Shrinkages and warpage in the processability of wood-filled polypropylene composite

Shrinkages and warpage in the processability of wood-filled polypropylene composite

Shrinkages and warpage in the processability of wood-filled polypropylene composite

Shrinkages and warpage in the processability of wood-filled

polypropylene composite thin-walled parts formed by injection


Shrinkages and warpage in the processability of wood-filled polypropylene composite

M.D.Azaman a ,b ,S.M.Sapuan a ,c ,⇑,S.Sulaiman a ,E.S.Zainudin a ,c ,A.Khalina d ,e


Department of Mechanical and Manufacturing Engineering,Universiti Putra Malaysia,43400UPM Serdang,Selangor,Malaysia b

School of Manufacturing Engineering,Universiti Malaysia Perlis,02600Arau,Perlis,Malaysia c

Laboratory of Biocomposite Technology,Institute of Tropical Forestry and Forest Products (INTROP),Universiti Putra Malaysia,43400UPM Serdang,Selangor,Malaysia d

Department of Biological and Agricultural Engineering,Universiti Putra Malaysia,43400UPM Serdang,Selangor,Malaysia e

Aerospace Malaysia Innovation Center,Universiti Putra Malaysia,43400UPM Serdang,Selangor,Malaysia

a r t i c l e i n f o Article history:

Received 9May 2013Accepted 21June 2013

Available online 1July 2013Keywords:

Lignocellulosic composite Thin-walled parts Injection molding Volumetric shrinkage Warpage

a b s t r a c t

Reducing volumetric shrinkages and warpage during the injection molding process is a challenging prob-lem in the production of molded thin-walled parts.In this study,the injection molding of shallow,thin-walled parts (thickness 0.7mm),composed of lignocellulosic polymer composites (polypropylene (PP)+50wt%wood),was simulated.The volumetric shrinkages and warpage in the thin-walled parts were evaluated under different process conditions,with varying post-filling parameters,such as mold temperature,cooling time,packing pressure and packing time.The analysis showed that the cooling time and packing time had less of an effect on the shrinkage and warpage;nevertheless the optimal levels for both parameters are required in the molding process for the thin-walled part to achieve the best results.The volumetric shrinkage was lower near the gate than at the end-of-fill location along the flow path.The results also showed that the volumetric shrinkage correlates with the warpage measured on the molded part.The optimum parameters ranges is 40–45°C for the mold temperature;20–30s for cooling time;0.85from injection pressure (P inject )for packing pressure;and 15–20s for the packing time to achieve the best results with the least amount of volumetric shrinkage and warpage.

Ó2013Elsevier Ltd.All rights reserved.


The current trend in the industry is to produce thin,light weight,small,and green mon efforts to reduce material costs and increase productivity (parts per hour)include reducing the thickness of parts [1–4]and using low-cost materials,such as lignocellulosic fillers [5–10],to replace fiber glass fillers in thermoplastic composites.

Azaman et al.[7]found that the use of shallow,thin-walled parts is preferable for molding composite materials with lignocel-lulosic polymers because of the resulting low residual stress and warpage.The design of shallow,thin-walled parts are showed structurally more rigid than flat,thin-walled parts and can be pro-cessed more economically using less material.Rahman et al.[8]studied the processability of a window frame using rice husk poly-mer composites through simulation.Solid and hollow designs were created to compare the advantages and disadvantages of each design in terms of the filling phase,manufacturing cost,and design durability.

Sanadi et al.[11]reported the lignocellulosic thermoplastic composites generally must be processed at temperatures below 230°C to minimize fiber degradation.Processing at low tempera-tures causes flow difficulties for the polymer melt in the mold cav-ities,which can ultimately result in inconsistent volumetric shrinkage and warpage in the molded products,particularly in thin parts.

According to Jacques [12],the warpage of a molded part results from the asymmetrical stress distribution over the thickness of the part from unbalanced cooling.Therefore,the thinnest part is the most sensitive to warpage.Several methods have been proposed by Cheng et al.[13]to reduce the warpage of thin-walled parts by considering the part design shape,mold system design,cooling system,and molding parameters.In designing thin-walled parts,the mold temperature should be kept uniform in the cavities and cores to ensure a consistent cooling rate within the entire part.The mold temperature was found to have the greatest effect on the occurrence of warpage in thin-walled parts.Hence,reducing warpage is one of the top priorities to improve the quality of injec-tion-molded parts.

0261-3069/$-see front matter Ó2013Elsevier Ltd.All rights reserved. /10.1016/j.matdes.2013.06.047

⇑Corresponding author at:Department of Mechanical and Manufacturing Engineering,Universiti Putra Malaysia,43400UPM Serdang,Selangor,Malaysia.Tel.:+60389466318;fax:+60386567122.

E-mail (S.M.Sapuan).