Polypropylene Injection Molding Problems and Solutions

some times when we try to inject plastic part, problems always occur, beginning with shot mold, flashing, and some times jetting.

data below i take from Exxon Mobil Chemical, will explain various Injection Molding Problems and how to solve that properly.

it's begin with

• sink mark
  
• void
  
• shrinkage
  
• weld line
  
• flash
  
• burning
  
• warp
  
• brittle part
  
• sticking in mold
  
• gate bush and various others
  

below if you prefer read it online

Injection Molding Problems and Solutions - Upload a Document to Scribd

but if you prefer download, link below maybe more useful to read and download
http://www.scribd.com/doc/6200927/Injection-Molding-Problems-and-Solutions

this paper is very good tutorial with simple and practice solution for those each problems

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Calculation and Designing Support Pin or Leader Pin

as we know in previous posting about main construction of mold, there was pin in leader component, this pin will support main construction and cavity plate (3) from deflection and guide the cavity part and core part when mold closing. here the picture of support pin or leader pin in 3 plate mold type.


Support Pin Main Part
we can divide main part of support pin into 3 type
1. Support pin it self, some mold engineer call support pin by leader pin or main support pin.
2. Bushings, how much bushing that we needed depend on type of mold base, for example for 3 plate type of mold base we need at least 2 bushings, one in stripper plate (2) and one another in cavity plate (3). bushings prevent support pin have direct contact with mold base plate, becouse mold base plate usually use more soft material and support pin is harder material, it can cause scratch if use in long time and mass production, bushings always use harder material with good tolerance to support pin. so the other advantage is mold can be opened and closed with precision. and it can use in mass production mold.
3. Collar, it to prevent plate of mold out from leader pin when puller bolt broken or others accident possibility, with collar give the mold construction more safe.
see the main part of support pin in picture below


Deflection Calculation
ok, to learn more how to calculate the deflection in support pin, when support pin receiving the weight of a mold base plate on their tips,support pins may
deflect due to a large amount of bending moment, to choose right diameter and how much the bending first we must know the force that affect support pin, see picture below


yes at 3 plate type minimum load is from weight of stripper plate and cavity plate, with simple calculation of deflection (although isn't precision) like formula below



to calculate more complex and precision add the force from support pin weight, collars weight, and 3 dimensional calculation methods.
below is some list and use full engineering link to calculate the deflection, radius gyration list, derivation of deflection formula and others
http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia
http://www.doitpoms.ac.uk/tlplib/beam_bending/printall.php

then by calculation we can get the fine radius, after calculation I, maximum deflection is about 0.05 mm, more than 50 micro millimeter, use more large radius of support pin. or place more precision guide, like block guide, linear guide and others between cavity plate and core plate, it should be more precision when mold in closed position.

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Considering Factor that affecting gate size

when we design a gate for mold injection, we need a proper size of gate, to large size of gate will make unbalanced flow and defect in product. to small size of gate make cycle time high, unfil l of product and sink mark is appear easily in product.

although there was various factor that affecting gate size, here i list some common factor that affecting gate size.

Mass of Part
of course large mass of part need more large gate size, i still doesn't have list a good relation graph or table which various material and mass of part. Herbert Ress in Mold Engineering 2nd edition page 159 listed some suggested dimension for gate cross section but it just very general list without material and other kind of gate.

Size of Part
size of part also have large effect when we design a proper gate,longer flow length, larger cavity surface in injection mold of course need larger size of gate.

Temperature of Melt
when we use possible maximum temperature then cavity still cannot be filled, use more smaller gate, because with smaller gate more heat from heater can be transfered in resin, so shear stress of resin will decrease and viscosity also decrease, then resin will flow easily with small value of viscosity.

Nozzle Tip Position
don't use smaller length between nozzle tip and gate, because it will make freeze premature, if the construction of mold does not enable to make longer length, you can prevent that by making the gate more smaller.

Viscosity of Resin
viscosity of resin and shear stress of resin have large effect in proper gate designing. if we buy a resin, shear stress and viscosity data also available, use those data to calculate proper gate size. we can calculate average plastic flow by dividing shear volume with injection time. always use average plastic flow speed more than speed of sound, more than 340 m/s, if your value below the speed of sound it large possibilities that your product will shot (unfinish fill of product).

although other factor beside i write above there are some large factor like cooling speed, cooling temperature and injection time also have valuable effect when designing the gate, the main point was i write all above. from those main point i think we should design a proper gate in mold fast and effective.

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