Ribs & Bosses

Ribs & Bosses

Injection Molding Ribs & BossesBoth ribs and bosses are functional features in many plastic part designs.  Ribs offer stability to side walls and standoffs such as bosses.

Bosses are used for screws or locators in assemblies.  It is important that when incorporating these into your design to follow one simple guideline.  Do not allow the rib or boss exceed 60% of the wall section of the mating surface of the part.  Failure to head this design tip will result in sink marks on the adjacent face.

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Gates & Runners

Gates and Runners are an essential part of the injection molding process.  There are three channels in which the plastic flows from the injection molding machine into the part cavity.


First, every mold has a sprue.  This is the contact point between the machine and the mold.  The sprue has a concave spherical seat where the molding machine nozzle tips seats off.  It is important that the nozzle tip and the sprue seat have the same radius.  If they do not seat off properly then material will leak at this point and parts will be inconsistent.  From here, the molten plastic travels through the sprue where it enters the runner system.  There are exceptions, on large parts the sprue can also act as a runner and a gate.  This is called direct sprue gating.

Injection Molding Sprue


Next, the plastic flows from the sprue into the runner system.  The runner is the channel that feeds directly into the gate of each part. If the Injection Mold only has one cavity then there will only be one branch to the runner.  If there are multiple cavities, then multiple branches will have to be engineered to ensure proper balance of flow.  This way each cavity gets the proper amount of material.  When designing a runner it is important to include a cold slug at every point there is a hard transition between branches.  The reason for this because as the molten plastic flows through the runner system, it begins to cool.  The cold slug is considered an area where this cold plastic is dumped in order to prevent it from entering the gate.

Injection Molding Runner


Last, the plastic enters into the gate.  This is where a lot of problems occur in the injection molding process.  Gates can be tricky, but understanding their purpose can help in designing a proper gate for a plastic injection molded part.  First, many think the larger the gate the better.  Although this may assist in getting the material in the cavity, it often leads to undesirable vestige that has to be hand worked as a secondary operation.  This is called gate trimming.


Gate Sizing for Plastic Injection Molding

The illustration above shows a gate land that is way too long.  In this scenario the process Tech will have to increase the molding pressure to overcome the frozen (plastic) gate at point of entry.  This will in turn cause the material to degrade due to too much shear heating (friction).  In addition, this gate will wear out over time.  To avoid this, it is best to use as small a gate as possible.  The trick to smaller gates is reducing the “Gate Land”.  This is the area where most toolmakers and injection molding companies make a mistake.  The reason for this is because the plastic virtually freezes off at the gate when the land is too large. Typically the land should be no more than .006″ wide.  This is not a typo, the runner should come all the way up to the part.

By following the guideline above, there will be virtually no shear as the plastic enters in through the gate and into the cavity.

Gate Location

The best gate location on any part is usually where the thickest wall section is.  However, it is important visually the plastic flowing into the cavity from the gate.  Best practice is to place a gate in front of a standing core on the tool.  This will cause the plastic to disburse evenly, preventing gate blemishes and jetting.  Jetting is when the plastic shoots through the gate and creates a snake like tail as it races to the far side of the cavity.  This material will freeze before the rest of the cavity is full, causing a snake like vestige in the part.  To over come this, a Fan Gate is used.  A Fan Gate will disperse the plastic material outward, thus preventing jetting.  Other gates include Sub gates, Cashew gates and Hot Runner Systems.

Injection Molding Gate Types

Edge Gates

Edge gates are the preferred gate for injection molding companies, as they offer a wide range of flexibility for filling a part.  Not only are edge gates affordable to manufacture, but they are easy to modify or change on the fly.  Following the guidelines above, will ensure success every time.  For high volume injection molders, a simple delayed ejection can be added to either the part or the runner to cause the gate to automatically detach.  This is a better replacement to the traditional Sub or tunnel Gate because it will never wear out and offers perfect shear conditions.

injection molding edge gate

Sub or Tunnel Gate

Sub Gates or tunnel are used in high volume automation where automatic de-gating is preferred.  These gates tunnel under the tool steel into a wall section.  As the part ejects the tunnel gate is sheared off as the ejector pin pushes up on the runner causing the gate to rip at the opening of the gate orifice.  These gates do exactly what they are intended to however, that does not come without a price.  Sub gates are expensive to manufacture and even more expensive to repair.  Furthermore, they often add challenges to the molder as it takes more pressure to overcome Bernoulli’s theorem.  “Pressures are least where velocities are greatest; likewise, pressures are greatest where velocities are least”.   As the plastic travels down the funnel velocity is decreased.  This in turn causes the plastic to freeze and pressure needs to be increased to over come.  Eventually leading to excessive wear.

injection molding tunnel gate

Cashew Gate

Cashew gate is another style of tunnel gate, where its shape resembles a cashew.  This allows for tunneling into a part where the surface is parallel to the runner.

Hot Tip

Hot tip gating is where a hot manifold keeps the material hot and molten ready for every shot.  They have a very tiny orifice and freeze off just enough to close the gate during cooling.  The gate land is usually a maximum of .006″ depending on the grade of material.  The benefit of this type of gate is that it reduces scrap produced by a runner and sprue and leave a very small gate vestige.

Other Gates

Over the years many have come up with some fancy gates and names.  They claim better fill conditions.  However, the rule is simple.  Keep it simple and follow the max gate land guideline above.

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Hinges, Snaps and Clips

Hinges, Snaps and clips designed into a plastic part can eliminate costly assembly labor.  However this does not come without a cost.  The upfront cost to integrate these features into a custom plastic injection molded part can be quite extensive.  Consider the following when integrating live in hinges, snaps and clips to your plastic parts;

Live In Hinges

Live in hinges are a great way to marry to haves of a component together.  However it should be noted that Live in hinges are not meant for multiple use or continuous duty.  Make sure that the material selection for your part supports live in hinges.  Many materials cannot flow through the thin wall section of the hinge thus causing the part to not be mold-able.  Furthermore, live in hinges require plastic materails that remain flexible when they are molded into thin wall sections.  If this important note is ignored, your live in hinge may crack and fail on first use.

 injection Molding Live In Hinge


Snaps and Clips

Snaps and clips are a great way to eliminate screws in the assembly of your project.  When incorporating snaps, consider the tooling cost that will surely rise with these features.  Often these features cause your part to have undercuts.  These undercuts will require additional tooling cost, that can mount really fast when multiple clips are integrated into your design.  Furthermore, these features often hang off a class a surface of the part often causing sink issues if the proper guidelines are not followed.

injection molding snap and latches

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OverMolding Service

OverMolding or Insert molding is the process of injection molding molten plastic around any substrate or insert.

Over-molding Services Provided

  • Plastic over Plastic Substrate
  • Rubber over Hard Substrate Overmolding
  • Plastic over Metal
  • Multi Color
  • Plastic Over Custom Inserts of any Material
  • Chemical Bond Overmolding
  • Plastic over Threaded Inserts
  • Mechanical Bond Overmolding
  • Medical Instrument Overmolding
  • Electronic Overmolding
  • Wire Overmolding

Custom Insert Overmolding.

The part below was created using a custom metal insert supplied by our customer.  Because of our skilled mold design engineers we were able to  mechanically place the insert into the mold and inject the plastic around it.

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Wall Thickness

  • Maintain a uniform wall thickness throughout your parts design
  • Thick wall design is prone to warp or other cosmetic issues
  • 10% Increase in wall thickness will provide 33% more stiffness with most materials.
  • Thick wall areas can sink, warp or contain voids resulting in undesired defects.
  • Using ribs can help to reduce thick wall sections while still giving the part strength.

Check with the material supplier before designing your part.

General Material Wall Thickness

  ABS 0.045 – 0.140
  Acetal 0.030 – 0.120
  Acrylic 0.025 – 0.500
  Liquid crystal polymer 0.030 – 0.120
  Long-fiber reinforced plastics 0.075 – 1.000
  Nylon 0.030 – 0.115
  Polycarbonate 0.040 – 0.150
  Polyester 0.025 – 0.125
  Polyethylene 0.030 – 0.200
  Polyphenylene sulfide 0.020 – 0.180
  Polypropylene 0.025 – 0.150
  Polystyrene 0.035 – 0.150
  Polyurethane 0.080 – 0.750


Need More Support?

At Xcentric our aim is to give the design engineer all the tools needed to make a fast educated decision.  That is why we have assigned a technical team to all of our accounts.  Furthermore, we believe that the fastest way to market is by preventing issues early on in the process.  Here are some key points that set us apart from other injection molding companies.

  • Online quote system – Our Online quote system gives our customers instant access to a technical team.  Your team is made up of a tool engineer and a sales rep.  Once a quote is submitted online, you will have a response within 24 hours.  In addition to that, your quote will be managed through our customer portal.
  • Customer Portal – Our online customer portal gives you 24-7 access to Xcentric from anywhere in the world.  In the portal you can;
    • Submit quotes
    • View and interact with a live interactive quote.
    • Purchase Tooling and Parts.
    • Instantly Re-order parts.
    • View history details such as invoices and Purchase Orders
    • Initiate Engineer Changes
    • Organize Parts by type.
    • Update and manage Account Information
    • And many more.  We are always adding features to support our customers needs
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Draft for Injection Molding

Also known as Angles or tapers on walls of plastic part features.  Draft angles are one of the most important design guides for injection molding of plastic parts.


Parts without draft can still be molded but they can and will have issues during  ejection from the mold.  As the plastic cools it shrinks around the mold core causing an enormous amount of friction.

Pin Push

pin push

While overcoming this friction the ejector pins push into the plastic resulting in pin push.  This results in undesirable marks and distortion of the plastic part.

Drag Marks

drag marks

Drag marks are caused by the plastic adhering to light scratches or textures in the mold side walls.  Thus, when the part is ejected the plastic peels out of these light scratches or texture causing drag marks.  When drag marks are present then the parts are often distorted whether  pin push is evident or not.

Minimum Draft

Regardless of how smooth the surface finish is, it is never a good idea to design a part for injection molding without draft.  There are no minimum draft requirements as each part has different features.  However as a rule of thumb a part that does not have in mold texture should have a minimum of 1 degree draft on side walls.

When adding texture to a cavity of the mold it is a good idea to find out the manufacture spec for minimum draft requirements before starting your part design.  This will eliminate the need to redesign your part as texture usually have a large draft angle requirement, usually 3 degrees and up.

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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.  Plastic Injection Molding Machine

The injection molding cycle is as follows;

  1. Material Enters Barrel
  2. Material melts and mixes
  3. Volume of material (Shot sizes in barrel is created)
  4. Mold closes
  5. Injection of the plastic into the mold cavity
  6. Molten material cooled (during this process steps 1-3 are preparing for next cycle)
  7. Mold Opens
  8. Part Ejects
  9. 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;

  • Material Selection
  • Shrink Rate
  • Draft
  • Ribs
  • Bosses
  • Undercuts
  • Integrated Fasteners
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Mold quality materials

With the modern industrials development, the plastic products industry, agriculture and daily life, and other fields are widely used; quality requirements have become more sophisticated. Plastic products, the production and quality of mold design, advanced mold manufacturing equipment and reasonable processing, mold quality materials and modern equipment are molding quality plastic parts forming an important condition.

In the Injection plastic mould process, hot molten plastic is forced under pressure by a hydraulic ram into a closed mold.  The mold is cooled to freeze the plastic in the desired shape, and no chemical reaction takes place.

Plastic injection mould Process, including pressure plastic, blow, extrusion, etc., plastic injection mouldprocessing is the most commonly used methods, apply to all parts of thermoplastic and thermosetting plastics. With the injection mold industry, mold cavity mold and shape the increasingly complex, precision die increasingly high demands, the production cycle requirements become increasingly short.

Plastic injection mould process demands precise control of melt temperature, melt viscosity, injection speed, injection follow-up pressure, switch over point from speed to pressure, cycle time. It is found that different polymers have different characteristics and different limitations in processing. Shear rate and shear stress influence melt temperature, viscosity, density and flow behavior of polymer. Some polymers are hygroscopic, some polymer have limited thermally stable time which is different at different temperature. Such polymers have limited residence time. The changes in each parameter has its own influences on other parameters.

Plastic injection mould process includes a number of factors, some of them are important. They play a decisive role for the quality in the plastic injection mould processing. Such as freezing time and injection time; maximum injection speed; maximum injection pressure; injection power; plasticizing rate; etc.

And plastic injection mould machine application shaping products directly affect the efficiency of production, quality and cost.

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Plastic mold polishing process

Plastic mold polishing the basic procedures in order to obtain high-quality polishing, the most important thing is to have a
Plastic mold
Quality of Whetstone, sandpaper and diamond grinding tools and polishing ointment aids.The polishing process depends on the choice of pre-processed surface conditions, such asmachining, EDM, grinding, and so on.
Plastic mold polishing the general process is as follows:
1, fine polishing
Fine polishing using diamond polishing paste key. If the polishing cloth with a mixture ofdiamond abrasive wheel abrasive powder or paste for grinding, then grind the usual order of9��m (# 1800) ~ 6��m (# 3000) ~ 3��m (# 8000). 9��m diamond polishing paste and polishing cloth wheel is used to remove the # 1200 and # 1500 sandpaper grinding marks left by the hairy. Then use the sticky carpet and polishing the diamond abrasive paste, in order of1��m (# 14000) ~ 1/2��m (# 60000) ~ 1/4��m (# 100000). Accuracy in more than1��m (including 1��m) of the polishing process in the mold shop in a clean room can bepolished. If a more sophisticated finish is absolutely necessary for a clean space. Dust,smoke, dandruff and saliva foam are likely to scrap a few hours after work to get the high-precision polishing surface.
2, rough polishing
After milling, EDM, surface grinding and other processes can be selected after the 35 000-40 000 rpm speed rotation of the surface grinding machine polishing machine or ultrasonicpolishing. Commonly used methods are the use of diameter ��3mm, WA # 400 of the sparkwheel to remove the white layer. And then grinding by hand Whetstone, Whetstone strip pluskerosene as coolant or lubricant. The use of general order # 180 ~ # 240 ~ # 320 ~ # 400 ~ #600 ~ # 800 ~ # 1000. Many mold makers in order to save time and the choice of startingfrom # 400.
3, semi-fine polishing
Semi-fine sandpaper and polishing the main use of kerosene. Sandpaper numbers were: #400 ~ # 600 ~ # 800 ~ # 1000 ~ # 1200 ~ # 1500. # 1500 sandpaper actually only suitable forhardened tool steel (52HRC above) does not apply to pre-hardened steel, as this may result in pre-hardened steel surface burns.

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Why Molds are Expensive


The simple answer is that Injection Molds are expensive because they are very complex mechanical systems. Molds require: Engineering and design, special materials, machinery and highly skilled personnel to manufacture, assemble and test them.
The injection molding process is one where molten plastic material is forced into a mold cavity under high pressure. The mold cavity is an exact hollow negative of the part to be produced. In order for the part to be released, the mold must open at the widest place on the part. The molten plastic pressure during injection ranges from 5,000 to over 20,000 psi. This pressure multiplied by the area of the part gives rise to huge forces seeking to open the mold. The mold must be constructed to withstand the very high clamping forces exerted by the injection molding machine to contain this pressure

The injection molding process is capable of rapidly producing large quantities of parts with very high precision. Tolerances of a few thousandths of an inch are routinely achieved. With the right combination of material, part design and mold construction, even sub one thousandth inch tolerances can be achieved for small features.

The cost of injection molds can range from a few thousand dollars to hundreds of thousands of dollars.


The materials used to construct injection molds range from aluminum to hardened steel:
Aluminum for simple low production prototypes.

The relative low strength of aluminum that makes it quicker to fabricate into molds likewise limits its useful life. Aluminum molds are typically intended to produce from a few thousand to a few hundred thousand parts with relatively simple features.
Prehardened tool steel for moderate production, more complex molds.
Prehardened tool steel molds are much stronger and more durable, yet still soft enough to be worked by conventional machining processes such as milling and turning. Prehardened tool steel molds are typically intended to produce from one hundred thousand to five hundred thousand parts, and can have a wide array of features such as slides and more intricate shapes that might break in an aluminum mold.
Hardened tool steel for high production, long life molds.
Hardened tool steel molds are the most durable and expensive because part way through fabrication their components are heat treated to achieve a hardness greater than can be machined. From that point on, the fabrication must continue using grinding and EDM processes.
Hardened steel molds are intended to produce one million or more parts. Their hardness enables them to resist wear from their own operation and the abrasion of the plastic material, particularly glass fiber reinforced materials. Hybrid construction is very common, where steel parts are used in an aluminum mold to add strength to a slender feature, or parts of a steel mold are hardened to prevent wear at a rotating or sliding mold feature.


Single cavity molds offer the lowest tooling costs and highest precision at the penalty of higher unit costs. Multi-cavity molds are utilized to increase capacity and lower unit costs.
Family molds, multi-cavity molds with different items together, offer both the lowest mold cost and low unit cost. However, they present other problems of matching the process conditions for each part and balancing supply when the product mix or yield at a later manufacturing step varies.

Engineering and Design:

The design of injection molds begins with a review of part specifications including: Aesthetics: color, clarity, high gloss, matte, special texture, etc. Material: strength, toughness, hardness, chemical and environmental resistance Interaction with mating parts: fits and tolerances Demand and unit cost goals
From this review process the mold design concept is evolved and decisions are made resulting in a mold specification:

Single, multiple cavity or family molds The grade of mold: aluminum, prehardened tool steel or hardened tool steel Material flow considerations Parting lines and gates Finish: high gloss, texturing, embedded text and graphics, etc. Accuracy and tolerances Cooling passages Ejection system Runners or runnerless system design

The next step is the actual design of the mold. Highly skilled designers using very complex and expensive computer software programs perform this. The design tasks include:

Modeling of the products and mold components in 3D. Mold flow analysis CNC tool path design and calculation Mold materials procurement list

Early in the design process, materials and components are ordered so that manufacturing can commence as soon as possible.


Once the design is completed manufacturing begins. Mold making involves many steps, most of which are very exacting work requiring highly skilled moldmakers. One mistake can ruin or cause major repair expense to a work piece that has undergone a series of manufacturing steps over several weeks. The processes employed in mold making include:
Milling and turning
Grinding and honing
Electrical discharge machining
Polishing and texturing
To save cost, common mold components are purchased from suppliers. Frequently, outside services are required from subcontractors, which use specialty equipment such as thread grinding, etc.
When all of the parts are completed the next step is to fit, assemble and test the mold. All of the mold component parts must fit together precisely to achieve an aesthetic result on the product and for the mold to not wear out rapidly or break. The mold must be fluid tight to contain the molten plastic. Yet, at the same time the mold must have venting features added to allow the air to escape. The behavior of the plastic material when molded has been anticipated, however there can be some variance in the actual result. The mold must be tested to insure the products are correct and that the mold is performing properly. Where high accuracy is required, the mold may intentionally be made “metal safe” with the final adjustments coming after the first molding trial.


As can be seen from the above, the engineering and creation of injection molds is a time consuming process. The work is demanding in terms of knowledge, skills and exacting attention to details. This will always be expensive, however this expense must be viewed in terms of what is achieved: Unsurpassed sophistication in part design and aesthetic appearance with low cost mass production.
Consider the Desk Telephone. The injection molds for these half a dozen parts likely costs a quarter of a million dollars. Amortizing that expense over the hundreds of thousands of units produced brings the mold cost to pennies per phone. For this type of product, no other manufacturing process can approach the level of design, functionality and cost effectiveness of an injection molded article.

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