In plastic injection molding troubleshooting, warpage is one of the most common quality issues.
So what is warpage?
Warpage is excessive change of shape of the part after it has been ejected from the injection mould. Also known as deformation, a warped part will twist or bend due to internal stresses in the part caused by uneven shrinkage rates or by some mechanically applied forced such as during the ejection phase.
Eliminating part warp can have huge cost savings and should be part of a lean manufacturing program.
But how to fix warp?
The root causes of warp can be grouped into 5 different areas:
1. Incorrect processing parameters
2. Mould issue
3. Injection moulding machine issue
4. Part design issue
5. Poor plastic material selection
Isolating which area is causing the warp can be quite difficult so it must be done in a step by step procedure eliminating each possible cause one at a time.
When doing plastic injection molding troubleshooting, keep an open mind because the cause might be different to what you think it is.
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When using this plastic injection molding troubleshooting guide below change one processing parameter at a time by a small amount (say 5%) until you get a result.
Explanation: Injecting with a lower rate will leave higher residual stresses in the moulded part. A lower injection rate causes a lower viscosity in the material which requires higher injection pressure to push the material into the mould cavity. A short shot might also result.
Action: Increase the injection rate by increasing the injection speed. Take note of the fill time parameter before and after increasing the injection speed. If it doesn't reduce then increase the injection pressure until the fill time reduces to the target time & part warp is eliminated.
Explanation: A high injection rate can result in a high shear rate of the material at the gate. If the shear rate is too high it will add to the internal stress in the part causing it to warp.
Action: Reduce injection rate.
Explanation: Short hold times don't allow the machine to compensate for the natural shrinkage that occurs as the part cools in the mould cavity. The machine needs to apply hold pressure to the part after first stage fill to compensate for most of the shrinkage.
Action: Increase hold time.
Explanation: Short cooling times don't allow sufficient heat to be extracted from the part while it is still held in the mould cavity. A part that is ejected hot is more likely to warp than a part that is ejected cold.
Action: Increase cooling time but also consider reducing barrel temperatures or screw back pressure during the plastizing stage.
Explanation: Low melt temperature will cause higher internal stresses in the part.
Action: Increase barrel temperature or screw back pressure during the plasticizing stage.
When plastic injection molding troubleshooting, the mould is usually the first area to be checked.
Explanation: Uneven cooling will cause different shrinkage rates within the part. This causes stresses in the part.
3 common reasons for uneven cooling are:
1. Poor mould design: For example: water channels might be spaced too far apart.
2. Poor mould setup resulting in low water flow rate: For example: looping too many water circuits externally or having small diameter hoses or fittings connected to the mould will restrict the flow.
3. Lack of mold maintenance.
Action: Check the moulding surfaces of the core and cavity for temperature difference. Large differences can be noticed by hand touch while smaller differences will need the aid of a temperature sensor. When checking make sure it is done immediately after the mould stops cycling. Any delay will give a false reading. Then modify mould design.
Alternatively, check that the water flow meters have a similar flow rate and measure the difference between the inlet and outlet water temperatures by placing a temperature sensor on each individual water fitting connected to the mould The temperature difference should be less than 4 degrees Celsius. Also,don't loop any water circuits; use individual water circuits. This might not fix the problem but it at least eliminates it as a cause.
Also consider descaling (cleaning) cooling channels.
Explanation: The ejection stage can warp products for the following reasons:
1. Poor design of ejector system: If the ejection system doesn't provide even force to the part then there is a possibility of distortion. For example , having too few ejector pins.
2. Sharp corners make the part hold tighter in the mould.
3. Polish: a high gloss finish will make it more difficult to eject the part especially on side walls with little draft. Also scratches or machining marks that are not in the direction of part ejection will cause the part to stick tighter in the mould.
Action: Radius sharp corners (R0.2mm minimum), repolish if necessary or modify ejector system.
Explanation: In most cases, the plastic material should be moving away from the gate at all times inside the mould cavity during the filling stage. If any portion of the flow front starts to move back in the direction of the gate then weld lines or voids due to air entrapment will setup extra stresses in the part.
In other words, the flow front should be as uniform as possible and gate position plays a critical role in this.
Action: Find new gate position. Seek the services of an engineer experienced in both plastic mold flow simulation software and the injection molding process.
Explanation: A small gate size can generate large shear rate in the material as it passes thru the gate. A high shear rate will stress the material resulting in warpage.
Action: Increase gate size but consider first changing to an easier flow material or trying another moulding machine with better capability. Increasing the gate size will also increase the gate mark on the part which can detract from the appearance.
Explanation: For multi-cavity moulds, if there is a slight difference in the physical size of the steel cavity or core moulding surface dimensions between cavities due to poor machining practices, then this will cause different cavity pressures.
Action: Measure all cavity and core moulding dimensions.
Explanation: Slow acceleration of the injection screw during the filling stage will increase the internal stresses in the part.
Action: Increase injection pressure or move mould to a better machine.
Explanation: Changes in wall thickness creates localised stresses due to different shrinkage rates. This can distort the part.
Action: Redesign part so that wall thickness change is gradual or make wall thickness even across the entire part then modify mould to suit.
Explanation #1 : If the wall thickness is very thin then the part will be naturally weak. A weak part can be distorted during ejection from the mould especially during short cycle times.
Explanation #2 :If the wall thickness is very thin compared to the flow length then high injection pressures and speeds are required to fill the mould cavity (this is required in thin wall injection molding). If injection times are too long then high internal stresses will be in the part causing warpage.
Action: Make wall section thicker or modify mould so that the ejector system is more reliable.
Explanation: Strengthening ribs improve rigidity of the part when placed in the right areas such as in corners.
Action: Modify part design so that ribs are included. The wall thickness of the ribs don't need to be as thick as the rest of the wall section to provide strength. 50%-80% of nominal wall thickness is adequate.
Explanation: Flat rectangular parts will warp more than round tall parts. An example of a flat rectangular part is some kind of lid for food packaging and an example of of a round tall part is a drinking cup.
Did you ever see a warped plastic drinking cup? I havent.
Action: Part designers should keep this fact in mind when designing.
Explanation: A low mfi material will leave more internal stress inside the moulded part.
This stress can cause the part to warp.
Action: Change to a higher mfi material.
Explanation: Crystalline materials such as polypropylene, polyethylene, PET & Nylon tend to warp more than amorphous materials such as polycarbonate, polystyrene, SAN and PVC.
Action: #1 Change to an amorphous material however, this is easier said than done because these materials have different shrinkage rates, properties, costs and may require a modified mold design in order to meet the end user part requirements.
Changing the material in most cases is unlikely to be the right solution.
Action: #2 Add fiber reinforcement. Fiber reinforced materials warp less than non-reinforced materials provided the part shape is symmetrical and gate location is correct.
Plastic injection molding troubleshooting can have a huge affect on cost savings and your production performance.
Although it can be costly to get the root causes of part warp repaired the question you have to ask yourself is:
What is the long term cost of not fixing warpage quality issues?