Internal & External Threading

Internal & External Threading

Injection Molding Threading

Adding threads to your injection molding project gives you the ability to fasten your plastic parts to other components securely with relative ease.

At Xentric we can handle any thread design.  Currently we are making threaded injection molded parts for all industries including; medical, aerospace, defense and Automotive.

Give us a shot on your next injection molding project.

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Surface Finishes

No matter what kind of surface finish you desire, Xcentric will be sure to exceed your expectations.

Available Surface Finishes from Xcentric

  • B3 320 Paper
  • B2 400 Paper
  • MT-11020 Heavy Bead Blast
  • MT-11010 Light Bead Blast
  • A3 Smooth Finish
  • A2 Optical Smooth Finish

During the part design consider the relationship between cost, lead time and surface finish. Typically higher level finishes like A3 or A2 will increase the tooling cost significantly.

Surface finishes play a key role in both function as well as the look and feel.  Parts that are hidden in the assembly may not require a surface finish at all.  In contrast, exposed components might need to be visually appealing.

If your parts will not be visible in the end product a lower finish option is typically selected.

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Flash

Parts with flash boil down to one or two issues.  First, Flash is when material seeps beyond the boundary of the part.  There are three causes. Either the tool is damaged,  the part is being over packed or the molding machine is too small to hold the mold halves closed while under pressure..

Tool damage requires an experienced tool engineer  to make a repair.  Sometimes this can cause extensive delays in delivery.

Over packing a part is generally not enough to cause it to flash.  However, when extra pressure is trying to force more plastic into the cavity, even a .003 gap in the shut off can leak.

Wrong sizing of the molding machine can result in the mold blowing open while under pressure.  Be sure that the correct tonnage is calculated.  As a  general rule of thumb, 2 to 5 ton of clamp tonnage per square inch of projected part area.  For example, if you have a part that is 3 inches x 5 (depth is not calculated in area) that is 15 square inches.  So that is between 30 and 75 tons of clamping pressure needed.

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Sink & Warp

Warp in injection molding is caused by the part wall sections being inconsistent.  As the part cools the thick sections will shrink more than the thin sections.  As a result the thin section will be pulled in toward the thick section thus resulting in warp.   Additionally, if the part is informally thick it can cause sink issues and unstable dimension caused by excessive shrink.

Sink Marks in injection molding can also be caused by features adjacent to nominal walls.  For example, if a wall has a rib or a boss protruding from it ant the width of the boss or rib is greater than 60% it can result in a witness or sink mark on the opposite side of the adjacent wall.  The above illustration is a part that has 2 bosses on the reverse side.  Bot of these bosses are larger than 60% of the wall section they are attached to.  Causing sink marks on the reverse side of the part.

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Close Tolerances

When designing parts with tight tolerances it is important to consider the tolerance of the plastic being used.  This is one mistake many design engineers make when designing a part for the injection molding process.  For example, if the shrink tolerance on a materiel is between .004 and .008 per inch.  Then it would make no sence to require your injection molder to hold less than .004 as the range is .004.  When tighter tolerances or repeatability is required try choosing a material  with low stable shrinkage.

If your project has tight tolerances usually less than .004 per inch or .1 mm.  It is advised to leave key dimensions stock safe and later groom those dimensions into tolerance.  This will save a lot of money in tooling cost as stock safe changes are much cheaper than adding stock.

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Insert Molding Service

Insert Molding Services Provided

  • Brass Bushings
  • Threaded Inserts
  • Magnets Overmolded
  • Bushings
  • Custom Inserts
  • Plastic over Plastic Substrate
  • Plastic Over Custom Inserts of any Material
  • Chemical Bond Overmolding
  • Plastic over Threaded Inserts
  • Mechanical Bond Overmolding
  • Medical Instrument Overmolding
  • Electronic Overmolding
  • Wire Overmolding

About Insert Molding

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

There are many substrates used in insert molding, but the most popular is brass threaded inserts and bushing.  In addition, many companies make custom inserts that can be insert molded.  It is really left up to the imagination when deciding what type of inserts can be over molded.

The benefit to insert molding is that parts can be mass produced faster than machining whole parts out of metal.  Even though insert molding adds to the cycle time (typically 15 seconds per insert) it is still much faster than cnc machining a part from a solid block.  Furthermore, by insert molding metals where necessary and replacing the rest of the part with plastic will in turn reduce the overall weight of the part.

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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.

Sprue

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

Runner

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

Gate

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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