Patterson & Rothwell

INJECTION MOULDING SPECIALISTS

In Pursuit Of The Perfect Injection Moulded Part – Flash

Flash

In Pursuit Of The Perfect Injection Moulded Part - FlashFlash is an excess of moulding material that appears as a thin lip or protrusion at the edge of a component. Flash appears because material has flowed outside of the intended flow channels and into the space between the tooling plates or at the injector. pin.

Flash is usually subtle but might be considered a major defect if particularly obvious on a product. The process for reworking a moulded product with flash often includes trimming the excess material.

Common causes of flash and prevention in moulded products

A poorly-designed or worn and degraded mould tool is the most common contributor to flash. Excessively high mould temperature or injection pressure can also cause flash. Material flowing through the mould cavities can force its way between plates when plate clamping force is inadequate. Common methods for addressing flash in moulded products include:

  • Retool or redesign the mould if plates don’t fit together properly or allow material to flow outside the channel
  • Increase plate clamping force to confine material flow to the channel
  • Adjust mould temperature, injection pressure and ventilation to improve material flow

Conclusion

It’s easy to follow a path that makes an injection mould tool cheaper to manufacture. But once you have a defect built into the tool created by cost cutting it becomes very expensive to fix the problem afterwards. It’s so much cheaper to make it right first time around and in our experience it’s better to bite the bullet. By all means if the part can live with a design change that makes tooling cheaper to do then that’s fine. But if it means your part can suffer from any points we have covered in our series it needs serious consideration. A £70,000 tool (that makes 1 million parts a year) that with a tweak here and a tweak there could become £50,000 is tempting. The downside is we have slowed it down by 10 seconds per cycle. This alone adds £20,000 to our part costs per year. Well your short term saving doesn’t look so good in year 2.

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Short Shot

Short Shot

In Pursuit Of The Perfect Injection Moulded Part - Short ShotA short shot occurs when the flow of molten material doesn’t completely fill the cavities in a mould. The result is that the moulded component is incomplete after cooling. Short shot might appear as incomplete compartments in plastic shelves of a display or missing prongs on a plastic fork, for example. Short shots are typically classified as a major defect that can inhibit the function or appearance of the moulded part.

Causes and remedies of short shot in moulded products

The most common cause of short shots is flow restriction resulting from narrow or blocked gates. Sometimes the material is too viscous or the mould is too cold to allow the molten material to completely fill the mould before cooling. And other times trapped air pockets may be hindering proper flow or injection pressure may be inadequate. Consider the following steps to prevent short shot:

  • Redesign the mould with wider channels or gates for better flow
  • Increase injection speed or pressure or choose a thinner base material to improve flow
  • Increase mould temperature to prevent material from cooling too rapidly
  • Add additional air vents or enlarge existing vents in the mould to allow trapped air to escape

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Delamination

Delamination

In Pursuit Of The Perfect Injection Moulded Part - DelaminationIf you find thin layers on the surface of a moulded part are easily separating or peeling off the underlying material, you’re seeing a moulding defect called delamination. Delamination is a defect characterised by a flaking surface layer, similar to what you’d commonly find on flake mica. This is generally regarded as a relatively serious defect because it reduces the strength of the component.

Causes and prevention of delamination in injection moulding

The most common cause of delamination is contamination of the resin pellets or other base material with a foreign material. Flaky separation results when the two materials cannot properly bond to each other. For example, you might combine a common base plastic like acrylonitrile butadiene styrene (ABS) with an incompatible plastic, such as polypropylene (PP). The resulting loss of strength of the material would be very dangerous if your part is intended for a safety-critical use.

Aside from material fed into the hopper, the contaminant could also be any excess release agents coating the mould for easier component separation. Excess moisture on the material, due to improper drying prior to use, can also cause delamination. Consider the following corrective actions to prevent recurrence if you discover delamination affecting your moulded parts:

  • Increase the mould temperature or pre-dry the material properly if excess moisture is an issue
  • Ensure workers are properly storing and handling the resin pellets or base material to prevent contamination
  • Consider redesigning the mould with a focus on the injection nozzle to limit your dependence on release agents

Injection moulding defects caused by poor mould design or maintenance

Defects can be introduced into moulded products by issues with the mould tooling itself. Certain defects are likely to occur when the mould is poorly maintained or designed. Especially in the latter case, these defects can be difficult or costly to address in future production runs when it’s necessary to completely overhaul the mould.

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Discoloration

Discoloration

In Pursuit Of The Perfect Injection Moulded Part - DiscolorationDiscoloration, or ‘colour streaking’, occurs when a moulded part is a different colour than intended. Often the discoloration is limited to a localised area or a few streaks of abnormal colour on a moulded part. This defect typically affects the appearance of the part without reducing its strength.

Causes of discoloration in moulded products and how to prevent it

A common cause of discoloration is leftover pellets in the hopper or residual resin in the nozzle or mould from a previous production run. Poor thermal stability of the colouring agent or improper mixing of the masterbatch are other potential causes. Take the following precautions to limit the risk of discoloration in your injection-moulded products:

  • Ensure that workers properly clean the hopper, nozzle and mould between production runs to eliminate any residual pellets or base material
  • Consider using a purging compound to remove excess colour from the machine
  • Ensure you or your supplier is using a colour agent with adequate thermal stability
  • Ensure that the masterbatch is evenly mixed for consistent colour output

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Jetting

Jetting

In Pursuit Of The Perfect Injection Moulded Part - JettingJetting refers to a kind of deformation in a moulded component that can occur when there’s an initial ‘jet’ of molten material injected into the mould cavity that starts to solidify before the cavity is filled. Jetting often appears as a squiggly line in the surface of the finished component, typically leading from the initial gate of injection. This visible flow pattern can result in part weakness.

Causes and remedies of jetting in moulded parts

The chief cause for jetting is excessive injection pressure.  When molten polymer or other material is injected through a small gate at high pressure it often squirts rapidly through the gate, rather than filling the mould cavity gradually. As this initial line of material cools against the mould walls and starts to harden, the remaining mould material pushes it, leaving impressions in the surface of the finished part. Avoid jetting in moulded parts by:

  • Reducing injection pressure to prevent rapid squirting of the material into the mould cavity
  • Increasing material and mould temperature to keep the initial jet of material from solidifying early
  • Designing the mould with the injection gate located such that the material is directed across the mould, rather than lengthwise

Defects typically related to material use or storage

Injection moulding defects can often originate from the material itself or how the manufacturer stores and handles the material prior to the production process. These defects can range from minor aesthetic issues to compromised strength of the finished component. Serious safety concerns may also result, depending on the intended application of the product affected.

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Weld Lines

Weld Lines

In Pursuit Of The Perfect Injection Moulded Part - Weld LinesWeld lines can appear on the surface of a moulded part where the molten material has converged after splitting off into two or more directions in a mould. The hair-like weld line is the result of weak material bonding, which lowers the strength of the part.

Common causes of weld lines and how to prevent them

Two or more fronts of polymer or other molten material need to maintain a certain temperature when colliding.  Otherwise, they become partially solidified and won’t sufficiently bond where they meet, resulting in weld lines. Common remedies for weld lines in moulded parts include:

  • Increase material temperature to prevent partial solidification
  • Raise injection speed and pressure to limit cooling before the material has filled the mould
  • Redesign the mould to eliminate partitions
  • Switch to a material with a lower melting temperature or viscosity to allow faster flow and prevent early cooling

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Sink Marks

Sink marks

In Pursuit Of The Perfect Injection Moulded Part - Sink MarksSink marks are small recesses or depressions in an otherwise flat and consistent surface of a moulded part. These can occur when the inner part of a moulded component shrinks, pulling material from the outside inward.

Causes and prevention of sink marks

Sink marks are similar to vacuum voids but are reversed in cause and effect.  Rather than the material cooling too rapidly near the exterior of the part, the material cools too slowly. The resulting shrinkage pulls the outside material inward before it’s had a chance to adequately cool, leading to a depression. As with voids, sink marks are more likely to occur in thicker parts of a component. Here are some steps you can take to prevent this defect:

  • Increase holding pressure and time to allow the material near the part’s surface to cool
  • Increase cooling time to limit shrinkage
  • Design your mould with thinner component walls to allow for faster cooling near the surface

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Vacuum Voids

It is very easy to make expensive errors when it comes to injection moulding. Quality issues in injection-moulded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the moulding process, material use, tooling design or a combination of all three.

This is the fourth on our series of posts where we look at a number of issues that can affect the quality of the end product and some of the common remedies. This post is about vacuum voids.

Vacuum voids / air pockets

Plastic Injection Moulding - Vacuum VoidsVacuum voids, or air pockets, are trapped air bubbles that appear in a finished moulded component. Quality control professionals typically consider voids to be a ‘minor’ defect.  But larger or more numerous voids can weaken the moulded part in some cases, as there’s air below the surface of the part where there should be moulded material.

Common causes of and steps to prevent vacuum voids in moulded parts

One of the chief causes of voids is inadequate moulding pressure to force trapped air out of the mould cavity. Other times, the material closest to the mould wall cools too quickly, causing the material to harden and pull the material toward the outside, creating an abscess. The material itself may be especially vulnerable to voids if its density changes significantly from the molten to hardened state. Voids are more difficult to avoid in moulded parts which are thicker than 6 mm. Common ways to prevent voids include:

  • Raise the injection pressure to force out trapped air pockets
  • Choose a grade of material with lower viscosity to limit the risk of air bubbles forming

Place gates close to the thickest parts of the mould to prevent premature cooling where the material is most vulnerable to voids.

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Warping

It is very easy to make expensive errors when it comes to injection moulding. Quality issues in injection-moulded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the moulding process, material use, tooling design or a combination of all three.

This is the third on our series of posts where we look at a number of issues that can affect the quality of the end product and some of the common remedies. This post is about Warping.

Warping

Plastic Injection Moulding - WarpingWarping is deformation that can occur in injection moulded products when different parts of a component shrink unevenly. Just as wood can warp when it dries unevenly, plastic and other materials can warp during the cooling process when uneven shrinkage puts undue stress on different areas of the moulded part. This undue stress results in bending or twisting of the finished part as it cools. This is evident in a part that’s meant to lie flat but leaves a gap when laid on a flat surface.

Causes and prevention of warping in moulded parts

One of the main causes for warping in injection-moulded plastic and similar materials is that cooling happens too quickly. Often excessive temperature or low thermal conductivity of the molten material can worsen the problem. Other times mould design can contribute to warping when the walls of the mould are not of uniform thickness—shrinkage increases with wall thickness. Here are some common ways to prevent warping in your moulded parts:

  • Ensure the cooling process is gradual and long enough to prevent uneven stresses on the material
  • Lower the temperature of the material or mould
  • Try switching to a material that shrinks less during cooling (e.g. particle-filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades)

Redesign the mould with uniform wall thickness and part symmetry to ensure greater stability in the part during cooling.

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.

In Pursuit Of The Perfect Injection Moulded Part – Burn Marks

It is very easy to make expensive errors when it comes to injection moulding. Quality issues in injection-moulded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the moulding process, material use, tooling design or a combination of all three.

This is the second on our series of posts where we look at a number of issues that can affect the quality of the end product and some of the common remedies. This post is about Burn Marks.

Burn marks

Injection Moulded Part - Burn MarksBurn marks typically appear as black or rust-coloured discoloration on an edge or surface of a moulded plastic part.  Burn marks generally don’t affect part integrity, unless the plastic is burned to the extent of degradation.

Causes and prevention of burn marks

The usual cause for burn marks in injection-moulded parts is trapped air, or the resin itself, overheating in the mould cavity during the injection process. Excessive injection speeds or heating of the material often lead to overheating that causes burns. Consider the following preventative measures to avoid burn marks in moulded components:

  • Lower the melt and mould temperature to prevent overheating
  • Reduce the injection speed to limit the risk of trapping air inside the mould
  • Enlarge gas vents and gates to allow trapped air to escape the mould
  • Shorten the mould cycle time so that any trapped air and resin don’t have a chance to overheat

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.