Very basic knowledge of injection molding design (Part 3)

Gates 

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 & warping.

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

Manually Trimmed Gates:

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

·         The gate is too bulky to be automatically sheared by the machine

·         Shear-sensitive materials such as PVC cannot be exposed to high shear rates

·         Flow distribution for certain designs that require simultaneous flow distribution across a wide front

Automatically Trimmed Gates

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:

·         Avoiding gate removal as a secondary operation, reducing cost

·         Maintaining consistent cycle times for all parts

·         Minimizing gate scars on parts

Common Gate Designs

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 of the most popular gate designs used by Quickparts customers:

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.

The Sub Gate is the only automatically trimmed gate on the list. Ejector pins will be 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.

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.

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 lower stressed and provide high strength. This gate leaves a large scar on the part at the point of contact.

Gate Location

To avoid problems from your gate location, below are some guidelines for choosing the proper gate location(s):

·         Place gates at the heaviest cross section to allow for part packing and minimize voids & sink.

·         Minimize obstructions in the flow path by placing gates away from cores & pins.

·         Be sure that stress from the gate is in an area that will not affect part function or aesthetics.

o    If you are using a plastic with a high shrink grade, the part may shrink near the gate causing “gate pucker” if there is high molded-in stress at the gate

·         Be sure to allow for easy manual or automatic degating.

·         Gate should minimize flow path length to avoid cosmetic flow marks.

·         In some cases, it may be necessary to add a second gate to properly fill the parts.

·         If filling problems occur with thin walled parts, add flow channels or make wall thickness adjustments to correct the flow.

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 time to mold or may need to have higher pressure to fill correctly.

Wall Thickness

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.

Proper Wall Thickness:

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.

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.

Uniform Wall Thickness:

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.

Alternatives:

If your part is so complex that you need variations on your wall thickness, look for an alternative. You may want to use design features such as coring or using ribs. 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.

Draft  

Most injection molded 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 referred to as “draft in the line of draw”. This draft allows the part to break free of the mold as soon as the mold opens. The amount of draft required can depend on the surface finish of the mold. A smooth, polished tool surface will allow the part to eject with less draft than a standard tool surface.

Consider the fabrication of the hollow plastic box seen to the right. Once the plastic has hardened around the mold, the mold must be removed. As the plastic hardens, it will contract slightly. By tapering the sides of the mold by an appropriate "draft angle", the mold will be easier to remove.

The amount of draft required (in degrees) will vary with geometry and surface texture requirements of the part. Below are several rules for using draft properly:

·         Be sure to add draft to your 3D CAD model before creating radii

·         Use at least 1 degree of draft on all "vertical" faces

·         1 ½ degrees of draft is required for light texture

·         2 degrees of draft works very well in most situations

·         3 degrees of draft is a minimum for a shutoff (metal sliding on metal)

·         3 degrees of draft is required for medium texture

Sink Marks

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 on 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.

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. Also, textures can be used to hide minor sink marks.

Textures

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 as well. Imperfect parts can be camouflaged by the right texture. Is the part designed for frequent handling? Texture can be used to hide finger prints and improve the grip for the end user. Texture can also be used to reduce part wear from friction.

A wide variety of textures are available for injection molded parts such as:

·         Natural/Exotic

·         Matte Finishes

·         Multi-Gloss Patterns

·         Fusions

·         Graphics

·         Leather Grains/Hides

·         Woodgrain, Slate & Cobblestone

·         Geometric & Linens

·         Layered Textures to Create New Looks

·         Images or Logos Incorporated into the Pattern

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.

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 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”.

Common Molding Defects 

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)

Molding Defects	Alternative Name	Descriptions	Causes
Blister	Blistering	Raised or layered zone on surface of the Plastic part	Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn marks	Air Burn/Gas Burn	Black or brown burnt areas on the plastic part located at furthest points from gate	Tool lacks venting, injection speed is too high
Color streaks (US)		Localized change of color	Plastic material and colorant isn't mixing properly, or the material has run out and it's starting to come through as natural only
Delamination		Thin mica like layers formed in part wall	Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond
Flash	Burrs	Excess material in thin layer exceeding normal part geometry	Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded contaminates	Embedded particulates	Foreign particle (burnt material or other) embedded in the part	Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marks	Flow lines	Directionally "off tone" wavy lines or patterns	Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)
Jetting		Deformed part by turbulent flow of material	Poor tool design, gate position or runner. Injection speed set too high.
Polymer degradation		polymer breakdown from oxidation, etc.	Excess water in the granules, excessive temperatures in barrel
Sink marks		Localized depression (In thicker zones)	Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high
Short shot	Non-Fill/Short Mold	Partial part	Lack of material, injection speed or pressure too low
Splay marks	Splash Mark/Silver Streaks	Circular pattern around gate caused by hot gas	Moisture in the material, usually when resins are dried improperly
Stringiness	Stringing	String like remain from previous shot transfer in new shot	Nozzle temperature too high. Gate hasn't frozen off
Voids		Empty space within part (Air pocket)	Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness).
Weld line	Knit Line/Meld Line	Discolored line where two flow fronts meet	Mold/material temperatures set too low (the material is cold when they meet, so they don't bond)
Warping	Twisting Part	Distorted part	Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)

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.