PUR-Foaming

There are basically 3 methods for PUR foaming.

RIM-process (reaction injection moulding) polyurethane is foamed to PU-compact and PU-integral skin foam. Application examples are: housing parts, dashboards, armrests.
SRIM-process (structural moulding reaction injection moulding) a glass-fibre mat is placed in the mould and is foamed around with a polyurethane hard foam. This technique is mostly used for interior linings or dashboards of coaches.
RRIM-process short glass fibres are blended with polyurethane constituents and injected in to the mould. Application examples are:for resistant bumpers.

The most common of the 3 is the RIM-process.
The plastics used are thermosets, like polyurethanes or foamed polyurethanes.
With the low viscosity and low injection pressures, large, complex parts can be produced more economically in low quantities.
Considerable design freedom is possible, including thick and thin wall sections that are not good for injection molding, due to the uniform shrink characteristics. Foamed polyurethanes are natural thermal and acoustic insulators.
Reaction injection molding is used in many industries for many types of parts. While bumpers for vehicles are produced in this process, most applications are for large, complex parts produced in quantities less than 10,000 units. Examples include panels, enclosures, and housings.

In some occasions as much as 70% savings can be achieved on tool cost when choosing a PUR process over a standard injection molding process.

PUR Advantages

  • Thick, light weight structural parts can be molded without sink
  • Walls can be thick and thin on same part
  • Tolerances approximate to Injection Molding
  • Can mold over metal, glass, wood, wiring, circuit boards, hardware, etc
  • Much better economies for lower volume parts

Injection Molding Advantages

  • Part cost lower in high volume
  • Plastic part can be colored; no need for paint
  • Finer part detail can be achieved
  • Greater range of material choices

Good luck and remember that there are often more than one way to get the result you are looking for.
Contact us at The Tool Hub and we will gladly help you with your project.

Turbulent Cooling

Injection moulding process is cyclic in characteristic. Cooling time is about 50 to 75% of the total cycle time. Therefore, optimising cooling time for best performance is very important from quality and productivity point of view.

cycletimechart

The real objective here is to control the cooling rate and temperature of the parts so they can be ejected at the earliest possible time, while maintaining the desired properties and dimensions.

Cooling channel design – location and size and type – should ensure that melt freezes uniformly inside the mould. Cooling channel design must be analyzed with the help of a Mold flow report.

Understanding Heat Exchange in the mould

During every injection moulding cycle following heat transfers take place:

  1. from the hot melt to mould steel (heat input to the mould) and
  2. from mould steel to coolant flowing through cooling channel of the mould. (heat removal from the mould)

If heat input is more than heat removal, then the mould temperature would keep on increasing from cycle to cycle. Therefore moulding quality would not be constant from cycle to cycle. The moulding quality would be erratic- i.e. varying from cycle to cycle. Therefore, there is a need to balance between the heat input and heat removal in the mould after the desired mould surface temperature is reached. In other words, removal of heat by circulating coolant through the mould cooling channel would arrest the rise of mould temperature above the desired value. In practice, it may not be possible maintain constant mould temperature with respect to time. However, the mould temperature would fluctuate between two values around the desired value.

Quick design tips

Cooling channel diameter should be more for thicker wall thickness:
For wall thickness up to 2 mm, channel diameter should be 8 – 10 mm.,
For wall thickness up to 4 mm, channel diameter should be 10 – 12 mm.,
For wall thickness up to 6 mm, channel diameter should be 10 – 16 mm.

The difference between the inlet and outlet water temperature should be less than 2 to 5 degrees C. However, for precision moulding, it should be 1 degree C or even 0.5 degree C.

It is often difficult to accommodate cooling channels in the smaller cores or cores with difficult geometry. In such case the core should be made of Beryllium copper or Ampco which has high thermal conductivity. These core inserts should be connected to a cooling channel to best dissipate the heat.

It is often a good idea to add thermocouples at one or two places in core as well as cavity to monitor the temperature of mould.

Turbulent Flow

Achieving a turbulent flow is a good way to increase the heat transfer without having to alter anything in an existing tool.

Studies have shown that for the same net flow through a cooling channel a turbulent flow can transfer as much as 150-500% more heat from the tool steel.

Turbulent flow begins when the velocity of fluid in a channel increases to a critical level. Above this critical velocity, vigorous internal mixing of the fluid occurs as it flows. This improves heat transfer by mixing warmer fluid near the wall of the cooling passage with the relatively cooler interior fluid. The precise velocity for turbulent flow depends on several variables, including the cooling passage geometry, fluid viscosity, and roughness of the pipe walls. The formula for a ratio known as Reynold’s number includes these variables. A Reynold’s number greater than 4000 denotes turbulent flow.

Boundary Layer

The boundary layer is defined as the area of the flow that has shear stress forces induced by the solid wall of the water block. What this basically means is that the boundary layer is the part of the moving water that is feeling the friction of the wall. The molecules of water that are closest to and touching the water block wall are not moving at all, but are stationary. As the distance from the wall increases, the molecules pick up speed until they are far enough away that the flow feels no effects from the wall.

The problem with having a boundary layer for heat transfer in a water block is that it is actually insulating the inner most layers of flow from being able to pick up the heat from the tool steel. This is especially true of laminar flow because the boundary layer is very thick. However, in turbulent flow the random action of the water molecules breaks up the boundary layer and disperses the majority of it, thus increasing the ability of all the water molecules to pick up heat from the water block wall.

Flow rate needed to achieve turbulent flow:

Pipe Size
ID of drilled passage (mm)
Min. flow rate for turbulent flow (L/min)
1/16 NPT
6.5
1.25
1/8 NPT
9
1.66
1/4 NPT
11
2.08
3/8 NPT
15
2.80
1/2 NPT
18
3.4

The best cooling system in the world won’t take away heat any faster than the molded part will give it up. Most unfilled resins transfer heat at a rate 1/10 to 1/25 that of steel. The outer walls of a thick part insulate the mold from the heat trapped in the center of the part. The message here is that for very thick part, the cooling system will have relatively little effect on cycle time.

Optimal flow economy

An increase high above the threshold for turbulent flow is not very beneficial. For optimal cooling economy we recommend to stay in the green zone.

Pressure Loss Analysis In Injection Molding

Understanding the pressure requirements to fill and pack a part is a key to developing a reasonable processing window and higher quality of part.
When using a Scientific Molding procedure, parts are filled to around 95% in the first injection stage (Just short).
When the part is short we know the plastic pressure at the end of flow is zero.

To find the plastic pressure in each stage, we need to multiply the pressure at the transfer point (psi) × the intensification ratio of the machine.
By doing this we can find the pressure needed to make a slightly short shot in PSI.

If we at this point use more pressure that we would like, what can be done?
The flow path is known: Plastic traveled through the nozzle, sprue, runner, gate, and finally filled the part.
How much “plastic” pressure did each of these paths use up?
To find out the pressure loss for each part of the flow path, we need to make a few short shots.

Adjust the cutoff or transfer position to push the flow through just the stages below:

• 0 (Always add an origo point of 0 pressure)
• Nozzle
• Sprue
• P Runner
• S Runner
• T Runner
• Through Gate
• 50% Part Fill
• EOF

Below is an example of a pressure study done before and after modifications of the tertiary runner and the gate:
Pressure drop

 

P-S-T Runners explained below:
P-S-T-Runner

Guiding rules of thumb:
The maximum required injection pressure should not be more than 80% of the maximum available pressure on the molding machine.
Avoid sudden increases in pressures between sections. The transitions should be as smooth as possible.

Normally you can not do anything about the part without your client’s permission, and the gate needs to be within normal limits.
The sprue and runner however are ripe for alterations.
But don’t simply open up the runner: The pressure loss will be less, but a larger runner also provides less shear, which will actually increase the viscosity of the polymer and wastes resin. You need to make a shear thinning analysis to make sure you take the correct decision.

At The Tool Hub we suggest using the FIMMTECH software to simplify the work with your pressure drop study.

Importing plastic parts and samples

Importing plastic products and samples from China is far from complicated or expensive.

The CPC (Commodity Code) number describes what the goods will be used for in the EU.
Most samples are duty and Vat free, however goods that are being tested are usually classified as only VAT free.

It is therefore important that we know if your parts will be used for testing or if they are only samples to judge production quality.

Duty VAT
Samples No No
Samples for testing Yes No
Production parts Yes Yes

Importing from outside the EU is subject to a third country duty of 6.50 %.
The CPC code for plastic products is: 3926909790

https://www.gov.uk/trade-tariff/commodities/3926909790

Good luck with your imports and please contact us at The Tool Hub if you have any questions.

Evaluate your gate balance

First step is to check the physical size of each gate through a dimensional report.

Gates should always be made by using multiple rough and fine electrodes to ensure that the electrode wear will not cause any imbalance.
We suggest never to spark more than 4 cavities with the same rough electrode, and to then follow up with a fine electrode.

EDM steps:

  • Rough spark all gates with minimum 1 electrode per 4 gates.
  • Check electrodes for wear.
  • Fine spark all gates with 1 fine electrode and check for electrode wear.
  • If wear is within acceptable limit the gates are done.
  • If the wear is outside of the acceptable limit go over all gates again in the opposite direction with a new fine electrode.
  • Repeat the process until wear is acceptable.

Verification steps:

  • Set the holding pressure to zero.
  • Set the holding time to zero.
  • Set the screw recovery delay time to about a value close to an estimated holding time.
  • Set the cooling time to a value such that you know that the part will be cool enough to eject.
  • Set your injection speed to the value obtained from the Viscosity Curve study.
    • (Viscosity = Peak Injection Pressure x Fill Time x Screw Intensification Ratio)
  • With the rest of the settings the same as you had in the viscosity study, start molding.
  • Only by adjusting the transfer position, mold parts that are just short. If there is a visible cavity imbalance, then the ‘biggest’ part should be just short.
  • Make three such shots and take the average weight of each cavity and plot a graph

fill balance

Check the %variation between the maximum and the minimum fill cavities. In most cases, the % variation should not be greater than 5%. For tight tolerance parts, the variation should not be more than about 3%. If the tolerances are large, variation more than 5% is acceptable. More importantly, it is the final quality of the part that should be checked to see it there is need to tighten up the cavity balance. For example, the Cpk values of all the parts from all the cavities is a good measure. The following should be considered:

  • Amorphous materials can tolerate more imbalance than crystalline materials.
  • Tighter the tolerances, lesser should be the variation.

Venting is a very big contributor to plastic fill and can have a big impact on the cavity balance although the gate and runner sizes are the same. Make sure that the venting is the same for all cavities.

In tools where the balance is of extra importance we recommend the use of dynamic rheological control inserts.
They add a great way to control the fill balance between cavities without even taking the tool down from the press.

The cause is the natural stratification of melt within a flow channel into zones, or “laminates,” of different temperature and viscosity. This results from the greater shear heating of melt close to the channel wall. When the melt channel divides in a runner system, the melt heading toward one cavity or set of cavities may be more fluid than the melt split off in the opposite direction. Thus, despite exact cavity replication and balance in the mold steel, parts are formed under different conditions, just as if they were molded at different melt temperatures, fill rates, and packing times and pressures.
temp-difference-in-melt-flow

 

Runner with central flow
The fast moving high shear material flows symmetrical along the edges of the runner
Runner with modified flow
The fast moving high shear material is set to one side of the runner steering the flow pattern in to the cavity
centre flow lower flow

Result before and after flow modification (no gate geometry was changed between the trials):
fill balance 2

iMarc single axis for balance between cavities
iMarc multi Axis for fill pattern within a single cavity

Flow insert

Fill-adjust-inserts

For any help with gate balance please feel free to contact us at The Tool Hub.

Eliminating Bubbles in plastic parts

Bubbles in plastic parts is one of the most common defects and it can be tricky to solve.
These are the steps we recommend at The Tool Hub.

  • Find out what type of bubble you are dealing with.

    Different Bubbles need different solutions

    (Is it trapped gas or is it a vacuum void)
    This can be done by carefully heating up the part with a heat gun
    The bubble expands after heating = trapped gas
    The bubble collapses after heating = vacuum void
  • If we find that we are dealing with trapped gas
    Start by examining your injection parameters.
    The first step in the procedure is to take off hold / second stage by adjusting the hold pressure down to a very low number and see if the bubbles are still there.
    Assuming you still see bubbles, the next check is to learn the filling pattern to determine if the gas is air trapped upon filling the part.
    Analyze where the bubbles form and look at the flow pattern to see if there are any hesitations that might form the bubble

Parameters

Make sure you maintain a proper temperature profile.
Make sure your injection speeds are not too high.
Try increasing the back pressure.

Tool

Make sure you vent at least 30% of the perimeter.
Thin out thick sections / features that might cause hesitations.
Add dynamic rheological control inserts to alter the flow pattern within the cavity.
(see our article on evaluating gate balance)

Material

Ensure that you have the correct drying conditions

  • If we find that we are dealing with vacuum voids
  • This defect is caused by insufficient packing or thick sections that form voids during material cooling.

Parameters

Try a slower fill rate.
Increase the back pressure.
Reduce the melt temperature.

Tool

Open up the gates slightly to allow a longer hold time.
Change the gate position to fill the problematic section sooner
Core out eventual thick sections.

For any help with these issues, please contact us at The Tool Hub

Understanding shrink rates

There are many factors that play a role in the shrinkage that you will experience in your finished molding.

Resin nature:
Semi-Crystalline (High shrink rate)
Amorphous (Low shrink rate)
Below you can see more information on Semi-Crystalline Vs Amorphous resin types, and how they group on the shrink rate scale.

15-11-2014 03-41-38
15-11-2014 03-48-36

Rein fillers:
Glass fibre
Glass beads
Minerals

Fibrous fillers tend to align their strands with the flow of the material.
This can give big differential shrinkage across your part and therefore increase warpage and uneven dimensions.

Minerals and beads are isotropic in their shape and are distributed evenly in the melt mass.
This gives much less differential shrinkage and improved planarity plus dimension stability (in semi-crystalline resins)

Dimensional stability comparison for a plate 120x80x3.5 mm with different resins and fillers.
15-11-2014 04-05-34

Part geometry

Thicker wall sections will increase the shrink rate
Longer flow lengths will increase the shrink rate

Processing parameters

Increased cavity pressure will decrease the shrink rate
Increased holding pressure time will decrease the shrink rate
Longer cooling time will decrease the shrink rate
Higher mould temperature will increase the shrink rate
Higher melt temperature will increase the shrink rate
Material Name Shinkage %
ABS 0.4 – 0.7
CA 0.3 – 0.7
CAB 0.2 – 0.5
CP 0.2 – 0.5
EVA 0.7 – 2.0
FEP 3.0 – 6.0
GPPS 0.2 – 0.8
HDPE 1.5 – 4.0
HIPS 0.2 – 0.8
LDPE 1.5 – 4.0
PA6 1.0 – 1.5
PA66 1.0 – 2.0
PBT 1.5- 2.0
PC 0.6 – 0.8
PES 0.6 – 0.8
PET 1.8 – 2.1
PMMA 0.2 – 1.0
POM 2.0 – 3.5
PP 1.0 – 3.0
PPO 0.5 – 0.7
PSU 0.6 – 0.8
PTFE 5.0 – 0.9
PVDF 2.0 – 3.0
SAN 0.2 – 0.6
PP/EPDM 1.0 – 2.0
PUR/TRU 0.5 – 2.0
SBS 0.4 – 1.0
SEBS 1.0 – 5.5
DMC 0.5 – 0.2
MF 0.6 – 1.0
PF 0.7 – 1.2

As toolmakers there are a few things we can do to help you control the shrink rates.

Good and uniform cooling are critical elements.
Tight shut off with the right amount of venting can help with the cavity pressure.
Gate size, placement and design will aid with increased holding pressure.

Below you can see a graph that shows how a bigger gate size can help decrease the shrink rate:

15-11-2014 06-51-17

Good luck!

If you have any questions or just want some quick support, please feel free to drop us a line or give us a call. We are always here to help.Contact us

Get in touch with us!

Feedback
Please contact us and tell us how we can support.

Email: info@thetoolhub.com
Phone UK: +44 1865 922 550
Phone SE: +46 31 30 88 200

We are always happy to hear from you!