Home » Plastic Injection Molding Basics
Shrinkage values for polypropylene (PP) molded parts can range from approximately 0.008" to approximately 0.025" per inch. This broad range of possible shrinkage values have been calculated from a multitude of injection molded parts having various thicknesses. Linear mold shrinkage is mostly dependent on the particular polypropylene resin type, the processing conditions used to make the plastic parts, the part design, and or including the material flow path direction that's partly imposed by gate location(s). In summary, shrinkage is a function of all of these variables. Thicker parts shrink more so, anisotropically, inch/inch, than do thinner parts. However, packing more weight of polymer into the geometry of an injection mold / molded part can generally yield slightly less shrinkage. Exceptions to conventional shrinkage rules lie with filled PP. Filler-modified PPs, i.e., calcium carbonate, talc, glass, etc., will not necessarily shrink proportionally or equally in relationship to any given weight % of filler that might be contained within the polypropylene material.

Rules-of-thumb:
  • General purpose PPs, including Homopolymer Polypropylene, Random Copolymer, Polypropylene, Impact Copolymer Polypropylene, HPP, RCP, ICP, respectively, shrink slightly slower than nucleated or clarified PP.
  • Nucleated PPs shrink faster and slightly more than general purpose PPs.
  • Clarified PPs (C-HPP & C-RCP) shrink the quickest in injection molding operations; to approximately 98 % of final dimensions and slightly more so in finished dimensions. Generally speaking, over time, shrinkage in relationship to the various components of PP can result in different shrinkage amounts (inch/inch).
Some of the basic questions that need to be answered before quoting or outsourcing the injection mold need to be answered.

Life of product, quantity and terms
Part design constraints, critical dimensional features and surface finish. Which features need to be cut steel safe for adjustment after process is established. Your customer will expect you to know.
How many spares should be purchased in the original injection mold build?
Resin, cost of resin and allowable regrind will dictate hot, hot to cold or cold runner. The escalating cost and availability of resin makes this critical to injection molders. We are all striving for zero waste at the lowest cost. What size press are they expecting to use, shot size, residence time and expected cycle. Can this be done in their selected press?
Monthly delivery quantities to determine cavitation.

This will give mold size, mold class, 101 etc.
We have talked about the basic practices of improving injection molding productivity and here we would like to add a couple of cents to it...

1) Allow specific injection molding machines to work on specific plastic parts. For this you can concentrate manpower and have specific trained people for specific parts. EG. components for Apple products must be halogen free so machines cannot be used for other products that have halogen in them and transparent plastic parts to be used on machines for transparent parts. So should also set aside machines for injection mold trials and machines for production.

2) Loading and unloading of injection molds can be time at 1 hour. This defers between big injection molds and small injection molds. How quickly this can be done really depend on facilities and the setup team. If you are running long term productions, the setting up time may not be a big issue.
First there are the general rules for joining the individual features of the part like reinforcement ribs, bosses, functional features, etc.
1. Too thick ribs causing sink marks.
2. Inconsistent/incorrect wall.
3. Incorrect draft angle or the combination with the defined texture roughness for proper demolding (specialty in astheatic parts).

On the other hand and more important there is combining these features into one part defining the way of demoulding (undercuts, etc) and the split line. This often is unknown by plastic part designers not familiar with injection mold design. It can cause a higher mold price than needed and/or more vulnerable mold inserts. Tooling must be involved in plastic part design in the early stage to correct demolding difficulties before lead time becomes critical.
When cooling time will be more material will lose its heat faster because metal (injection mold has high thermal conductivity (Ability to transfer heat) as compared to air. So when we remove component after long cooling time material lost most of its heat inside the injection mold and material atom will not have sufficient minimum energy so they can not contract further. This is why dimension is stable due to high cooling time.

When the cooling time is less, plastic material lost its heat less inside the injection mold. And also outside the injection mold component cooling rate is slow as compared inside the injection mold because air thermal conductivity less as compared to metal. Now material atom has high thermal energy and for more time. So now atom will have more tendency to contract and have more ability to come back to its equilibrium position. (Also shrinking time on core is less). This is how when cooling time is less chances of dimension disturbances increases.
It's old school to try and use the process to push the plastic part around and compensate for injection mold or plastic part design deficiencies. Scientific Molding defines the process capability giving you an optimum window for a stable and repeatable process. Using processing gimmicks will tend to give you grief as it does not compensate for the natural variations all injection molding has and instead you are adding an un-natural variation making it difficult for future processing. The trend in process development is to find and fix these deficiencies.

Injection molding cooling time is often the forgotten child in a lot of ways. The plastic part stability is a good way to look at the problem. Given the rest of the process is correct and stable then releasing the plastic part prematurely will result in fairly unpredictable dimensional and form changes from the volumetric differential shrinkage. Every short is different in some may. Processing is the attempt to control that natural variation to produce acceptable plastic parts. If you neglect this and "muck" around with once confirmed stable settings, especially if you do not apply scientific molding, you will get trouble.
Cycle time reduction has been my passion for better part of last 20 years, so I might be able to add some value.
  • Do not be scared of PP!
  • Have injection mold conformably cooled, e.g. 6 mm. cooling channel distance to injection mold wall, 12-15 mm. pitch, 7 mm. cooling channel diameter/ width if milled.
  • If possible use RHCM, so part fills without any stress and change to cooling quickly. You may be able to use 20 bar pressurized water heated to say 180C for heating cycle followed by cooling with 12C chilled water.
  • Must have zoned cooling for each thickness variation-have unique temperature so that each plastic part of molding is having identical part thickness average temperature at end of cooling. Since part shrinkage is proportional to part thickness average temperature, equal average temperature means equal shrinkage and once differential shrinkage is eliminated part will be very flat, believe me, it works.
  • Getting into further detail, you may need a small differential between cavity and core as heat transfer to cavity decreases as air-gap develops. Lower cavity temperature by 5-15 C is necessary. For this study you need to see parabola of temperature through thickness (even Mouldflow has a feature for this graph).
You can try to improve the cooling method using two cooling supply to the injection mold, 1st of supply chilled water into the mold cavity plate, this is to make sure the injection mold temperature are meet the injection processing request, 2nd of cooling is use normal water (the water from cooling tower) supply to the injection mold base so the mold surface temperature will be much more close to the open air temperature will help you to reduce the mold sweating issue. And make sure your injection mold is maintain properly by the schedule, any dirty or rusty will cause to block the air release line in the injection mold, when injection speed build up will cause to air trapping and burn the surface sometime looks like moisture.

Basic facts already outlined well. Multi-cavity molding of PET preforms typically involves huge quantities of cold water (600 liters per minute not unheard of) and extreme turbulence are needed to give the low levels of crystallinity and clarity. Local A/C enclosures are certainly used in environments where condensation would otherwise be a problem.
For a good mold flow analysis you must manually select and calculate as follow:
Runner geometry, the ideal runner cross section is circular as this ensures favorable melt flow and cooling.
Runner dimensions, the diameter of a runner highly depends on its length in addition to the part volume, part flow length, machine capacity, and gate size. Generally they must never be smaller than the largest wall thickness of the plastic parts and usually lie within the range 3 mm to 15 mm.

Initially runner diameters can be calculated with the this formula
D= w^1/2 x L^1/4/3.7
Where
D= runner Diameter (mm),
W= part weight (g),
L= runner length (mm).
If the material has glass fiber and carbon fiber reinforced LNP, composites. The cold runner sizes have to be 30% to 60 % larger than a runner for an unfilled material. An alternative way for determining the runner dimensions is Reverse Engineering.
We perform a complete mold flow analysis on every new injection mold we build. The moldflow software today is very accurate at predicting potential injection molding problems and bring benefits. By optimizing the cooling circuitry valuable cycle time can be reduced. Warpage data can be used to play critical features steel safe or modify the part model by adding windage. Injection molders and mold makers who neglect this important step are losing a lot of potential profit in lost time and wasted material.
Mold flow analysis
Many plastic injection mold makers can attempt to make your mold but any small error makes the mold useless and impossible to fix. Also, injection mold shrinkage can be different on the various part axis (x,y,z) and can make the part not function properly. You have two choices:
1) go with an experienced gear design molder who may even tell you your project is too high tolerance for success or make you aware that you can succeed but only with expensive tooling and exacting production control..
OR
2) you can work with lower cost inexperienced plastic injection mold makers and have a project with all kinds of problems and final failure. I have done both, even a simple polycarbonate ring gear we made, had a host of production problems before we got it right.
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