate, and smiley gate to name a few. The sub gate allows you to gate away from the parting line, giving more flexibility to place the gate at an optimum location on the part. This gate leaves a pin sized scar on the part.

Core Outs Refers to the portion of a part that is gutted out in order to achieve uniform wall thickness. This portion of the part has no end use function other than lightening the part and reducing warp

When applying a texture to a part, the CAD drawing must be adjusted to accommodate for this surface variance. If the texture is on a surface that is perpendicular or angled away from the mold opening, then no draft changes are necessary. If the texture is on a parallel surface with the mold opening, however, increased draft is necessary to prevent scraping and drag marks that could occur during part ejection. Different textures have different impacts on the molded part. The rule-of-thumb when designing for texture is to have 1.5 degrees of draft for each 0.001" of texture finish depth.

Following some basic rules of injection molded part design will result in parts that are easier to manufacture and assemble, and are typically much stronger in service.

The largest factor to consider when choosing the proper gate type for your application is the gate design. There are many different gate designs available based on the size and shape of your part. Below are four popular gate designs:

Warp Refers to area of a injection molded part that distorts during cooling or molding, causing undesired results in the finished product. Usually caused by un-uniform wall sections

Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding).

The main enemy of any injection molded plastic part is stress. When a plastic resin (which contains long strains of molecules) is melted in preparation for molding, the molecular bonds are temporarily broken due to the heat and shear force of the extruder, allowing the molecules to flow into the mold. Using pressure, the resin is forced into the mold, filling in every feature, crack, and crevice of the mold. As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part. Turning hard or sharp corners exerts more stress on the molecules than taking gentle turns with generous radii. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to.

The direct or sprue gate is a manually trimmed gate that is used for single cavity molds of large cylindrical parts that require symmetrical filling. Direct gates are the easiest to design and have low cost and maintenance requirements. Direct gated parts are typically less stressed and provide high strength. This gate leaves a large scar on the part at the point of contact.

A parting line is the line of separation on the part where the two halves of the mold meet. The line actually indicates the parting "plane" that passes through 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 part’s outer silhouette. Part lines can also occur where any two pieces of a mold meet. This can include side action pins, tool inserts and shutoffs. Parting lines cannot be avoided; every part has them. Keep in mind when designing your part, that the melt will always flow towards the parting line because it is the easiest place for the displaced air to escape or vent.

Gates vary in size and shape depending upon the type of plastic being molded and the size of the part. Large parts will require larger gates to provide a bigger flow of resin to shorten the mold time. Small gates have a better appearance but take longer to mold or require higher pressure to fill correctly.

Prior to ejection from the mold, injection molded parts are cooled down from manufacturing temperatures so that they hold their shape when ejected. During the part cooling step of the molding process, changes in pressure, velocity and plastic viscosity should be minimized to avoid defects. Few aspects are more crucial during this period than wall thickness. This feature can have major effects on the cost, production speed and quality of the final parts.

These type of gates incorporate features in the tool to break or shear the gates when the tool opens to eject the part. Automatically trimmed gates are used for several reasons:

The sequence of events during the injection molding of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected.

Undercuts Refers to the portion of the designed component where a slide or hand pull is required to create holes, windows or clips that are not in the line of draw (#1 in Figure 1 below)

Keep these factors in mind when designing your injection molded part, and remember that it is easier to avoid problems in the beginning than change your design down the line.

There are tens of thousands of different materials available for injection molding. Most polymers may be used, including all thermoplastics (such as nylon, polyethylene, and polystyrene) and some elastomers. Materials are chosen based on the strength and function required for the final part, but each material also has different parameters for molding that must be considered. Mixing the available materials with alloys or blends of previously developed materials enables product designers to choose from a vast range of materials to find the one with exactly the right properties.

Texturing is a process used to apply patterns to a mold surface. This process allows flexibility in creating the final appearance of your parts. Texturing is an integral piece in overall product development and should be considered during the design process to achieve the desired results. Texture can be a functional component of design (for example, to improve grip), as well as a strategy for camouflaging imperfect or frequently handled parts. Texture can also be used to reduce part wear from friction.

Draft Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. Generally all plastic components should be designed with draft where possible

Steel Safe Refers to the amount of metal left on the mold in order to tweak in a dimension. For example, if you have an inside diameter that is supposed to be .500 you may leave the mold at .505 in case you get excessive shrink

Order steel/mold base, machining process includes CNC, DME, wire-cut, milling, grinding, polishing, assembly and mold fitting.

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